Mechanisms of NaCl-tolerance in acidophilic iron-oxidizing bacteria and archaea: Comparative genomic predictions and insights

For most acidophilic iron-oxidizing bioleaching microorganisms, chloride ions are toxic even at low concentrations since they are able to induce an acidification of the cytoplasm and osmotic stress. To identify molecular determinants for osmotic tolerance of these microorganisms, a genomic analysis based on the complete genomes of twelve iron oxidizers was carried out. Genes for a complete or partial repertoire of K+ transporters Kdp and Ygg were found in most of the genomes. Representatives of Nitrospirae, Firmicutes and Actinobacteria harbor genes for the biosynthesis of (5-hydroxy)ectoine and trehalose. Members of the phylum Nitrospirae appeared to be better equipped with classical mechanisms for osmotic tolerance than representatives of other phyla. The moderately halotolerant bacteria Acidihalobacter prosperus V6 and Leptospirillum ferriphilum Sp-Cl did not show overrepresentation of these mechanisms. In agreement with the bioinformatics results, minimal inhibitory concentrations (MICs) of NaCl showed that Sulfobacillus thermosulfidooxidans, Ferrimicrobium acidiphilum and Leptospirillum ferriphilum were more tolerant than Acidithiobacillus ferrooxidans. Furthermore, external supplementation of ectoine led to an increase in NaCl MIC for L. ferriphilum and F. acidiphilum. These results may provide useful insights for designing chloride-based technologies to bioprocess minerals.