Aphotic N2 Fixation in the Eastern Tropical South Pacific Ocean

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.

Filtration via Conventional Glass Fiber Filters in 15N2 Tracer Assays Fails to Capture All Nitrogen-Fixing Prokaryotes

“…Taken together, our data suggest that by using Advantec filters, detection of low BNF rates is more likely. This finding is important because BNF rates in mesopelagic waters and other "unusual" habitats are often equal to or even lower than our rates (Fernandez et al, 2011;Bonnet et al, 2013;Dekaezemacker et al, 2013;Rahav et al, 2013;Benavides et al, 2015;Löscher et al, 2016). Consequently, it would be desirable to test whether rates determined for these environments (and the biogeochemical significance derived from such rates) are underestimates due to the use of 0.7 µm pore size GF/F filters.…”

“…However, nitrogenase (nifH) gene sequencing has uncovered an almost ubiquitous distribution of diverse and presumably heterotrophic diazotrophs (Zehr et al, 1998;Farnelid et al, 2011). Notably, these organisms are even found in high-latitude, deep, cold, or coastal waters where cyanobacteria are few or absent (e.g., Fernandez et al, 2011;Blais et al, 2012;Bonnet et al, 2013) and their lifestyles and how they impact biogeochemical element cycling seems fundamentally different from photoautotrophic diazotrophs (Rahav et al, 2013;Farnelid et al, 2014;Moisander et al, 2014;Benavides et al, 2015;Bentzon-Tilia et al, 2015a). This has inspired many investigators to hypothesize that global N inputs by BNF could potentially be much higher than previously thought.…”

Five decades of N2 fixation research in the North Atlantic Ocean

“…However, it must be borne in mind that different diazotroph phylotypes have different responses to daily fluctuations in irradiance (e.g., Church et al, 2005). Finally, the presence and activity of heterotrophic diazotrophs in aphotic waters (e.g., Hewson et al, 2007;Bonnet et al, 2013;Rahav et al, 2013) challenges this strict control and defines physiological differences among diazotrophic species.…”

“…Recent culture and field studies indicate that diazotrophy can continue upon exposure to dissolved inorganic nitrogen (nitrate and ammonium) as long as phosphate is available (reviewed in Knapp, 2012). Furthermore, active N 2 fixation in dissolved inorganic nitrogen-rich environments suggests that restricting diazotrophy studies to the upper ocean layer of the tropics and subtropics can result in large underestimations of N 2 fixation rates (Raimbault and Garcia, 2008;Benavides et al, 2011;Sohm et al, 2011b;Bonnet et al, 2013). Foster et al (2009) found UCYN-A over the Equatorial upwelling region in the Gulf of Guinea.…”

“…Despite this, other facets of diazotrophy are not well-constrained yet. The discovery of new organisms with surprising metabolisms like the unicellular diazotrophic cyanobacterium UCYN-A (Candidatus Atelocyanobacterium thalassa; Zehr et al, 2008;Thompson et al, 2012), the measurement of significant N 2 fixation rates in unexpected habitats (e.g., Bonnet et al, 2013), and the possibility that global N 2 fixation rates have been systematically underestimated in the past (Großkopf et al, 2012) has left room for plenty of new questions still to be answered, including: how much do heterotrophic diazotrophs and UCYN contribute to total N 2 fixation?, how much does N 2 fixation contribute to new production and what is its role in the biological carbon pump?, can the methodological underestimation of available N 2 fixation rates explain the current differences between denitrification and nitrogen fixation at a global scale?, to what extent published N 2 fixation rates are affected by 15 N 2 gas contamination?, and, how will diazotrophs respond to climate change? The advent of new molecular biology methods, high resolution mass spectrometry techniques and single-cell techniques, together with more powerful modeling approaches should provide new insights and answers to these questions.…”