Syntrophic Oxidation of Propionate in Rice Field Soil at 15 and 30°C under Methanogenic Conditions

ABSTRACTPropionate is one of the major intermediary products in the anaerobic decomposition of organic matter in wetlands and paddy fields. Under methanogenic conditions, propionate is decomposed through syntrophic interaction between proton-reducing and propionate-oxidizing bacteria and H2-consuming methanogens. Temperature is an important environmental regulator; yet its effect on syntrophic propionate oxidation has been poorly understood. In the present study, we investigated the syntrophic oxidation of propionate in a rice field soil at 15°C and 30°C. [U-13C]propionate (99 atom%) was applied to anoxic soil slurries, and the bacteria and archaea assimilating13C were traced by DNA-based stable isotope probing.Syntrophobacterspp.,Pelotomaculumspp., andSmithellaspp. were found significantly incorporating13C into their nucleic acids after [13C]propionate incubation at 30°C. The activity ofSmithellaspp. increased in the later stage, and concurrently that ofSyntrophomonasspp. increased. AceticlasticMethanosaetaceaeand hydrogenotrophicMethanomicrobialesandMethanocellalesacted as methanogenic partners at 30°C. Syntrophic oxidation of propionate also occurred actively at 15°C.Syntrophobacterspp. were significantly labeled with13C, whereasPelotomaculumspp. were less active at this temperature. In addition,Methanomicrobiales,Methanocellales, andMethanosarcinaceaedominated the methanogenic community, whileMethanosaetaceaedecreased. Collectively, temperature markedly influenced the activity and community structure of syntrophic guilds degrading propionate in the rice field soil. Interestingly,Geobacterspp. and some other anaerobic organisms likeRhodocyclaceae,Acidobacteria,Actinobacteria, andThermomicrobiaprobably also assimilated propionate-derived13C. The mechanisms for the involvement of these organisms remain unclear.