# New chemical and microbial perspectives on vitamin B1 and vitamer dynamics of a coastal system

## TL;DR

This study explores how vitamin B1 and related compounds cycle in coastal waters, identifying key microbes involved in their production and use.

## Contribution

The paper provides new insights into the dynamics of vitamin B1 and vitamer cycling in coastal systems, identifying specific microbial players and seasonal patterns.

## Key findings

- Dissolved B1 concentrations remained stable across seasons, suggesting a balance between supply and demand.
- Vitamer concentrations and related gene transcripts varied seasonally, indicating their use by bacterioplankton like Pelagibacterales.
- Up to 78% of bacterioplankton taxa were B1 auxotrophs, with de novo B1 production limited to a few groups like Vulcanococcus and picocyanobacteria in summer.

## Abstract

Vitamin B1 (thiamin, B1) is an essential micronutrient for cells, yet intriguingly in aquatic systems most bacterioplankton are unable to synthesize it de novo (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: 4-amino-5-hydroxymethyl-2-methylpyrimidine, HET: 4-methyl-5-thiazoleethanol, FAMP: N-formyl-4-amino-5-aminomethyl-2-methylpyrimidine) in the particulate and dissolved pool in a temperate coastal system. Dissolved B1 was not the primary limiting nutrient for bacterial production and 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. Pelagibacterales. Genomic and transcriptomic analyses showed that up to 78% of the bacterioplankton taxa were B1 auxotrophs. Notably, de novo B1 production was restricted to a few abundant bacterioplankton (e.g. Vulcanococcus, BACL14 (Burkholderiales), Verrucomicrobiales) 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.