Coastal Vitamin B1 dynamics - Roskilde fjord

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。目前学界尚未完全阐明该珍贵代谢物在水生系统中的循环过程，且维生素B1相关代谢物（vitamers）的检测与纳入B1循环研究仅刚刚起步。本研究通过对温带沿岸系统中颗粒态与溶解态组分内的B1及其相关代谢物——4-氨基-5-羟甲基-2-甲基嘧啶（HMP）、4-甲基-5-噻唑乙醇（HET）、N-甲酰基-4-氨基-5-氨甲基-2-甲基嘧啶（FAMP）——进行定量检测，明确了B1的潜在关键生产者与消费者，并为解析B1循环动力学提供了全新认知。溶解态B1并非细菌生产力的主要限制营养因子，其浓度在74–117 pM范围内随季节保持相对稳定，表明供需处于动态平衡。不过，相关代谢物的浓度随季节发生显著变化，与代谢物补救合成及转运相关的转录本表达也随之改变，提示部分浮游细菌（如远洋杆菌目（Pelagibacterales））可利用这类代谢物。基因组与转录组分析显示，多达78%的浮游细菌类群为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全年优势浮游细菌类群，例如沃尔卡诺球菌属（Vulcanococcus）、伯克霍尔德菌目（Burkholderiales）的BACL14类群以及疣微菌目（Verrucomicrobiales）。夏季时，基于转录活性判断，优势微微型蓝细菌成为重要的潜在B1来源，推动了B1库的丰度上升。本研究结果为沿岸水域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，并明确了未来研究需优先关注的特定类群与过程。