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 mmol photons m22 s21 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 1 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.