Which bacterivorous flagellates are major grazers in summer plankton of freshwaters?
Small bacterivorous eukaryotes play a cardinal role in aquatic food webs and their taxonomic classification and ecological traits are currently hot topics in aquatic microbial ecology. Despite increasing interest in their diversity, core questions regarding predator–prey specificity remain largely unanswered, e.g., which heterotrophic nanoflagellates (HNFs) are the main bacterivores in freshwaters and which prokaryotes support the growth of these small protists.
To answer these questions, Vesna Grujčić and her colleagues from the Biology Centre CAS, performed an experiment where they fed natural communities of HNFs from the Římov reservoir (Czech Republic) with five different bacterial strains of the ubiquitous betaproteobacterial genera Polynucleobacter and Limnohabitans. They combined amplicon sequencing and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) targeting eukaryotic 18S rRNA genes to track responses of the natural HNF community to prey amendments. Moreover, they developed a double-hybridization technique that allows simultaneous phylogenetic identification of both predator and prey.
The results showed that community composition of HNFs is strongly dependent upon prey type. Surprisingly, Cryptophyta were the most abundant bacterivores, although this phylum has been so far assumed to be mainly autotrophic and thus typical algal members of phytoplankton. Furthermore, the growth of a small lineage of Cryptophyta (CRY1 clade) was strongly stimulated by one Limnohabitans strain in their experiment. Thus, this study is the first report that colorless Cryptophyta are major bacterivores in summer plankton samples and can play a key role in the carbon transfer from prokaryotes to higher trophic levels.
Each image is an overlay of three pictures of the same HNF cell observed via epiflourescence microscopy under different light excitations. Both bacterial and HNF cells are labeled with specific genetic probes (applying the CARD-FISH method). Those probes are targeting different groups of bacterivorous flagellates and bacteria in their food vacuoles. Scale bar is 2 µm.
More info:
Grujčić V., Nuy J. K., Salcher M., Shabarova T., Kasalický V., Boenigk J., Jensen M. , Šimek K. (2018) Cryptophyta as major bacterivores in freshwater summer plankton ISME Journal 12: 1668-1681.
DOI: 10.1038/s41396-018-0057-5
