Kinetics of NH₃‐oxidation, NO‐turnover, N₂O‐production and electron flow during oxygen depletion in model bacterial and archaeal ammonia oxidisers

Abstract

Ammonia oxidising bacteria (AOB) are thought to emit more nitrous oxide (N O) than ammonia oxidising archaea (AOA), due to their higher N O yield under oxic conditions and denitrification in response to oxygen (O ) limitation. We determined the kinetics of growth and turnover of nitric oxide (NO) and N O at low cell densities of Nitrosomonas europaea (AOB) and Nitrosopumilus maritimus (AOA) during gradual depletion of TAN (NH + NH4+) and O . Half-saturation constants for O and TAN were similar to those determined by others, except for the half-saturation constant for ammonium in N. maritimus (0.2 mM), which is orders of magnitudes higher than previously reported. For both strains, cell-specific rates of NO turnover and N O production reached maxima near O half-saturation constant concentration (2-10 μM O ) and decreased to zero in response to complete O -depletion. Modelling of the electron flow in N. europaea demonstrated low electron flow to denitrification (≤1.2% of the total electron flow), even at sub-micromolar O concentrations. The results corroborate current understanding of the role of NO in the metabolism of AOA and suggest that denitrification is inconsequential for the energy metabolism of AOB, but possibly important as a route for dissipation of electrons at high ammonium concentration.

Nitrous oxide production by ammonia oxidizers: Physiological diversity, niche differentiation and potential mitigation strategies

“…Bakken, 1999;Shaw et al, 2006), due to differences in available NH + 4 and oxygen concentrations that affect enzymatic reactions. N 2 O yields from AOA cultures are below or in the lower range of those observed for AOB, that is, 0.04%-0.3%, and NH + 4 and oxygen concentration have no or little effect on yield Hink, Lycus, et al, 2017;Kim et al, 2012;Löscher et al, 2012;Santoro et al, 2011;Stieglmeier, Mooshammer, et al, 2014;Qin et al, 2017). An alternative explanation is redox balancing, in which electrons generated by hydroxylamine dehydrogenase are shuttled to denitrification enzymes when they exceed the capacity of terminal oxidases (Hink, Lycus, et al, 2017).…”

“…Acetylene is a potent inhibitor of AMO of all AO, but 1-octyne has recently been established as a specific inhibitor of AOB Taylor et al, 2013). Higher yields are observed with increasing NH 4 + concentration, potentially due to a lower reaction rate of hydroxylamine dehydrogenase than AMO, leading to accumulation of hydroxylamine, which is subsequently transformed abiotically to N 2 O (Hink, Lycus, et al, 2017). Bakken, 1999;Shaw et al, 2006), due to differences in available NH + 4 and oxygen concentrations that affect enzymatic reactions.…”

“…The NO-scavenger PTIO has been used as a specific inhibitor of AOA (Kozlowski, Stieglmeier, et al, 2016;Shen et al, 2013;Yan et al, 2012), but also inhibits comammox bacteria (Kits et al, 2019), and simvastatin can specifically inhibit AOA (Zhao, Bello, Meng, Prosser, & Gubry-Rangin, in press). An alternative explanation is redox balancing, in which electrons generated by hydroxylamine dehydrogenase are shuttled to denitrification enzymes when they exceed the capacity of terminal oxidases (Hink, Lycus, et al, 2017). Higher yields are observed with increasing NH 4 + concentration, potentially due to a lower reaction rate of hydroxylamine dehydrogenase than AMO, leading to accumulation of hydroxylamine, which is subsequently transformed abiotically to N 2 O (Hink, Lycus, et al, 2017).…”

Dynamic investigation and modeling of the nitrogen cometabolism in Methylococcus capsulatus (Bath)

“…One can only speculate if the physical phenomenon described by Equation is correct as numerous reactions of both biotic (Hink et al, ; Prosser, ) and abiotic (Damschen & Martin, ; Ignarro, Fukuto, Griscavage, Rogers, & Byrns, ; Park & Lee, ) nature, could account for the observed production of nitrate. A small experiment where a 2 ml pulse of a 10 g·L −1 NaNO 2 stock solution was added to the steady state culture resulted in no observable production of nitrate, suggesting that the intracellular matrix may play a role in the nitrate production, but further research is clearly necessary.…”

Systems biology approaches towards predictive microbial ecology

“…For example, higher N 2 O production is associated with low pH conditions, an observation that is consistent with field observations (SImek & Cooper, ). Certain ammonia oxidizers have been characterized in‐depth, including the ammonia‐oxidizing archaeon (AOA) Nitrosopumilus maritimus and the ammonia‐oxidizing bacterium (AOB) Nitrosomonas europaea (Hink et al, ). While the AOA and AOB carry out the same nitrogen transformation, the two groups show important differences with respect to their ammonia‐oxidizing phenotype, including higher N 2 O emissions by the AOB (Hink et al, ).…”