The energetic cost of facing cyanotoxins: a case study on Daphnia magna

Under the effects of global change, toxic cyanobacterial proliferations increase and subsequently threaten freshwater ecosystems and the services they provide. Zooplankton consumers, a key trophic level of freshwater ecosystems, are particularly impaired by cyanotoxins, but populations regularly exposed to cyanobacteria can develop tolerance against toxins. While the physiological processes involved in this tolerance have been extensively studied, their consequences for consumers’ energetics remain poorly explored, impeding predictions of energy flow from zooplankton up to higher trophic levels. Here, we explored the metabolic response of Daphnia magna exposed to toxic and non-toxic strains of cyanobacteria to define the energy dedicated to cyanotoxins resistance mechanisms. We showed that resting metabolic rate (RMR) of individuals exposed to toxins increased by up to 60%, reflecting the energy requirement involved by cyanotoxin resistance processes. By quantifying the energy dedicated to resistance mechanisms, RMR constitutes an interesting metric to estimate overall capacity of individual zooplankters to actively handle cyanotoxins. Moreover, RMR responds more promptly to cyanotoxins than growth thus being adequate to assess short-term cyanotoxins constraints (< 72 h). Overall, we showed that cyanotoxins resistance constitutes an energy leak in freshwater ecosystems. Quantifying this leak of energy may help anticipate alterations of energy flow within food web in response to cyanobacterial proliferation and, ultimately, enhance our predictions of freshwater ecosystem structure and functions in a context of global change.