Impacts of varying durations of passive oxygen exposure on freshwater denitrifier community structure and function

Abstract

Fertilizer use has dramatically increased the availability of nitrate (NO3 −) in aquatic systems. Microbe-mediated denitrification is one of the predominant means of NO3 − removal from freshwaters, yet oxygenation (O2)-induced disruptions—e.g., extreme precipitation events—can occur, resulting in a disproportional increase in nitrous oxide (N2O) production and efflux as facultative anaerobic bacterial populations use of O2 as a terminal electron acceptor increases. We examined the effects of 12- and 24-h passive O2 exposure on previously anaerobic bacterial communities focusing on denitrification enzyme activity (DEA), N2O production, and bacterial community 16S rRNA and nitrous oxide reductase gene (nosZ) profiles after 12, 24, and 48 h of anaerobic recovery. Treatments experiencing 24-h O2 exposure had significantly higher DEA 12 h into anaerobic recovery than treatments undergoing 12-h O2 exposure. Initial N2O emissions were significantly lower in the 24-h O2 exposure treatments although by 24 h a dramatic spike (tenfold relative to the 12-h O2 exposure treatments) in N2O concentrations was observed. However, within 6 h (30-h anaerobic recovery) these differences were gone. Community nosZ profiles experiencing 24-h O2 exposure exhibited reduced diversity after 24-h recovery, which corresponded with an increase in N2O emissions. However, after 48 h of anaerobic recovery, nosZ diversity had recovered. These observations highlight the effects of short-term aerobic disruption on denitrification, as well as the effects on the denitrifier community profile. Together, these data suggest that recovery to ambient N cycling is exacerbated by disturbance length due to increased lag time and subsequent loss of denitrifier community diversity.