Coordination Funds

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Marine bacteria play an essential role in global carbon cycling: Whilst algae convert carbon dioxide into carbohydrates at a rate of 50 gigatons per year, it is bacterial enzymes that degrade the algal biomass and recycle this carbon – in hitherto unknown quantities. Bacterial action thus dictates how much carbon dioxide is removed from the atmosphere and stored in the ocean (carbon sink) and how much is rechannelled into the carbon cycle. Shedding light on this black box, i.e., elucidating the underlying mechanisms, genes and enzyme repertoires that determine the fate of algal glycans is therefore mandatory for our overall understanding of the ocean’s role in the climate emergency. With these premisses in mind, we bundled our synergistic expertise in bacterial diversity, microbial ecology, (meta)genomics, (meta)proteomics, biochemistry, enzymology, and structural biology, and set out to unravel the physiologies and functions of key bacterial populations during spring phytoplankton blooms. Our studies revealed that algal polysaccharides are degraded by the successive action of numerous highly specialized bacterial strains, each of which targets a different subset of sugar intermediates. We could demonstrate that hundreds of carbohydrate-active enzymes are involved in the cycling of individual algal glycans. Moreover, we recently discovered specific intricate algal glycan structures which, unlike less complex polysaccharides, are difficult to digest and therefore hardly degraded by bacteria during algal blooms. These research efforts of FOR 2406 resulted in more than 25 joint peer-reviewed publications to date. In the third and final phase of the research unit POMPU we will strengthen our focus on protein functions involved in central polysaccharide utilization mechanisms of marine Bacteroidetes. In-depth molecular biological analyses of so-called polysaccharide utilization loci (PULs) and biochemical characterizations of identified key proteins will allow us to determine marine strategies of alpha-mannan-, sulfated xylan- and laminarin utilization. These proteogenomic analyses of cultivable, environmentally relevant key bacteria under defined in vitro conditions will complement our comparative in situ studies on particle-associated and free-living bacteria during phytoplankton blooms. We will elucidate the composition, interaction and function of complex microbiomes of marine particles and their specific glycan-cycling processes. Our interdisciplinary and holistic approach in FOR 2406 is crucial to unravel fundamental molecular mechanisms of marine polysaccharide utilization and will thus ultimately enable us to decipher hitherto unknown facets of global carbon cycling.