O(2) versus N(2)O respiration in a continuous microbial enrichment

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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00253-018-9247-3) contains supplementary material, which is available to authorized users.