Diversity, phylogeny and ecological role of heterotrophic protists in the pelagic redoxcline of the central Baltic Sea

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The water column of the Baltic Sea, but also of several other marginal and coastal seas, is characterized in the deeper basins by pronounced vertical gradients in oxygen and redox conditions. Our knowledge regarding the diversity and function of microorganisms in pelagic redoxclines is based mainly on data from prokaryotic communities (bacteria, archaea) which are mediating important biogeochemical processes in the transition zone from suboxic to anoxic waters (redoxcline). Comparable studies on microbial eukaryotes (protists) are still largely lacking despite the general assumption that they exert a strong impact on the prokaryotic assemblages. The aim of this project is to investigate the composition and functional role of phagotrophic protists (mainly flagellates and ciliates) along the redox gradients. The major hypotheses are: (1) that there occurs a fundamental shift in the structure of the microbial food web from oxic to anoxic/sulfidic waters, with a shift from heterotrophic nanoflagellates (HNF) to microaerophilic ciliates and bacteriophages as major agents of bacterial losses; (2) that interactions between protists and prokaryotes, which comprise both bacterivory and symbiotic associations, impact the structure and activity of the prokaryotic communities and modulate biogeochemical processes mediated by functional guilds of prokaryotes (e.g., denitrifiers, ammonia oxidizers, sulfur oxidizers etc.). To this aim, the quantitative species composition of phagotrophic protists will be examined by a combination of molecular techniques (18S DNA/RNA fingerprints and clone libraries, design and application of specific gene probes for in situ hybridization and quantitative PCR), classical microscopical methods, and incubation experiments with natural assemblages and isolated organisms. In order to understand the impact of the oxygen gradients on the major mortality factors for planktonic prokaryotes, we will quantify mortality rates due to protist grazing and viral lysis along the gradients. Different processes (dark CO2 fixation, nitrification, denitrification, sulfur oxidation etc.) will be assessed in experimental incubations to examine whether these rates are modulated by protist-prokaryote interactions. The Baltic Sea redoxcline will serve as a model system to reveal a general understanding of bacteria-protist interactions at oxic-anoxic interfaces. Samples and experiments from other marine redoxclines (e.g., Black Sea, Norwegian fjords, Cariaco Basin) will serve to test whether the patterns observed in the Baltic Sea can be generalized.