Identification of key bacteria for ecological dynamics in a coastal marine observatory

Bacteria are essential for ecosystem maintenance, drive biogeochemical cycles, and influence responses to climate change. Additionally, many are involved in blooms and toxin production, affecting environmental quality. Despite their crucial role, some of their characteristics are only now being understood with the development of molecular techniques independent of cultivation. Thus, the recent effort to unify classical and microbial ecology studies has led to a better understanding of their dynamics. Here, we investigate the temporal processes that influence bacterial abundance and persistence, as well as their role in promoting or inhibiting the presence of certain organisms in the core of a metacommunity. Using molecular approaches such as amplicon and metagenomic analyses, we retrieved information on key bacteria (those with high abundance and/or persistence) from the South Atlantic Microbial Observatory (SAMO) to assess their interactions with environmental and biological factors. Bimodality was hardly observable, while the community structuring reflected the influence of large scale environmental processes in a recurrent pattern that clusters winter-spring and summer-autumn communities, reinforcing previous findings of intense influence of marine currents in this region. Among the key organisms, SAR11 and Flavobacteriaceae stand out. Flavobacteriaceae is known for its adaptive capacity and large genomes, while SAR11 has a reduced genome, making it dependent on compounds produced by other organisms. We identified an annual fluctuation in its abundance linked to Synechococcus blooms. The significant co-occurrence of ASVs from both groups reinforces the evidence of a biological interaction that sustains SAR11 through the exchange of these compounds. Through this study, we contribute to clarifying the factors that locally influence the geographic patterns of marine bacteria and identifying the pathways that promote their dynamics and functions in the ecosystem.