Genomic Analysis Unravels Reduced Inorganic Sulfur Compound Oxidation of Heterotrophic AcidophilicAcidicaldussp. Strain DX-1

Although reduced inorganic sulfur compound (RISC) oxidation in many chemolithoautotrophic sulfur oxidizers has been investigated in recent years, there is little information about RISC oxidation in heterotrophic acidophiles. In this study,Acidicaldussp. strain DX-1, a heterotrophic sulfur-oxidizing acidophile, was isolated. Its genome was sequenced and then used for comparative genomics. Furthermore, real-time quantitative PCR was performed to identify the expression of genes involved in the RISC oxidation. Gene encoding thiosulfate: quinone oxidoreductase was present inAcidicaldussp. strain DX-1, while no candidate genes with significant similarity to tetrathionate hydrolase were found. Additionally, there were genes encoding heterodisulfide reductase complex, which was proposed to play a crucial role in oxidizing cytoplasmic sulfur. Like many heterotrophic sulfur oxidizers,Acidicaldussp. strain DX-1 had no genes encoding enzymes essential for the direct oxidation of sulfite. An indirect oxidation of sulfite via adenosine-5′-phosphosulfate was proposed inAcidicaldusstrain DX-1. However, compared to other closely related bacteriaAcidiphilium cryptumandAcidiphilium multivorum, which harbored the genes encoding Sox system, almost all of these genes were not detected inAcidicaldussp. strain DX-1. This study might provide some references for the future study of RISC oxidation in heterotrophic sulfur-oxidizing acidophiles.

Utilisation of Inorganic Substrates by Alicyclobacillus - Like Strain FP1

As Alicyclobacillus-like strain FP1 was isolated from copper heap process water (pH 1.5), this research was directed towards its bioleaching attributes, specifically ferrous ion (Fe (II)) and reduced inorganic sulfur compound (RISC) oxidation, and bioleaching of sulfide minerals. Strain FP1 oxidised iron (II) but not tetrathionate or elemental sulfur in growth media containing yeast extract as growth factor. The addition of tetrathionate (2.5 mM) suppressed iron (II) oxidation. Strain FP1 grew on pyrite, arsenopyrite, chalcopyrite, sphalerite and pentlandite in BSM-YE medium at 30 °C and pH 1.8 (35 days), enhancing Zn, Co (in cobaltiferous pyrite), Fe and As recovery, but not Cu or Ni, relative to abiotic controls.

Gene Identification and Substrate Regulation Provide Insights into Sulfur Accumulation during Bioleaching with the Psychrotolerant Acidophile Acidithiobacillus ferrivorans

ABSTRACTThe psychrotolerant acidophileAcidithiobacillus ferrivoranshas been identified from cold environments and has been shown to use ferrous iron and inorganic sulfur compounds as its energy sources. A bioinformatic evaluation presented in this study suggested thatAcidithiobacillus ferrivoransutilized a ferrous iron oxidation pathway similar to that of the related speciesAcidithiobacillus ferrooxidans. However, the inorganic sulfur oxidation pathway was less clear, since theAcidithiobacillus ferrivoransgenome contained genes from bothAcidithiobacillus ferrooxidansandAcidithiobacillus caldusencoding enzymes whose assigned functions are redundant. Transcriptional analysis revealed that thepetA1andpetB1genes (implicated in ferrous iron oxidation) were downregulated upon growth on the inorganic sulfur compound tetrathionate but were on average 10.5-fold upregulated in the presence of ferrous iron. In contrast, expression ofcyoB1(involved in inorganic sulfur compound oxidation) was decreased 6.6-fold upon growth on ferrous iron alone. Competition assays between ferrous iron and tetrathionate withAcidithiobacillus ferrivoransSS3 precultured on chalcopyrite mineral showed a preference for ferrous iron oxidation over tetrathionate oxidation. Also, pure and mixed cultures of psychrotolerant acidophiles were utilized for the bioleaching of metal sulfide minerals in stirred tank reactors at 5 and 25°C in order to investigate the fate of ferrous iron and inorganic sulfur compounds. Solid sulfur accumulated in bioleaching cultures growing on a chalcopyrite concentrate. Sulfur accumulation halted mineral solubilization, but sulfur was oxidized after metal release had ceased. The data indicated that ferrous iron was preferentially oxidized during growth on chalcopyrite, a finding with important implications for biomining in cold environments.

Elemental Sulfur Oxidation in Acidiphilium spp.

The alpha-proteobacterial genus Acidiphilium consists of several acidophilic species, generally known as a part of the mesophilc microbial flora of leaching biotopes. All of them can grow chemoorganotrophically on carbon sources like sugars and many express additional photosynthetic pigments. Thus far, only Ap. acidophilum is known to be capable of chemolithotrophic growth on elemental sulfur oxidation. The oxidation potential of inorganic sulfur species by the other strictly heterotrophic species has not yet been thoroughly investigated. Here, we demonstrate the unequivocal evidence of inorganic sulfur compound oxidation by strains of Ap. cryptum and other Acidiphilium species. Evolutionary and biochemical aspects of this new feature among the heterotrophic Acidiphilium spp. are discussed. This finding will possibly help to solve the long-standing question about the biochemical nature of elemental sulfur oxidation in mesophilic leaching bacteria.