O2 versus N2O respiration in a continuous microbial enrichment

Abstract

Despite its ecological importance, essential aspects of microbial N 2 O reduction—such as the effect of O 2 availability on the N 2 O sink capacity of a community—remain unclear. We studied N 2 O vs. aerobic respiration in a chemostat culture to explore (i) the extent to which simultaneous respiration of N 2 O and O 2 can occur, (ii) the mechanism governing the competition for N 2 O and O 2 , and (iii) how the N 2 O-reducing capacity of a community is affected by dynamic oxic/anoxic shifts such as those that may occur during nitrogen removal in wastewater treatment systems. Despite its prolonged growth and enrichment with N 2 O as the sole electron acceptor, the culture readily switched to aerobic respiration upon exposure to O 2 . When supplied simultaneously, N 2 O reduction to N 2 was only detected when the O 2 concentration was limiting the respiration rate. The biomass yields per electron accepted during growth on N 2 O are in agreement with our current knowledge of electron transport chain biochemistry in model denitrifiers like Paracoccus denitrificans . The culture’s affinity constant ( K S ) for O 2 was found to be two orders of magnitude lower than the value for N 2 O, explaining the preferential use of O 2 over N 2 O under most environmentally relevant conditions.