Nineteen fungal strains having an ability to oxidize elemental sulfur in mineral salts medium were isolated from deteriorated sandstones of Angkor monuments. These fungi formed clearing zone on agar medium supplemented with powder sulfur due to the dissolution of sulfur. Representative of the isolates, strain THIF01, was identified as Fusarium solani on the basis of morphological characteristics and phylogenetic analyses. PCR amplification targeting 16S rRNA gene and analyses of full 16S rRNA gene sequence indicated strain THIF01 harbors an endobacterium Bradyrhizobium sp.; however, involvement of the bacterium in the sulfur oxidation is still unclear. Strain THIF01 oxidized elemental sulfur to thiosulfate and then sulfate. Germination of the spores of strain THIF01 was observed in a liquid medium containing mineral salts supplemented with elemental sulfur (rate of germinated spores against total spores was 60.2%), and the culture pH decreased from pH 4.8 to 4.0. On the contrary, neither germination (rate of germinated spores against total spores was 1.0%) nor pH decrease was observed without the supplement of elemental sulfur. Strain THIF01 could also degrade 30 ppmv and ambient level (approximate 500 pptv) of carbonyl sulfide.
To characterize sulfate-producing microorganisms from the deteriorated sandstones of Angkor monuments in Cambodia, strains of Mycobacterium spp. were isolated from most probable number-positive cultures. All five strains isolated were able to use both elemental sulfur (S0) for chemolithoautotrophic growth and organic substances for chemoorganoheterotrophic growth. Results of phylogenetic and phenotypic analyses indicated that all five isolates were rapid growers of the genus Mycobacterium and were most similar to Mycobacterium cosmeticum and Mycobacterium pallens. Chemolithoautotrophic growth was further examined in the representative strain THI503. When grown in mineral salts medium, strain THI503 oxidized S0 to thiosulfate and sulfate; oxidation was accompanied by a decrease in the pH of the medium from 4.7 to 3.6. The link between sulfur oxidation and energy metabolism was confirmed by an increase in ATP. Fluorescence microscopy of DAPI-stained cells revealed that strain THI503 adheres to and proliferates on the surface of sulfur particles. The flexible metabolic ability of facultative chemolithoautotrophs enables their survival in nutrient-limited sandstone environments.
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