Warming accelerates termination of a phytoplankton spring bloom by fungal parasites

Abstract

Climate change is expected to favour infectious diseases across ecosystems worldwide. In freshwater and marine environments, parasites play a crucial role in controlling plankton population dynamics. Infection of phytoplankton populations will cause a transfer of carbon and nutrients into parasites, which may change the type of food available for higher trophic levels. Some phytoplankton species are inedible to zooplankton, and the termination of their population by parasites may liberate otherwise unavailable carbon and nutrients. Phytoplankton spring blooms often consist of large diatoms inedible for zooplankton, but the zoospores of their fungal parasites may serve as a food source for this higher trophic level. Here, we investigated the impact of warming on the fungal infection of a natural phytoplankton spring bloom and followed the response of a zooplankton community. Experiments were performed in ca. 1000 L indoor mesocosms exposed to a controlled seasonal temperature cycle and a warm (+4 °C) treatment in the period from March to June 2014. The spring bloom was dominated by the diatom Synedra. At the peak of infection over 40% of the Synedra population was infected by a fungal parasite (i.e. a chytrid) in both treatments. Warming did not affect the onset of the Synedra bloom, but accelerated its termination. Peak population density of Synedra tended to be lower in the warm treatments. Furthermore, Synedra carbon: phosphorus stoichiometry increased during the bloom, particularly in the control treatments. This indicates enhanced phosphorus limitation in the control treatments, which may have constrained chytrid development. Timing of the rotifer Keratella advanced in the warm treatments and closely followed chytrid infections. The chytrids' zoospores may thus have served as an alternative food source to Keratella. Our study thus emphasizes the importance of incorporating not only nutrient limitation and grazing, but also parasitism in understanding the response of plankton communities towards global warming.

Fungal parasites of a toxic inedible cyanobacterium provide food to zooplankton

“…Zoospore density at the end of the experiment was lower in the treatments where Keratella was present, as compared to the treatments where Keratella was absent. These results thus support our hypothesis that Keratella could directly feed on chytrid zoospores (Frenken et al ), and confirm that the mycoloop may transfer nutrients from large inedible algae to zooplankton (Kagami et al ). As a result, cyanobacterial blooms may not represent trophic bottlenecks, since chytrid parasites recouple primary producers to secondary consumers.…”

“…Through lysis of inedible phytoplankton cells, parasites may unlock nutrients that otherwise would remain unavailable to higher trophic levels. Fungal parasites belonging to the phylum Chytridiomycota, usually referred to as chytrids, are often involved in the decline of blooms of diatoms (Van Donk and Ringelberg ; Van Donk ; Frenken et al ) or filamentous cyanobacteria (Rasconi et al ; Gerphagnon et al , ). They are host‐specific zoosporic fungi that parasitize on phytoplankton cells and completely rely on them for nutrition and energy.…”

“…As zoospores can be grazed by zooplankton, they may allow the transfer of nutrients from primary producers to higher trophic levels (Kagami et al , , b ; Agha et al ) and as such alleviate trophic bottlenecks associated to blooms of diatoms or filamentous cyanobacteria (Rasconi et al ; Frenken et al ). Through the provisioning of zoospores, chytrid infections may thus support higher trophic levels during blooms of inedible algae (Agha et al ; Frenken et al ), also referred to as the “mycoloop” (Kagami et al ). Additionally, these zoospores can complement the zooplankton diet, since they are rich in polyunsaturated fatty acids and sterols (Kagami et al ).…”

Warming advances top‐down control and reduces producer biomass in a freshwater plankton community

“…The epidemic ended 18 days earlier in the warm treatment (Weibull; P < 0.05; Table , see also Frenken et al. ), and the dynamics in prevalence of infection differed between treatments (LME; P < 0.05; Table , Fig. a).…”

“…The phytoplankton spring bloom by Synedra , a large diatom in the size range of 75–115 μm (Frenken et al. ), was most probably unsuitable as a food source for the predominant rotifer Keratella , which has a body size range of 83–113 μm (Stemberger and Gilbert ) and can ingest food particles of 0.5–20 μm (Pourriot ). A chytrid epidemic infected up to 40% of the Synedra population in both treatments, and the end of this epidemic was advanced in the warm treatment.…”