Oxygen at Nanomolar Levels Reversibly Suppresses Process Rates and Gene Expression in Anammox and Denitrification in the Oxygen Minimum Zone off Northern Chile

ABSTRACT

A major percentage (20 to 40%) of global marine fixed-nitrogen loss occurs in oxygen minimum zones (OMZs). Concentrations of O₂ and the sensitivity of the anaerobic N₂-producing processes of anammox and denitrification determine where this loss occurs. We studied experimentally how O₂ at nanomolar levels affects anammox and denitrification rates and the transcription of nitrogen cycle genes in the anoxic OMZ off Chile. Rates of anammox and denitrification were reversibly suppressed, most likely at the enzyme level. Fifty percent inhibition of N₂ and N₂O production by denitrification was achieved at 205 and 297 nM O₂, respectively, whereas anammox was 50% inhibited at 886 nM O₂. Coupled metatranscriptomic analysis revealed that transcripts encoding nitrous oxide reductase (nosZ), nitrite reductase (nirS), and nitric oxide reductase (norB) decreased in relative abundance above 200 nM O₂. This O₂ concentration did not suppress the transcription of other dissimilatory nitrogen cycle genes, including nitrate reductase (narG), hydrazine oxidoreductase (hzo), and nitrite reductase (nirK). However, taxonomic characterization of transcripts suggested inhibition of narG transcription in gammaproteobacteria, whereas the transcription of anammox narG, whose gene product is likely used to oxidatively replenish electrons for carbon fixation, was not inhibited. The taxonomic composition of transcripts differed among denitrification enzymes, suggesting that distinct groups of microorganisms mediate different steps of denitrification. Sulfide addition (1 µM) did not affect anammox or O₂ inhibition kinetics but strongly stimulated N₂O production by denitrification. These results identify new O₂ thresholds for delimiting marine nitrogen loss and highlight the utility of integrating biogeochemical and metatranscriptomic analyses.

IMPORTANCE The removal of fixed nitrogen via anammox and denitrification associated with low O₂ concentrations in oceanic oxygen minimum zones (OMZ) is a major sink in oceanic N budgets, yet the sensitivity and dynamics of these processes with respect to O₂ are poorly known. The present study elucidated how nanomolar O₂ concentrations affected nitrogen removal rates and expression of key nitrogen cycle genes in water from the eastern South Pacific OMZ, applying state-of-the-art ¹⁵N techniques and metatranscriptomics. Rates of both denitrification and anammox responded rapidly and reversibly to changes in O₂, but denitrification was more O₂ sensitive than anammox. The transcription of key nitrogen cycle genes did not respond as clearly to O₂, although expression of some of these genes decreased. Quantifying O₂ sensitivity of these processes is essential for predicting through which pathways and in which environments, from wastewater treatment to the open oceans, nitrogen removal may occur.