Carbon isotope fractionation factors reported for aerobic bacterial oxidation of CH4 (CH4-CO2) range from 1.003 to 1.039. In a series of experiments designed to monitor changes in the carbon isotopic fractionation of CH4 by Type I and Type II methanotrophic bacteria, we found that the magnitude of fractionation was largely due to the first oxidation step catalyzed by methane monooxygenase (MMO). The most important factor that modulates the (CH4-CH3OH) is the fraction of the total CH4 oxidized per unit time, which strongly correlates to the cell density of the growth cultures under constant flow conditions. At cell densities of less than 0.1 g/L, fractionation factors greater than 1.03 were observed, whereas at cell densities greater than 0.5 g/L the fractionation factors decreased to as low as 1.002. At low cell densities, low concentrations of MMO limit the amount of CH4 oxidized, while at higher cell densities, the overall rates of CH4 oxidation increase sufficiently that diffusion of CH4 from the gaseous to dissolved state and into the cells is likely the rate-determining step. Thus, the residual CH4 is more fractionated at low cell densities, when only a small fraction of the total CH4 has been oxidized, than at high cell densities, when up to 40% of the influent CH4 has been utilized. Therefore, since Rayleigh distillation behavior is not observed, 13C values of the residual CH4 cannot be used to infer the amount oxidized in either laboratory or field-studies. The measured (CH4-CH3OH) was the same for both Type I and Type II methanotrophs expressing particulate or soluble MMO. However, large differences in the 13C values of biomass produced by the two types of methanotrophs were observed. Methylosinus trichosporium OB3b (Type II) produced biomass with 13C values about 15 higher than the dissimilated CO2, whereas Methylomonas methanica (Type I) produced biomass with 13C values only about 6 higher that the CO2. These effects were independent of the magnitude of the initial carbon isotope fractionation caused by MMO and were relatively constant despite changing ratios of assimilatory to dissimilatory carbon transformation by the organisms. This suggests that the difference in biomass carbon isotopes is primarily due to differences in the fractionation effect at the formaldehyde branch point in the metabolic pathway, rather than assimilation of CO2 by Type II methanotrophs. 2005 Elsevier Inc. All rights reserved.
