Stable carbon isotope discrimination in rice field soil during acetate turnover by syntrophic acetate oxidation or acetoclastic methanogenesis

Rice fields are an important source for the greenhouse gas methane. In Italian rice field soil CH4 is produced either by hydrogenotrophic and acetoclastic methanogenesis, or by hydrogenotrophic methanogenesis and syntrophic acetate oxidation when temperatures are below and above about 40–45 °C, respectively. In order to see whether these acetate consumption pathways differently discriminate the stable carbon isotopes of acetate, we measured the δ13C of total acetate and acetate-methyl as well as the δ13C of CO2 and CH4 in rice field soil that had been pre-incubated at 45 °C and then shifted to different temperatures between 25 and 50 °C. Acetate transiently accumulated to about 6 mM, which is about one-third of the amount of CH4 produced, irrespective of the incubation temperature and the CH4 production pathway involved. However, the patterns of δ13C of the CH4 and CO2 produced were different at low (25, 30, 35 °C) versus high (40, 45, 50 °C) temperatures. These patterns were consistent with CH4 being exclusively formed by hydrogenotrophic methanogenesis at high temperatures, and by a combination of acetoclastic and hydrogenotrophic methanogenesis at low temperatures. The patterns of δ13C of total acetate and acetate-methyl were also different at high versus low temperatures, indicating the involvement of different pathways of production and consumption of acetate at the two temperature regimes. Isotope fractionation during consumption of the methyl group of acetate was more pronounced at low (α = 1.010–1.025) than at high (α = 1.0–1.01) temperatures indicating that acetoclastic methanogenesis exhibits a stronger isotope effect than syntrophic acetate oxidation. Small amounts of propionate also transiently accumulated and were analyzed for δ13C. The δ13C values slightly increased (by about 10‰) during production and consumption of propionate, but were not affected by incubation temperature. Collectively, our results showed distinct isotope discrimination for different paths of acetate (and propionate) production and consumption, albeit differences were only small, and discrimination between methanogenic and syntrophic acetate consumption in nature may be difficult to detect.