Dinitrogen fixation in aphotic oxygenated marine environments

We measured NO₂ fixation rates from oceanic zones that have traditionally been ignored as sources of biological NO₂ fixation; the aphotic, fully oxygenated, nitrate (NO₃^-)-rich, waters of the oligotrophic Levantine Basin (LB) and the Gulf of Aqaba (GA). NO₂ fixation rates measured from pelagic aphotic waters to depths up to 720 m, during the mixed and stratified periods, ranged from 0.01 nmol N L^-1 d^-1 to 0.38 nmol N L^-1 d^-1. NO₂ fixation rates correlated significantly with bacterial productivity and heterotrophic diazotrophs were identified from aphotic as well as photic depths. Dissolved free amino acid amendments to whole water from the GA enhanced bacterial productivity by 2-3.5 fold and NO₂ fixation rates by ~2-fold in samples collected from aphotic depths while in amendments to water from photic depths bacterial productivity increased 2-6 fold while NO₂ fixation rates increased by a factor of 2 to 4 illustrating that both BP and heterotrophic NO₂ fixation were carbon limited. Experimental manipulations of aphotic waters from the LB demonstrated a significant positive correlation between transparent exopolymeric particle (TEP) concentrations and NO₂ fixation rates. This suggests that sinking organic material and high carbon (C): nitrogen (N) micro-environments (such as TEP-based aggregates or marine snow) could support high heterotrophic NO₂ fixation rates in oxygenated surface waters and in the aphotic zones. Indeed, our calculations show that aphotic NO₂ fixation accounted for 37 to 75% of the total daily integrated NO₂ fixation rates at both locations in the Mediterranean and Red Seas with rates equal or greater to those measured from the photic layers. Moreover, our results indicate that that while NO₂ fixation may be limited in the surface waters, aphotic, pelagic NO₂ fixation may contribute significantly to new N inputs in other oligotrophic basins, yet it is currently not included in regional or global N budgets.