Functioning of deep-sea nano- and microfauna food webs fostered by sedimentation of organic matter

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Although the deep sea makes up most of the biosphere, little is known about the structure and function of its benthic microbial food webs. This is especially true for the smallest eukaryote fraction of the benthos, the protists (heterotrophic nanoflagellates, naked amoebae, and small ciliates). This contrasts with the potential importance that microorganisms may have on global carbon fluxes in the deep sea. Among the results obtained in the first phase were the following results, most of which are new to science and have been published in international journals: 1) We found an extremely high diversity of protists in deep-sea sediments, indicating the need to understand their function; 2) Diversity is specific at small and large spatial scales; 3) Specific distribution patterns were exemplified for percolocoans, cafeteriaceans, and placidid flagellates; 4) Deep-sea communities differ significantly from upper bathyal regions, indicating their isolation; 5) Sedimentation of large organic particles alters protist community structure, albeit in a relatively slow process. A renewal proposal is made here to shed light on a particular aspect, namely the role of sinking of large organic particles (exemplified by Sargassum brown algae sunk to the seafloor) on the deep-sea food web. We hypothesize that macrophyte sinking acts as a potentially important part of the biological pump, structuring communities in the deep sea. We hypothesize that Sargassum aggregates are colonized by microorganisms while still en route to the deep sea. Furthermore, we hypothesize that the deep-sea nano- and microbenthos may enhance the food quality of nutrient-poor brown algae (e.g., through PUFA enrichment), which may then provide a suitable food source for the deep-sea metazoobenthos. And we hypothesize that benthic megafauna and macrofauna are attracted to the presence of sedimented Sargassum aggregates. To address these research questions, we intend to use a dedicated small ROV system and subsequent molecular analyses of sedimented brown algae to study colonization; we intend to conduct long-term laboratory experiments with cultures of deep-sea protists and associated bacteria isolated in the first phase and added to the brown algae in common garden experiments; and we will undertake a re-analysis of existing high-resolution AUV seafloor photos of regions characterized by strong Sargassum sinking events at 5000 m depth to study the potential attraction of microbial-colonized macrophytes to benthic large macro- and megafauna.