Abstract
Sulfur oxidation is an essential component of the earth’s sulfur cycle. Acidithiobacillus spp. can oxidize various reduced inorganic sulfur compounds (RISCs) with high efficiency to obtain electrons for their autotrophic growth. Strains in this genus have been widely applied in bioleaching and biological desulfurization. Diverse sulfur-metabolic pathways and corresponding regulatory systems have been discovered in these acidophilic sulfur-oxidizing bacteria. The sulfur-metabolic enzymes in Acidithiobacillus spp. can be categorized as elemental sulfur oxidation enzymes (sulfur dioxygenase, sulfur oxygenase reductase, and Hdr-like complex), enzymes in thiosulfate oxidation pathways (tetrathionate intermediate thiosulfate oxidation (S4I) pathway, the sulfur oxidizing enzyme (Sox) system and thiosulfate dehydrogenase), sulfide oxidation enzymes (sulfide:quinone oxidoreductase) and sulfite oxidation pathways/enzymes. The two-component systems (TCSs) are the typical regulation elements for periplasmic thiosulfate metabolism in these autotrophic sulfur-oxidizing bacteria. Examples are RsrS/RsrR responsible for S4I pathway regulation and TspS/TspR for Sox system regulation. The proposal of sulfur metabolic and regulatory models provide new insights and overall understanding of the sulfur-metabolic processes in Acidithiobacillus spp. The future research directions and existing barriers in the bacterial sulfur metabolism are also emphasized here and the breakthroughs in these areas will accelerate the research on the sulfur oxidation in Acidithiobacillus spp. and other sulfur oxidizers.
Highlights
Acidithiobacillus, the gram-negative sulfur-oxidizing chemolithotrophic bacteria in the proteobacterial class Acidithiobacillia, formerly belonged to the genus “Thiobacillus” (Vishniac and Santer, 1957; Kelly and Wood, 2000; Williams and Kelly, 2013)
All these results indicated that the absence of Sulfur Dioxygenase (SDO) in A. ferrooxidans and A. caldus neither impaired their elemental sulfur oxidation activities nor caused lethal effects on their growth rates in S0media
These results suggest that the existence of the thiosulfate dehydrogenase (TSD)-dependent thiosulfate-oxidation pathway is probably a typical character for sulfur- and ferrous-oxidizing bacteria (A. ferrooxidans, A. ferridurans, and A. ferrivorans)
Summary
Acidithiobacillus, the gram-negative sulfur-oxidizing chemolithotrophic bacteria in the proteobacterial class Acidithiobacillia, formerly belonged to the genus “Thiobacillus” (Vishniac and Santer, 1957; Kelly and Wood, 2000; Williams and Kelly, 2013). The ability of heavy metal leaching has expanded the application of Acidithiobacillus spp. from hydrometallurgy to the treatment of wastes containing heavy metals, such as sewage sludge, spent household batteries, mine tailings, and printed circuit boards (Pathak et al, 2009; Bayat and Sari, 2010; Arshadi and Mousavi, 2014; Ijadi Bajestani et al, 2014; Nguyen et al, 2015; Rastegar et al, 2015) These bacteria have been widely studied in microbial desulfurization of coal and gas (Azizan et al, 2000; He et al, 2012; Charnnok et al, 2013). The key points are summarized to provide an overall picture of sulfur oxidation in Acidithiobacillus spp
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