Grazing by zooplankton can regulate bloom-forming cyanobacteria but can also transfer toxin-producing cells, as well as toxic metabolites, to the food web. While laboratory investigations have provided extensive knowledge on zooplankton and toxic cyanobacteria interactions, information on zooplankton feeding on toxin-producing cyanobacteria in natural water bodies remains scarce. In this study, we quantified Microcystis-specific mcyE synthase genes from the gut contents of various cladoceran and copepod taxa to assess the in situ crustacean community and taxon-specific ingestion of potentially toxic Microcystis in Lake Peipsi, a large eutrophic lake in Estonia, Northern Europe. Microcystis cells with mcyE genes were found in all crustaceans examined. However, some species, such as the cyclopoid copepod Mesocyclops leuckarti, were more efficient in ingesting potentially toxic Microcystis than other co-occurring cladocerans (Daphnia spp., Bosmina spp., Chydorus sphaericus) and copepods (Eudiaptomus gracilis). The amount of toxigenic Microcystis cells grazed by crustacean population changed temporarily, and copepods were the predominant consumers of toxigenic Microcystis during several months of the 5-month study period. Crustacean ingestion of toxigenic Microcystis was not related to Microcystis biomass or mcyE gene copy numbers in the environment but was instead related to the abundance of major crustacean grazers. Our findings emphasize the close interaction between crustacean zooplankton and toxigenic Microcystis, indicating that some species may play a more significant role in linking toxic cells within the food web than others.

Chydorus sphaericus is often a dominant cladoceran zooplankton species in water bodies experiencing harmful cyanobacterial blooms. However, its relationship with toxin-producing algae remains largely unexplored. In this study, the feeding behavior of C. sphaericus on colonial cyanobacteria and potentially toxic Microcystis was investigated in a temperate, shallow, eutrophic lake. Liquid chromatographic analyses of phytoplankton marker pigments in C. sphaericus gut content revealed that pigments characteristic of cyanobacteria (identified a zeaxanthin, echinenone, and canthaxanthin) comprised the majority of its diet. Among them, colonial cyanobacteria (marked by the pigment canthaxanthin) were the highly preferred food source despite their minor contribution to phytoplankton biomass. qPCR targeting Microcystis genus-specific mcyE synthase genes, which are involved in microcystin biosynthesis, indicated that potentially toxic strains of Microcystis were present in C. sphaericus gut content throughout its temporal and spatial presence in the lake. The results suggest that the common small cladoceran in eutrophic waters, C. sphaericus, has a close trophic interaction with colonial cyanobacteria (including Microcystis) and may represent an important vector for transferring toxigenic Microcystis to the food web, even under conditions of low Microcystis biomass in the lake water.