Molecular characterization of the aggregate formation-inducing interaction between Marinobacter adhaerens HP15 and the diatom, Thalassiosira weissflogii

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In light-exposed marine environments, phytoplankton cells such as diatoms photosynthetically fix atmospheric carbon dioxide and channel it into the biological pump. Most of thus-by fixed carbon re-enters the atmosphere via consumers while 1-10% of phytoplankton-fixed CO2 is transported to the deep sea via fast-sinking marine snow aggregates. Marine snow formation is significantly fostered by bacteria and by polysaccharide-containing coatings, which connect diatom cells. Dissecting the cellular interplay between diatoms and bacteria was the topic of the initial funding period. For this, a bilateral diatom-bacteria model system in combination with hypotheses-driven mutagenesis, an in vivo expression technology (IVET) screen, and a proteomics approach were used to systemically investigate this interaction and to identify bacterial genes and gene products required for it. The model system consisted of the diatom, Thalassiosira weissflogii, and the gamma-proteobacterium, Marinobacter adhaerens HP15, which showed reproducible and specific attachment to diatom cells, induced diatom-borne exopolymer secretion, and gave rise to marine snow formation. Using gene-specific mutagenesis, it could be demonstrated that both, flagellum- and pilus-mediated bacterial chemotaxis were required for initial attachment to diatom cells and aggregate formation. It could be further demonstrated that the chemical composition of transparent exopolymeric particles produced by diatoms is impacted by the presence of the bacterium. Surprisingly, IVET screen and proteomics approach revealed that M. adhaerens seems to benefit from diatom-released amino acids but not carbohydrates in a potentially phosphate limitation-dependent manner. The IVET approach furthermore showed that the bacterium might adhere to diatom surfaces via a plasmid-borne, tight adhesion locus-encoded Type IVb pilus, and might profit from its ability to withstand strongly elevated and toxic concentrations of zinc. This complex but not-yet fine-resolved picture revealed exciting new research questions on the precise functions and interplay of the above genetic traits during the interaction and the specific response(s) of the diatom which should be addressed in the extension of the research project.