LAUSR

Abstract Vitamin B12 (cobalamin), a large and complex organometallic molecule composed by a corrin ring with a central cobalt atom, is an essential coenzyme for nearly half of phytoplankton species but its production in marine ecosystems is restricted to prokaryotic organisms. Vitamin B 12 auxotrophy is widespread in the marine microbial community not being linked to any species lineage. Knowledge about the role of this organic compound on the control of the structure, composition and growth of natural marine microbial assemblages is scarce, but recent studies carried out in shelf waters off NW Iberian Peninsula have demonstrated that growth of phytoplankton and heterotrophic bacteria can be affected by the availability of this vitamin. Liquid Chromatography coupled to tandem mass spectrometry (LC-MS/MS) has been set up as an analytical tool for the first direct determination of vitamin B 12 present in these coastal waters enabling the separation of two different B 12 analogs: hydroxocobalamin (HyB12) and cyanocobalamin (CyB12). In this study, samples were collected from shelf (coastal station) and shelf-break waters of the NW Iberian Peninsula in February, April and August 2016 in order to encompass both the spatial and temporal variability of vitamin B 12 concentrations in seawater. Concentrations of dissolved vitamin B 12 varied from 0.15 pM, at the shelf-break station, to 2.7 pM at the coastal station, HyB12 being the most abundant congener. Overall, the low concentrations of dissolved vitamin B 12 and the significant and positive relationship between depth-weighted water column averages of Chl-a and vitamin B 12 (r 2 = 0 . 80 , p>0.001), suggest that this micronutrient might play a significant role in phytoplankton biomass production during the spring productive period of this coastal ecosystem. The temporal cycle elucidated from this data described an exhaustion of vitamin B 12 winter inventory by direct consumption during the phytoplankton productive period and a stock recovery in August.