Effects of Plant Residue Extract and Cobalamin on Growth and Propionate Production of Propionicimonas paludicola Isolated from Plant Residue in Irrigated Rice Field Soil

Since most of the anaerobic bacterial isolates from rice plant residue in irrigated rice field soil grew slowly or weakly in the medium (PY medium) used, growth factors for the isolates were investigated. Plant residue extract (RE) was prepared by autoclaving plant residue collected from the soil, and RE was added to the medium as a possible source of growth factors. With the addition, growth of the slowly-growing, propionate-producing strains of Propionicimonas paludicola was considerably improved. Lactate was the dominant product of these strains in the presence of lower concentrations of RE. Moreover, amounts of acetate and propionate produced increased in proportion to the RE concentration added. The factor in RE affecting the growth of these strains appeared to be cobalamin, and addition of cobalamin to the medium remarkably improved their growth. With a sufficient amount of cobalamin added, propionate was the dominant product from the onset of fermentation, while lactate was only a minor product. Concentrations of cobalamin in RE were determined using the microbiological method. Almost exactly the same amount of cobalamin was detected in RE prepared from plant residue samples collected in different years. Cobalamin was also detected in extracts of the rice field soil, although the concentrations were much lower than those in RE. It was suggested that these cobalamin-requiring, propionateproducing bacteria survive in rice field soil by using cobalamin supplied by other microbes, which endogenously produce cobalamin and release it into the environment.