From adverse to beneficial – contrasting dietary effects of freshwater mixotrophs on zooplankton

Abstract

The importance of mixotrophic algae as key bacterivores in microbial food webs is increasingly acknowledged, but their effects on consumers is less understood, with previous studies having revealed contrasting results. In freshwater, this may be related to fundamental differences in the nutritional quality of two major mixotrophic groups. While cryptophytes are generally considered as high-quality food for zooplankton, chrysophytes (golden algae) are often referred to be toxic.Using four chrysophyte species, we performed a comparative study as an attempt to generalize their dietary quality by (1) revealing their stoichiometric and biochemical profiles, and (2) quantifying their dietary effects in feeding bioassays with Daphnia longispina. We compared the observed effects to a known high-quality reference food (Cryptomonas sp.) and a starvation control as a reference for potential toxicity.We found dramatic differences in survival and growth of D. longispina depending on the chrysophyte species provided as food. Even within the same genus, dietary quality ranged from deleterious to high. As this was not reflected in differences in cellular stoichiometry and fatty acid profiles, we suggest that toxicity may be the underlying mechanism.Our results suggest that the dietary effects of chrysophytes cannot be generalised. Besides, the fact that a species previously reported to be deleterious turned out to be a beneficial food source suggests that toxic effects may dynamically vary depending on environmental cues, mode of nutrition or the investigated strain.To fully understand the nutritional value of mixotrophic algae in aquatic food webs, representatives of multiple taxa need to be tested under a range of environmental conditions. This is also needed for a better predictive capability of climate change effects, as it may not only promote the dominance of mixotrophic algae, but also induce functional changes related to their nutritional quality.

Contribution of silica-scaled chrysophytes to ecosystems services: a review

“…As a closely related subject to mixotrophy, foodweb studies involving chrysophytes are one of the most inspiring headings, both in freshwater and marine environments (Kristiansen & Škaloud, 2017;Vad et al, 2021;Cormier et al, 2022). Moreover, as an exciting supplement of nutrition mode studies, the molecular basis of nutritional diversification, accompanying pathway reduction and degradation of plastid structures help to understand the evolution of heterotrophy in chrysophytes (Beisser et al, 2017;Graupner et al, 2018;Kim et al, 2020).…”