Oxygen as Intermediate in Anoxic Environments: Nitrite-Dependent Methane Oxidation and Beyond

Abstract:

In recent years the known diversity of hydrocarbon activation mechanisms under anaerobic conditions has been extended by intra-aerobic denitrification, a process in which oxygen is derived from NO and used for substrate activation. For two phylogenetically unrelated bacterial species, the freshwater NC10 phylum bacterium Methylomirabilis oxyfera [1] and the marine γ-proteobacterial strain HdN1 [2] it has been shown that, under anoxic conditions with nitrate and/or nitrite, mono-oxygenases are used for methane and hexadecane oxidation, respectively. No degradation was observed with nitrous oxide (N₂O) only. In the anaerobic methanotroph M. oxyfera, which lacks apparent nitrous oxide reductase in its genome, substrate activation in the presence of nitrite was directly associated with both O₂ and N₂ formation. These findings strongly argue for the role of nitric oxide (NO), or an oxygen species derived from it, in the activation reaction of methane. Although intracellular oxygen generation has been experimentally documented and elegantly explains the utilization of ‘aerobic’ pathways under anoxic conditions, research about the underlying molecular mechanism has just started. The proposed candidate enzymes for oxygen (or possibly another another reactive intermediate) production from NO, an NO dismutase (NOD) [3], related to quinol-dependent NO reductases (qNORs), is present and highly expressed in both M. oxyfera and strain HdN1. Besides that, several recently sequenced species from the Cytophaga-Flavobacterium-Bacteroides group harbor Nod/Nor genes, but experimential evidence is needed to show if these have NOD activity, are unusual but functional qNORs, or represent transition states between the two. Additionally, for several anaerobic hydrocarbon-degrading organisms the biochemical mechanism of substrate activation has not been elucidated yet: whereas signature genes of anaerobic degradation are missing, monooxygenase genes are present. Also these microorganisms may, analogously to chlorate-reducing bacteria, produce their own supply of oxygen to activate recalcitrant hydrocarbons in anoxic environments.

[1] Ettwig et al. (2010) Nature 464: 543-548. 

[2] Zedelius et al. (2011) Env Microb Rep 3(1): 125-135.

[3] Ettwig et al. (2012) Front Microb 3(273): 1-8.