Individual stages of bacterial dichloromethane degradation mapped by carbon and chlorine stable isotope analysis.

The fractionation of carbon and chlorine stable isotopes of dichloromethane (CH2Cl2, DCM) upon dechlorination by cells of the aerobic methylotroph Methylobacterium extorquens DM4 and by purified DCM dehalogenases of the glutathione S-transferase family was analyzed. Isotope effects for individual steps of the multi-stage DCM degradation process, including transfer across the cell wall from the aqueous medium to the cell cytoplasm, dehalogenase binding, and catalytic reaction, were considered. The observed carbon and chlorine isotope fractionation accompanying DCM consumption by cell supensions and enzymes was mainly determined by the breaking of CCl bonds, and not by inflow of DCM into cells. Chlorine isotope effects of DCM dehalogenation were initially masked in high density cultures, presumably due to inverse isotope effects of non-specific DCM oxidation under conditions of oxygen excess. Glutathione cofactor supply remarkably affected the correlation of variations of DCM carbon and chlorine stable isotopes (Δδ13C/Δδ37Cl), increasing corresponding ratio from 7.2-8.6 to 9.6-10.5 under conditions of glutathione deficiency. This suggests that enzymatic reaction of DCM with glutathione thiolate may involve stepwise breaking and making of bonds with the carbon atom of DCM, unlike the uncatalyzed reaction, which is a one-stage process, as shown by quantum-chemical modeling.

Estimation of microbial methane generation and oxidation rates in the municipal solid waste landfill of Kaluga city, Russia.

Using a theoretical model and mass isotopic balance, biogas (methane and CO(2)) released from buried products at their microbial degradation was analysed in the landfill of municipal and non-toxic industrial solid organic waste near Kaluga city, Russia. The landfill contains about 1.34 x 10(6) tons of waste buried using a 'sandwich technique' (successive application of sand-clay and waste layers). The delta(13)C values of biogenic methane with respect to CO(2) were-56.8 (+/-2.5) per thousand, whereas the delta(13)C of CO(2) peaked at+9.12 per thousand (+1.4+/-2.3 per thousand on average), reflecting a virtual fractionation of carbon isotopes in the course of bacterial CO(2) reduction at the landfill body. After passing through the aerated soil layers, methane was partially oxidised and characterised by delta(13)C in the range of-50.6 to-38.2 per thousand, evidencing enrichment in (13)C, while the released carbon dioxide had delta(13)C of-23.3 to-4.04 per thousand, respectively. On the mass isotopic balance for the delta(13)C values, the methane production in the landfill anaerobic zone and the methane emitted through the aerated landfill surface to the atmosphere, the portion of methane oxidised by methanotrophic bacteria was calculated to be from 10 to 40% (averaged about 25%). According to the theoretical estimation and field measurements, the annual rate of methane production in the landfill reached about 2.9(+/-1.4)x10(9) g C CH(4) yr(-1) or 5.3(+/-2.6)x10(6) m(3) CH(4) yr(-1). The average rates of methane production in the landfill and methane emission from landfill to the atmosphere are estimated as about 53 (+/-26) g C CH(4) m(-2) d(-1) (or 4 (+/-2) mol CH(4) m(-2) d(-1)) and 33 (+/-12) g C CH(4) m(-2) d(-1) (or 2.7 (+/-1) mol CH(4) m(-2) d(-1)), respectively. The calculated part of methane consumed by methanotrophic bacteria in the aerated part of the landfill was 13(+/-7) g C CH(4) m(-2) d(-1) (or 1.1(+/-0.6) mol CH(4) m(-2) d(-1)) on average.