Syntrophobacteraceae-affiliated species are major propionate-degrading sulfate reducers in paddy soil.

Methane is an important greenhouse gas and propionate is next to acetate the main intermediate (average 23%) of the carbon flow to CH4 in paddy fields. Sulfate (e.g., gypsum) application can reduce CH4 emissions up to 70%. However, the effect of gypsum application on propionate degradation and the microbial communities involved are not well understood. Therefore, we studied propionate-dependent sulfate reduction in anoxic microcosms of paddy soils from Italy and the Philippines, combining 16S rRNA and dissimilatory sulfite reductase (dsrB) gene profiling and co-occurrence network analysis. Sulfate was stoichiometrically reduced in treatments with propionate addition, while CH4 production was partially suppressed. Methane production but not sulfate reduction were suppressed and acetate accumulated after addition of methyl fluoride or fluoroacetate. With methyl fluoride in the presence of sulfate, the accumulated acetate was consumed after the depletion of propionate. Simultaneously, the relative abundances of Syntrophobacteraceae and Desulfovibrionaceae were significantly enhanced, while fluoroacetate repressed Desulfobulbaceae in both soils. Syntrophobacter 16S rRNA and dsrB gene copy numbers were also remarkably increased with gypsum amendment. Network analysis of both 16S rRNA and dsrB genes illustrated a strong co-occurrence of operational taxonomic units belonging to Syntrophobacteraceae, Desulfovibrionaceae and Desulfobulbaceae. In summary, Syntrophobacteraceae affiliated species were identified as the major propionate-dependent sulfate reducers in paddy soil. They (together with Desulfobulbaceae) oxidized propionate directly to acetate and CO2 , or coupled the oxidation syntrophically to H2 /formate-utilizing Desulfovibrionaceae. The transiently accumulating acetate was preferentially consumed by acetoclastic Methanosarcinaceae.