Syntrophic acetate oxidation under thermophilic methanogenic condition in Chinese paddy field soil. 
The aim of the present work was to determine and compare the degradation of acetate in a Chinese rice field soil at 25°C and 50°C, respectively, and to identify specifically the active organisms involved in syntrophic acetate oxidation.  Soil was preincubated anaerobically for 30 days to reduce alternative electron acceptors other than CO(2).  The [2-(13)C] acetate (99% (13)C) was added twice: 0 day and 19 days after preincubation.  Addition of [2-(13)C] acetate resulted in an immediate increase of (13)C labeled CH(4) but non-labeling of CO(2) at 25°C.  The methanogen community was dominated by Methanosarcinaceae and Methanocellales at 25°C.  In contrast, the addition of [2-(13)C] acetate at 50°C resulted in a rapid increase of (13)CO(2).  The (13)C labeling of CH(4) gradually increased and reached a similar value to CO(2) (13% (13)C) at the end of incubation (40 days).  Nearly all archaeal 16S rRNA genes detected at 50°C belonged to hydrogenotrophic Methanocellales.  DNA-based stable isotope probing analysis revealed that the organisms related to Thermacetogenium lineage and the unclassified Thermoanaerobacteraceae group were intensively labeled with (13)C in the incubations at 50°C.  Thus, acetate was converted to CH(4) and CO(2) through aceticlastic methanogenesis at 25°C, while syntrophic acetate oxidation occurred at 50°C.