B Vitamins and Their Congeners as Potential Drivers of Microbial Community Composition in an Oligotrophic Marine Ecosystem

Determining the factors that influence marine microbial growth and community structure are critical for the understanding of global carbon cycling. Since the early twentieth century, it has been known that B vitamins play an important role in phytoplankton community dynamics. Limited oceanic dissolved B vitamin distributions indicate that these important coenzymes are present at picomolar levels, which could be too low to support maximal phytoplankton growth, and vast regions of the ocean exist where they are undetectable. Despite their importance, particulate B vitamin concentrations of field microbial populations are unknown. Here we report B vitamin concentrations measured in both the particulate and dissolved fractions, including multiple biochemically relevant B vitamin congeners. We establish their spatial distributions spanning distinct biogeographic and oceanographic regimes in the Mediterranean Sea and the Eastern Atlantic Ocean and show that all congeners are present both dissolved in seawater and in suspended particles. We observe that B vitamins cooccur in patches defined by regional biogeographic and oceanographic features. Additionally, distinct patterns of congener relative abundance in the dissolved and particulate pools provide insight to biological and chemical cycling of these compounds between and within the dissolved and particulate pools. Finally, linear model results demonstrate that model fits of microbial assemblages are strongest when they include both inorganic nutrients and dissolved B vitamin concentrations. We believe that these findings represent an advance in our understanding of B vitamin oceanographic distributions and point to interesting hypotheses of their influence on marine microbial ecology.