Bonnet Sophie auteur aut IRD Webb E. A. auteur aut IRD Panzeca C. auteur aut IRD Karl D. M. auteur aut IRD Capone D. G. auteur aut IRD Sanudo-Wilhelmy S. A. auteur aut IRD text journalArticle eng text/pdf reformatted digital access 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 similar to 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. specialized 036 55 5 1959-1964 2010 0024-3590 https://www.documentation.ird.fr/hor/fdi:010081605 10.4319/lo.2010.55.5.1959 0024-3590 [F B010081605] https://www.documentation.ird.fr/hor/fdi:010081605 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers21-03/010081605.pdf IRD - Base Horizon / Pleins textes 2011-01-10 2021-04-01 fdi:010081605 fre