Abstract:
In suboxic or anoxic environments, nitrous oxide (N2O) can be produced by ammonia oxidizing bacteria (AOB) as a potent greenhouse gas. Although N2O producing inventory and pathways have been well-characterized using archetypal AOB, there is little known about their adaptive responses to oxic-anoxic cycling, which is a prevalent condition in soil, sediment, and wastewater treatment bioreactors. In this study, cellular responses of Nitrosomonas europaea 19718 to sustained anoxic-oxic cycling in a chemostat bioreactor were evaluated at transcriptomic, proteomic, and fluxomic levels. During a single oxic-anoxic transition, the accumulations of major intermediates were found at the beginning of anoxia (nitric oxide, NO) and post anoxia (hydroxylamine, NH2OH, and N2O). Anoxic-oxic cycling over thirteen days led to significantly reduced accumulations of NH2OH, NO and N2O. Distinct from short-term responses, which were mostly regulated at the mRNA level, adapted cells seemed to sustain energy generation under repeated anoxia by partially sacrificing the NO detoxification capacities, and such adaptation was mainly regulated at the protein level. The proteomic data also suggested the potential contributions of the newly discovered cytochrome P460-mediated NH2OH oxidation pathway to N2O productions. Flux balance analysis was performed based on a metabolic network model consisting of 49 biochemical reactions involved in nitrogen respiration, and changes in metabolic fluxes after the anoxic-oxic cycling were found to be better correlated with intracellular protein concentrations rather than mRNA levels. Previous studies focusing on single anoxic-oxic transition might have overlooked the adaptive responses of nitrifiers to anoxic-oxic cycling, and thus overestimated NO and N2O emission levels from natural and engineered nitrification systems.
