Stability of methane oxidation capacity to variations in methane and nutrient concentrations
Uptake rate constants for atmospheric methane consumption by the 4–6 cm depth interval of a forest soil did not change during 4 months incubation in vitro, even though atmospheric methane concentrations were significantly higher than in situ concentrations. Uptake rate constants were also unaffected by continous incubation at a constant methane concentration of 17 ppm for 2 weeks and 170 ppm for 3 weeks. Uptake rates during incubation with 1000 ppm methane increased 176-fold when assayed with 1000 ppm methane and 5.5-fold when assayed with 1.7 ppm methane. These enhancements were lost after subsequent incubation with atmospheric methane. The ratio of methane oxidized to carbon dioxide produced varied from 49–53% at methane concentrations up to 170 ppm. Incorporation of 14C-methane into phospholipids was 0.35% and 0.22% at atmospheric and 170 ppm methane concentrations, respectively, suggesting that patterns of assimilation were independent of methane concentrations. Addition of several carbon substrates (glucose, starch, yeast extract, methanol, ethanol, formate, acetate, malate, or lactate) to soils incubated at 1.7 or 100 ppm methane did not stimulate methane oxidation. Addition of copper, nitrate or a mineral medium also did not affect methane oxidation. However, incubations with 0.2 or 2% oxygen resulted in lower activity than with ambient air. The methane-consuming capacity of soil decreased exponentially with time when starved for methane by continuous incubation with air containing < 0.03 ppm methane. After 6.3 days of starvation, the soil lost 50% of its original activity; activity was not recoverable after further incubation with atmospheric methane. Methane uptake by soil was rapidly inhibited by the addition of antibacterial antibiotics (streptomycin, chloro-tetracycline, chloramphenicol, ampicillin) as well as by the eukaryotic antibiotic, cycloheximide. Culture suspensions of Methylosinus trichosporium OB3b showed a similar sensitivity to both types of antibiotics. Cell suspensions of Methylosinus trichosporium OB3b and Methylobacter albus consumed atmospheric methane, but consumption rates decreased continuously over a period of 15 days. In contrast, methane consuuption by soil incubated under the same conditions was temporally stable. However, cell suspensions of both cultures showed higher consumption rates for atmospheric methane when sprayed on sand relative to incubations in liquid media.