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Nitrogen fixation within the water column associated with two hypoxic basins in the Southern California Bight

We measured pelagic dinitrogen (N2) fixation by incorporation of 15 N2 during regular cruises over 4 yr to deep hypoxic basins in the Southern California Bight, USA. N2 fixation in the photic zone was dominated (80%) by nanoplankton (<10 µm). N2 fixation rates in surface waters were near the upper range measured for nanoplankton in tropical waters, averaging 5.8 µmol m -3 d -1 at the San Pedro Ocean Time Series (SPOTS) station, and 2.4 µmol m -3 d -1 at the Santa Monica Bay Obser- vatory (SMBO) station, with a maximum at SPOTS of 35 µmol m -3 d -1 . Quantitative polymerase chain reaction (qPCR) assays and nested PCR-based clone libraries targeting the nitrogenase gene nifH indicate that the uncultivated unicellular cyanobacterial group A (UCYN-A) is an abundant dia- zotroph in the photic zone. Although N2 fixation rates were highest at the surface, mean N2 fixation averaged 0.07 µmol m -3 d -1 at depths of 500 and 885 m within hypoxic basin waters (<10% O2 satu- ration). When integrated over the aphotic water column, this deep N2 fixation may account for as much as one-third of the total areal N2 fixation, estimated at 150 µmol N m -2 d -1 . These deep hypoxic N2 fixers were an assemblage of heterotrophic bacteria, including Alpha- and Gammaproteobacteria and putative sulfate-reducing bacteria. Our results suggest that N2 fixation could play a role in other hypoxic, high-nitrate waters.