Some properties of methane oxidation in a thermally stratified lake

Profiles of (CH4)-C-14 and (CO)-C-14 oxidation and concentrations of CH4, NH4+, and nitrogen oxides (N2O, NO2-, NO3-) were studied in Lake St. George, Ontario, during late summer stratification. Metalimnetic maxima of the nitrogen oxides occurred and high rates of oxidation of CH4 and CO were limited to a narrow band (<1 m) below the nitrogen oxide peaks at the oxic-anoxic interface. Oxidation of CH4 and CO was strongly inhibited by the nitrification inhibitor allylthiourea. The affinity of the oxidizers for CH4 (approximate K-m = 4.6 mu M), their sensitivity to picolinic acid, and the ratio of CH4 oxidation to allylthiourea-sensitive CO oxidation together suggest that methanotrophs were responsible for the observed CH4 oxidation. Pure cultures of Nitrosomonas europaea, although less sensitive to picolinic acid than the CH, oxidizers of Lake St. George,were affected by the inhibitor (approximate K-i for inhibition of NH4+ oxidation = 51 mu M).