Abstract
Abstract The importance of mixotrophic algae as key bacterivores in microbial food webs is increasingly acknowledged, but their effects on the next trophic level remain poorly understood. Their high stoichiometric food quality is contrasted by anti‐grazing strategies. We tested the quality of freshwater mixotrophs as prey for zooplankton, using four non‐colonial chrysophyte species and a cryptophyte as a high quality reference food. We (1) analyzed the stoichiometric and biochemical (fatty acid) composition of the mixotrophs, and (2) quantified their dietary effects on Daphnia longispina survival. Survival of D. longispina significantly depended on the identity of species provided as food, ranging from higher to lower as compared to starvation. This was not reflected in differences in cellular stoichiometry or fatty acid profiles of the mixotrophs. We suggest that toxicity may be the driver for the observed differences. Generalization of the dietary effects of mixotrophic chrysophytes does not appear straightforward. Besides fundamental species‐specific differences, potential toxic effects may vary depending on environmental cues or physiological strategies. Notably in our study, Ochromonas tuberculata, a species previously reported to be deleterious, turned out to be a beneficial food source in terms of enabling high survival of D. longispina. We challenge the generality of the assumption that chrysophytes are of low value as food for zooplankton. We recommend that future studies test how environmental conditions and physiological strategies shape the quality of mixotrophs as food for consumers at higher trophic levels, specifically focusing on effects of dietary toxicity.
Highlights
Mixotrophic protists are common in the plankton of freshwater and marine ecosystems, and can represent a dominant component of phytoplankton in the surface waters of oceans (Hartmann et al, 2012; Leles et al, 2019; Zubkov & Tarran, 2008) and lakes (Pålsson & Granéli, 2004; Ptacnik et al, 2008; Watson et al, 1997)
This experiment consisted of six treatments, each representing a different algal food source: (1) O. danica; (2) Ochromonas sp.; (3) O. tuberculata; (4) P. malhamensis; (5) Cryptomonas sp., used as a high-quality reference food; and (6) a starvation treatment as control (Lampert, 1981)
The feeding experiment clearly showed that dietary effects of mixotrophic chrysophytes can vary considerably even among taxonomically closely related species, from beneficial to deleterious consequences for Daphnia
Summary
Mixotrophic protists are common in the plankton of freshwater and marine ecosystems, and can represent a dominant component of phytoplankton in the surface waters of oceans (Hartmann et al, 2012; Leles et al, 2019; Zubkov & Tarran, 2008) and lakes (Pålsson & Granéli, 2004; Ptacnik et al, 2008; Watson et al, 1997). Most harmful (eukaryotic) algal blooms, with detrimental effects on secondary production, are caused by mixotrophic species (Burkholder et al, 2008; Flynn et al, 2018; Watson et al, 2015), suggesting a link between mixotrophy and toxicity Whether this contrasting evidence on the food quality of mixotrophs are specific to major taxonomic groups or are related to inter-specific differences, is an important question to be solved to better understand the functional roles of mixotrophs in both microbial and associated food webs. Assuming predominantly detrimental dietary effects of mixotrophic chrysophytes on consumers, as suggested by previous experimental studies (Table 1), a shift in the phytoplankton assemblage towards this taxonomic group may have profound effects on zooplankton secondary production and higher trophic levels (e.g. fish). Based on the available evidence, we hypothesise that chrysophytes will represent lower-quality food sources, being in general detrimental compared to Cryptomonas, used as a high- quality reference food
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