Diversity, phylogeny and ecological role of heterotrophic protists in the pelagic redoxcline of the central Baltic Sea
Zusammenfassung der Projektergebnisse
Oxic-anoxic transition zone (redoxclines) in marine oxygen deficient water columns (e.g. in the Baltic and Black Sea) are documented sites of high microbial activity and important biogeochemical transformations. Much less is known about the diversity and function of microbial eukaryotes (protists) and microbial food web structures. Therefore the aim of this project was to investigate the composition and functional role of phagotrophic protists in pelagic redoxclines by conducting field research in the central Baltic and the Black Sea, accompanied by laboratory experiments with isolated organisms. Only the combination of molecular techniques (18S rRNA gene and transcript analyses) with different microscopical approaches resulted in a comprehensive picture of the stratified protist communities along the redox gradients, from suboxic to sulfidic water depths, in the Baltic and Black Seas. Newly developed primers (V4 region of the 18S rRNA gene), which cover most of the protist diversity and which can be used for high-throuput sequencing techniques, were successfully tested with model communities and field samples. Different functional groups and specific key taxa among ciliates and flagellates dominated at different oxygen concentrations, with an astonishing diversity also in anoxic and even sulfidic water layers. Besides the identification of protist groups which seem to occur globally in marine oxygen-deficient systems, also system-specific differences became apparent. The microbial food web structure in oxygendeficient systems differs fundamentally from the surface waters as metazoans are missing, several trophic groups within the protist assemblage occur, and bacterivory is mainly governed by ciliates and dinoflagellates instead of nanoflagellates. The vertical distribution pattern of the different protist groups was also reflected in their grazing pressure on prokaryotes which was assessed by incubations with fluorescently labelled bacteria. The obtained grazing rates were compared to estimates of viral lysis, derived from an assessment of virally infected prokaryotic cells. Significant protist grazing pressure could be measured for suboxic layers and around the oxic-anoxic interface, where 50-100 % of prokaryotic standing stocks are removed per day due to grazing. In contrast, sulfidic waters seem to constitute a grazing refuge for prokaryotic cells due to low protist abundance. Interestingly, protist taxa are characterized here by abundant bacterial endosymbionts. The hypothesis, that viral lysis becomes the major mortality factor in sulfidic waters was not supported from the experimental results, leaving the identity of the major prokaryotic mortality factor at these depths unresolved. Protist grazing also affects prokaryotes which are key players for biogeochemical transformations in pelagic redoxclines. This could be demonstrated for the first time by in situ grazing experiments with labelled cells of a cultured representative of the Sulfurimonas group (Epsilonproteobacteria) which is responsible for chemoautotrophic denitrification and sulfide oxidation. Protist grazing can balance the cell production of Sulfurimonas spp. and constitutes a major regulating factor for its abundance, growth and vertical distribution in pelagic redox gradients, thereby also significantly influencing biogeochemical cycling. For assessing the diversity of hitherto uncultured heterotrophic nanoflagellates (HNF), unamended incubations proved to be suitable in which growth of HNF followed bacterial growth. The 18S rRNA gene sequence analysis of dominating HNF in these incubations provided valuable information for the development of specific probes that can be used for in situ quantifications. Further, efforts were successful to isolate and describe new taxa of flagellates derived from oxygen-deficient water depths in the Baltic Sea. Isolates of choanoflagellates, which showed in culture considerable growth at very low oxygen concentrations, are especially interesting with regard to their mitochondrial ultrastructures which might constitute adaptations to varying oxygen concentrations. These cultures open up new directions for the study of the physiology, adaptations and ecological functions of protists in hypoxic marine waters.
Projektbezogene Publikationen (Auswahl)
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(2011): Protist diversity in suboxic and sulfidic waters of the Black Sea. Environ. Microbiol. 13: 2939-2956
Wylezich, C., Jürgens, K.
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(2012): Ecologically relevant choanoflagellates collected from hypoxic water masses of the Baltic Sea have untypical mitochondrial cristae. BMC Microbiology 12:271
Wylezich, C., Karpov, S.A., Mylnikov, A.P., Anderson, R., Jürgens, K.
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(2012): Protist grazing and viral lysis as prokaryotic mortality factors at Baltic Sea oxic-anoxic interfaces. Mar. Ecol. Prog. Ser. 467: 1-14
Anderson, R., Winter, C., Jürgens, K.
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(2012): Unveiling trophic functions of uncultured protist taxa by incubation experiments in the brackish Baltic Sea. PLOS One 7: e41970
Weber, F., Campo, J.d., Wylezich, C., Massana, R., Jürgens, K.
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(2013): Impact of protist grazing on a key bacterial group for biogeochemical cycling in Baltic Sea pelagic oxic/anoxic interfaces. Environ. Microbiol. 15: 1580-1594
Anderson, R., Wylezich, C., Glaubitz, S., Labrenz, M., Jürgens, K.
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(2014): Morphological and molecular approaches reveal highly stratified protist communities in Baltic Sea pelagic redox gradients. Aquat. Microb. Ecol. 73: 1-16
Weber, F., Anderson, R., Foissner, W., Mylnikov, A.P., Jürgens, K.
