Microbial manganese(II) oxidation in the marine environment: a quantitative study

A great number of important chemical reactions that occur in the environment are microbially mediated. In order to understand the kinetics of these reactions it is necessary to develop methods to directly measure in situ reaction rates and to develop models to help elucidate the mechanisms of microbial catalysis. The oxidation of Mn(II) in a zone above the O₂/H₂S interface in Saanich Inlet, B.C., Canada is one such reaction. We present here a method by which in situ rates of microbial Mn(II) oxidation are measured and a model based on our experimental results to describe the general mechanism of Mn(H) oxidation. We propose a two step process in which Mn(II) is first bound by a site on the bacterial surface and then oxidized. The model is analogous to the Langmuir isotherm model for surface catalyzed gas reactions or the Michaelis-Menten model for enzyme kinetics. In situ Mn(II) oxidation rates were measured during five cruises to Saanich Inlet during the summers of 1983 and 1984. We use the model to calculate the apparent equilibrium binding constant (K_s ≈ 0.18 μM), the apparent half saturation constant for biological Mn(H) oxidation (K_m = 0.22 to 0.89 μM), the maximum rate of Mn(II) oxidation (V_max = 3.5 to 12.1 nM·h^-1) and the total microbial surface binding site concentration (Σ E ≈ 51 nM). V_max for Mn(II) oxidation agrees with the rates calculated from the value of the flux of Mn(II) to the oxidizing zone using the Mn(II) gradient and estimates of the eddy diffusion coefficient. This consistancy verifies our methodology and indicates that the rate of Mn(II) oxidation is nearly equal to the (V_max for the reaction. We conclude that in this environment the Mn(II) oxidation rate is more a function of the total number of surface binding sites than the Mn(H) concentration.