Sulfur oxidation in rice field soil: Activity, enumeration, isolation and characterization of thiosulfate-oxidizing bacteria

In rice paddy fields the bulk soil is anoxic, but oxygenated zones occur in the surrounding of the rice roots to where oxygen is transported via the a erenchyma system of the rice plants. In the anaerobic soil compartments sul fate is consumed by sulfate-reducing bacteria. In the rhizosphere the reduc ed sulfur compounds can be reoxidized by sulfur-oxidizing bacteria. Measure ments of the potential activity of thiosulfate-oxidizing bacteria in soil s lurries derived from planted rice soil microcosms showed turnover rates of 2-6 mu mol d(-1) g-dw(-1). Thiosulfate was oxidized to sulfate with tetrath ionate as intermediate. Most probable number (MPN) enumeration with three a erobic media and one anaerobic nitrate-amended medium showed that thiosulfa te-oxidizing bacteria were abundant in paddy soil and in rhizosphere soil a t numbers of 10(5) to 10(6) per gram dry weight soil. Nine isolates of S-ox idizing bacteria were obtained from enrichment cultures or from the highest dilutions of the MPN series and were affiliated to four different phylogen etic groups. These isolates were characterized by physiological properties and by comparative 16S rDNA sequence analysis. Three isolates (TA1-AE1, TA1 -A1 and TA12-21) were shown to be facultatively chemolithoautotrophic strai ns of Ancylobacter aquaticus. Three further isolates (Tv6-2b, Z2A-6A and Z4 A-2A) were also facultatively chemolithoautotrophic and were affiliated wit h the Xanthobacter sp. group, probably representing new strains of X. flavu s or X. tagetidis. Strain 5Z-2111 was phylogenetically related to Bosea thi ooxidans. However, the genus Bosea is described as obligately heterotrophic , whereas strain 5Z-2111 was able to grow autotrophically. The isolates 5Z- C1 and TBW3 were obligate chemolithoautotrophs and were closely affiliated with Thiobacillus thioparus. Our results showed that S-oxidizing bacteria w ere abundant and active in rice paddy soil and consisted of physiologically and phylogenetically diverse populations.