Fungal symbionts of Arctic zooplankton induce mortality and reduce swimming activity

Zooplankton are key intermediaries between microbes and higher trophic levels. Elevated temperatures in the Arctic are expected to increase metabolic rates, which in turn will result in increased rates of zooplankton production and foraging. We postulate that this may also be tied to elevated incidence of fungi parasitism in key zooplankton species. We present data of in-situ fungal parasitism in the key Arctic zooplankton species complex Calanus spp. We found that >10% of Calanus specimens were internally parasitized by fungi that had 100% molecular identity to Penicillium chrysogenum. Experimental studies with two strains of P. chrysogenum and Calanus spp. revealed an increased mortality effect suggesting that fungi decrease the survival of overwintering zooplankton, which may have cascading effects on higher trophic levels that feed on Calanus. Moreover, Calanus demonstrated reduced swimming activity in the presence of P. chyrsogenum, indicating that fungi could affect predator avoidance and vertical migration patterns of zooplankton. Semi-thin sectioning of live zooplankton incubated with P. chrysogenum revealed the presence of hyphae throughout the body, underscoring the parasitic nature of P. chrysogenum in Calanus. We hypothesize that as the Arctic warms, the balance between host and pathogen could shift in favor of pathogen reproduction, resulting in a greater effect of fungal pathogens on zooplankton.