Copper Biogeochemistry: A cornerstone in aerobic methanotrophic bacterial ecology and activity?

Two distinct enzymatic pathways are implicated in the key step whereby methane is converted to methanol by the aerobic methane oxidizing bacteria (methanotrophs). These two enzymes, soluble and particulate methane monooxygenases (sMMO and pMMO, respectively), are evolutionarily unrelated. However, the activities of these enzymes are tightly linked to copper, which is central to the switch responsible for regulating MMO expression. When bioavailable copper exceeds a certain threshold relative to cell biomass, pMMO is expressed and its activity maintained by available copper. Below this threshold or when copper is entirely absent, sMMO catalyses methane oxidation. The individual forms of MMO degrade methane and hydrocarbon pollutants at different rates and efficiencies. Typically, pMMO is by up to 30% more efficient at methane degradation as opposed to sMMO which is more effective in the transformation of a wide range of hazardous hydrocarbons than pMMO. Consequently, the type of MMO expressed influences the ability of methanotrophs to effectively act as biological filters curtailing methane input into the atmosphere and for bioremediation. Because of the crucial requirement of copper some methanotrophs produce chalkophores specifically involved in copper trafficking. These chalkophores can in addition bind a variety of earth metals with varying affinities. Methanotrophs can also extract copper from various minerals thereby implicating them in weathering processes. The abundance of the methanotrophic bacteria in nature implies a significant amount of copper and iron in the geobiosphere that form the core of the MMOs, is regularly cycled through these organisms. This discussion is focused on methanotrohphic bacterial population dynamics observed during growth on various copper species, to extrapolate their impact on geomicrobiological processes