Introduction

Epicontinental aquatic ecosystems, more than other natural environments, are exposed constantly to pollutants, because freshwater is used for various industrial and urban activities and receives a wide variety of wastes (Demirak et al. 2006). In addition, runoffs during the rainy season of the irrigated lands transport toxicants, which indirectly affect aquatic systems (Mateo-Sagasta et al. 2017). Aquatic ecosystems an withstand toxicants, but excessive discharge of pollutants interferes with biological interactions (Beeby 1993). Many pollutants threaten the health of different groups of aquatic organisms from bacteria to fishes; detergents, fertilizers, insecticides and heavy metals affects aquatic life. Plastic pollution also deteriorates water quality (Abowei and Sikoki 2005).

Global plastic production has been steadily increasing, almost exponentially for the last six decades, leading to 6300 million tons of plastic waste of which only < 10% has been recycled (Greyer et al. 2017; Plastics Europe 2019). Polystyrene is one of the main components of plastics, and because of mismanagement or lack of appropriate technology to recycle plastic wastes, this pollutant reaches aquatic ecosystems as large and small microplastics (Dris et al. 2018; Eerkes-Medrano et al. 2015).

Microplastics are emerging pollutants consisting of a wide variety of small particles and synthetic polymer fibers with diameters varying from 100 nm to about 5 mm (Lusher et al. 2017). Because of their small size, microplastics are ingested by a great diversity of aquatic fauna at different trophic levels (Hall et al. 2015; Alfonso et al. 2020). Birds that feed on fish or other aquatic prey suffer from plasticosis (Charlton-Howard et al. 2023). An additional impact of microplastics on aquatic fauna is associated with their ability to adsorb organic and inorganic chemicals from the ambient waters (Bakir et al. 2014; Llorca et al. 2018).

Cadmium (Cd) is one of the most toxic heavy metals with no known biological role. Despite this, it is common in the aquatic environment after being released from alloys, galvanization, pigments, batteries, and plastics (Wright and Welbourn 2002; Yang et al. 2018). Because species of zooplankton in nature are exposed to both microplastics and cadmium compounds, ecotoxicological evaluations are needed using their combined effects and such studies are rare in literature (Snell and Joaquim-Justo 2007).

The interaction between contaminants can vary in different ways. Antagonistic interactions between selenium and mercury have been reported where the former, an essential metal, reduces the toxicity of the latter (Khan and Wang 2009). In some other cases, the contaminants produce a synergistic effect reported in study on microplastics contaminated with BP-3 (Na et al. 2020). Microplastics with other toxicants combined may have an additive effect. Hence, microplastics affect the aquatic organisms depending on the other contaminants with which they interact (Beiras et al. 2018).

Rotifers are an important component of freshwater zooplankton; they belong to the phylum Rotifera with more than 2100 species (Wallace et al. 2019). Given their ecological importance in the aquatic food chain and ease of handling in the laboratory, they are widely used as bioassay organisms (Wallace et al. 2006). Behavioral traits such as the feeding rates and demographic variables such as lifespan and reproductive rates are sensitivite to toxicant stress. Sublethal toxicity tests mainly consider life history variables (Snell and Joaquim-Justo 2007; Guo et al. 2012).

Though most rotifers are primary consumers, there are also a few secondary consumers in the phylum Rotifera. For example, predatory species of the genus Asplanchna are common in zooplankton  communities (Gilbert 2014). Asplanchna is not a visual predator, but has mechano-receptors to sense its prey. Hence, its feeding behavior is essentially a function of encounter probability. Hence, the relationships among the number of attacks, captures and ingestions that follow random prey encounters, as well as the selectivity based on the chemoreceptors on the ciliary corona often determine the survival of Asplanchna in nature (Gilbert 1980a; Iyer and Rao 1996).

Contaminants also interfere in the feeding process of non-visual predators. For example, inert substances such as soil particles, and detritus consisting of fragments of diatoms are known to decrease the feeding efficiency of rotifers (Gilbert 2022). Different studies also show that microplastics have an indirect effect on food ingestion, assimilation and population growth of planktonic rotifers (Drago and Weithoff 2021; Morgalev et al. 2022). Species of Asplanchna are also affected by the presence of large (< 30 µm) inorganic particles and microplastics that offer no energy to the predators, but occupy space in gut filling. From an ecological perspective, quantitative changes in the feeding behavior of Asplanchna indicate the consequences of its demographic responses when the predators encounter prey contaminated with heavy metals such as cadmium and microplastics. Therefore, Asplanchna exposed to combined effects of microplastics, and cadmium would present quantifiable changes in the demography over time. The life table approach allows quantifying the numerical characteristics of mortality and reproduction in an age-specific way, and this helps to detect non-random sources of death of a cohort population (Krebs 1985).

Therefore, the objective of this study was to evaluate the effect of prey (Plationus patulus (Müller, 1786)) previously exposed to cadmium and microplastics on the feeding behavior and demography of Asplanchna sieboldii (Leydig, 1854) using the life table method.

Materials and methods

Isolation and culture of test species

The herbivorous prey rotifer P. patulus has been cultured in the laboratory for many years. The original strain was isolated from Lake Xochimilco (Mexico City, Mexico). Monoclonal culture of P. patulus was established from a single parthenogenetic female and fed the single-celled alga, Chlorella vulgaris as food. The density of alga chosen for maintaining stock cultures of P. patulus and for the experiments was 1 × 106 cells mL−1 and moderately hard water as medium (EPA medium). The EPA medium was prepared by dissolving 1.9 g of NaHCO3, 1.2 g of CaSO4, 1.2 of MgSO4 and 0.04 g of KCl in 20 L of distilled water (Weber 1993). Chlorella vulgaris was cultured in transparent 2 L culture jars using Bold basal medium (Borowitzka and Borowitzka 1988) supplemented with 0.5 g L−1 of NaHCO3 after every 3rd day. When the alga was in the logarithmic growth phase, it was harvested, centrifuged at 3000 rpm for 5 min. and then resuspended in a small volume distilled water. The concentrated alga was stored at 4 °C until use. To maintain favorable conditions for P. patulus growth, the culture medium was 100% replaced after every 2 days. The pH of the culture medium was maintained at 7.0–7.3. Temperature 25 ± 1 °C and continuous and diffuse light were set for rotifer cultures and for experiments.

The predatory rotifer Asplanchna sieboldii was also isolated from Lake Xochimilco and a monoclonal culture was established starting with a parthenogenetic female providing mixed diets of two brachionid rotifers (Brachionus havanaensis Rousselet, 1911 and Brachionus calyciflorus Pallas, 1766) as food. To prevent sexual reproduction of A. sieboldii in the stock cultures, the predator densities were kept low and the culture medium was changed daily. The other test conditions were similar to those of P. patulus.

Polystyrene microspheres

Polystyrene (PS) microparticles with a diameter of 30 μm were acquired from Sigma-Aldrich (Batch BCCC0789). Using the appropriate dilution using distilled water, the quantity of PS was determined using a Sedgewick–Rafter chamber following the safety instructions of the manufacturer. A stock solution of 40 mg L−1 was prepared. The stock solution was subject to ultrasonication at 20 kHz 10 watts for 3 min for homogenization. This procedure was carried out every time the solution was used for the experiments.

Cadmium stock solution

A stock solution of 1 g L−1 of CdCl2 2.5 H2O (Fermont, Batch: 80,702, purity 99.7%) was prepared in a 100 mL volumetric flask with distilled water so that each mL of the solution contained 1 mg of Cd. Previously, all the glassware that was to be used for the tests was cleaned thoroughly (Moody and Lindstrom 1977).

Feeding behavior of A. sieboldii

The feeding behavior of Asplanchna sieboldii was evaluated using twenty-one individuals of P. patulus as prey (7 ind. mL−1) and one predator in petri dishes containing 3 mL of EPA medium. Low density (0.01 × 106 cells mL−1 ) of C. vulgaris added to the medium to keep the prey individuals active. Pre-starved (3 h) adult A. sieboldii, campanulate morphotype, individuals were used as predators. The prey individuals were exposed for 1 h to one of the two sublethal concentrations of Cd (10 and 20% of LC50, 0.009 and 0.018 mg L−1, respectively) and one concentration of microplastics (5 mg L−1). For each treatment, there were 5 replicates. Ten-minute feeding observations were made for quantifying the following parameters: Encounter (E), when there is detection by the predator of the prey; Attack (A), when the predator tries to capture the prey; Capture (C), when the predator holds the prey; Ingestion (I), when the predator consumes the prey (Gilbert 1980a).

The number of E, A, C and I events was statistically evaluated with a one-way ANOVA and Tukey post-hoc tests.

Life table study

Life table demography experiments were conducted in 25-mL capacity glass jars containing 20 mL of the medium. In all, there were 90 test jars (3 prey densities × 6 treatments × 5 replicates). Into each jar we introduced the prey P. patulus of chosen density (1, 2 or 4 ind. mL−1) that was previously exposed to the selected Cd levels (10 or 20% of LC50) for 24 h and a neonate of A. sieboldii (< 3 h) that was never exposed to either microplastics or Cd. The test jars also received alga at 1 × 104 cells mL−1 to keep the prey active. These experiments were conducted at 25 ± 1 °C with continuous, diffuse illumination. Following initiation of life table study, after every 12 h, neonates born if any, from each replicate were counted and discarded. After every 24 h, the surviving predators were transferred to fresh jars containing appropriate test conditions. The experiment was discontinued when the predator in each replicate died. Demographic variables were calculated according to Krebs (1985). For lx and mx data, we derived mean ± SE values using 5 replicates. However, for other variables (average lifespan, life expectancy at birth (E0), gross reproductive rate (GRR), net reproductive rate (NRR), generation time (GT) and the rate of population increase (r per day), we used jackknife method (Meyer et al. 1986) to derive mean ± SE. This was necessary because Asplanchna sieboldii is cannibalistic and feeds on congeners, so life table experiments were initiated with single individuals (1980a), after which the jackknife method was used to pool and analyze the data. Replicates where no reproduction occurred were eliminated from the analyses.

The life table variables were statistically tested using one-way ANOVA for each prey density. For multiple comparisons within a prey density treatment, we used post-hoc Tukey tests. Statistical evaluations were carried out following Kaptein and van den Heuvel (2022) and SigmaPlot version 11.

Results

Feeding rates

The presence of microplastic spheres in the body of P. patulus was observed. However, only a few microplastic spheres were seen in the rotifer gut. Data on the frequency of feeding events by A. sieboldii (mean ± SE), Encounter, Attack, Capture and Ingestion are presented in Fig. 1. The number of encounters and attacks by the predator on its prey P. patulus exposed to one or the combination of contaminants (microplastics or Cd) was not significantly different. However, the capture decreased significantly (p < 0.05, ANOVA; Table 1) with prey previously exposed to sublethal concentrations of cadmium and microplastics. With the prey exposed to both pollutants, the lowest values of capture and ingestion were obtained, which were lower by 71 and 61%, respectively, as compared to the control.

Fig. 1
figure 1

Feeding behavior of A. sieboldii offered the prey P. patulus previously exposed to cadmium (0.009 and 0.018 mg L−1), microplastics (5 mg L−1) and the combination of both contaminants. Shown are the mean ± SE of 5 replicate observations. * = p < 0.05 One-way ANOVA)

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Table 1 One-way analysis of variance (ANOVA) on the feeding behavior of A. sieboldii fed P. patulus previously exposed to sublethal concentrations of cadmium, microplastics and the combination of both contaminants. DF: Degrees of Freedom; SS: Sum of Squares; MS: Mean Squares (α = 0.05)
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Life table data

Survivorship curves of A. sieboldii showed that when fed on the prey previously exposed to 10%Cd, its survival increased compared to the higher concentration of Cd or controls. In treatments where prey were exposed previously to both Cd and microplastics, the predator population died earlier than in other treatments (Fig. 2). The offspring production in A. sieboldii started after 24 h. In general, in controls and treatments with Cd and microplastics, the offspring production increased as the available prey numbers increased. Compared to controls, the fecundity rate of the predator decreased when the contaminated prey was offered as food (Fig. 3).

Fig. 2
figure 2

Age-specific survivorship curves of A. sieboldii fed on 1, 2 and 4 ind. mL−1 of P. patulus previously exposed separately to 10%Cd (0.009 mg L−1), 20%Cd (0.018 mg L−1), MPs (5 mg L−1) or their combinations. Values represent mean ± SE 5 replicates

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Fig. 3
figure 3

Age-specific fecundity curves of A. sieboldii fed 1, 2 and 4 ind. mL−1 of P. patulus previously exposed to 10%Cd (0.009 mg L−1), 20%Cd (0.018 mg L−1), MPs (5 mg L−1) and the combination of both contaminants. Values represent mean ± SE based on 5 replicates

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The average lifespan of A. sieboldii ranged from 3.3 to 5.2 days and life expectancy at birth ranged from 2.8 to 4.5 days. Under all the three offered prey densities, both average lifespan and life expectancy at birth of the predator were significantly affected (p < 0.05, Tukey test) when fed prey previously exposed to 20%Cd + MPs or 20%Cd only. However, at the prey densities of 1 and 2 ind. mL−1 and at lower levels of Cd (10%) level, there was a significant increase in both these variables. The reproductive period decreased with increasing concentration of cadmium (from 10 to 20%). Both gross (GRR) and net reproductive (NRR) rates of predators ranged from 1 to 8 offspring female−1 lifespan−1. Higher GRR and NRR were recorded in controls when fed prey at the density of 4 ind. mL−1. Generation time (GT) varied from 2–3 days, depending on treatment. Single and combined contaminants caused significant decrease (< 0.05, Tukey test) in the reproductive rates (GRR and NRR) of the predator even at the highest offered prey density (4 ind. mL−1). The population growth rate of A. sieboldii in controls ranged from 0.12 to 0.55 day−1 while in treatments with contaminant-exposed prey, it ranged from 0.15 to 0.99 d−1 (Table 2).

Table 2 Demographic variables of A. sieboldii fed 1, 2 and 4 ind. L−1 of P. patulus previously exposed to 10%Cd (0.009 mg L−1), 20%Cd (0.018 mg L−1), MP (microplastics) (5 mg L−1) and the combination of both the contaminants. The average of 5 replicates is shown with ± standard error. Data bearing a different alphabet letter in superscript are statistically significant (p < 0.05, Tukey test). ALS: Average lifespan, days; E0: Life expectancy, days; GRR: Gross reproductive rate, neonates female−1; NRR: Net reproductive rate, survival- weighted neonates female−1; GT: Generational time, days and r: Population growth rate, days
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Discussion

The effects of interactions between contaminants such as metals (Sarma et al. 2006), pesticides (Faust et al. 1994) and microplastics with other contaminants (Rehse et al. 2018) are commonly analyzed by rapid acute toxicity tests (LC50) and at a single food chain level, usually herbivores (Wallace et al. 2006). The LC50 24 h of Cd obtained in this work was 0.9 mg L−1, which was close to that reported in literature (Snell and Joaquim-Justo 2007). With only MPs, P. patulus had a LC50 of 90 mg L−1. Therefore, we used for cadmium 10 and 20% of LC50 and for MPs 10% of the LC50. Despite the useful information generated from chronic trials (Liang et al. 2021), there are few studies that have evaluated the ecotoxicological effects on diet exposed to pollutants (Sarma et al. 1998). Although some studies on the food chain effects of toxicants are available in literature (Van Kirk and Hill 2007), less information is available on the effects of two pollutants at higher trophic levels such as the results of the present study.

The prey rotifer P. patulus had only a few microspheres probably because the size of the spheres used here, although large-bodied rotifers of Brachionidae are capable of feeding on food particles up to 50 µm (Gilbert 2022). We did not quantify the MPs in the gut of either prey or the predator.

Regarding the interactions of A. sieboldii and its contaminated prey, there are five important variables which determine the feeding behavior of an aquatic predator: (1) the swimming speed of prey, (2) the size of the prey, (3) the swimming speed of the predator, (4) density of the food and (5) the visual field of the predator (Salt 1987). There were no significant differences in the encounter rates between the predator and its prey that was exposed to the two contaminants at the concentrations used in this study. This implies that there was possibly no significant change in the swimming pattern of P. patulus. As for the swimming speed of the predator, A. sieboldii, all individuals were of bell-shaped morphotype and were not directly exposed to contaminants. So, the changes in the number of attacks, captures and ingestion could depend on the visual field of the predator. However, Asplanchna is a non-visual predator, and its receptive field depends on the sensory organs that detect the vibrations in the medium produced by the prey (Dumont 1977). The chemo- and mechano-receptors are located around the ciliary corona of the predator have a role in these feeding responses (Gilbert 1980b). Decreased capture and ingestion of the prey treated with cadmium and microplastics was most evident when the predator was offered P. patulus with combined contaminants, as reported in previous studies (Joanidopoulos and Marwan 1998; Liess et al. 2020). It has been shown that A. sieboldii did not capture its clones and congeners despite constantly encountering them (Gilbert 1977, 1980a). However, our study is one of the few studies where the possible action of these chemo- and mechano-receptors failed to capture and ingest prey when contaminated with toxicants. This might be due to the lower levels of contaminants used here.

In the life table variables, it was recorded that survival increased with 10% Cd and was affected by the presence of MPs and the combination of both contaminants. Although fecundity of the predator increased when fed on the prey exposed to the combination of pollutants, this had generated a negative effect on further survival, i.e., a higher energy cost of reproduction leading to reduced life expectancy. This response has been reported for several herbivorous rotifer species, from both freshwater and marine ecosystems (Xiao-Ping et al. 2020; Zhang et al. 2021). Population growth rates showed that A. sieboldii fed on prey exposed to the combination of pollutants possibly accelerated their metabolism, increased their offspring but reduced life expectancy. The higher offspring production under lower stressful levels of toxicants, including heavy metals, is the hormetic effect possibly to overcompensate the stress effects (Tang et al. 2019). Hormetic effects have some important consequences in the community structure of zooplankton, which are often under-reported (Calabrese and Mattson 2017). These variations make populations more susceptible, especially across generations (González-Pérez et al. 2021).

The statistical analysis showed that the toxicants interactions (Cd and/or MPs) with prey density affected the survival and reproduction of predator A. sieboldii. This suggests that possibly P. patulus transfers Cd, MPs or their combination to its predators (Drago and Weithoff 2021). Information on the accumulation and/or the transfer of pollutants within a single zooplankton group such as rotifers suggests the ecological relevance of Asplanchna. This work highlights that microplastics and heavy metals together affect the ingestion and demography of predatory A. sieboldii through the prey P. patulus. Gama-Flores et al. (2007) have found a similar response, as reported here, where the demography of the predatory A. brightwellii was affected through ingesting prey exposed to copper and cadmium. Sarma et al. (1998) have also reported that Asplanchna brightwellii was negatively affected when fed on prey previously exposed to a pesticide.

Conclusions

For Mexican freshwater ecosystems, the Official Mexican Standard NOM-001-ECOL-1996 allows a maximum discharge of 200 μg L−1 of Cd, and for microplastics, there is still no maximum permissible limit. The Cd concentrations used in this work have been recorded in freshwater ecosystems with values < 20 μg L−1 of Cd (Bervoets et al. 2009), and 10 mg L−1 of microplastics (Dong et al. 2020). For the experiments conducted in this work, the prey P. patulus was exposed to concentrations that are within the permissible limits of Cd and natural levels of microplastics. It is evident that there are adverse effects of microplastics and cadmium on the feeding behavior and demography of the predator A. sieboldii. This study further suggests that Cd and MPs affect the prey capturability by Asplanchna. Predator’s survival, reproduction and population growth rates were affected due to the presence of Cd, microplastics or their mixture. Further studies are needed to quantify the biotransfer/bioaccumulation of the chosen contaminants in the predator–prey interactions within the Rotifera. For such studies, life table approach may not provide enough material for chemical analysis. Other population level studies such as population dynamics of predatory rotifers will yield sufficient biomass for bioaccumulation/biotransfer studies.