Exogenous nitrate attenuates nitrite toxicity to anaerobic ammonium oxidizing (anammox) bacteria

Abstract

Anaerobic ammonium oxidizing bacteria (anammox) can be severely inhibited by one of its main substrates, nitrite (NO2-). At present, there is limited information on the processes by which anammox bacteria are able to tolerate toxic NO2-. Intracellular consumption or electrochemically driven (transmembrane proton motive force) NO2- export are considered the main mechanisms of NO2- detoxification. In this work, we evaluated the potential of exogenous nitrate (NO3-) on relieving NO2- toxicity, putatively facilitated by NarK, a NO3-/NO2- transporter encoded in the anammox genome. The relative contribution of NO3- to NO2- detoxification was found to be pH dependent. Exposure of anammox cells to NO2- in absence of their electron donating substrate, ammonium (NH4+), causes NO2- stress. At pH 6.7 and 7.0, the activity of NO2- stressed cells was respectively 0 and 27% of the non-stressed control activity (NO2- and NH4+ fed simultaneously). Exogenous NO3- addition caused the recovery to 42% and 80% of the control activity at pH 6.7 and 7.0, respectively. The recovery of the activity of NO2- stressed cells improved with increasing NO3- concentration, the maximum recovery being achieved at 0.85 mM. The NO3- pre-incubation time is less significant at pH 7.0 than at pH 6.7 due to a more severe NO2- toxicity at lower pH. Additionally, NO3- caused almost complete attenuation of NO2- toxicity in cells exposed to the proton gradient disruptor carbonyl cyanide m-chlorophenyl hydrazone at pH 7.5, providing evidence that the NO3- attenuation is independent of the proton motive force. The absence of a measurable NO3- consumption (or NO3- dependent N2 production) during the batch tests leaves NO3- dependent active transport of NO2- as the only plausible explanation for the relief of NO2- inhibition. We suggest that anammox cells can use a secondary transport system facilitated by exogenous NO3- to alleviate NO2- toxicity.