A Mutant Fitness Assay Identifies Bacterial Interactions in a Model Ocean Hot-Spot

Jeremy E. Schreier, Christa B. Smith, Thomas R. Ioerger, Mary Ann Moran

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Ecological interactions that occur between phycosphere-associated bacteria at the micron scale have the potential to influence a major fraction of annual carbon flux at the global scale. Despite the importance of microbial carbon flow, studying the ecology of these microenvironments remains challenging. We identified genes that mediate bacterial interactions in phycosphere communities by culturing a transposon mutant library of Ruegeroai pomeroyi DSS-3 with the diatom Thalassiosira pseudonana CCMP1335 as the sole source of organic matter in the presence or absence of other heterotrophic bacterial species. The function of genes that significantly impacted R. pomeroyi fitness indicated explicit cell-cell interactions occurring in the multi-bacterial phycospheres. We found that R. pomeroyi simultaneously competed for shared substrates while increasing reliance on substrates that did not support the other species’ growth. Fitness outcomes also elucidated bacteria competition for nitrogen, cross-feeding of amino acids, purines, pyrimidines, and cofactors, antagonistic interactions between bacterial species, and evidence for altered oxygen and superoxide levels driven by microbial activity. These results provided insights into the potential mechanisms of interspecies interactions in a model marine hot-spot of biogeochemical importance.