Marine microbial communities are fundamental to nutrient and biogeochemical cycling, with intricate networks of metabolic interdependencies influencing their structure and dynamics. Among these, vitamins B₁ (thiamin) and B₁₂ (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 B₁/B₁₂ auxotrophs (need for an exogenous vitamin source) were present throughout the year. Among B₁ auxotrophs, those requiring the thiamin precursor HMP were the most prevalent, with peak abundances in summer. Distinct metagenome-assembled genome co-abundance patterns between B₁ and B₁₂ producers/auxotrophs across seasons suggested mutualistic relationships. Double B₁/B₁₂ vitamin complementarities were more common in summer, and single vitamin complementarity was dominant in winter. As previously shown for vitamin B₁₂, which is limiting during winter, bioassays revealed variable availability of vitamin B₁ in winter seawater despite the abundance of its producers, suggesting potential transfer of vitamin B₁ among microorganisms. Finally, microcosm experiments showed that B₁ and B₁₂ amendments significantly influenced the composition of microbial communities, with temporal variations in their impact. In some cases, B₁₂ and B₁ amendments favored both vitamin auxotrophs and producers, highlighting complex interdependencies between B₁ and B₁₂ producers and consumers. Our findings highlight the complexity of B vitamin-mediated 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.

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