Isolation of salt-tolerant, iron-oxidising, acidophilic bacteria and assessment of their bioleaching potential at high salinity

Abstract

The occurrence of chemoautotrophic, acidophilic bacteria in the marine environment has been widely noted and they have been implicated in the biogeochemical cycling of iron and biodeterioration of iron-containing structures in the oceans. However, the isolation, molecular ecology, growth profiles and physiological responses of these bacteria at elevated salt levels have rarely been described, despite widespread interest in their unique metabolic capacity. These bacteria may have a potential application in the extraction of metals via bioleaching of salt contaminated ores or to facilitate the use of seawater in the bioleaching process. Traditional bioleaching microorganisms cannot be used in these cases due to the toxicity of elevated salt concentrations. In this study, three strains of halotolerant gram-positive, rod shaped, acidophilic bacteria were isolated from estuarine and coastal areas, two of which were novel species. Enrichment cultures were set up using pyrite medium of different salinities with sediment and seawater samples from a variety of metal contaminated areas exposed to the sea or brackish water. These enrichment cultures were then further purified using end-point dilution culture methods and the 16S rRNA genes were sequenced and phylogeny assessed. The growth characteristics, morphology and growth profiles on a variety of metaliferrous ore samples of the strains were characterised. The strains exhibited autotrophic growth on a variety of iron and sulphur-containing compounds, heterotrophic growth on yeast extract medium as well as mixotrophic growth on a combination of these substrates. The strains grew optimally with 30gl" sea salts added to the medium, at a pH of 2.0 and a temperature of 37"C. Two of these isolated bacteria represent novel species in the genera Suffibbacillus and Alicyclobacillus. High final iron dissolution levels were demonstrated after biooxidation of Lihir gold ore and Escondida Copper ore in medium with 30 gl" sea salts and 2% ore for 30 days. Bacterium 4G mediated 66.10%, 5C 100% and Cligga 88.86% dissolution of the total iron present in the Lihir sample after 30 days, while bacterium 4G mediated 52.63%, 5C 60% and Cligga 49.75% dissolution of the total iron present in the Escondida samples after 30 days. The growth characteristics displayed by these bacterial strains demonstrate their potential application in high salinity bioleaching operations