Salt Stress-Induced Loss of Iron Oxidoreduction Activities and Reacquisition of That Phenotype Depend onrusOperon Transcription inAcidithiobacillus ferridurans

ABSTRACTThe type strain of the mineral-oxidizing acidophilic bacteriumAcidithiobacillus ferriduranswas grown in liquid medium containing elevated concentrations of sodium chloride with hydrogen as electron donor. While it became more tolerant to chloride, after about 1 year, the salt-stressed acidophile was found to have lost its ability to oxidize iron, though not sulfur or hydrogen. Detailed molecular examination revealed that this was due to an insertion sequence, ISAfd1, which belongs to the ISPepr1subgroup of the IS4family, having been inserted downstream of the two promoters PI and PII of therusoperon (which codes for the iron oxidation pathway in this acidophile), thereby preventing its transcription. The ability to oxidize iron was regained on protracted incubation of the culture inoculated onto salt-free solid medium containing ferrous iron and incubated under hydrogen. Two revertant strains were obtained. In one, the insertion sequence ISAfd1had been excised, leaving an 11-bp signature, while in the other an ∼2,500-bp insertion sequence (belonging to the IS66family) was detected in the downstream inverted repeat of ISAfd1. The transcriptional start site of therusoperon in the second revertant strain was downstream of the two ISs, due to the creation of a new “hybrid” promoter. The loss and subsequent regaining of the ability ofA. ferriduransTto reduce ferric iron were concurrent with those observed for ferrous iron oxidation, suggesting that these two traits are closely linked in this acidophile.IMPORTANCEIron-oxidizing acidophilic bacteria have primary roles in the oxidative dissolution of sulfide minerals, a process that underpins commercial mineral-processing biotechnologies (“biomining”). Most of these prokaryotes have relatively low tolerance to chloride, which limits their activities when only saline or brackish waters are available. The study showed that it was possible to adapt a typical iron-oxidizing acidophile to grow in the presence of salt concentrations similar to those in seawater, but in so doing they lost their ability to oxidize iron, though not sulfur or hydrogen. The bacterium regained its capacity for oxidizing iron when the salt stress was removed but simultaneously reverted to tolerating lower concentrations of salt. These results suggest that the bacteria that have the main roles in biomining operations could survive but become ineffective in cases where saline or brackish waters are used for irrigation.