Inhibitory effect of light on methane oxidation in the pelagic water column of a mesotrophic lake (Lake Biwa, Japan)

Methane oxidation was studied in mesotrophic lake water (Lake Biwa, Japan) under thermally stratified conditions. Methane oxidation rates at in situ concentrations were very low in lake water from the epilimnion and thermocline but were high in hypolimnetic water. Incubation under light conditions ranging from 4.1 to 57 mu mol photons m(-2) s(-1) resulted in decreased methane oxidation in hypolimnetic water. This inhibition was more severe as the light intensity increased. Addition of inorganic nitrogen (ammonium and nitrate) did not promote methane oxidation in the thermocline but inhibited it in the hypolimnion. Methane oxidation activity in the thermocline was observed after I month of incubation under dark conditions. Our results suggest that the inhibitory effect of light was bacteriostatic for the methanotrophic population. The different rates of methane oxidation between the hypolimnion and epilimnion/thermocline may explain the surface maximum of dissolved methane during the period of thermal stratification.