Axenic cultures of the N 2 ‐fixing marine cyanobacterium Crocosphaera watsonii exhibit very high B 12 excretion rates (up to 40 times higher per cell), compared to the smaller non‐N 2 ‐fixing strain of marine Synechococcus . The effect of N availability on vitamin synthesis is also evident in the non‐diazotrophic strain, as they release five times more B 12 under N‐replete conditions relative to N‐limiting conditions, although...