Hydrogen isotopic composition of bacterial methane: CO2/H2 reduction and acetate fermentation

In order to discern the δDwater-δDCH4 relationship between C02/H2 reduction and acetate fermentation in freshwater environments where CH4 is produced from both C02/H2 and acetate, incubation experiments of a paddy soil were made, using water with four different SD values. From the observed δDwater-δDCH4 relationships and a fractional contribution of acetate estimated from δ13C of CH4, δDwater-δDCH4 relationships were obtained for C02/H2 reduction and for acetate fermentation, respectively. The relationships were expressed as follows; δDCH4(co2) = (0.683 ± 0.020)δDwater − (317 ± 20) δDCH4(Ac) = (0.437 ± 0.045) δDwater - (302 ± 15). The obtained difference in δD between freshwater and CH4 from C02/H2 (317%) was much larger than that reported in marine sediments (180%). Accordingly, no significant difference in δD of CH4 between CO2/H2 reduction and acetate fermentation is expected when the δD of water is close to SMOW, and δD of CH4 from C02/H2 is lower than that from acetate when the δD of water is low. The large fractionation obtained for C02/H2 reduction may be caused by a high H2 concentration in the freshwater soil which is rich in easily decomposable organic matters. The estimated slope (0.437) of the δDwater- δDCH4 relationship for acetate fermentation indicates that some of the hydrogen atoms in the methyl group of acetate exchanged with water during the acetate production. On the basis of the experimental results, the range of δD-δ13C for CH4 from C02/H2 reduction and that for acetate fermentation were calculated for the freshwater environments. Although only a negative correlation between δD and δ13C of CH4 has been expected, our results indicate the possibility of a positive correlation between them in freshwater areas.