Abstract
We examined rates of N2 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 N2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 µmol N m−2 d−1 in 2010 and 24±14 to 118±87 µmol N m−2 d−1 in 2011. In the aphotic layers, volumetric N2 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 α-proteobacteria), 1G (predominantly comprised of γ-proteobacteria), and 3 (sulfate reducing genera of the δ-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N2 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 N2 fixation in the ETSP.
Highlights
The efficiency of oceanic carbon (C) sequestration depends upon many factors, among which is the availability of nutrients to support phytoplankton growth in the illuminated surface ocean
The coastal waters of this region of the Eastern Tropical South Pacific (ETSP) are characterized by a permanent wind-driven upwelling of cool nutrient-replete water (Fig. 1), which supports high primary productivity and a persistent subsurface oxygen minimum zones (OMZs), where O2 concentrations are low enough to induce the anaerobic processes of the N cycle, such as denitrification and anammox [30,31,32]
This study provides one of the first estimates of N2 fixation rates in aphotic waters of the ETSP
Summary
The efficiency of oceanic carbon (C) sequestration depends upon many factors, among which is the availability of nutrients to support phytoplankton growth in the illuminated surface ocean. Dissolved N in the form of nitrate (NO32) or ammonium (NH4+) is directly usable for growth, but concentrations of fixed N are low (,1 mmol L21) and often growth-limiting in most of the open ocean euphotic zone [1]. Dinitrogen (N2) gas dissolved in seawater, on the other hand, is very abundant in the euphotic zone 450 mmol L21) and could constitute a nearly inexhaustible N source for the marine biota. N source since they can break the triple bond between the two N atoms of the N2 molecule, and convert it into a usable form
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