Cyanobacteria facilitate parasite epidemics in Daphnia

Abstract

The seasonal dominance of cyanobacteria in the phytoplankton community of lake ecosystems can have severe implications for higher trophic levels. For herbivorous zooplankton such as Daphnia, cyanobacteria have poor nutritional value and some species can produce toxins affecting zooplankton survival and reproduction. Here, we present another, hitherto largely unexplored aspect of cyanobacteria, namely that they can increase Daphnia susceptibility to parasites. In a 12-year monthly time-series analysis of the Daphnia community in Greifensee (Switzerland) we observed that cyanobacteria density correlated significantly with the epidemics of a common gut parasite of Daphnia, Caullerya mesnili, regardless of what cyanobacteria species was present or whether it was colonial or filamentous. The temperature from the previous month also affected the occurrence of Caullerya epidemics, either directly or indirectly by the promotion of cyanobacterial growth. A laboratory experiment confirmed that cyanobacteria increase the susceptibility of Daphnia to Caullerya, and suggested a possible involvement of cyanotoxins or other chemical traits of cyanobacteria in this process. These findings expand our understanding of the consequences of toxic cyanobacterial blooms for lake ecosystems and might be relevant for epidemics experienced by other aquatic species.

Parasites driving host diversity: Incidence of disease correlated withDaphniaclonal turnover*

“…It has been recently shown that Caullerya prevalence in Greifensee is correlated with cyanobacterial blooms, and experimental evidence has supported this facilitation (Tellenbach et al. ). However, if clones of Daphnia are differently susceptible to cyanobacteria (Hietala et al.…”

“…; Tellenbach et al. ). Moreover, in a previous three‐month survey of a Daphnia population infected with Caullerya in lake Rimov (Czech Republic), random and infected subsamples of Daphnia population significantly differed in their clonal composition (Yin et al.…”

“…; Tellenbach et al. ). This strong parasite seasonality as well as its high abundance offers a unique possibility of the comparison of clonal turnover between epidemic and nonepidemic months.…”

“…; Tellenbach et al. ), were genotyped using allozyme markers from 1998 until 2011, and microsatellite markers from 2008 until 2013. First, however, we tested an important requirement of the Red Queen hypothesis, namely the genetic specificity of infection.…”

“…Moreover, recent work has shown that Daphnia seem to be more susceptible to Caullerya infection while being simultaneously exposed to cyanobacteria (Tellenbach et al. ). Other factors tested were oxygen and temperature, both having a known potential to induce clonal selection (Weider and Lampert ; Paul et al.…”