Methanotrophic bacteria and their activity on submerged aquatic macrophytes

The presence and activity of methanotrophic bacteria on submerged freshwater macrophyte organs were examined for 13 species in 10 lake and river sites in New Zealand. Methanotrophic bacteria were first identified qualitatively using polymerase chain reaction (PCR) analysis. PCR successfully located the presence of the particulate methane monooxygenase gene associated with both shoot and root tissue of eight species found in habitats with organic, methane-rich sediments, but not on plants growing in oligotrophic habitats with low sediment methane concentrations. The rates of methane oxidation associated with the macrophyte organs were studied in vitro over a range of methane concentrations from 0 to 3000 ppm. Methanotrophic activity varied considerably between sites, with negligible activity in plants growing in oligotrophic sites, but maximum rates up to 16.2 μmol CH 4 g −1 (plant dry weight) h −1 in plants from eutrophic habitats. The response of methanotrophy to methane concentration was hyperbolic, and half-saturation constants were similar for all plant organs, ranging from 3.4 to 7.8 μmol CH 4 l −1. The submerged macrophyte-associated methanotrophic activity showed anoxia and acetylene sensitivity, similar to methanotrophs from other freshwater habitats. Activity also varied considerably between sites, and within species. In Potamogeton crispus L., the activity associated with the below-ground material was greater for stolons than for roots, whilst for shoots it declined from shoot base to apex. These patterns appeared to be related to differences in the availability of oxygen and methane between sediment, plant and water phases of the gas transport pathway.