Coral cavity sponges depend on reef-derived food resources: stable isotope and fatty acid constraints

Cryptic encrusting sponges living in coral cavities on the fore-reef slope of fringing reefs sequester most of their organic matter from the passing water. They efficiently filter the pico- and nanoplankton, but dissolved organic matter (DOM) appears to be the major resource of various sponge species living on the open reef and in coral cavities (Yahel et al. 2003; De Goeij et al. 2008a). The origin and source of this organic matter is unknown. It may be DOM released from phytoplankton, DOM supplied to the reef from outside or DOM released from the reef benthos. Gradients in DOM concentrations over the reef suggest that there is a net release of DOM from coral reef waters to adjacent open waters (Van Duyl and Gast 2001). Only 10–20% of the DOM in reef water appears to be available for sponges (De Goeij et al. 2008a). By analyses of dual stable isotope signatures (δ¹³C_org and δ¹⁵N) and FA patterns of sponges and their potential food sources, we obtained circumstantial evidence that the reef itself mainly feeds her cavity sponges. This remarkable finding implies that cryptic sponges support reef internal nutrient recycling and conservation of reef organic matter.

It was surprising that none of the 12 sponge species examined appeared to live mainly on suspended particulate organic matter or DOM derived from this source. The δ¹³C_org values of the SPM fractions were in the range reported for marine suspended matter (Peterson and Fry 1987). Bacterioplankton values were within 2‰ of SPM in open water, which indicates that bacteria were the primary consumers of SPM. This makes it highly likely that SPM represents predominantly phytoplankton and that the bacterioplankton reflects the δ¹³C isotope ratio of reactive DOM released by phytoplankton (e.g., Harvey et al. 2006; Van den Meersche et al. 2009). The δ¹³C of SPM did not change during passage over the reef, for instance by obtaining a typical δ¹³C SPM reef signal (−17 to −20‰: Land et al. 1975; Fry et al. 1982; Yamamuro et al. 1995). The δ¹⁵N of SPM did not change either and remained ~4‰. Moreover, at two of the four stations (CT and B1) comparable δ¹³C_org values of predominantly heterotrophic bacterioplankton (−26, −27‰, Table 2) between open water and reef bottom water suggest that this bacterioplankton remained dependent on phytoplankton DOC. Such low δ¹³C values have been reported, for e.g., Synechococcus (δ¹³C −33 to −25‰, Brutemark et al. 2009) and Prochlorococcus, both abundant in reef and open water along Curaçao (Van Duyl et al. 2002). Residence time of the SPM and bacterioplankton in reef overlying waters may have been too short for changes in isotopic ratios either due to rapid water exchange with the open water and/or due to SPM removal by benthic suspension feeders. Phytoplankton concentrations in reef surrounding waters in Curaçao are usually low (van Duyl et al. 2002), and the organic matter demand of cryptic biota, cryptic sponges in particular, is high. Cavity biota between 10 and 15 m depth on the fore-reef slope in Curaçao consume on average 350 mmol C m⁻² d⁻¹ of which cavity sponges consume on average 256 mmol C m⁻² d⁻¹ (73%) with the DOC fraction comprising >90% of the diet (De Goeij et al. 2008a). This organic matter supply is orders of magnitude higher than the supply of pelagic primary production to reefs (Richter et al. 2001; Genin et al. 2009). It is evident that the supply of open water-derived phytoplankton and its DOM release is insufficient to satisfy the total organic carbon requirements of coral reef cavity sponges. In accordance, the isotope mixing model results confirmed a SPM contribution of less than <50% in the diet of most sponges. This implies that the examined sponges with average δ¹³C_org of −18.9‰ and δ¹⁵N of 4.6‰ mainly assimilate other sources than pelagic primary production.

Benthic primary production is the most likely other source of DOM for sponges. Presence of distinct reactive DOM in reef bottom water compared to open water was demonstrated by the increased δ¹³C value of bacterioplankton at two of the four stations (LJT and SB). Bacterioplankton apparently assimilated a source of carbon not available in the open water, a source, which may also be available to the DOM feeding cavity sponges. Benthic primary producers usually have higher δ¹³C_org values than pelagic primary producers (France 1995; Fry 2006). Thus, assimilation of benthic algae-derived reactive DOC may explain the elevated δ¹³C values in bacterioplankton in reef bottom water at LJT and SB. Bacteria respond quickly when exposed to changing resources in the vicinity of the reef bottom. Enhanced bacterioplankton production in reef bottom waters, cavity waters, and coral mucus point to the reef bottom as the source of inorganic nutrients and reactive DOM (Van Duyl and Gast 2001; Scheffers et al. 2005). Sponges and bacteria may compete for the released DOM. However, the δ¹³C reef signal of bacterioplankton (−17.9 to −15.4‰) was higher than that of most examined sponges (−20.8 to −17.2‰) suggesting that sponges and bacterioplankton may not fully depend on the same source of reef-derived DOM. Sponges may exploit a wider size range of the operationally defined DOM fraction than bacterioplankton (De Goeij et al. 2008a).

Benthic primary producers such as corals and benthic macroalgae have been reported to release reactive DOM (e.g., Crossland 1983, 1987; Wild et al. 2004, 2010; Haas et al. 2010; Naumann et al. 2010). The δ¹³C values of the benthic substrates (coral mucus of two dominant coral species and crustose coralline algae) were on average higher in δ¹³C than the SPM and open water bacterioplankton and were more in range of the sponge stable isotope signals. The δ¹⁵N values of reef-derived substrates with values 3–4‰ lower than those of sponges on average suggest a direct trophic link (one trophic level distance). The lower δ¹⁵N values of reef substrates versus open water substrates may be due to enhanced N₂ fixation by the reef benthos (Davey et al. 2008; Lesser et al. 2007) compared with N₂ fixation by microorganisms in open waters. Discharge from land or pollution may counteract this effect by introducing organic and inorganic N enriched in ¹⁵N. This may partly explain the variations in δ¹⁵N in mucus, coralline algae, and sponges between different stations.

The range of δ¹³C_org in crustose coralline algae (CCA) of −25.2 to −12.1‰ was unexpectedly large, possibly due to the unique ontogeny of CCA. Younger portions of the CCA may have lower δ¹³C_org values than older portions possibly because rapid photosynthesis in younger parts depends more on respired CO₂ than photosynthesis in older parts (Lee and Carpenter 2001). Anyway, the lower range of δ¹³C_org values of CCA was within 2‰ of our sponges. Moreover, the δ¹⁵N of CCA (1.3–4.0‰) make it a likely food source for the majority of cavity sponges with 3–4‰ higher δ¹⁵N values than the CCA. Little is known of exudation of organic matter by CCA apart from the releases of allelochemicals (e.g., Kim et al. 2004). Total DOM release by CCA has not been determined as far as we know, but their productivity equals that of fleshy macroalgae and turf algae on reefs (Chrisholm 2003). Reported δ¹³C values of −18.7 to −16.8‰ for Dictyota sp.,−19.3‰ for Halimeda sp. (Raz-Guzman Macbeth and De la Lanza Espina 1991; Lepoint et al. 2000), and δ¹⁵N values of 0.5–3‰ of macroalgae on coral reefs (Yamamuro et al. 1995; France et al. 1998) fall in the same range as CCA (δ¹³C_org −25.2 to −12.1‰, δ¹⁵N 1.3–4‰). Therefore, DOM released from these brown and green macroalgae may have also contributed to the isotope signal in sponges. Trophic transfers of organic matter derived from benthic macroalgae and mangroves, to sponges have been reported before (Behringer and Butler IV 2006; Granek et al. 2009) and appear to be important in the nutrition of sponges.

Unlike CCA, coral mucus has already been described as an important food source on coral reefs (Wild et al. 2004, 2010). About 56–80% of coral mucus dissolves in sea water (Crossland et al. 1980; Wild et al. 2004) and may be a suitable source of DOM for cavity sponges. The δ¹³C of the mucus samples of Montastraea annularis and Madracis mirabilis (range −17.9 to −14.2‰) is higher than that of most cavity sponges, implying that sponges do not feed exclusively on mucus or mucus-derived DOM. Coral-derived organic matter is a reactive substrate for bacterioplankton growth (Ferrier-Pagès et al. 2000) and may have contributed to the δ¹³C reef signal of bacterioplankton at LJT (−15.4‰). The average δ¹⁵N of mucus (1.9‰) and that of cavity sponges (4.6‰) are consistent with mucus/mucus-derived DOM as a food source for most cavity sponges.

It is evident that individually, neither phytoplankton, bacterioplankton, CCA (benthic macroalgae) nor coral mucus can account for the sponge dual isotope signatures. The isotope mixing model analysis showed that cavity sponges incorporate from half to three quarters of their organic matter from mucus and CCA (benthic macroalgae) with mucus as the dominant source of reef-derived organic matter (up to 66% of the diet). It appears that with decreasing coral cover the contribution of coral mucus to the diet of sponges decreases and shifts to a larger contribution of CCA (benthic macroalgae) and phytoplankton. At station CT with a lower coral cover than at the other reef stations, mucus only contributed 17% (mode) to the diet of sponges while the bulk food was provided by CCA including other benthic macroalgae (31%) and small phytoplankton (34%, GFF fraction). This was different at B1, where coral cover on the slope is relatively high (Van Duyl 1985; Bak and Nieuwland 1995) and the sponges reflect a diet of mainly mucus (60%, mode) with 13% CCA. This suggests that the sponge community is opportunistic and consumes OM according to availability irrespective of the DOM source. This does not imply that all examined sponge species at a particular reef site have similar diets. Significant differences in stable isotope values of different sponge species already suggest distinct diets. This is further corroborated with our FA profiles of sponges at B1.

The overall similarity in FA composition pattern of six examined sponge species at B1 was low. The three groups within these six sponge species (based on significantly different FA compositions) also had different diets according to the isotope mixing model analysis, implying differences in the proportional use of mucus, benthic algae (including CCA), and phytoplankton-derived DOM. This suggests that there is trophic niche segregation among sponges which may be an important factor facilitating co-existence of different sponge species (Thurber 2007). Furthermore, the differences in diet may to some extent be reflected in the FA compositions of sponge groups suggesting that the selected food resources may indeed form part of the diet of sponges.

The fatty acid profiles of our coral mucus and CCA sources resemble those reported for coral tissue and CCA (e.g., Latyshev et al. 1991; Viso and Marty 1993; Papina et al. 2003; Bachok et al. 2006). By assimilating these substrates, distinct FAs may conservatively be transferred to the consumer (Dalsgaard et al. 2003). Although many dominant FAs such as 14:0, 16:0, and 18:0 (Table 2) are common to many organisms, some FAs have been identified as potential tracer of food source in sponges. For this study area specifically, for example, De Goeij et al. (2008b) conclude from isotope tracer experiments that the dominant FA 20:4ω6 found in the sponge Halisarca caerulea has an exogenous source. Interestingly, 20:4ω6 is found dominant in all 6 sponge species examined and it is also dominant in CCA and M. mirabilis mucus. The high cover of CCA in cavities (30% of the hard bottom surface in cavities, Scheffers et al. 2004), its close proximity to cavity sponges and the fact that CCA was rich in 20:4ω6, may have contributed to the 20:4ω6 content in sponges. In sponge groups 2 and 3, this FA was the second most abundant (>10% of total FAs) and the diet of these sponge groups consisted for 19–22% (mode) of CCA (including other macroalgae). In sponge group 1, the predominantly mucus feeding group, 20:4ω6 was the third most abundant FA with a lower relative abundance than in the other groups (<10%). This coincided with a lower contribution of CCA in their diet (10% mode). Trophic transfer of this FA may be directly linked to the consumption of CCA as part of macroalgal-derived organic matter.

Phytanic acid, which can be considered as a sponge biomarker (De Goeij et al. 2008b) was the second most abundant FA in group 1. For its synthesis, degradation products of chlorophyll a are required, possibly delivered to the sponge by sponge associated phototrophic microbes, considering the relatively high amount of the likely bacterial specific FA 16:1ω7c as well as the overall high % of bacteria specific FAs in group 1 (up to 29% compared with up to 10% of total FAs in the other groups, Table 4). Sponge species in group 1 indeed harbor-rich microbial communities (Hill et al. 2005; Erwin and Thacker 2007). Mutualism between sponges and their associated microorganisms may have influenced the diet of group 1 compared with groups 2 and 3. Weisz et al. (2007) show that sponges with high versus low abundances of associated microbes may have distinct diets. In line, we show that the diet of sponges (based on stable isotopes as well as FAs) varies with abundance of bacteria specific FAs. Interestingly, sponges with a relatively high abundance of associated microbes (group 1) appear to be more dependent on reef-derived food than other sponges.

The FA 20:5ω3, a dominant FA in (benthic) diatoms and the mucus of corals, particularly in mucus of M. annularis was most abundant in S. ruetzleri (group 3). The SIAR model analysis showed that the contribution of phytoplankton-derived food was highest in this sponge (mean 49% and mode 57%, both plankton size fractions together), which may suggest that it feeds on diatoms or diatom-derived OM rich in 20:5ω3. The other sponge groups (1 and 2) may obtain 20:5ω3 predominantly by consuming mucus considering the higher contribution of mucus and lower plankton-derived food in the diet of these groups than in the diet of S. ruetzleri (group 3). Diatom-derived OM as well as mucus may have contributed to the abundance of 20:5ω3 in sponges.

Considering the congruencies between the FA compositions and the dual stable isotope signals in the trophic transfer and relations between sources and consumers, it was not surprising that the matrices (based on Euclidean distances) of the reef sources and consumers of both approaches were significantly correlated. Both methods apparently lead to comparable results with FA composition and biomarkers identifying the food items of consumers, and evidencing trophic transfer and with dual stable isotope analyses estimating the contribution of the different sources to the diet of consumers. Results of both methods strongly support a trophic transfer from zooxanthellate corals and CCA to cavity sponges. The quantitative contribution of CCA to the diet of sponges remains unresolved, because its dual isotope signal overlaps with that of other benthic macroalgae.