The abundance of total bacteria, total archaea, and the three major cultured cellulolytic species of bacteria was quantified to assess the impact from the added nitrate. Total bacteria, calculated as bacterial rrs gene copies per ml culture, was not affected except for an increase (nearly 2-fold) in the cultures that received 24 μmol ml−1 nitrate ( Fig. 2a). This contradicts the finding of a previous study ( Marais et al., 1988), in which total bacteria was reduced based on plate counts. The addition of nitrate might have caused succession of microbiota and reduced the number of bacteria that can be cultured on the media used. Nitrate at 24 μmol ml−1 doubled the population of total bacteria, while total bacterial was not affected at other concentrations. It is not known why 24 μmol ml−1 nitrate increased the population of total bacteria, but the larger total bacterial population corresponded positively to the higher total VFA concentration observed at 24 μmol ml−1 nitrate than at other nitrate concentrations ( Table 1). On the contrary, methanogens were greatly reduced, by at least 97%, at all the nitrate concentrations tested ( Fig. 2b). No significant difference in methanogen abundance was seen among the four nitrate concentrations, suggesting that 12 μmol ml−1 is high enough to inhibit methanogens. Nitrate at 12 μmol ml−1 significantly reduced the population of R. albus, but not at the higher nitrate concentrations ( Fig. 2c). R. flavefaciens was inhibited by nitrate at all the concentrations, with 12 μmol ml−1 inhibiting to the greatest extent (>99%) ( Fig. 2d). F. succinogenes was almost completely (>99%) inhibited by nitrate at all the four tested concentrations