Non‐denitrifier nitrous oxide reductases dominate marine biomes
Anthony D. Bertagnolli, Konstantinos T. Konstantinidis, Frank J. Stewart
Abstract
Summary Microbial enzymes often occur as distinct variants that share the same substrate but differ in substrate affinity, sensitivity to environmental conditions, or phylogenetic ancestry. Determining where variants occur in the environment helps identify thresholds that constrain microbial cycling of key chemicals, including the greenhouse gas nitrous oxide (N 2 O). To understand the enzymatic basis of N 2 O cycling in the ocean, we mined metagenomes to characterize genes encoding bacterial nitrous oxide reductase (NosZ) catalyzing N 2 O reduction to N 2 . We examined data sets from diverse biomes but focused primarily on those from oxygen minimum zones where N 2 O levels are often elevated. With few exceptions, marine nosZ data sets were dominated by ‘atypical’ clade II gene variants. Atypical nosZ has been associated with low oxygen, enhanced N 2 O affinity, and organisms lacking enzymes for complete denitrification, i.e., non‐denitrifiers. Atypical nos Z often occurred in metagenome‐assembled genomes (MAGs) with nitrate or nitrite respiration genes, although MAGs with genes for complete denitrification were rare. We identified atypical nos Z in several taxa not previously associated with N 2 O consumption, in addition to known N 2 O‐associated groups. The data suggest that marine environments generally select for high N 2 O‐scavenging ability across diverse taxa and have implications for how N 2 O concentration may affect N 2 O removal rates.