Thiobacillus

***Thiobacillus*** is a genus of bacteria which obtain energy through oxidisation of reduced sulfur compounds. This is sulfur with extra electrons. _Thiobacillus_ species (spp.) exist in any environment with a source of reduced sulfur such as soils, hydrothermal vents, and sludge disposal sites. _Thiobacillus_ spp. grow best in temperatures from 28-32 °C, and pHs 2-8.

To identify _Thiobacillus_ spp., they can be viewed under a microscope. First, they need to be coloured with a gram stain. After staining, _Thiobacillus_ spp. appear rod-shaped, and pink. Pink means gram-negative and indicates no outer cell wall.

## Classification

As of date, there are three established species of _Thiobacillus_ genus: _T._ _denitrificans_, _T._ _thioparus_, _T._ _thiophilus_, and two candidates: _T._ _plumbophilus_, _T._ _sajanensis_. These candidate species need to have their research papers verified. Historically, many species were classified under this genus. However, after the development of a new RNA sequencing technology known as “16S ribosomal RNA sequencing”, most were reclassified into new or existing genera. Confusingly, the older names of these reclassified bacteria are still used in the literature. For example, _Acidithiobacillus ferooxidans_ is sometimes referred to as _Thiobacillus_ _ferooxidans_. However, they are no longer part of the _Thiobacillus_ genus.

Many of the former members were mixotrophs. However, the remaining three species are completely reliant on carbon fixation, meaning the entire _Thiobacillus_ genus can now be described as obligate autotrophs.

## Sulfur oxidisation

_Thiobacillus_ spp. get their food by using the carbon from CO<sub>2</sub> and turning it into other compounds. All _Thiobacillus_ spp. use energy from sulfur oxidisation to provide themselves with energy.

Sulfur oxidisation is a reaction involving reduced sulfur and an electron acceptor - either oxygen or nitrate. This process has two important outcomes: (1) the extra electrons in the reduced sulfur are freed, releasing energy, and (2) sulphate is produced. An example source is the gas hydrogen sulfide (H<sub>2</sub>S).

## Applications

_Thiobacillus_ spp. are good for industrial use due to their sulfur oxidisation ability, low nutrient requirements, and their resilience to environmental extremes including high sulfide concentrations. One application of _Thiobacillus_ spp. is the removal of hydrogen sulfide. H<sub>2</sub>S is undesirable in biogas plants because they are: toxic, damage equipment, slow down biogas production, and contribute to the formation of sulfur dioxide (toxic gas). _Thiobacillus_ spp. can oxidise the sulfide and turn it into a safer compound known as sulphate. This also reduces the level of sulfide. Similarly, _Thiobacillus_ spp. is used in wastewater treatment. Wastewater contains sulfide, making it smelly and toxic. Conversion to sulphate by _Thiobacillus_ means the water can be safely deposited into the sea.

Another application is to produce sulphate in soil to help in agriculture. Sulfur is the fourth most essential plant nutrient. However, reduced sulfur is not usable by plants. Sulphate is one of the two forms of sulfur that plants can take up. So, the bacterial oxidisation of sulfide to sulphate allows plants to take up sulfur through their roots. To deal with low sulphate levels, it is possible to add sulphate directly to the soil. However, a longer-term solution is to promote the activity of _Thiobacillus_ spp.  Furthermore, _Thiobacillus_ spp. can oxidise elemental sulfur. This produces sulfuric acid, which lowers the pH of the soil. This is especially important in alkaline soils as it improves the ability of crops to take up other nutrients such as phosphorous, zinc, and iron.