Vitamin B 12 excretion by cultures of the marine cyanobacteria Crocosphaera and Synechococcus

Abstract

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 this value is still an order of magnitude smaller than the amount produced by the diazotroph. The cyanobacterial contribution to the oceanic B 12 supply may be ~ 50 times higher than the contribution of heterotrophic bacteria. Oceanic cyanobacteria do not compete for exogenous B 12 with other prokaryotes and eukaryotes, but instead are obligate producers, thereby defining a unique dual ecological role for N 2 ‐fixing cyanobacteria in the oligotrophic ocean. They provide both a source of “new” bioavailable nitrogen and the B 12 needed to support the growth of larger auxotrophic eukaryotic phytoplankton important for the biological carbon pump.

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