Abstract Vitamin B 12 (cobalamin) is a major cofactor required by most marine microbes, but only produced by a few prokaryotes in the ocean, which is globally B 12 ‐depleted. Despite the ecological importance of B 12 , the seasonality of B 12 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 12 ‐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 12 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 12 synthesis potential through the anaerobic pathway ( cbi genes). In turn, Alphaproteobacteria (HIMB11, UBA8309, Puniceispirillum ) contributed to B 12 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 12 synthesis genes were replaced by others having the same gene, thus maintaining the potential for B 12 production. Such ecological insurance could contribute to the long‐term functional resilience of marine microbial communities exposed to contrasting inter‐annual environmental conditions.