Taxonomic diversity and abundance profiles of denitrification genes nirK and nirS in the Saanich Inlet water column

Abstract

Oxygen minimum zone (OMZ) expansion throughout the global ocean prompts investigation into progressive deoxygenation effects on marine microbial communities. Nitrogen cycling in OMZs is of particular interest because in the absence of oxygen, nitrate is used as an alternative electron acceptor by various microorganisms resulting in biological nitrogen loss through denitrification.  In the denitrification pathway, two functionally equivalent but structurally different enzymes, nirK and nirS, execute the step of reducing nitrite to nitric oxide. Saanich Inlet, a seasonally anoxic fjord on the Vancouver Island coast provides a model ecosystem for microbial community responses to OMZ expansion including the distribution of nitrogen cycling genes along defined water column oxygen gradients. Previous research has highlighted the importance of considering both nitrite reductase genes when assessing denitrification potential in marine ecosystems, and several studies have implicated Gammaproteobacteria as playing a prominent role in Saanich Inlet denitrification. However, more research on less prominent taxonomic groups involved in specific denitrification steps is needed to better understand pathway distribution patterns. In this study, we applied metagenomic approaches to analyze the differences in diversity and abundance of nirK and nirS across three depths of the Saanich Inlet, 100, 120, and 200 meters. Consistent with previous results, both genes, particularly nirS, were primarily associated with the Proteobacteria phylum and Thaumarcheota had the highest abundance for nirK. These results highlight differential abundance patterns of nirK and nirS genes in the water column relevant to the study of both ammonia oxidation and denitrification processes at different oxygen concentrations within OMZs.