Effects of methane concentration (a), nitrite concentration (b), and salinity (c) on denitrifying methanotroph-specific activity. The denitrifying methanotroph-specific activity was characterized by the methane consumption (•) or nitrite reduction (■) rates. When they were not adjusted, the methane concentration, nitrite concentration, and salinity levels were 0.11 mM, 0.2 mM, and 20.5‰, respectively.