The role of marine fungi in the microbial loop

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Marine food webs are largely driven by the interplay of carbon fixation, primary production, and remineralization of organic matter. These organic substances are chemically diverse and occur in dissolved, adsorbed or aggregated forms. They are degraded by a wide range of heterotrophic microorganisms. The role of bacteria in this process is comparatively well understood. They form metabolic subpopulations with complementary decomposition strategies, resulting in so-called metabolic guilds. Although marine fungi are ubiquitously present in pelagic microbial communities, their role in the microbial loop is still largely unknown. Whether and how fungi interact with bacteria in the microbial loop influences the efficiency of marine carbon transformation and is thus of high scientific value. Here we hypothesize that (1) fungi and bacteria form specific metabolic guilds depending on the available organic carbon source, and (2) thus increase the degradation potential of the guild via interkingdom interactions. This is investigated with time series in-vitro experiments. For this purpose, seawater will be sampled during geographically different phytoplankton bloom events and concentrated to a microbial inoculum of fungi and bacteria. The inoculum will be used to inoculate different dissolved organic substrates, representative of phytoplankton exudates, to establish specific metabolic guilds. These guilds will then be studied for phylogenetic analysis by tag-sequencing and chemical analyses by LC-MS and NMR of their degradation products.