Abstract
Shifts in plant functional groups associated with climate change have the potential to influence peatland carbon storage by altering the amount and composition of organic matter available to aquatic microbial biofilms. The goal of this study was to evaluate the potential for plant subsidies to regulate ecosystem carbon flux (CO2) by governing the relative proportion of primary producers (microalgae) and heterotrophic decomposers (heterotrophic bacteria) during aquatic biofilm development in an Alaskan fen. We evaluated biofilm composition and CO2 flux inside mesocosms with and without nutrients (both nitrogen and phosphorus), organic carbon (glucose), and leachates from common peatland plants (moss, sedge, shrub, horsetail). Experimental mesocosms were exposed to either natural sunlight or placed under a dark canopy to evaluate the response of decomposers to nutrients and carbon subsidies with and without algae, respectively. Algae were limited by inorganic nutrients and heterotrophic bacteria were limited by organic carbon. The quality of organic matter varied widely among plants and leachate nutrient content, more so than carbon quality, influenced biofilm composition. By alleviating nutrient limitation of algae, plant leachates shifted the biofilm community toward autotrophy in the light-transparent treatments, resulting in a significant reduction in CO2 emissions compared to the control. Without the counterbalance from algal photosynthesis, a heterotrophic biofilm significantly enhanced CO2 emissions in the presence of plant leachates in the dark. These results show that plants not only promote carbon uptake directly through photosynthesis, but also indirectly through a surrogate, the phototrophic microbes.
Similar content being viewed by others
Highlights
-
Plant subsidies governed autotrophic vs. heterotrophic microbial biofilm composition.
-
Biofilm composition determined the direction of net ecosystem CO2 exchange.
-
By alleviating nutrient limitation of algae, plant subsidies reduced CO2 emissions.
Introduction
Biofilms play an essential role in the structure and functioning of aquatic ecosystems. They have well-established influences on ecosystem processes, including aspects of nutrient cycling and energy flow (Hurst 2019; Halvorson and others 2020), and they form the basis of aquatic food webs (Ferguson and others 2021; Rober and others 2022). More recently, biofilms have become broadly recognized for their contribution to global carbon fluxes (Battin and others 2016; DelVecchia and others 2019; Jassey and others 2022). In stream networks, for example, biofilms are responsible for outgassing large amounts of carbon dioxide (CO2) to the atmosphere (Battin and others 2008; Raymond and others 2013). Depending on light availability, biofilms can also increase CO2 uptake, especially in shallow aquatic environments where biofilm primary production exceeds that of heterotrophic respiration (Hamard and others 2021a; Wyatt and others 2021; Jassey and others 2022). Many of these features are linked to biofilm structure (Battin and others 2016; Hamard and others 2021a, b) and the community of organisms that make up biofilms are sensitive to environmental change (Wyatt and others 2019). Therefore, environmental perturbations have the potential to influence aspects of ecosystem function by altering biofilm composition (for example, Sklar and others 2005), especially disturbance events that shift the biofilm community in favor of one trophic structure over another (Lougheed and others 2008; Myers and others 2021).
Biofilms are a complex community of microorganisms, which include both autotrophic and heterotrophic components that grow in close association on submerged substrata (Carr and others 2005; Scott and others 2008; Flemming and Wingender 2010). The autotrophic community is made up of microalgae (including cyanobacteria) that produce organic compounds (that is, leachates) during photosynthesis. Some of these compounds (for example, carbohydrates, amino acids) are released extracellularly where they are used as an energy source by the neighboring heterotrophs, namely bacteria and fungi (Cole 1982; Haack and McFeters 1982; Ylla and others 2009; Wyatt and Turetsky 2015; Halvorson and others 2019; Francoeur and others 2020). The heterotrophs in return release respiratory CO2 and break down organic nutrients into inorganic forms, which can be assimilated by microbial autotrophs (Daufresne and Loreau 2001; Kuehn and others 2014; Mesquita and others 2019). This reciprocal exchange of resources is assumed to be a primary reason for the close association (that is, microbial coupling) between autotrophs and heterotrophs in aquatic biofilms (Daufresne and Loreau 2001).
The level of microbial coupling within aquatic biofilms is not constant but instead, depends to a large extent on resource availability (Kalscheur and others 2012; Koedooder and others 2019). This is due, in part, to the asymmetrical arrangement between producers and decomposers where producers rely on decomposers to recycle nutrients and decomposers rely on producers for carbon energy but both compete for the same inorganic nutrients (Cotner and Wetzel 1992; Daufresne and Loreau 2001; Danger and others 2007). Although decomposers are better competitors for inorganic nutrients than producers (Rhee 1972; Currie and Kalff 1984a,b; Jansson 1993; Joint and others 2002; Liu and others 2012), both producers and decomposers can continue to coexist even in low nutrient environments (Scott and Doyle 2006; Scott and others 2008). However, when external carbon supplies are available, decomposers tend to outcompete producers for available nutrients (Joint and others 2002; Hasegawa and others 2005; Klug 2005; Stets and Cotner 2008; Bechtold and others 2012; Myers and others 2021), indicating that carbon limitation is a precursor to prevent competitive exclusion in low nutrient environments (Wyatt and others 2019).
Our knowledge of microbial interactions within wetlands is lacking compared to most other environments (Battin and others 2016; Bechtold and others 2012; Wagner and others 2017; Ozersky and others 2018; Wyatt and others 2019; Halvorson and others 2020). This knowledge gap is particularly evident in peatlands, a common landscape feature at northern latitudes (Kolka and others 2018). Northern peatlands have relatively low nutrient availability and plants such as mosses that can tolerate those conditions produce a large fraction of annual biomass and tend to form litter that decomposes slowly. Over time, this imbalance between primary production and decomposition leads to the accumulation of organic matter as peat. Within this context, plant necromass operates primarily as an agent of carbon storage. Although this paradigm may well represent dry conditions, it does not fully represent periods of time when peatlands are inundated with water. In these conditions (that is, a wet phase), an autotrophic biofilm develops on peat surface layers that contributes significantly to ecosystem carbon uptake (Wyatt and others 2021). Plants, instead of acting solely as agents of carbon storage, have the potential to facilitate decomposition by providing carbon subsidies that disrupt microbial coupling, shifting the metabolic balance in favor of heterotrophy (Robroek and others 2016; Sahar and others 2022). Given that some plant subsidies are more labile to heterotrophs than others (Wickland and others 2007; Rupp and others 2019; Sahar and others 2022), the ability for plants to facilitate microbial activity may depend on plant community composition, which varies among peatlands and is susceptible to environmental change (Dorrepaal and others 2007; Dieleman and others 2014; Churchill and others 2015).
Our primary research objective was to evaluate the extent to which organic carbon subsidies from plant communities may govern ecosystem CO2 flux by regulating the composition of microbial biofilms in northern peatlands. To do this, we used a combination of nutrient and organic matter manipulations to examine how dissolved organic matter released by plants regulate carbon losses from the system. Instead of using the plants directly, we developed a method for lyophilizing (that is, freeze drying) plant leachates that were added to the water column using slow-release agar pellets. We hypothesized that plant subsidies regulate carbon flux by determining the relative proportion of autotrophic and heterotrophic components within the aquatic microbial community (Figure 1). We predicted that: (1) Shifts in plant composition that favor labile carbon subsidies facilitate CO2 emissions by promoting heterotrophic activity and reducing algal photosynthesis (uncoupling producer–consumer co-dependency) within the aquatic biofilm. (2) Alternatively, nutrient-rich plant subsidies could alleviate biofilm nutrient limitation and increase algal biomass and subsequently increase CO2 uptake.
Materials and Methods
Study Site and Experimental Design
This study was conducted in a fen peatland located in central interior Alaska, approximately 35 km southwest of Fairbanks (64° 42 N, 148° 18 W). The fen is affiliated with the Alaska Peatland Experiment (APEX), a long-term study site positioned within the Tanana River floodplain just outside the Bonanza Creek Experimental Forest. This region of Alaska has a relatively short growing season (≤ 135 days/year) with more than 21 h of daylight during the study period in June (Hinzman and others 2006). Topography at the site is flat with no tree cover and the plant community is composed of a mixture of Sphagnum and brown moss species and emergent vascular plants (mainly Carex atherodes (Sprengel), Equisetum fluviatile (Linneaus), and Potentilla palustris (Linneaus). Water-table position at this site is highly variable, ranging from 0 to 45 cm above the peat surface (Ferguson and others 2021). Surface water concentrations of nitrate (NO3−) and phosphate (PO4−) are typically less than 23 and 5 µg l−1, respectively, and pH ranges from 5.5 to 6.9 (Rober and others 2014). Dissolved organic carbon (DOC) in the water column ranges between 15 and 50 mg l−1 (Gu and Wyatt 2016). The site has a permanently raised boardwalk that allows access to the fen complex with minimal disturbance.
Experimental Design
We evaluated biofilm composition and CO2 flux inside mesocosms with and without nutrients (both nitrogen and phosphorus), organic carbon (glucose), and leachates from common peatland plants (moss, sedge, shrub, horsetail). To quantify heterotrophic responses to resources with and without microbial autotrophs, experimental enclosures were exposed to either natural sunlight or dark treatments, respectively. The mesocosm experiment was conducted within an open-water area of the fen (approximately 120 m2 area). A total of 48 mesocosms, each 50 cm in diameter, were constructed by rolling welded wire mesh into a cylinder and then wrapping each cylinder with a thin layer (0.1 mm thick) of polyvinylidene film (ShurTech, Avon, OH) that transmitted greater than 90% of photosynthetically active radiation (Gu and Wyatt 2016). Mesocosms were evenly spaced throughout the fen and the bottom was pushed into the peat so that the open top extended approximately 10 cm above the water surface. The open-bottom design allowed for water inside enclosures to be in contact with the peat to maintain hydrologic connectivity. The open-top design allowed for the placement of a portable gas analyzer for measurements of ecosystem carbon flux (CO2) (Kane and others 2021). We deployed six, 5 × 5 cm unglazed ceramic tiles as a standard inorganic substratum for sampling biofilm composition inside treatment enclosures. Tiles were suspended attached to a wire shelf that could be repositioned to maintain a consistent depth of 10 cm below the water surface inside each enclosure (Rober and others 2011). Inorganic substrates were used so that we could evaluate aspects of carbon limitation on the biofilm community without the confounding effects of substrate composition.
Each mesocosm enclosure was randomly assigned to one of sixteen amendment treatments: nutrients (nitrogen and phosphorus, NP), glucose (G), a combination of NP and G (NPG), a mixture of Sphagnum and brown moss species leachates (hereafter moss treatment), sedge leachates (Carex spp.), shrub leachates (Potentilla spp.), horsetail leachates (Equisetum spp.), or a control (agar only), with and without sunlight (light-transparent and dark treatments, respectively), with three replicates for each treatment (n = 48 total mesocosms). The water column was constantly enriched with each resource subsidy by diffusing nutrients or plant leachates into the water column using a slow-release agar pellet (see detailed procedures below). The top and sides of the dark treatment enclosures were covered with a black shroud made of polyester fabric that blocked more than 99% of incoming PAR (hereafter dark treatments) and light-transparent treatment enclosures were left uncovered to allow for passage of ambient sunlight to evaluate the effects of plant subsidies on the heterotrophic biofilm community and ecosystem carbon flux with and without microbial autotrophs, respectively. Treatments were initiated after snowmelt in late May 2021 and maintained until water dropped below the peat surface in June.
Prior to the initiation of the study, plant leachates were collected from dried peatland plants and lyophilized (freeze dried) before use in diffusing agar pellets. Aboveground biomass of four vascular plant genera (Carex, Potentilla, Equisetum) and the top 10 cm of a mixture of Sphagnum and brown moss species were collected from peatlands near the study site and dried for 48 h at 60 °C in a drying oven. Leachates were produced by leaching dried plant material from each genus in beakers containing 1500 ml of nanopure water for 12 h. The resulting leachate was filtered through a fine metal sieve to remove large particulate debris and then through a 0.45-µm filter (VacuCap, Pall Life Sciences, Ann Arbor, MI, USA) before being lyophilized using a Labconco® Freezone 6 benchtop freeze dryer (Labconco, Kansas City, MO, USA) for approximately 48 h. The resulting coarse powdered dried leachate was then placed in sterile amber bottles and stored in a desiccator until use. Analysis of leachate powder was conducted to ascertain the chemical composition and concentration of dissolved organic matter in each plant leachate (Table 1).
We made nutrient-diffusing agar pellets using 60 ml polyethylene canisters filled with agar + 0.5 m KNO3 + 0.5 m KH2PO4 (NP treatment), agar + 0.5 m glucose (G treatment), agar + all three (NPG treatment), agar + one of four plant leachates (3 g l−1; moss, Carex, Potentilla, Equisetum) or a control with agar only (Rier and Stevenson 2002; Tank and others 2017). Our goal with NP and G enrichments was to alleviate resource limitation of the biofilm while maintaining environmentally relevant concentrations such as from the release of nutrients during re-flooding events (DeColibus and others 2017) or permafrost thaw (Abbott and others 2014). Leachate amendments were selected to be consistent with mineral enrichment and emulate background carbon and nutrient levels upon release. Further, we assumed that previously reported diffusion rates (Wyatt and others 2015) would be growth saturating because they exceeded levels shown to alleviate resource limitation of other wetland biofilms (Wyatt and Turetsky 2015) without inhibiting heterotrophic microbes (sensu Treseder 2008). We used a similar design as the common method used for the deployment of nutrient-diffusing substrates (Tank and others 2017) except that the agar pellets were removed from the plastic containers and allowed to diffuse directly into the water column. In the field, six agar pellets were removed from canisters so that the agar pellet was positioned on the peat surface within each respective mesocosm and allowed to diffuse continuously for 16 days (beginning on June 2) to allow for biofilm colonization. We confirmed during laboratory assays following methods described by Rugenski and others (2008) and Wyatt and others (2015) that nutrient pellets would diffuse continuously into solution during this timeframe. We expected this period of time would allow us to observe biofilm development that is characteristic of an ephemeral photic zone while also minimizing the potential for desiccation associated with variable hydrology observed within the larger fen complex (DeColibus and others 2017). The relative proportion of microalgae and heterotrophic bacteria were collected from tiles within mesocosm enclosures and processed for algal and heterotrophic biomass (see detailed procedures below). Measures of algal and heterotrophic biomass and autotrophic index (AI) were concurrent with measurements of ecosystem CO2 flux.
Sampling and Analytical Methods
Physiochemical conditions were measured within each mesocosm during biofilm and gas flux measurements (see detailed procedures below). Water depth (cm) was measured with a meter stick and measurements of water temperature (°C), pH, conductivity (µS), and dissolved oxygen (DO; mg l−1) were made with a Hach model 40d multiprobe (Hach Company, Loveland, CO, USA). Light was measured inside mesocosm enclosures at 12-h intervals (measured as lux and converted to µmol photons m−2 s−1 photosynthetically active radiation (PAR) according to the manufactures specifications) using data loggers (Onset Computer Corporation, Cape Cod, MA, USA). Water samples for dissolved nutrient analysis (NO3− and PO43−) and DOC were collected with a syringe and filtered through a 0.45-μm filter (Millipore Corporation, Bedford, MA, USA) into 60-ml acid-washed polyethylene bottles. Dissolved nutrient samples were stored on ice in the field and frozen until analysis using ion chromatography (Dionex Corporation, Sunnyvale, CA, USA). Dissolved organic carbon was analyzed using a Shimadzu TOC analyzer (Shimadzu Scientific Instruments, Columbia, MD, USA).
Following the completion of the study, the attached biofilm was removed from tile substrates with a toothbrush and the resulting slurry was homogenized and split for analysis of chlorophyll a, ash-free dry mass (AFDM), and bacterial cell density. Autotrophic biofilm colonization was quantified as chlorophyll a (a proxy for algal biomass) from a subsample collected on a 0.7-µm glass fiber filter (GF/F; Whatman, Maidstone, UK) following 24 h extraction with 90% ethanol in the dark. Chlorophyll a concentration was measured from the extract with a Cary 60 UV–Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) at 665 and 750 nm after acidification to correct for pheopigments (APHA 2005). A separate aliquot was poured into pre-weighed aluminum pans, dried at 105 °C for 24 h and then ashed at 500 °C for 1 h for measures of dry and ash mass, respectively, which were used to determine AFDM (APHA 2005). The remaining aliquot was preserved with a 2% formalin solution to quantify bacterial cell density. Sample aliquots were stained with 4′, 6-diamino-2-phenylindole (DAPI) (Porter and Feig 1980) and vacuum filtered onto a 0.2-µm pore-size black filter. A minimum of 300 cells or 25 fields were counted per filter at 1000× magnification using a Leica DM 4000 microscope with fluorescence. Bacterial biomass was calculated by a bacterial abundance/biomass conversion factor of 35 fg C cell−1 (Theil-Nielsen and Søndergaard 1998).
Autotrophic index (AI) was used to quantify the ratio of microbial autotrophs to heterotrophs among treatments (Steinman and others 2006). Autotrophic index was determined by dividing AFDM (a measure of the total autotrophic and heterotrophic biomass accumulated) by the concentration of chlorophyll a (a measure of algal biomass) using standard methods (APHA 2005). Lower values of the index indicate a higher proportion of autotrophy in the microbial community (Bechtold and others 2012).
Ecosystem CO2 flux was measured at the same time as biofilm collection using a CYP-4 canopy assimilation chamber (PP Systems, Amesbury, MA, USA) placed on a stainless-steel collar that was sealed with a neoprene gasket within each mesocosm (Wyatt and others 2021). The CO2 flux rate (µmol CO2 m−2 s−1) was calculated as the slope of the linear relationship between headspace CO2 concentration and time using a portable infrared gas analyzer (IRGA; PP Systems EGM-4, Amesbury, MA, USA). Net ecosystem exchange (NEE) was measured under ambient light conditions and positive NEE values indicated carbon release to the atmosphere while negative values indicated carbon uptake (Wyatt and others 2021).
Characterization of Plant Subsidies
Leaching experiments were conducted using aboveground biomass of moss, Carex, Potentilla, and Equisetum that had been dried at 60 °C (as described above). Dried plant material (1 g of each plant) was soaked in 1 l of nanopure water for 24 h at 7 °C (n = 4 for each plant type). The resulting extracts were filtered through syringe-driven 0.45 µm Whatman filters and characterized for nutrient content and dissolved organic matter (DOM) composition. Filtered samples were analyzed for dissolved nutrients (NO2−, NO3−, NH4+, PO43−, and K) with an ion chromatograph equipped with IonPac AS18-Fast and CS12A analytical columns for anions and cations, respectively (Dionex Corporation, Sunnyvale, CA, USA) and for DOC and total dissolved nitrogen (TDN) concentration with a Shimadzu TOC-V carbon analyzer with a TN unit (Shimadzu Scientific Instruments, Columbia, MD, USA). Dissolved organic N (DON) was calculated by subtracting measured inorganic N (NO2−, NO3−, and NH4+) from TDN.
Absorbance Measurements
Filtered samples were analyzed for ultraviolet absorption at 254 nm using an Agilent Cary 60 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). Specific UV absorbance at 254 nm (SUVA254) was calculated by dividing ultraviolet absorption at 254 nm by DOC concentration. SUVA254, reported in units of l mg C−1 m−1, gives an average molar absorptivity for all the molecules contributing to the DOC in a sample and has been shown to be a useful measure of aromatic content (Weishaar and others 2003) and molecular weight (Chowdhury 2013).
Spectral slopes (S) and spectral slope ratios (SR) were determined following Helms and others (2008) and Hansen and others (2016). Absorbance spectra were performed on filtered samples at room temperature in acid-cleaned 1 cm quartz cuvettes using an Agilent Cary 60 spectrophotometer with a dual beam and Xenon lamp with a 5 nm bandpass, and a 0.5 s integration time at wavelengths of 275–400 nm. To ensure that absorbance measurements were within the linear response range of the spectrophotometer (1 cm pathlength), samples with absorbance greater than 0.6 at 254 nm were diluted with nanopure water prior to absorption measurements. Absorbance units were converted to absorption coefficients as follows:
where a = absorption coefficient, A = absorbance, and l = path length (m).
Spectral slopes for the intervals of 275–295 nm (S275–295), 290–350 (S290–350), and 350–400 nm (S350–400) were calculated using linear regression of the log-transformed a spectra following Helms and others (2008). Slopes are reported as positive numbers. Higher (or steeper) slopes indicate a more rapid decrease in absorption with increasing wavelength and therefore lower values are generally indicative of higher molecular weight dissolved organic matter. The SR was calculated as the ratio of S275–295 to S350–400 (Helms and others 2008; Hansen and others 2016).
Statistical Analyses
Two-way general linear models (GLM) were used to evaluate the effect of subsidies with and without light on algal biomass, bacterial biomass, and CO2 emissions. Multivariate GLMs were used to evaluate differences among physiochemical conditions within mesocosm enclosures and in absorbance and chemical characteristics among plant subsidies. When GLM indicated significant differences among treatments, Tukey’s post hoc comparison of means tests were used to discriminate between treatments. Linear regression analysis was used to evaluate the relationship between biofilm composition (measured as AI) and net ecosystem CO2 exchange (NEE) among treatments. Statistical analyses were performed with SPSS 20 (IBM Statistics, Chicago, IL, USA).
Results
Peatland Physiochemical Conditions
Inside mesocosm enclosures (overall mean ± SD), water temperature (15.3 ± 1.15 °C; F7,32 = 0.30, P = 0.95), depth (17.5 ± 2.16 cm; F7,32 = 0.57, P = 0.77), pH (6.63 ± 0.23; F7,32 = 1.21, P = 0.32), NO3− (0.11 ± 0.04 mg l−1; F7,32 = 0.67, P = 0.70), and DOC (31.3 ± 3.28 mg l−1; F7,32 = 1.06, P = 0.41) were similar among treatments. The concentration of DO was higher in all light-transparent treatments (6.72 ± 1.15 mg l−1), where algal photosynthesis was elevated, compared to dark treatments (2.57 ± 1.11; P = 0.03). Conductivity (µS m−2 s−1) was elevated in all dark treatments enriched with plant subsidies (62.8 ± 2.04) or a combination of NPG (66.4 ± 0.90) compared to all other treatments (40.9 ± 4.78; P < 0.001). Phosphate was elevated in the NPG light treatment (0.84 ± 0.05 mg l−1) compared to all other treatments (P < 0.001), and was higher in both the NPG dark (0.34 ± 0.21 mg l−1) and NP light (0.31 ± 0.18 mg l−1) treatments compared to the light or dark control (P ≤ 0.02), but was similar among all treatments (0.18 ± 0.21 mg l−1). Photosynthetically active radiation (µmol photons m−2 s−1) was 5.86 ± 0.50 in dark and 276.4 ± 15.1 in light-transparent treatments.
Resource Limitation of the Biofilm
Algal biomass was limited by nutrients and bacterial biomass was limited by both nutrients and carbon (Figure 2A, B). In the control (without nutrient enrichment), algal biomass was less than 2.0 mg cm−2 chlorophyll a and was not significantly different in the presence of glucose (P = 0.45; Figure 2A). Algal biomass increased (fourfold increase in chlorophyll a) with nutrient enrichment (NP or NPG) compared to the control (P ≤ 0.001; Figure 2A). In the dark (without algal photosynthesis), organic carbon enrichment (glucose alone) stimulated a twofold increase in bacterial biomass compared to the control (P ≤ 0.001; Figure 2B). The effect of nutrients alone on bacterial biomass was not significantly different compared to the control (P = 0.13). The combined effect of nutrients and glucose (NPG treatment) nearly doubled bacterial biomass compared to glucose alone (P ≤ 0.001). Bacterial biomass in the NPG treatment was fourfold greater compared to the control (P ≤ 0.001; Figure 2B). Heterotrophic bacteria outcompeted algae (elevated AI; Figure 3) when organic carbon was supplemented and nutrient levels remained low (G light treatment). This effect was overturned (reduced AI) when organic carbon and nutrients were both elevated (NPG light treatment), owing to a subsequent increase in algal biomass in the absence of nutrient limitation (Figure 3).
Characterization of Plant Subsidies
Equisetum contributed the greatest amount of DOC (mean ± SD; 125.7 ± 0.16 mg l−1) to the DOM pool, followed closely by Potentilla (111.3 ± 1.31 mg l−1; Table 1). Carex contributed approximately half the amount of DOC (69.5 ± 2.62 mg l−1) to the DOM pool as Equisetum or Potentilla, while moss contributed the least (27.9 ± 0.46 mg l−1). Despite releasing large amounts of DOC, the composition of Equisetum carbon subsidies were aromatic (SUVA254 = 1.78 ± 0.008 l mg C−1 m−1) and high molecular weight (lowest SR), indicative of recalcitrant carbon compounds. However, Equisetum subsidies were the most nutrient-rich (PO43−, TDN, and K) of all the plant types and the majority of the N pool was DON (~ 54%) and approximately 33% NO3− and 14% NH4+. Potentilla subsidies had the highest SUVA254 (3.10 ± 0.02 l mg C−1 m−1) compared to all other plant types suggesting high aromaticity, but the spectral slope (S) values and SR were indicative of low molecular weight compounds (Potentilla subsidies were composed of low molecular weight aromatic compounds). Potentilla subsidies were rich in PO43−, K, and the majority (~ 75%) of the N pool was comprised of NO3− and the remaining 25% was NH4+. Despite the small amount of DOC released by moss, the composition of the carbon was the least aromatic (SUVA254 0.96 ± 0.02 l mg C−1 m−1) and lowest molecular weight (SR), suggesting it was a small but labile subsidy (Table 1). Moss subsidies were more depleted in nutrients (PO43− and TDN) compared to the other plant types (highest N:P), but the N pool was comprised of nearly equal parts NO3−, NH4+ and DON. Carex subsidies had intermediate aromaticity (SUVA254) and molecular weight (SR) compared to all other plant types (lower than Potentilla and Equisetum, but higher than moss). Carex subsidies were rich in PO43− and TDN (lowest N:P) with the N pool comprised almost entirely (89%) of DON and no more than 5% of each NO3−, NH4+ and NO2−, yet was the only plant subsidy with NO2− concentrations above detection (Table 1).
Influence of Plant Subsidies on Biofilm Composition and CO2 Emissions
In the dark (without algal photosynthesis), bacterial biomass increased across a gradient of plant subsidies (F4,10 = 51.3, P ≤ 0.0001; Figure 4A). Moss subsidies had the least effect followed by Potentilla (shrubs), Equisetum (horsetails), and Carex (sedges) (Figure 4A–C, Table 1). As expected (because there were no algae present in the dark), AI was elevated (more heterotrophic) among all treatments compared to the control (F4,10 = 11.0, P = 0.001) and increased along the gradient of plant subsidies in-step with bacterial biomass (Figure 4B). Consequently, CO2 emissions to the atmosphere followed a similar trend and increased along the same gradient (F4,10 = 8.35, P = 0.003; Figure 4C). All dark treatments were a source of CO2 to the atmosphere, but CO2 emissions were not different from the control in treatments where plant subsidies were low in both dissolved nutrients and organic matter (that is, moss; P = 0.80), and increased in treatments with greater concentrations of both nutrients and organic matter, such as Equisetum or Carex (P ≤ 0.003; Figure 4C, Table 1).
By alleviating nutrient limitation of algae, plant leachates promoted elevated levels of chlorophyll a compared to the control (F4,10 = 16.8, P ≤ 0.001; Figure 5A), thereby shifting the biofilm community toward autotrophy (low autotrophic index) in the light-transparent treatments (F4,20 = 11.2, P ≤ 0.001; Figure 5B). Enhanced autotrophy promoted CO2 uptake among all treatments relative to the control (F4,10 = 146.9, P ≤ 0.001; Figure 5C). Treatments with the highest levels of algal biomass (Potentilla and Carex) were a sink of CO2 from the atmosphere (Figure 5C), neutralizing the effects of heterotrophs on autotrophic index and CO2 emissions (Figure 4C).
Relationship Between Net Ecosystem Exchange and Biofilm Composition
Biofilm composition determined the direction of CO2 exchange (r2 = 0.56, F1,47 = 57.9, P ≤ 0.0001; Figure 6). Treatments with lower measures of AI indicated a more autotrophic biofilm and corresponded with lower values of NEE (greater CO2 uptake) than treatments with a more heterotrophic biofilm (Figure 6). Among light treatments, plant subsidies had a stronger influence on biofilm composition than nutrient enrichment (F7,32 = 35.7, P ≤ 0.001), but only biofilms grown in the presence of Carex or Potentilla subsidies resulted in CO2 uptake (negative NEE values). The remaining treatments enriched with plant subsidies (Equisetum or moss) were still a source of CO2 to the atmosphere, but had significantly reduced CO2 emissions compared to the control (P ≤ 0.01) or in the absence of algal photosynthesis (dark treatments) (F1,32 = 276.5, P ≤ 0.001). In the dark, all forms of subsidies (plants, carbon, or nutrients) promoted more heterotrophy (higher AI and greater NEE) than the control (F7,32 = 7.83, P ≤ 0.001) but the degree of heterotrophy was determined by a gradient of resource quality (Figure 6). For example, biofilms subsidized by Carex, Equisetum, or a combination of nutrients and glucose (NPG) were the most heterotrophic and therefore released the greatest amount of CO2 to the atmosphere (P ≤ 0.003). However, biofilms subsidized by Potentilla, moss, glucose, or nutrients alone (NP) were not significantly different from the control (P ≤ 0.76; Figure 6).
Discussion
By regulating the availability of limiting resources, plants structured peatland biofilm composition, which in turn governed CO2 flux. Previous research within this larger peatland complex has demonstrated that (1) autotrophic components of the biofilm are typically limited by nutrients (nitrogen and phosphorus in combination) (Wyatt and others 2015) and the heterotrophic biofilm is limited by organic carbon (Wyatt and Turetsky 2015), and (2) heterotrophic bacteria are able to outcompete algae for available nutrients in the presence of carbon enrichment when nutrient levels are low but not when both nutrients and organic carbon are elevated simultaneously (Myers and others 2021). Similarly, studies in other aquatic environments have demonstrated that heterotrophic bacteria have an affinity for nutrients and can outcompete algae for available nutrients when carbon requirements are met by outside sources (Klug 2005; Stets and Cotner 2008; Bechtold and others 2012; Wyatt and others 2019). Given that plants release compounds that are rich in carbohydrates (Farjalla and others 2009; Hansen and others 2016; Rupp and others 2019), we expected that plant leachates would promote a heterotrophic aquatic biofilm and elevate ecosystem CO2 emissions. Instead of favoring heterotrophy, the biofilm community shifted toward autotrophy in the presence of plant leachates in the light, resulting in a significant reduction in net ecosystem exchange compared to dark treatments without algae. It is worth noting that while we used aboveground plant biomass in this study (thereby replicating the organic matter pool most readily available to the aquatic biofilm), the chemical composition of plant leachates likely differs from root leachates (Weinhold and others 2021). As such, the effect of plant subsidies on biofilm composition observed here may have been different in comparison to root leachates which tend to be more easily degradable (Robroek and others 2016). This finding is relevant as many low-lying landscapes in northern regions are expected to become wetter (with a saturated photic zone) in the future (Douglas and others 2020; Jorgenson and others 2020). Our results suggest that wet surfaces may be buffered against net heterotrophy (carbon loss to the atmosphere) by algae. The extent to which this occurs will likely depend on the composition of resources delivered to surface waters with future thawing (Abbott and others 2014; Wickland and others 2018) as well as the changing physical aspects of northern peatlands (Freeman and others 2004; Fenner and others 2007), including light attenuation associated with elevated levels of dissolved organic matter (Gu and Wyatt 2016).
By alleviating nutrient limitation, plants eliminated competitive exclusion by heterotrophs and shifted the biofilm in favor of autotrophy. Results from our organic carbon (glucose) enrichment supported this hypothesis, showing that the biofilm community shifted toward heterotrophy in the presence of carbon enrichment alone but enrichment with both nutrients and organic carbon promoted an autotrophic biofilm. We have observed in previous studies that aquatic biofilms tend to take up carbon even when there is a substantial heterotrophic presence unless microbial autotrophs are absent (Wyatt and others 2021), such as the case in dark environments or if the dissolved organic matter pool is deficient in nutrients (Wyatt and others 2019; Myers and others 2021). Interestingly, bacterial biomass was slightly elevated in light treatments (with algae) than in the dark (without algae). These differences in heterotrophic biofilm responses to plant subsidies between light and dark treatments (that is, higher bacterial biomass in the light than the dark) indicate possible mutualistic interactions (microbial coupling) between producers and decomposers in the light. The ability for microbial autotrophs to coexist with heterotrophs in the presence of elevated nutrient and carbon availability has been demonstrated in previous studies (Myers and others 2021) and may provide evidence of negative priming, whereby heterotrophic microbes use labile carbon from algal leachates for biomass accrual instead of toward decomposition (Halvorson and others 2019). Nevertheless, reduced CO2 emissions in the light (despite elevated bacterial biomass) revealed that microbial autotrophs had the competitive advantage within the biofilm matrix.
By blocking algae growth in the dark, we were able to observe the effects of plant subsidies on the heterotrophic biofilm and CO2 flux without the counterbalance from algal photosynthesis. In the dark, without algae, plant leachates (particularly Equisetum and Carex) promoted a heterotrophic biofilm and CO2 emissions to the atmosphere. We observed a similar increase in heterotrophic biofilm development following enrichment with a combination of nutrients and glucose (NPG treatment) in the dark. Our results are similar to other studies, showing that conditions of greater nutrient availability can stimulate organic matter decomposition in northern peatlands (Bragazza and others 2006; Bubier and others 2007). Our work also further demonstrates that elevated CO2 emissions associated with enhanced decomposition (in dark treatments) can be offset by autotrophic biofilm development (in light treatments). In a previous study simulating nutrient release from permafrost thaw, we estimated that nutrient enrichment would increase peatland CO2 emissions by approximately 300 g CO2 m−2 y−1 in the absence of a counterbalance from algal photosynthesis (Wyatt and others 2021). The magnitude of this effect is similar to our current results and highlights the importance of algae as regulators of net ecosystem carbon exchange during wet periods in northern peatlands. Similarly, periods of drought can increase nutrient mineralization, which provides a nutrient subsidy when water-table position is restored (legacy effects of drainage; DeColibus and others 2017). This has also been correlated with algal abundance and increased carbon uptake in this rich fen ecosystem (Kane and others 2021). Considering that open-water areas of northern peatlands release between 23 and 419 g CO2 m−2 y−1 (Waddington and Roulet 2000; Pelletier and others 2014), the presence or absence of autotrophic biofilms could determine whether an individual peatland is a carbon source or sink and possibly offset carbon loss from warming (for example, Jassey and others 2022).
Nutrient availability and organic matter quality varied among peatland plants and leachates from some plant species were more easily metabolized by the microbial community than others. Previous studies have shown that the composition of organic matter varies among peatland plant functional groups (Wickland and others 2007; Ward and others 2010; Rupp and others 2019), and some leachates are rich in carbohydrates, which can create hotspots of heterotrophic microbial activity (Findlay and others 1986; Farjalla and others 2009; Sahar and others 2022). Here, we found that nutrient availability played a stronger role than organic matter composition in regulating biofilm composition and some plant leachates (for example, Carex and Potentilla) promoted more autotrophy than others. Collectively, plant subsidies increased the level of autotrophy (lower autotrophic index values) compared to the control. In the light, the highest autotrophic index values (more heterotrophy) were associated with horsetail (Equisetum) leachates, which had high nutrient content (N and P) and released the highest concentration of DOC (providing nutrients and carbon in combination to the microbial biofilm). This is consistent with previous research showing that horsetail have the ability to transport essential nutrients, such as phosphorus, from the subsurface to their tissues, which are subsequently leached into the organic matter pool (Marsh and others 2000; Rupp and others 2019). By comparison, sedge (Carex) and shrub (Potentilla) leachates were rich in N and P but released lower concentrations of DOC, perhaps explaining greater autotrophy and subsequently CO2 uptake in these treatments (in the light). This finding is interesting given that sedges and shrubs are favored under opposing hydrologic conditions, flooding and drought, respectively (Churchill and others 2015; McPartland and others 2019). The fact that these different plant functional groups both promoted autotrophy may indicate resilience in the carbon sink strength of this system (Olefeldt and others 2017; Euskirchen and others 2020; Kane and others 2021). Therefore, changes in plant communities, such as those expected with climate change, have the potential to influence ecosystem carbon flux by altering the amount and composition of resources available for biofilm microorganisms.
Conclusions
Despite covering less than 5% of the global land area, northern peatlands store about 20% of the world’s soil organic carbon (Yu 2012). The ability for northern peatlands to store carbon in the future is uncertain as carbon flux rates are sensitive to environmental change and can vary from one year to the next depending on the balance between rates of primary production and decomposition (Frolking and others 2014; Evans and others 2021; Loisel and others 2021). There is a growing concern that climate change may alter environmental conditions in a way that favors carbon release, with subsequent climate feedback implications (Ward and others 2009; Gallego-Sala and others 2018; Treat and others 2021). For example, altered hydrologic regimes associated with ongoing climate change are causing shifts in plant functional groups across northern peatland landscapes (Laiho and others 2003; Pedrotti and others 2014; McPartland and others 2019). Most notable has been an increase in shrub cover (for example, Potentilla) in response to drier conditions, with subsequent reductions in moss dominance (for example, Sphagnum) (Fenner and others 2007; Dieleman and others 2014; Churchill and others 2015; Hobbie and others 2017), while wetter conditions favor increased sedge cover (for example, Carex) (Potvin and others 2015; Rupp and others 2019). Owing to the recalcitrant characteristics of mosses which are fundamental to carbon sequestration (Turetsky and others 2012), reductions in moss cover are anticipated to decrease long-term peatland carbon storage capacity (Jassey and others 2013; Ward and others 2013). While much work has been done to better understand the factors driving peatland carbon release, most studies have focused on abiotic factors, such as temperature and soil moisture (Ward and others 2009; Euskirchen and others 2020). Comparatively, the ways in which secondary or biotic factors (for example, vegetation composition) influence ecosystem carbon flux have been less explored. This is especially true for periods of time when peatlands are inundated with water and the biotic community includes microalgae (Wyatt and others 2012; Rober and others 2013; DeColibus and others 2017; Myers and others 2021; Wyatt and others 2021). By exposing the aquatic biofilm to a range of plant subsidies (decoupled from plant photosynthesis), we show that common peatland plants can govern ecosystem CO2 flux by alleviating resource limitation of the aquatic microbial community. We anticipated that the most labile carbon subsidies from plants would accelerate CO2 emissions by shifting the microbial community toward heterotrophy. Instead, most plant leachates reduced CO2 emissions by alleviating nutrient limitation of primary producers. In doing so, our results show that plants not only promote carbon uptake directly through photosynthesis, but they can also promote carbon uptake indirectly by stimulating carbon uptake by phototrophic microbes.
Data Availability
Data from this manuscript are available at http://www.lter.uaf.edu.
References
Abbott BW, Larouche JR, Jones JB, Bowden WB, Balser AW. 2014. Elevated dissolved organic carbon biodegradability from thawing and collapsing permafrost. J Geophys Res Biogeosci 89:65. https://doi.org/10.1002/2014JG002678.
APHA (American Public Health Association). 2005. Standard methods for the examination of water and wastewater. Washington, D.C.: American Public Health Association.
Battin TJ, Kaplan LA, Findlay S, Hopkinson CS, Marti E, Packman AI, Newbold JD, Sabater F. 2008. Biophysical controls on organic carbon fluxes in fluvial networks. Nat Geosci 1:95–101.
Battin TJ, Besemer K, Bengtsson MM, Romani AM, Packmann AI. 2016. The ecology and biogeochemistry of stream biofilms. Nat Rev Microbiol 14:251–63.
Bechtold HA, Marcarelli AM, Baxter CV, Inouye RS. 2012. Effects of N, P, and organic carbon on stream biofilm nutrient limitation and uptake in a semi-arid watershed. Limnol Oceanogr 57:1544–54.
Bragazza L, Freeman C, Jones T, Rydin H, Limpens J, Fenner N, Ellis T, Gerdol R, Hajek M, Lacumin P, Kutnar L, Tahvanainen T, Toberman H. 2006. Atmospheric N deposition promotes carbon loss from peat bogs. Proc Natl Acad Sci 103:19386–9.
Bubier JL, Moore TR, Bledzki LA. 2007. Effects of nutrient addition on vegetation and carbon cycling in an ombrotrophic bog. Glob Change Biol 13:1168–86.
Carr GM, Morin A, Chambers PA. 2005. Bacteria and algae in stream periphyton along a nutrient gradient. Freshw Biol 50:1337–50.
Chowdhury S. 2013. Trihalomethanes in drinking water: effect of natural organic matter distribution. Water SA 39:1–7.
Churchill AC, Turetsky MR, McGuire AD, Hollingsworth TN. 2015. Response of plant community structure and primary productivity to experimental drought and flooding in an Alaskan fen. Can J For Res 45:185–93.
Cole JJ. 1982. Interactions between bacteria and algae in aquatic ecosystems. Annu Rev Ecol Syst 13:291–314.
Cotner JB, Wetzel RG. 1992. Uptake of dissolved inorganic and organic phosphorus compounds by phytoplankton and bacterioplankton. Limnol Oceanogr 37:232–43.
Currie DJ, Kalff J. 1984a. A comparison of the abilities of freshwater algae and bacteria to acquire and retain phosphorus. Limnol Oceanogr 29:298–310.
Currie DJ, Kalff J. 1984b. Can bacteria outcompete phytoplankton for phosphorus? A chemostat test. Microb Ecol 10:205–16.
Danger M, Leflaive J, Oumarou C, Ten-Hage L, Lacroix G. 2007. Control of phytoplankton–bacteria interactions by stoichiometric constraints. Oikos 116:1079–86.
Daufresne T, Loreau M. 2001. Ecological stoichiometry, primary producer-decomposer interactions, and ecosystem persistence. Ecology 82:3069–82.
DeColibus DT, Rober AR, Sampson AM, Shurzinske AC, Walls JT, Turetsky MR, Wyatt KH. 2017. Legacy effects of drought alters the aquatic food web of a northern boreal peatland. Freshw Biol 62:1377–1388.
DelVecchia AG, Balik JA, Campbell SK, Taylor BW, West DC, Wissinger SA. 2019. Carbon dioxide concentrations and efflux from permanent, semi-permanent, and temporary subalpine ponds. Wetlands 30:955–69.
Dieleman CM, Branfireun BA, McLaughlin JW, Lindo Z. 2014. Climate change drives a shift in peatland ecosystem plant community: implications for ecosystem function and stability. Glob Change Biol 21:388–95.
Dorrepaal E, Cornelissen JHC, Aerts R. 2007. Changing leaf litter feedbacks on plant production across contrasting sub-arctic peatland species and growth forms. Oecologia 151:251–61.
Douglas TA, Turetsky MR, Koven CD. 2020. Increased rainfall stimulates permafrost thaw across a variety of Interior Alaskan boreal ecosystems. NPJ Clim Atmos Sci 3:1–7.
Euskirchen ES, Kane ES, Edgar CW, Turetsky MR. 2020. When the source of flooding matters: divergent responses in carbon fluxes in an Alaskan rich fen to two types of inundation. Ecosystems 23:1138–53.
Evans CD, Peacock M, Baird AJ, Artz RR, Burden A, Callaghan N, Chapman PJ, Cooper HM, Coyle M, Craig E, Cumming A. 2021. Overriding water table control on managed peatland greenhouse gas emissions. Nature 593(7860):548–52.
Farjalla VF, Marinho CC, Faria BM, Amado AM, de Esteves F, A, Bozelli RL, Giroldo G. 2009. Synergy of fresh and accumulated organic matter to bacterial growth. Microb Ecol 57:657–66.
Fenner N, Freeman C, Lock MA, Harmens H, Reynolds R, Sparks T. 2007. Interactions between elevated CO2 and warming could amplify DOC exports from peatland catchments. Environ Sci Technol 41:3146–52.
Ferguson HM, Slagle EJ, McCann AA, Walls JT, Wyatt KH, Rober AR. 2021. Greening of the boreal peatland food web: periphyton supports secondary production in northern peatlands. Limnol Oceanogr 66:1743–58.
Findlay S, Carlough L, Crocker MT, Gill HK, Meyer JL, Smith PJ. 1986. Bacterial growth on macrophyte leachate and the fate of bacterial production. Limnol Oceanogr 31:1335–41.
Flemming HC, Wingender J. 2010. The biofilm matrix. Nat Rev Microbiol 8:623–33.
Francoeur SN, Neely RK, Underwood S, Kuehn KA. 2020. Temporal and stoichiometric patterns of algal stimulation of litter-associated heterotrophic microbial activity. Freshw Biol 65:1223–38.
Freeman C, Fenner N, Ostle NJ, Kang H, Dowrick DJ, Reynolds B, Lock MA, Sleep D, Hughes S, Hudson J. 2004. Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels. Nature 430:195–8.
Frolking S, Talbot J, Subin ZM. 2014. Exploring the relationship between peatland net carbon balance and apparent carbon accumulation rate at century to millennial time scales. Holocene 24(9):1167–1173.
Gallego-Sala A, Charman D, Brewer S, Page S, Prentice C, Friedlingstein P, Moreton S, Amesbury M, Beilman D, Björck S, Blyakharchuk T, Bochicchio C, Booth R, Bunbury J, Camill P, Carless D, Chimner R, Clifford M, Cressey E, Courtney-Mustaphi C, De Vleeschouwer F, de Jong R, Fialkiewicz-Koziel B, Finkelstein S, Garneau M, Githumbi E, Hribjlan J, Holmquist J, Hughes P, Jones C, Jones M, Karofeld E, Klein E, Kokfelt U, Korhola A, Lacourse T, Le Roux G, Lamentowicz M, Large D, Lavoie M, Loisel J, Mackay H, MacDonald G, Makila M, Magnan G, Marchant R, Marcisz K, Martínez Cortizas A, Massa C, Mathijssen P, Mauquoy D, Mighall T, Mitchell F, Moss P, Nichols J, Oksanen P, Orme L, Packalen M, Robinson S, Roland T, Sanderson N, Sannel A, Silva-Sánchez N, Steinberg N, Swindles G, Turner T, Uglow J, Väliranta M, van Bellen S, van der Linden M, van Geel B, Wang G, Yu Z, Zaragoza-Castells J, Zhao Y. 2018. Latitudinal limits to the predicted increase of the peatland carbon sink with warming. Nat Clim Change 8:907–13.
Gu L, Wyatt KH. 2016. Light availability limits the response of algae and heterotrophic bacteria to elevated nutrient levels and warming in a northern boreal peatland. Freshw Biol 61:1442–53.
Haack TK, McFeters GA. 1982. Nutritional relationships among microorganisms in an epilithic biofilm community. Microb Ecol 8:115–26.
Halvorson HM, Barry JR, Lodato MB, Findlay RH, Francoeur SN, Kuehn KA. 2019. Periphytic algae decouple fungal activity from leaf litter decomposition via negative priming. Funct Ecol 33:188–201.
Halvorson HM, Wyatt KH, Kuehn KH. 2020. Ecological significance of autotrophic-heterotrophic microbial interactions in freshwaters. Freshw Biol 65:1183–8.
Hamard S, Céréghino R, Barret M, Sytiuk A, Lara E, Dorrepaal E, Kardol P, Küttim M, Lamentowicz M, Leflaive J, le Roux G, Tuittila E, Jassey VE. 2021a. Contribution of microbial photosynthesis to peatland carbon uptake along a latitudinal gradient. J Ecol 109:3424–41.
Hamard S, Küttim M, Céréghino R, Jassey VE. 2021b. Peatland microhabitat heterogeneity drives phototrophic microbes distribution and photosynthetic activity. Environ Microbiol 23:6811–27.
Hansen AM, Kraus TEC, Pellerin BA, Fleck JA, Downing BD, Bergamaschi BA. 2016. Optical properties of dissolved organic matter (DOM): effects of biological and photolytic degradation. Limnol Oceanogr 61:1015–32.
Hasegawa T, Fukuda H, Koike I. 2005. Effects of glutamate and glucose on N cycling and the marine plankton community. Aquat Microb Ecol 41:125–30.
Helms JR, Stubbins A, Ritchie JD, Minor EC, Kieber DJ, Mopper K. 2008. Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter. Limnol Oceanogr 53(3):955–69.
Hinzman LD, Viereck LA, Adams PC, Romanovsky VE, Yoshikawa K. 2006. Climate and permafrost dynamics of the Alaskan boreal forest. In: Chapin FS, Oswood MW, Van Cleve K, Viereck LA, Verbal DL, Eds. Alaska’s changing boreal forest. Oxford University Press. pp 39–61.
Hobbie JE, Shaver GR, Rastetter EB, Cherry JE, Goetz SJ, Guay KC, Gould WA, Kling GW. 2017. Ecosystem responses to climate change at a low arctic and a high arctic long-term research site. Ambio 46:160–173. https://doi.org/10.1007/s13280-016-0870-x
Hurst CJ. 2019. The structure and function of aquatic microbial communities. Advances in Environmental Microbiology 7, Springer Nature Switzerland AG
Jansson M. 1993. Uptake, exchange, and excretion of orthophosphate in phosphate-starved Scenedesmus quadricauda and Pseudomonas. Limnol Oceanogr 38:1162–78.
Jassey VE, Chiapusio G, Binet P, Buttler A, Laggoun-Défarge F, Delarue F, Bernard N, Mitchell EA, Toussaint ML, Francez AJ. 2013. Above-and belowground linkages in Sphagnum peatland: climate warming affects plant-microbial interactions. Glob Change Biol 19:811–23.
Jassey VE, Walcker R, Kardol P, Geisen S, Heger T, Lamentowicz M, Hamard S, Lara E. 2022. Contribution of soil algae to the global carbon cycle. New Phytol 234:64–76.
Joint I, Henriksen P, Fonnes GA, Bourne D, Thingstad TF, Riemann B. 2002. Competition for inorganic nutrients between phytoplankton and bacterioplankton in nutrient manipulated mesocosms. Aquat Microb Ecol 29:145–59.
Jorgenson MT, Douglas TA, Liljedahl AK, Roth JE, Cater TC, Davis WA, Frost GV, Miller PF, Racine CH. 2020. The roles of climate extremes, ecological succession, and hydrology in repeated permafrost aggradation and degradation in fens on the Tanana Flats. J Geophys Res Biogeosci. https://doi.org/10.1029/2020JG005824.
Kalscheur KN, Rojas M, Peterson CG, Kelly JJ, Gray KA. 2012. Algal exudates and stream organic matter influence the structure and function of denitrifying bacterial communities. Microb Ecol 64:881–92.
Kane ES, Dieleman CM, Rupp D, Wyatt KH, Rober AR, Turetsky MR. 2021. Consequences of increased variation in peatland hydrology for carbon storage: Legacy effects of drought and flooding in a boreal fen ecosystem. Front Earth Sci 8:577746.
Klug JL. 2005. Bacterial response to dissolved organic matter affects resource availability for algae. Can J Fish Aquat Sci 62:472–81.
Koedooder C, Stock W, Willems A, Mangelinckx S, De Troch M, Vyverman W, Sabbe K. 2019. Diatom-bacteria interactions modulate the composition and productivity of benthic diatom biofilms. Front Microbiol 10:1–11.
Kolka R, Trettin C, Tang W, Krauss K, Bansal S, Drexler J, Wickland K, Chimner R, Hogan D, Pindilli EJ, Benscoter B, Tangen B, Bridgham ES, Richardson C. 2018. Terrestrial wetlands. Cavallaro N, Shrestha G, Birdsey R, Mayes MA, Najjar RG, Reed SC, Romero-Lankao P, Zhu Z, Eds. Second state of the carbon cycle report: a sustained assessment report. Washington, DC: U.S. Global Change Research Program. pp 507–567
Kuehn KA, Francoeur SN, Findlay RH, Neely RK. 2014. Priming in the microbial landscape: periphytic algal stimulation of litter-associated microbial decomposers. Ecology 95:749–62.
Laiho R, Vasander H, Penttilä T, Laine J. 2003. Dynamics of plant-mediated organic matter and nutrient cycling following water-level drawdown in boreal peatlands. Glob Biogeochem Cycles. https://doi.org/10.1029/2002GB002015.
Liu H, Zhou Y, Xiao W, Ji L, Cao X, Song C. 2012. Shifting nutrient-mediated interactions between algae and bacteria in a mesocosm: evidence from alkaline phosphatase assay. Microbiol Res 167:292–8.
Loisel J, Gallego-Sala AV, Amesbury MJ, Magnan G, Anshari G, Beilman D, Benavides JC, Blewett J, Camill P, Charman DJ, Chawchai S, Hedgpeth A, Kleinen T, Korhola A, Large D, Mansilla CA, Müller J, van Bellen S, West JB, Yu Z, Bubier J, Garneau M, Moore T, Sannel ABK, Page S, Väliranta M, Bechtold M, Brovkin V, Cole LES, Chanton JP, Christensen TR, Davies MA, De Vleeschouwer F, Finkelstein SA, Frolking S, Gałka M, Gandois L, Girkin N, Harris L, Heinemeyer A, Hoyt AM, Jones MC, Joos F, Juutinen S, Kaiser K, Lacourse T, Lamentowicz M, Larmola T, Leifeld J, Lohila A, Milner A, Minkkinen K, Moss P, Naafs BDA, Nichols J, O’Donnell J, Payne R, Philben M, Quillet A, Ratnayake AS, Roland T, Sjogersten S, Sonnentag O, Swindles GT, Swinnen W, Talbot J, Treat C, Valach AC, Wu J, Piilo S. 2021. Future vulnerability of the global peatland carbon sink. Nat Clim Change 11:70–7.
Lougheed VL, Mcintosh MD, Parker CA, Stevenson RJ. 2008. Wetland degradation leads to homogenization of the biota at local and landscape scales. Freshw Biol 53:2402–13.
Marsh AS, Arnone JA, Bormann BT, Gordon JC. 2000. The role of Equisetum in nutrient cycling in an Alaskan shrub wetland. J Ecol 88:999–1011.
McPartland MY, Kane ES, Falkowski MJ, Kolka R, Turetsky MR, Palik B, Montgomery RA. 2019. The response of boreal peatland community composition and NDVI to hydrologic change, warming and elevated carbon dioxide. Glob Change Biol 25:93–107.
Mesquita MMF, Crapez MAC, Teixeira VL, Cavalcanti DN. 2019. Potential interactions bacteria-brown algae. J Appl Phycol 31:867–83.
Myers JM, Kuehn KA, Wyatt KH. 2021. Carbon subsidies shift a northern peatland biofilm community towards heterotrophy in low but not high nutrient conditions. Freshw Biol 66:589–98.
Olefeldt D, Euskirchen ES, Harden J, Kane E, McGuire AD, Waldrop M, Turetsky MR. 2017. Greenhouse gas fluxes and their cumulative response to inter-annual variability and experimental manipulation of the water table position in a boreal fen. Glob Change Biol 23:2428–40.
Ozersky T, Volkova EA, Bondarenko NA, Timoshkin OA, Malnik VV, Domysheva VM, Hampton SE. 2018. Nutrient limitation of benthic algae in Lake Baikal, Russia. Freshw Sci 37:472–82.
Pedrotti E, Rydin H, Ingmar T, Hytteborn H, Turunen P, Granath G. 2014. Fine-scale dynamics and community stability in boreal peatlands: revisiting a fen and a bog in Sweden after 50 years. Ecosphere 5:1–24.
Pelletier L, Strachan IB, Garneau M, Roulet NT. 2014. Carbon release from boreal peatland open water pools: Implication for the contemporary C exchange. J Geophys Res Biogeosci 119:207–22.
Porter KG, Feig Y. 1980. The use of DAPI for identification and enumeration of bacteria and blue green algae. Limnol Oceanogr 25:943–8.
Potvin LR, Kane ES, Chimner RA, Kolka RK, Lilleskov EA. 2015. Effects of water table position and plant functional group on plant community, aboveground production, and peat properties in a peatland mesocosm experiment (PEATcosm). Plant Soil 387:277–94.
Raymond PA, Hartmann J, Lauerwald R, Sobek S, McDonald C, Hoover M, Butman D, Striegl R, Mayorga E, Humborg C, Kortelainen P, Durr H, Meybeck M, Ciais P, Guth P. 2013. Global carbon dioxide emissions from inland waters. Nature 503:355–9.
Rhee GY. 1972. Competition between an alga and an aquatic bacterium for phosphate. Limnol Oceanogr 17:505–14.
Rier ST, Stevenson RJ. 2002. Effects of light dissolved organic carbon, and inorganic nutrients on the relationship between algae and heterotrophic bacteria in stream periphyton. Hydrobiologia 489:179–84.
Rober AR, Wyatt KH, Stevenson RJ. 2011. Regulation of algal structure and function by nutrients and grazing in a boreal wetland. J N Am Benthol Soc 30:787–96.
Rober AR, Wyatt KH, Turetsky MR, Stevenson RJ. 2013. Algal community response to experimental and interannual variation in hydrology in an Alaskan boreal fen. Freshw Sci 32:1–11.
Rober AR, Wyatt KH, Stevenson RJ, Turetsky MR. 2014. Spatial and temporal variability of algal community dynamics and productivity in floodplain wetlands along the Tanana River, Alaska. Freshw Sci 33:765–777.
Rober AR, McCann KS, Turetsky MR, Wyatt KH. 2022. Cascading effects of predators on algal size structure. J Phycol 58:308–17.
Robroek BJM, Albrecht RJH, Hamard S, Pulgarin A, Bragazza L, Buttler A, Jassey VEJ. 2016. Peatland vascular plant functional types affect dissolved organic matter chemistry. Plant Soil 407:135–43.
Rugenski AT, Marcarelli AM, Bechtold HA, Inouye RS. 2008. Effects of temperature and concentration on nutrient release rates from nutrient diffusing substrates. J N Am Benthol Soc 27:52–7.
Rupp DL, Kane ES, Dieleman C, Keller J, Turetsky MR. 2019. Plant functional group effects on peat carbon cycling in a boreal rich fen. Biogeochemistry 144:305–27.
Sahar N, Robroek BJM, Mills RTE, Dumont MG, Barel JM. 2022. Peatland plant functional type effects on early decomposition indicators are non-pervasive, but microhabitat dependent. Wetlands 42:98. https://doi.org/10.1007/s13157-022-01626-7.
Scott JT, Doyle RD. 2006. Coupled photosynthesis and heterotrophic bacterial biomass production in a nutrient-limited wetland periphyton mat. Aquat Microb Ecol 45:69–77.
Scott JT, Back JA, Taylor JM, King RS. 2008. Does nutrient enrichment decouple algal-bacterial production in periphyton? J N Am Benthol Soc 27:332–44.
Sklar FH, Chimney MJ, Newman S, McCormick PV, Gawlik D, Miao S, McVoy C, Said W, Newman J, Coronado C, Crozier G, Korvela M, Rutchey K. 2005. The ecological-societal underpinnings of Everglades restoration. Front Ecol Environ 3:161–9.
Steinman AD, Lamberti GA, Leavitt PR. 2006. Biomass and pigments of benthic algae. In: Hauer FR, Lamerti GA, Eds. Methods in stream ecology, 2nd edn. Academic Press. pp 357–379.
Stets EG, Cotner JB. 2008. The influence of dissolved organic carbon on bacterial phosphorus uptake and bacteria-phytoplankton dynamics in two Minnesota lakes. Limnol Oceanogr 53:137–47.
Tank JL, Reisinger AJ, Rosi-Marshall EJ. 2017. Nutrient limitation and uptake. In: Hauer FR, Lamerti GA, Eds. Methods in stream ecology, 3rd edn. San Diego: Elsevier. pp 147–71.
Theil-Nielsen J, Søndergaard M. 1998. Bacterial carbon biomass calculated from biovolumes. Archiv Für Hydrobiologie 141:195–207.
Treat CC, Jones MC, Alder J, Sannel AB, Camill P, Frolking S. 2021. Predicted vulnerability of carbon in permafrost peatlands with future climate change and permafrost thaw in Western Canada. J Geophys Res Biogeosci 126(5):e2020JG005872.
Treseder KK. 2008. Nitrogen additions and microbial biomass: a meta-analysis of ecosystem studies. Ecol Lett 11:1111–20.
Turetsky MR, Bond-Lamberty B, Euskirchen E, Talbot J, Frolking S, McGuire AD, Tuittila ES. 2012. The resilience and functional role of moss in boreal and arctic ecosystems. New Phytol 196:49–67.
Waddington JM, Roulet NT. 2000. Carbon balance of a boreal patterned peatland. Glob Change Biol 6:87–98.
Wagner K, Bengtsson MM, Findlay RH, Battin TJ, Ulseth AJ. 2017. High light intensity mediates a shift from allochthonous to autochthonous carbon use in phototrophic stream biofilms. J Geophys Res Biogeosci 122:1806–20.
Ward SE, Bardgett RD, McNamara NP, Ostle NJ. 2009. Plant functional group identity influences short term peatland ecosystem carbon flux: evidence from a plant removal experiment. Funct Ecol 23:454–62.
Ward SE, Ostle NJ, McNamara NP, Bardget RD. 2010. Litter evenness influences short-term peatland decomposition processes. Oecologia 164:511–20.
Ward SE, Ostle NJ, Oakley S, Quirk H, Henrys PA, Bardgett RD. 2013. Warming effects on greenhouse gas fluxes in peatlands are modulated by vegetation composition. Ecol Lett 16:1285–93.
Weinhold A, Döll S, Liu M, Schedl A, Pöschl Y, Xu X, Neumann S, van Dam NM. 2021. Tree species richness differentially affects the chemical composition of leaves, roots and root exudates in four subtropical tree species. J Ecol 110:97–116.
Weishaar JL, Aiken GR, Depaz E, Bergamaschi B, Fram M, Fujii R. 2003. Evaluation of specific ultra-violet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon. Environ Sci Technol 37:4702–8.
Wickland KP, Neff JC, Aiken GR. 2007. Dissolved organic carbon in Alaskan boreal forest: sources, chemical characteristics, and biodegradability. Ecosystems 10:1323–40.
Wickland KP, Waldrop MP, Aiken GR, Koch JC, Jorgenson MT, Striegl RG. 2018. Dissolved organic carbon and nitrogen release from boreal Holocene permafrost and seasonally frozen soils of Alaska. Environ Res Lett. https://doi.org/10.1088/1748-9326/aac4ad.
Wyatt KH, Turetsky MR. 2015. Algae alleviate carbon limitation of heterotrophic bacteria in a boreal peatland. J Ecol 103:1165–71.
Wyatt KH, Turetsky MR, Rober AR, Kane ES, Giroldo D, Stevenson RJ. 2012. Contributions of algae to GPP and DOC production in an Alaskan fen: effects of historical water table manipulations on ecosystem responses to a natural flood. Oecologia 169:821–32.
Wyatt KH, Bange JS, Fitzgibbon AS, Bernot MJ, Rober AR. 2015. Nutrients and temperature interact to regulate algae and heterotrophic bacteria in an Alaskan poor fen peatland. Can J Fish Aquat Sci 72:447–53.
Wyatt KH, Seballos RC, Shoemaker MN, Brown SP, Chandra S, Kuehn KA, Rober AR, Sadro S. 2019. Resource constraints highlight complex microbial interactions during lake biofilm development. J Ecol 107:2737–46.
Wyatt KH, McCann KS, Rober AR, Turetsky MR. 2021. Trophic interactions regulate peatland carbon cycling. Ecol Lett 24:781–90.
Ylla I, Borrego C, Romani AM, Sabater S. 2009. Availability of glucose and light modulates the structure and function of a microbial biofilm. FEMS Microbiol Ecol 69:27–42.
Yu ZC. 2012. Northern peatland carbon stocks and dynamics: a review. Biogeosciences 9:4071–85.
Acknowledgements
This research was supported by the National Science Foundation (DEB-2141285) and the Bonanza Creek Long-Term Ecological Research Program (USDA Forest Service, Pacific Northwest Research Station grant number RJVA-PNW-01-JV-11261952-231 and National Science Foundation grant number DEB-1636476).
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Conflict of interest
None declared.
Additional information
Author Contributions: ARR, KHW, ESK, and MRT contributed to the conception and design of the study. ARR and KHW performed research. ARR and AJL conducted laboratory work and analyzed data. ARR and KHW co-wrote the manuscript with input from the other authors.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Rober, A.R., Lankford, A.J., Kane, E.S. et al. Structuring Life After Death: Plant Leachates Promote CO2 Uptake by Regulating Microbial Biofilm Interactions in a Northern Peatland Ecosystem. Ecosystems 26, 1108–1124 (2023). https://doi.org/10.1007/s10021-023-00820-w
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10021-023-00820-w