Résumé
Marine microbial communities are fundamental to nutrient and biogeochemical cycling, with intricate networks of metabolic interdependencies influencing their structure and dynamics. Among these, vitamins B 1 (thiamin) and B 12 (cobalamin) play crucial roles as enzymatic cofactors in central metabolic pathways. Despite their importance, the temporal dynamics of vitamin production, bioavailability, and associated microbial interactions remain poorly understood. Using a 7-year monthly metagenomic time series from the NW Mediterranean Sea (SOLA station), we found that vitamin B 1 /B 12 auxotrophs (need for an exogenous vitamin source) were present throughout the year. Among B 1 auxotrophs, those requiring the thiamin precursor pyrimidine were the most prevalent, with peak abundances in summer. Distinct metagenome-assembled genome co-abundance patterns between B 1 and B 12 producers/auxotrophs across seasons suggested mutualistic relationships. Double B 1 /B 12 vitamin complementarities were more common in summer, and single vitamin complementarity was dominant in winter. As previously shown for vitamin B 12 , which is limiting during winter, bioassays revealed variable availability of vitamin B 1 in winter seawater despite the abundance of its producers, suggesting potential transfer of vitamin B 1 among microorganisms. Finally, microcosm experiments showed that B 1 and B 12 amendments significantly influenced the composition of microbial communities, with temporal variations in their impact. In some cases, B 12 and B 1 amendments favored both vitamin auxotrophs and producers, highlighting complex interdependencies between B 1 and B 12 producers and consumers. Our findings highlight the complexity of B vitaminmediated metabolic interactions that shape microbial community dynamics and underscore the need for long-term, high-resolution studies to better understand vitamin-driven ecological processes in marine systems.