Effects of nitrate- and sulfate-amendment on the methanogenic populations in rice root incubations.

Abstract Potential interactions between methanogenic archaea and nitrate- and sulfate-reducing bacteria were studied in anoxic incubations of excised rice roots as a model system, by following changes in the concentrations of electron donors and acceptors and in the methanogenic community structure. The relative abundances of methanogenic groups were determined by analyses of terminal restriction fragment length polymorphism targeting archaeal SSU rRNA. Nitrate and sulfate amendment both resulted in suppression of CH(4) production. Suppression by nitrate was initially due to substrate competition for H(2) but eventually also resulted in the suppression of rRNA synthesis of methanogens, in particular of Methanosarcina spp., which was most probably caused by toxic N-compounds (e.g. nitrite). Sulfate-reducing bacteria also successfully competed with methanogens for H(2) and retarded the growth (and/or rRNA synthesis) of the methanogenic populations belonging to the hydrogenotrophic Rice cluster I/Methanomicrobiaceae-group. Our results thus show the potential effect of competition and substrate toxicity on growth and/or synthesis of ribosomes in the two major functional groups of methanogens in a natural habitat.