Preliminary

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.

维生素B1（硫胺素，B1）是细胞必需的微量营养素，但有趣的是，在水生生态系统中绝大多数浮游细菌无法从头合成该物质（营养缺陷型），需依赖外源来源获取。这种高价值代谢物在水生系统中的循环过程尚未得到充分研究，而B1类维生素衍生物（vitamer）的相关测量与纳入B1循环的工作也才刚刚起步。本研究通过对温带近岸海域颗粒相与溶解相中维生素B1及其衍生物（HMP：4-氨基-5-羟甲基-2-甲基嘧啶，HET：4-甲基-5-噻唑乙醇，FAMP：N-甲酰基-4-氨基-5-氨甲基-2-甲基嘧啶）的定量检测，明确了维生素B1的潜在关键生产者与消费者，并为解析B1循环动力学机制提供了新认知。研究发现，溶解态维生素B1并非细菌生产力的主要限制性营养盐，其浓度在各季节相对稳定，范围为74~117皮摩尔（pM），表明供需处于动态平衡。然而，B1类衍生物的浓度随季节发生显著变化，与衍生物补救合成及转运相关的转录本表达模式亦呈现季节差异，这表明部分浮游细菌（如Pelagibacterales菌目）可利用此类衍生物。基因组与转录组分析显示，多达78%的浮游细菌类群为维生素B1营养缺陷型。值得注意的是，从头合成维生素B1的能力仅局限于少数丰度较高的浮游细菌类群（如Vulcanococcus球菌属、BACL14（伯克霍尔德菌目）、疣微菌目（Verrucomicrobiales）），且这一特征在各季节均保持一致。夏季时，基于转录活性分析，丰度较高的微型蓝藻被认定为重要的潜在维生素B1来源，这推动了水体中B1库的扩容。本研究结果为近岸海域维生素B1循环的时序动态参与者与过程提供了全新视角，并明确了未来研究需重点关注的类群与过程。