LAUSR

Denitrifying bacteria have evolved elaborate regulatory networks securing their respiratory metabolism in environments with fluctuating oxygen concentrations. Here, we provide new insight regarding their bet-hedging in response to hypoxia, which minimizes their N2O emissions because all cells express NosZ, reducing N2O to N2, while a minority express NirS + Nor, reducing NO2− to N2O. ABSTRACT Adaptation to anoxia by synthesizing a denitrification proteome costs metabolic energy, and the anaerobic respiration conserves less energy per electron than aerobic respiration. This implies a selective advantage of the stringent O2 repression of denitrification gene transcription, which is found in most denitrifying bacteria. In some bacteria, the metabolic burden of adaptation can be minimized further by phenotypic diversification, colloquially termed “bet-hedging,” where all cells synthesize the N2O reductase (NosZ) but only a minority synthesize nitrite reductase (NirS), as demonstrated for the model strain Paracoccus denitrificans. We hypothesized that the cells lacking NirS would be entrapped in anoxia but with the possibility of escape if supplied with O2 or N2O. To test this, cells were exposed to gradual O2 depletion or sudden anoxia and subsequent spikes of O2 and N2O. The synthesis of NirS in single cells was monitored by using an mCherry-nirS fusion replacing the native nirS, and their growth was detected as dilution of green, fluorescent fluorescein isothiocyanate (FITC) stain. We demonstrate anoxic entrapment due to e−-acceptor deprivation and show that O2 spiking leads to bet-hedging, while N2O spiking promotes NirS synthesis and growth in all cells carrying NosZ. The cells rescued by the N2O spike had much lower respiration rates than those rescued by the O2 spike, however, which could indicate that the well-known autocatalytic synthesis of NirS via NO production requires O2. Our results bring into relief a fitness advantage of pairing restrictive nirS expression with universal NosZ synthesis in energy-limited systems. IMPORTANCE Denitrifying bacteria have evolved elaborate regulatory networks securing their respiratory metabolism in environments with fluctuating oxygen concentrations. Here, we provide new insight regarding their bet-hedging in response to hypoxia, which minimizes their N2O emissions because all cells express NosZ, reducing N2O to N2, while a minority express NirS + Nor, reducing NO2− to N2O. We hypothesized that the cells without Nir were entrapped in anoxia, without energy to synthesize Nir, and that they could be rescued by short spikes of O2 or N2O. We confirm such entrapment and the rescue of all cells by an N2O spike but only a fraction by an O2 spike. The results shed light on the role of O2 repression in bet-hedging and generated a novel hypothesis regarding the autocatalytic nirS expression via NO production. Insight into the regulation of denitrification, including bet-hedging, holds a clue to understanding, and ultimately curbing, the escalating emissions of N2O, which contribute to anthropogenic climate forcing.