substrates. Bottles were incubated (39 +/- 1 degrees C for 56 days), and the gas production was measured (mbar) and converted to the volume. A sample of produced gas was taken and analyzed for methane concentration using a gas chromatography. Bottles were opened at days 0, 25, and 56 to determine total bacteria, total, and hydrogenotrophic methanogens archaea (HMA) concentrations using quantitative polymerase chain reaction and population biodiversity using denaturing gradient gel electrophoresis (DGGE). Incubation time reduced the titers of total bacteria and archaea (P<0.01) but did not modify HMA population. Doses of Co-i showed a positive correlation with HMA titers, although interacted with an incubation period (P<0.001); at 5% Co-inoculation, total bacteria decreased significantly (0-25 days) but remained steady until day 56 (P>0.05), whereas at 10% Co inoculation, titers decreased constantly. Most of the archaeal DGGE bands were observed in all samples, suggesting a common microbial population origin but Co-i supply altered the DGGE structure of archaea populations