Source Data for main figures of publication Bittner et al.

These are source data files for the main figures of manuscript Bittner et al.For description of methods and data see Bittner et al.Vitamin B1 (thiamin, B1) is an essential micronutrient for cells, yet intriguingly in aquatic systems most bacterioplankton are unable to synthesize it <i>de novo</i> (auxotrophy), requiring an exogenous source. Cycling of this valuable metabolite in aquatic systems has not been fully investigated and vitamers (B1-related compounds) have only begun to be measured and incorporated into the B1 cycle. Here, we identify potential key producers and consumers of B1 and gain new insights into the dynamics of B1 cycling through measurements of B1 and vitamers (HMP, HET, FAMP) in the particulate and dissolved pool in a temperate coastal system. Dissolved B1 was relatively stable across seasons with concentrations ranging from 74-117 pM, indicating a balance of supply and demand. However, vitamer concentration changed markedly with season as did transcripts related to vitamer salvage and transport suggesting use of vitamers by certain bacterioplankton, e.g. <i>Pelagibacterales</i>. Genomic and transcriptomic analyses showed that up to 78% of the bacterioplankton taxa were B1 auxotrophs. Notably, <i>de novo</i> B1 production was restricted to a few abundant bacterioplankton (e.g. <i>Vulcanococcus</i>, BACL14 (<i>Burkholderiales</i>), <i>Verrucomicrobiales</i>) across seasons. In summer, abundant picocyanobacteria were important putative B1 sources, based on transcriptional activity, leading to an increase in the B1 pool. Our results provide a new dynamic view of the players and processes involved in B1 cycling over time in coastal waters, and identify specific priority populations and processes for future study.