Methane is an important greenhouse gas, and emissions from termites are a significant yet highly uncertain contribution from tropical ecosystems . Methanogenesis in the anoxic termite hindguts has been studied thoroughly, yet little is known about potential microbial methane oxidation (MOX) in the generally oxic nests and mounds of termite colonies. Similar environments with steep counter-gradients of methane and oxygen (e.g. landfill-cover soils) typically harbor an active and diverse population of methanotrophs . However, for termite hindguts, nests and mounds there is some controversy on methanotroph presence and activity [3,4]. Hard evidence of in-situ MOX in termite mounds is still lacking, with the main challenge being the separation of MOX from methane production and physical-transport processes.
Here we present initial results of a comprehensive study to quantify MOX and describe the methanotrophic community in Northern Australian termite mounds. We investigated mounds of three termite species with different mound architectures and representing the dominant feeding groups (wood-, grass- and soil-interface-feeder). Microbial function in the termite mounds was assessed using gas push-pull tests, a tracer method to quantify activity and enzyme-kinetic parameters in situ. The application of a simplified model framework, facilitated by the use of a photogrammetric approach to characterise the physical structure of termite mounds, allowed us to calculate a mass balance of methane turnover for each mound. Despite exhibiting net methane emissions, all investigated mounds consumed methane. Typically 40-60 percent of termite-produced methane was oxidised before reaching the atmosphere. The microbial nature of the consumption was verified using difluoromethane, an inhibitor of the methane-monooxigenase enzyme catalysing MOX. Incubation experiments with excavated mounds using difluoromethane and stable-isotope labelling corroborated the results. Estimated Michaelis-Menten parameters were classified into low and high enzyme-to-substrate affinities that could indicate the presence of two methane-monooxigenase variants specific for high- and low methane concentrations, respectively.
Investigations are under way to identify the community structure of mound-dwelling methanotrophs based on high-throughput sequencing of their molecular marker gene pmoA.