Isolation and characterization of sulphur oxidizing bacteria (Halothiobacillus sp.) from aquaculture farm soil

ABSTRACT

Aim: Isolation and characterization of Halothiobacillus sp. from the shrimp aquaculture farm soil and their sulphur oxidation ability and utilization of H2S in in-vitro model. Methodology: Starkeys mineral salt medium was used to screen autotrophic sulphur oxidizing bacteria. For the qualitative screening, bacterial isolates were inoculated in mineral salt medium containing bromo phenol blue indicator to monitor change in pH. The isolates were studied further for their sulphate ion production, sulphur oxidase enzyme production and utilization of Na2S. Identification was carried out by conventional biochemical and molecular methods. Results: Fifty isolates showed distinct sulphur oxidizing ability on the mineral salt medium. The pH reduction test revealed that out of fifty isolates six could efficiently reduce the pH of the medium to 3.0 from an initial pH of 7 within 96 hr of incubation at 30oC. Maximum sulphate ion (12.65 mg ml-1) and sulphur oxidase enzyme (16.64 mM sulphate hr-1 ml-1) was produced by a bacterial isolate, Halothiobacillus sp. strain rk3. All the six isolates efficiently utilized Na2S in in-vitro conditions. Conventional and molecular identification (16S rRNA sequence analysis) revealed that the sulphur oxidizing bacterial isolates belonged to Halothiobacillus spp. Furthermore, sequencing similarity calculation showed an average nucleotide identity (ANI) values higher than 99% which suggests that the isolates were not genetically different. Interpretation: The present investigation revealed the presence of Halothiobacillus sp. as natural microflora of farm soils in shrimp aquaculture.