Acidithiobacillus caldus
Classification
Bacteria; Proteobacteria; Acidithiobacillia; Acidithiobacillales; Acidithiobacillaceae
Genus Species
Acidithiobacillus; A. caldus
Description and Significance
A. caldus is a short, rod-shaped bacterium that moves using a single flagellum. It is gram-negative and can often be found in pairs. A. caldus typically lives within rocks found in a coal mine, and it is one of the most common microbes involved in biomining. A. caldus oxidizes reduced inorganic sulfur compounds and elemental sulfur to create the acidity needed for biomining to occur.
Genome
The genomic sequence of A. caldus has a length of 2,932,335 base pairs and is rich in GC. Strain KU has a GC content of 63.1-63.9%, while strain BC13 has a GC content of 61.7%. DNA hybridization studies have shown that the two strains exhibit 100% homology with one another, yet showed between 2-20% DNA hybridization with other species in the genus. A. caldus strains have been differentiated from other acidithiobacilli using PCR amplification to analyze the sequences of the 16S-23S rDNA intergenic spacer and restriction fragment length polymorphism.
Cell Structure, Metabolism and Life Cycle
A. caldus has the capabilities to reduce a number of substrates, such as reduced inorganic sulfur compounds, molecular hydrogen, formate, and other sulfide minerals. It will grow in a chemolithotrophic fashion when exposed to sulfur, and it uses reduced sulfur compounds in environments where light is not present to support autotrophic growth. Strain BC13 of A. caldus is capable of growing on a glucose growth medium if it has not previously been exposed to sulfur. Elemental sulfur and tetrathionate are key intermediates in the metabolism of A. caldus. Sulfite oxidizes the elemental sulfur into sulfate and tetrathionate hydrolase breaks tetrathionate down into pentathionate, thiosulfate, and sulfur. The generation time of A caldus depends on the pH of its environment. At its fastest, the generation time of A caldus can occur in 2 to 3 hours. At its longest, the generation time can be around 45 hours.
Ecology and Known Roles in Symbiosis
A. caldus is typically found in a biomining or mineral-rich environment. The optimal growth temperature for the bacterium is around 40-45 degrees Celsius. Strain KU has previously exhibited growth in temperatures as low as 32 degrees Celsius and as high as 52 degrees Celsius. Strain BC13 has been known to grow in temperatures as high as 55 degrees Celsius. A. caldus grows best in acidic environments, with a preferred pH of between 2.0 and 2.5. Strains KU and BC13 have been able to grow in a wider pH range. In a pH of 4.0, A. caldus has been found to have a generation time of about 45 hours, while in a pH of 1.0 the generation time is around 6 to 7 hours. The shortest generation time, 2 to 3 hours, occurs at a pH of 2.0-2.5. No growth was observed at a pH of 0.5. A. caldus has not been known to have a symbiotic relationship with any other organisms. The main contribution of A. caldus is the oxidation of sulfuric compounds to produce an acidic environment and the removal of elemental sulfur.
Fun Facts
Biomining is the process of extracting metals from ores using microbes. The microbes secrete compounds that bond with the metals and bring it towards the cell. Large amounts of these microbes can leach a significant amount of metals from the rock, which can be harvested and transformed into usable metal products. A. caldus is one such microbe that can be used in biomining. The use of thermophilic sulfur-oxidizing bacteria to extract metals can drastically increase recovery rates and reduce the cost of collecting such metals. Therefore, the use of A. caldus can reduce the costs associated with purchasing copper products.
References
1. Dopson, Mark, Holmes, David S., Mangold, Stefanie, Valdés, Jorge. "Sulfur Metabolism in the Extreme Acidophile Acidithiobacillus caldus." Frontiers in Microbiology, 10 Feb. 2011, https://www.frontiersin.org/articles/10.3389/fmicb.2011.00017/full.
2. Xiao-Yan You et al. "Unraveling the Acidithiobacillus caldus Complete Genome and its Central Metabolisms for Carbon Assimilation." Science Direct, 20 June 2011, https://www.sciencedirect.com/science/article/abs/pii/S1673852711000786?via%3Dihub.
3. Kelly, D.P., Wood, A.P. "Reclassification of Some Species of Thiobacillus to the Newly Designated Genera Acidithiobacillus gen. nov., Halothiobacillus gen. nov. and Termithiobacillus gen. nov." International Journal of Systematic and Evolutionary Biology, 1 March 2000, https://www.microbiologyresearch.org/content/journal/ijsem/10.1099/00207713-50-2-511.
4. Hallberg, Kevin B., Lindström, E. Börje. "Characterization of Thiobacillus caldus sp. nov., a Moderately Thermophilic Acidophile." Microbiology, 1 Dec. 1994, https://www.microbiologyresearch.org/content/journal/micro/10.1099/13500872-140-12-3451.
5. "What is Biomining?" American Geosciences Institute, https://www.americangeosciences.org/critical-issues/faq/what-biomining.
Author
This page was authored by Nicole Jonah as part of the 2020 UM Study USA led by Dr. Erik Hom at the University of Mississippi.
