Activity of sulphur-oxidizing microorganisms in some Australian soils

Abstract

The percentage oxidation of sulphur and the activity of Thiobacillus thiooxidans in a red-brown earth and a chernozem were followed at monthly intervals over a period of a year, but no obvious seasonal pattern emerged, and neighbouring sites rarely synchronized. Th. thiooxidans occurred more often in the red-brown earth than in the chernozem. In a survey of 273 composite samples of Australian soils oxidation of sulphur was good in approximately one-third only, moderate to poor in a half, and nil in the remaining one-sixth. There was no obvious correlation between sulphofication and several soil properties tested; but it was often higher in soils containing sulphides and lower in more alkaline soils. Th. thiooxidans was not detected in one-third of 288 soils examined, and it was seldom very active in the remaining two-thirds. There was no relationship between the incidence of these bacteria and the Great Soil Group or the locality from which the sample came, and only poor correlations between initial pH, final pH, and titratable acidity of incubated samples. The most numerous sulphur-oxidizing organisms were heterotrophs, including nine bacterial genera and two yeasts. Facultative autotrophic thiobacilli were encountered more often than obligately autotrophic species. Green or purple sulphur bacteria were seldom isolated from arable soils. Seventy-five isolates were tested for their ability to oxidize thiosulphate and sulphur, with and without peptone, and in general heterotrophs were inferior to autotrophs and facultative autotrophs. Autoclaved soils when reinoculated with autotrophic or heterotrophic bacteria or mixtures of both types oxidized most sulphur when Th. thiooxidans and Th, thioparus were present. Soil inoculum could be imitated by a mixture of autotrophic and heterotrophic bacteria. No photochemical oxidation of sulphur was detected in soil exposed to sunlight. After enrichment of soil with seven inorganic sulphur compounds only sodium thiosulphate and sodium sulphite stimulated subsequent oxidation of elementary sulphur, while two organic sulphur compounds, cysteine and particularly methionine, were inhibitory. Starch supplement had no effect on sulphur oxidation, but asparagine depressed it. Preliminary incubation of soil for 1 to 4 weeks prior to the addition of sulphur accelerated its oxidation. The incidence of Th. thiooxidans was unaffected by eight sulphur compounds, was reduced by methionine, also by asparagine and starch, and was increased by periods of incubation prior to the addition of elemental sulphur.