Carbon metabolism and sulfate respiration by a non-conventional Citrobacter freundii strain SR10 with potential application in removal of metals and metalloids

Pollution by heavy metals is a serious environmental problem and removal methods have gained more and more attention from potential applications. Precipitation of heavy metals through sulfate reduction by sulfate reducing bacteria (SRB) is a promising biotechnological solution. A Citrobacter freundii species sulfate-reducing bacterium (SR10) was isolated from an up-flow anaerobic sludge bed for treatment of acid mine drainage. In this study, the ecophysiology of SR10 was investigated, then carbon metabolism and sulfate respiration were discussed. Growth of SR10 occurred with a doubling time of about 5.0 h, removal of both COD and sulfate were observed simultaneously in the culture medium. The precipitation produced by SR10 was most likely contributed by Fe (III) polysulfide, and production of elemental sulfur as intermediate. Strain SR10 was identified to be Citrobacter freundii based on 16S rRNA gene, but the dissimilatory sulfite reductase (dsrAB) gene of SR10 was highly (99%) affiliated with Desulfobulbus elongatus species. Strain SR10 is a mixotroph, being able to perform both autotrophic and heterotrophic sulfate reduction. Sulfate, sulfite, thiosulfate, sulfur and lauryl sulfate could all be utilized by strain SR10 to produce sulfide, suggesting that an adapted trithionate-like sulfate reduction pathway is operative in strain SR10. Moreover, the reduction of sulfate to sulfite might be the limiting step during the entire sulfate reduction process. Because of the quick growth and metabolism, SR10 could be applied as a biocatalyst for removal of both heavy metals and metalloids in bioremediation.