Effects of Copper Availability on Methanotroph Growth and Activity in Rice Paddy Soil

Methanotrophs (aerobic, obligate methane-oxidizing bacteria) represent the largest sink for methane in the terrestrial environment, oxidizing a significant fraction of methane produced in anoxic regions of flooded soils and sediments before it reaches the atmosphere. Two unrelated methane monooxygenase (MMO) enzymes observed in methanotrophs have different kinetics, and have isotope fractionation effects for methane C which vary by about 12 per mil. Expression of the two enzymes in methanotroph cell cultures is controlled by copper, leading to the hypothesis that copper availability in soils could influence methane isotope fractionation in wetlands and rice paddies. Soil incubations using added copper chelate indicate that methanotroph growth and methane oxidation rates in a California rice soil are not limited by copper. Copper to methanotroph biomass ratios were in the range predicted for the expression of the particulate form of MMO (pMMO), the enzyme associated with the larger isotope fractionation factor (approx. -25 per mil).