Vitamin B12 excretion by cultures of the marine cyanobacteria Crocosphaera and Synechococcus

Abstract

Axenic cultures of the N2-fixing marine cyanobacterium Crocosphaera watsonii exhibit very high B12 excretion rates (up to 40 times higher per cell), compared to the smaller non-N2-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 B12 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 B12 supply may be ∼ 50 times higher than the contribution of heterotrophic bacteria. Oceanic cyanobacteria do not compete for exogenous B12 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 B12 needed to support the growth of larger auxotrophic eukaryotic phytoplankton important for the biological carbon pump. © 2010, by the American Society of Limnology and Oceanography, Inc.