Interrelating the availability of particulate and dissolved B-vitamins with marine microbial diversity in the Mediterranean Sea

Abstract

Determining the factors that control phytoplankton growth and community structure is critical for understanding global carbon cycling. Since the early 20th century, it has been known that B-vitamins play a major role in phytoplankton community growth, production, and composition. However, the limited oceanic dissolved B-vitamin measurements that exist indicate that these important coenzymes are present at pM concentrations, which are too low to support maximal phytoplankton growth. In addition, there are vast regions of the ocean where these B-vitamins are undetectable and potentially limiting to the microbial community. Despite their importance, particulate B-vitamin quotas in field microbial populations are unknown, as are the distributions of important biochemical B-vitamin congeners. Here we present B-vitamin concentrations measured in both the particulate and dissolved fractions, including multiple biochemically relevant B-vitamin congeners (B1, TMP, TPP, HMP, B7, AB12, CB12, MB12, HB12, and the amino acid methionine). Spatial distributions of B-vitamins spanning distinct microbiological and oceanographic regimes in the Mediterranean Sea and the eastern Atlantic Ocean show that all congeners are present both dissolved in seawater and within suspended particles. We observed that B-vitamins co-occur in distinct biogeographic patches. Additionally, B-vitamin quotas were observed to be highest at depths where Chl-a and bacterial activity were the greatest, indicating that microbial B-vitamin requirements are potentially related to microbial growth. Finally, linear regression models demonstrated strong, statistically significant, correlations between the concentrations of dissolved B-vitamins and the abundances of microbes with obligate B-vitamin requirements. These findings represent a substantial advance in our understanding of how B-vitamins influence marine microbial ecology, and suggests that B-vitamin availability shapes microbial community structure, thus affecting the strength of the biological carbon pump