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进行从头合成（de novo）以获取该物质，属于营养缺陷型（auxotrophy），必须依赖外源来源。这类珍贵代谢物在水生系统中的循环尚未得到充分研究，而维生素体（vitamer，即B1相关化合物）的检测与纳入B1循环的工作才刚刚起步。本研究通过对温带近岸系统中颗粒态与溶解态库内的B1及维生素体（vitamer）——包括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——进行检测，鉴定了B1的潜在关键生产者与消费者，并为解析B1循环的动态特征提供了新见解。溶解态B1并非细菌生产力的主要限制性营养盐，其浓度在74~117皮摩尔（pM）范围内随季节相对稳定，表明供需处于平衡状态。然而，维生素体浓度随季节发生显著变化，与维生素体补救及转运相关的转录本亦是如此，这表明部分浮游细菌（如远洋杆菌目（Pelagibacterales））会利用维生素体。基因组与转录组分析显示，多达78%的浮游细菌类群属于B1营养缺陷型。值得注意的是，从头合成B1的能力仅局限于少数丰度较高的浮游细菌类群（如火山球菌属Vulcanococcus、BACL14（伯克霍尔德氏菌目Burkholderiales）、疣微菌目Verrucomicrobiales），且这一特征在全季节中保持稳定。夏季时，基于转录活性分析，丰度较高的微型蓝细菌是重要的推定B1来源，这推动了B1库的增加。本研究结果为近岸水域B1循环随时间变化的参与类群与过程提供了全新的动态视角，并确定了未来研究需重点关注的特定类群与过程。