Recent findings indicate that N-2 fixation is significant in aphotic waters, presumably due to heterotrophic diazotrophs depending on organic matter for their nutrition. However, the relationship between organic matter and heterotrophic N-2 fixation remains unknown. Here we explore N-2 fixation in the deep chlorophyll maximum and underneath deep waters across the whole Mediterranean Sea and relate it to organic matter composition, characterized by optical and molecular methods. Our N-2 fixation rates were in the range of those previously reported for the euphotic zone of the Mediterranean Sea (up to 0.43nmolNL(-1)d(-1)) and were significantly correlated to the presence of relatively labile organic matter with fluorescence and molecular formula properties representative for peptides and unsaturated aliphatics and associated with the presence of more oxygenated ventilated water masses. Finally, and despite that the aphotic N-2 fixation contributes largely to total water column diazotrophic activity (>50%), its contribution to overall nitrogen inputs to the basin is negligible (<0.5%).
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