Anaerobic conversion of carbon dioxide to methane, acetate and propionate on washed rice roots

Washed excised roots of rice (Oryza sativa) produced H(2), CH(4), acetate, propionate and butyrate when incubated under anoxic conditions. Acetate production was most pronounced with a maximum rate (mean+/-standard error; four different root preparations) of 3.4+/-0.6 µmol h(-1) g-dry weight(-1) roots, compared to 0.45+/-0.13, 0.06+/-0.03, and 0.04+/-0.01 µmol h(-1) g-dw(-1) for propionate, butyrate and CH(4)1 kPa after one day of incubation. Then it decreased and reached more or less constant concentrations of about 50-80 Pa after about 7-8 days. Hydrogen partial pressures were always high enough to allow exergonic methanogenesis (DeltaG=-67 to -98 kJ mol(-1) CH(4)) and exergonic homoacetogenesis (DeltaG=-18 to -48 kJ mol(-1) acetate) from H(2) plus CO(2). Radioactive bicarbonate/CO(2) was incorporated into CH(4), acetate and propionate. The specific radioactivities of the products indicated that CH(4) was exclusively produced from H(2)/CO(2) confirming a previous study. The contribution of CO(2) to the production of acetate and propionate was 32-39% and 42-61%, respectively, assuming that each carbon atom was equally labeled. Propionate also became radioactively labeled, when the roots were incubated with either [1-(14)C]acetate or [2-(14)C]acetate accounting for 60-76% of total propionate production. Reductive formation of propionate was thermodynamically favorable both from H(2) plus acetate plus CO(2) (DeltaG=-15 to -38 kJ mol(-1) propionate) and from H(2) plus CO(2) (DeltaG=-34 to -85 kJ mol(-1) propionate). A substantial fraction of propionate was apparently reductively formed from acetate and/or CO(2). In conclusion, our results demonstrate an intensive anaerobic dark metabolism of CO(2) on washed rice roots with reduction of CO(2) contributing significantly to the production of acetate, propionate and CH(4). The CO(2) reduction seemed to be driven by decay and fermentation of root material.