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Abstract

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 aerenchyma system of the rice plants. In the anaerobic soil
compartments sulfate is consumed by sulfate-reducing bacteria. In the
rhizosphere the reduced sulfur compounds can be reoxidized by
sulfur-oxidizing bacteria. Measurements of the potential activity of
thiosulfate-oxidizing bacteria in soil slurries 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 tetrathionate as intermediate.
Most probable number (MPN) enumeration with three aerobic media and one
anaerobic nitrate-amended medium showed that thiosulfate-oxidizing
bacteria were abundant in paddy soil and in rhizosphere soil at numbers
of 10(5) to 10(6) per gram dry weight soil. Nine isolates of S-oxidizing
bacteria were obtained from enrichment cultures or from the highest
dilutions of the MPN series and were affiliated to four different
phylogenetic 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 strains of Ancylobacter aquaticus. Three further
isolates (Tv6-2b, Z2A-6A and Z4A-2A) were also facultatively
chemolithoautotrophic and were affiliated with the Xanthobacter sp.
group, probably representing new strains of X. flavus or X. tagetidis.
Strain 5Z-2111 was phylogenetically related to Bosea thiooxidans.
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 were abundant and active in rice paddy soil and
consisted of physiologically and phylogenetically diverse populations.