The soluble B vitamins (B1, B7, and B12) have long been recognized as playing a central metabolic role in marine phytoplankton and bacteria; however, the importance of these organic external metabolites in marine ecology has been largely disregarded, as most research has focused on inorganic nutrients and trace metals. Using recently available genomic data combined with culture-based surveys of vitamin auxotrophy (i.e., vitamin requirements), we show that this auxotrophy is widespread in the marine environment and occurs in both autotrophs and heterotrophs residing in oligotrophic and eutrophic environments. Our analysis shows that vitamins originate from the activities of some bacteria and algae and that taxonomic changes observed in marine phytoplankton communities could be the result of their specific vitamin requirements and/or vitamin availability. Dissolved vitamin concentration measurements show that large areas of the world ocean are devoid of B vitamins, suggesting that vitamin limitation could be important for the efficiency of carbon and nitrogen fixation in those regions.

We have developed a new method to directly measure the concentration of the particulate and dissolved pools of B-vitamins in seawater. B-vitamins are coenzymes required for life, yet many organisms cannot synthesize these compounds de novo and must scavenge them from the environment. It has been shown that B-vitamins can control the marine microbial community's structure and activity; however, the actual B-vitamin requirements/quotas for marine microbes are not well studied due to the lack of analytical protocols. This method will enable the study of B-vitamin cellular quotas as well as their environmental cycling. With this method, we can also simultaneously determine the biochemical congeners of vitamins B 1 and B 12 , as well as vitamins B 2 , B 6 , B 7 , and the amino acid methionine in both the particulate and dissolved pool, using liquid chromatography-mass spectrometry after a chemical extraction (particulate) or resin preconcentration (dissolved). Particulate and dissolved B-vitamin concentrations were simultaneously determined from a microbial community in the Atlantic Ocean. Particulate B-vitamin concentrations in the Atlantic ranged from 0.01 pM (cyano-B 12 ) to 46.4 pM (thiamin monophosphate, a B 1 congener) while dissolved B-vitamin concentrations ranged from 0.07 pM (adenosyl-B 12 ) to 679.4 pM (B 7 ). The application of this technique in different marine systems has the potential to shed new light on previously unknown biochemical processes occurring within the oceanic microbial community, and the resulting roles B-vitamins play in regulating global biogeochemical cycles.

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.

Vitamin B1 (thiamin) is an essential coenzyme for all cells. Recent findings from experimental cell biology and genome surveys have shown that thiamin cycling by plankton is far more complex than was previously understood. Many plankton cells cannot produce thiamin (are auxotrophic) and obligately require an exogenous source of thiamin or one or more of 5 different thiamin-related compounds (TRCs). Despite this emerging evidence for the evolution among plankton of complex interactions related to thiamin, the influence of TRCs on plankton community structure and productivity are not understood. We report measurements of three dissolved TRCs 4-amino-5-aminomethyl-2-methylpyrimidine (AmMP), 5-(2-hydroxyethyl)-4-methyl-1,3-thiazole-2-carboxylic acid (cHET), and 4-methyl-5-thiazoleethanol (HET) that have never before been assayed in seawater. Here we characterize them alongside other TRCs that were measured previously [thiamin and 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP)], in depth profiles from a latitudinal transect in the north Atlantic in March 2018. TRC concentrations ranged from femptomolar to picomolar. Surface depletion relative to a maximum near the bottom of the euphotic zone and low concentrations at deeper depths were consistent features. Our observations suggest that when bacterial abundance and production are low, TRC concentrations approach a steady state where TRC production and consumption terms are balanced. Standing stocks of TRCs also appear to be positively correlated with bacterial production. However, near the period of peak biomass in the accumulation phase of a bloom we observed an inverse relationship between TRCs and bacterial production, coincident with an increased abundance of Flavobacteria that comparative genomics indicates could be vitamin B1 auxotrophs. While these observations suggest that the dissolved pool of TRCs is often at steady state, with TRC production and consumption balanced, our data suggests that bloom induced shifts in microbial community structure and activity may cause a decoupling between TRC production and consumption, leading to increased abundances of some populations of bacteria that are putatively vitamin B1 auxotrophs.