The influence of anoxia and substrate availability on N₂O cycling by denitrification in the upper boundary of the oxygen minimum zone off northern Chile

Abstract

Strong accumulations of N 2 O at oxyclines are some of the most conspicuous features of the world's oceans. However, the origin of these maxima, and the relative contribution of nitrification and denitrification in N 2 O cycling, remains unclear. In order to gain insight into the importance of denitrification and factors regulating N 2 O cycling at upper oxyclines in the eastern South Pacific, the production and consumption of N 2 O by denitrification were measured using a classical acetylene method under induced anoxia with the addition of an electron acceptor (nitrite) and donors (sodium acetate and glucose). The results indicated that decreased O 2 clearly affected the ratio in which N 2 O is reduced to N 2 at the midoxycline (∼40 m depth) and at the oxycline's base (∼80 m depth). Under induced anoxia, higher N 2 O production (from NO − 2 to N 2 O of 67.2 nM d −1 ) occurred at 40 m depth, with half of the total quantity being consumed by denitrification (from N 2 O to N 2 of 32 nM d −1 ); conversely, 100% of the N 2 O was reduced to N 2 at 80 m depth. In comparison with previously reported results at the base of the oxycline at an offshore station, the addition of NO − 2 (as sodium nitrite) along with dissolved organic carbon (as sodium acetate and glucose) doubled the net N 2 O production by denitrification (∼20 nM d −1 ). Our results suggest that decreasing O 2 levels along with an increased availability of NO − 2 and organic compounds in the upper oxycline may impact the N 2 O/N 2 ratio and, therefore, the N 2 O efflux to the atmosphere.

Ecological Energetic Perspectives on Responses of Nitrogen-Transforming Chemolithoautotrophic Microbiota to Changes in the Marine Environment

“…N 2 O is produced by many microbial processes and it is also the precursor of NO radicals that cause ozone destruction in the stratosphere ( Carpenter et al, 2012 ; Schreiber et al, 2012 ; Voss et al, 2013 ; Mellbye et al, 2016 ). Hypoxic and anoxic environments that are rich in labile organic matter facilitate heterotrophic denitrification and the production of N 2 O as a metabolic intermediate ( Gilly et al, 2013 ; Townsend-Small et al, 2014 ; Babbin et al, 2015 ; Castro-González and Farías, 2015 ). Autotrophic denitrification that couples denitrification with anaerobic chemolithoautotrophic sulfide oxidation also contributes to fixed nitrogen removal and N 2 O production in these environments ( Shao et al, 2010 ; Ulloa et al, 2012 ).…”