Basin-wide aphotic N2 fixation in the Mediterranean Sea

Abstract

Original research paperRecent findings indicate that N2fixation is significant in aphotic waters, presumably due toheterotrophic diazotrophs depending on organic matter for their nutrition. However, the relationshipbetween organic matter and heterotrophic N2fixation remains unknown. Here we explore N2fixation inthe deep chlorophyll maximum and underneath deep waters across the whole Mediterranean Sea andrelate it to organic matter composition, characterized by optical and molecular methods. Our N2fixation rateswere in the range of those previously reported for the euphotic zone of the Mediterranean Sea (up to0.43 nmol N L1d1) and were significantly correlated to the presence of relatively labile organic matter withfluorescence and molecular formula properties representative for peptides and unsaturated aliphatics andassociated with the presence of more oxygenated ventilated water masses. Finally, and despite that theaphotic N2fixation contributes largely to total water column diazotrophic activity (>50%), its contribution tooverall nitrogen inputs to the basin is negligible (<0.5%).Project HOTMIX (CTM2011-30010-C02-01 and CTM2011-30010-C02-02) ; the MarieSklodowska-Curie Actions of theEuropean Union’s Seventh FrameworkProgramme (FP7/2007-2013) undergrant agreement 625185; the Institut de Recherchepour le Développement (IRD); FPI and short stay fellowships from the Spanish Ministry ofEconomy and Competitiveness; the CSIC Program“Junta para la Ampliación de Estudios”cofinanced by the ESF and a collaborative grant with S.B.from MOST Israel and the High Councilfor Science and Technology (HCST)Versión del editor4,78