Activity of methanotrophic bacteria in Green Bay sediments

Sediment pore water samples obtained from a 19 m station in Green Bay in Lake Michigan were examined for levels of ambient dissolved methane and copper, and for the potential for in situ methane oxidation by methanotrophs found within surface sediments. The in situ methane concentration in the upper oxic sediment layer ranged from 20–150 μmol · 1−1 at this station. The activity of methanotrophs and the kinetics of methane oxidation in these sediments were demonstrated by the uptake of radiolabeled methane. Ks values varied between 4.1–9.6 nmol · cm3 of sediment slurry. High Vmax values (12.7–35.2 nmol · cm−3 · h−1) suggest a large population of methanotrophs in the sediments. An average methane flux to the oxic sediments of 0.24 mol · m−2 · year−1 was calculated from the pore water methane gradients. Pore water concentrations of copper in the upper sediment layer ranged from 10–120 nmol · 1−1. Based upon the copper concentration, other measured parameters, and equilibrium conditions defined by WATEQF4, an estimate for dissolved free Cu2+ concentration of 5–38 nmol · 1−1 pore water was obtained. Several factors control the rate of methane oxidation, including oxygen, methane, and the bioavailability of free Cu2+.