Vitamin B (cobalamin) is a major cofactor required by most marine microbes, but only produced by a few prokaryotes in the ocean, which is globally B -depleted. Despite the ecological importance of B , the seasonality of B metabolisms and the organisms involved in its synthesis in the ocean remain poorly known. Here we use metagenomics to assess the monthly dynamics of B -related pathways and the functional diversity of associated microbial communities in the coastal NW Mediterranean Sea over 7 years. We show that genes related to potential B metabolisms were characterized by an annual succession of different organisms carrying distinct production pathways. During the most productive winter months, archaea (Nitrosopumilus and Nitrosopelagicus) were the main contributors to B synthesis potential through the anaerobic pathway (cbi genes). In turn, Alphaproteobacteria (HIMB11, UBA8309, Puniceispirillum) contributed to B synthesis potential in spring and summer through the aerobic pathway (cob genes). Cyanobacteria could produce pseudo-cobalamin from spring to autumn. Finally, we show that during years with environmental perturbations, the organisms usually carrying B synthesis genes were replaced by others having the same gene, thus maintaining the potential for B production. Such ecological insurance could contribute to the long-term functional resilience of marine microbial communities exposed to contrasting inter-annual environmental conditions.