We examined rates of N-2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niño (2010) and La Niña (2011). Replicated vertical profiles performed under oxygen-free conditions show that N-2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 mu mol N m(-2) d(-1) in 2010 and 24 +/- 14 to 118 +/- 87 mu mol N m(-2) d(-1) in 2011. In the aphotic layers, volumetric N-2 fixation rates were relatively low (< 1.00 nmol N L-1 d(-1)), but when integrated over the whole aphotic layer, they accounted for 87-90% of total rates (euphotic+ aphotic) for the two cruises. Phylogenetic studies performed in microcosms experiments confirm the presence of diazotrophs in the deep waters of the Oxygen Minimum Zone (OMZ), which were comprised of non-cyanobacterial diazotrophs affiliated with nifH clusters 1K (predominantly comprised of alpha-proteobacteria), 1G (predominantly comprised of gamma-proteobacteria), and 3 (sulfate reducing genera of the delta-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N-2 fixation in the core of the OMZ at all stations tested and as did simple carbohydrates at stations located nearest the coast of Peru/Chile. The episodic supply of these substrates from upper layers are hypothesized to explain the observed variability of N-2 fixation in the ETSP.
