Community Composition of Nitrous Oxide Consuming Bacteria in the Oxygen Minimum Zone of the Eastern Tropical South Pacific.

Xin Sun,Amal Jayakumar,Bess B Ward

doi:10.3389/fmicb.2017.01183

Abstract

The ozone-depleting and greenhouse gas, nitrous oxide (N2O), is mainly consumed by the microbially mediated anaerobic process, denitrification. N2O consumption is the last step in canonical denitrification, and is also the least O2 tolerant step. Community composition of total and active N2O consuming bacteria was analyzed based on total (DNA) and transcriptionally active (RNA) nitrous oxide reductase (nosZ) genes using a functional gene microarray. The total and active nosZ communities were dominated by a limited number of nosZ archetypes, affiliated with bacteria from marine, soil and marsh environments. In addition to nosZ genes related to those of known marine denitrifiers, atypical nosZ genes, related to those of soil bacteria that do not possess a complete denitrification pathway, were also detected, especially in surface waters. The community composition of the total nosZ assemblage was significantly different from the active assemblage. The community composition of the total nosZ assemblage was significantly different between coastal and off-shore stations. The low oxygen assemblages from both stations were similar to each other, while the higher oxygen assemblages were more variable. Community composition of the active nosZ assemblage was also significantly different between stations, and varied with N2O concentration but not O2. Notably, nosZ assemblages were not only present but also active in oxygenated seawater: the abundance of total and active nosZ bacteria from oxygenated surface water (indicated by nosZ gene copy number) was similar to or even larger than in anoxic waters, implying the potential for N2O consumption even in the oxygenated surface water.

Highlights

N2O is a major ozone-depleting substance and a greenhouse gas whose radiative forcing per mole is 298 times that of carbon dioxide (IPCC, 2007; Ravishankara et al, 2009)
Sampling depths were chosen to represent water column features defined by oxygen concentration, as measured with the in situ STOX sensor: oxygenated surface water, upper oxycline [characterized by sharp O2 concentration gradient ranging from saturation to below detection limit (
At station BB1, the abundance of both total and active nosZ genes was highest in a sample from the surface water, and decreased with depth

Summary

Introduction

N2O is a major ozone-depleting substance and a greenhouse gas whose radiative forcing per mole is 298 times that of carbon dioxide (IPCC, 2007; Ravishankara et al, 2009). Oxygen minimum zones (OMZs) are the most intense marine sources of N2O and are hot spots of rapid N2O cycling (Martinez-Rey et al, 2015). OMZs are marine regions with a strong O2 gradient (oxycline) overlying an oxygen deficient zone (ODZ) where O2 concentration is low enough to induce anaerobic processes. The dominant microbial process for N2O cycling is denitrification, the sequential reduction of NO3− to NO2−, NO, N2O and to N2 (Zumft, 1997; Naqvi et al, 2000). Denitrification could stop at an intermediate step before N2 if O2 concentration exceeds the threshold for the latter step or if electron donors are depleted (Ward et al, 2008; Dalsgaard et al, 2012; Babbin et al, 2014). While multiple processes can produce N2O, reduction by N2O consuming bacteria is the only known biological N2O sink

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