Detection and phylogenetic analysis of the membrane‐bound nitrate reductase (Nar) in pure cultures and microbial communities from deep‐sea hydrothermal vents

Over the past few years the relevance of nitrate respiration in microorganisms from deep‐sea hydrothermal vents has become evident. In this study, we surveyed the membrane‐bound nitrate reductase ( N ar) encoding gene in three different deep‐sea vent microbial communities from the E ast P acific Rise and the M id‐ A tlantic Ridge. Additionally, we tested pure cultures of vent strains for their ability to reduce nitrate and for the presence of the N ar G ‐encoding gene in their genomes. By using the narG gene as a diagnostic marker for nitrate‐reducing bacteria, we showed that nitrate reductases related to G ammaproteobacteria of the genus M arinobacter were numerically prevalent in the clone libraries derived from a black smoker and a diffuse flow vent. In contrast, N ar G sequences retrieved from a community of filamentous bacteria located about 50 cm above a diffuse flow vent revealed the presence of a yet to be identified group of enzymes. 16 S r RNA gene‐inferred community compositions, in accordance with previous studies, showed a shift from A lpha ‐ and G ammaproteobacteria to E psilonproteobacteria as the vent fluids become warmer and more reducing. Based on these findings, we argue that N ar‐catalyzed nitrate reduction is likely relevant in temperate and less reducing environments where A lpha‐ and G ammaproteobacteria are more abundant and where nitrate concentrations reflect that of background deep seawater.