Horizon 1 1 17 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES Limnology and Oceanography 1959-1964 2010 fdi:010081605 ENG Limnology and Oceanography 0024-3590 ISI:000283667100014 5 10.4319/lo.2010.55.5.1959 https://www.documentation.ird.fr/hor/fdi:010081605 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers21-03/010081605.pdf 55 Horizon (IRD) 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. 036 UR213