Acidithiobacillus ferrooxidans

The genus name of the bacteria, Acidithiobacillus, contains Latin and Greek roots. It can be broken up into three words consisting of acidus (meaning “acidic”), thios (meaning “sulfur”), and bacillus (meaning “rod”), which highlight the rod-shaped organism's affinity for acid and ability to oxidize sulfur. The species name of the bacteria, ferrooxidans, comes from the Latin word ferrum (meaning “iron”) and oxidare (meaning “to oxidize”), characterizing its distinct ability to oxidize ferrous iron. == Taxonomic hierarchy ==

Acidithiobacillus ferrooxidans is classified within the domain Bacteria, kingdom Pseudomonadati, phylum Pseudomonadota, class Acidithiobacilla, order Acidithiobacillales, family Acidithiobacillaceae, and genus Acidithiobacillus. There are currently 8 identified species within the Acidithiobacillus genus, with A. ferrooxidans serving as one of the most well-studied amongst the known species. == Phylogenetic relationships ==

Acidithiobacillus ferrooxidans is most closely related to other iron and sulfur-oxidizing species within the Acidithiobacillus genus, including Acidithiobacillus caldus, Acidithiobacillus thiooxidans, and Acidithiobacillus ferrivorans. The species listed were organized into distinct clades to represent major evolutionary branches by comparing their 16S rRNA gene sequences — a gene sequence commonly used to study evolutionary relationships due to their presence in all bacteria and slow mutation rate. The genomes for the species were obtained from a publicly available database of nucleotide sequences. == Discovery process ==

The bacterium, Acidithiobacillus ferrooxidans, and its ability to oxidize ferrous iron was first reported by Colmer and Hinkle in 1947. Reclassification In 2000, the bacterium was reassigned to the Acidithiobacillus genus based on information obtained from 16S rRNA gene sequence analysis. Phylogenetic studies revealed that the organism was genetically distinct from other well-studied species within the Thiobacillus genus, forming a separate cluster and lineage with a few closely related species. In response, the genus Acidithiobacillus was created to reflect the shared physiological traits, such as iron and sulfur oxidation, as well as the additional acid-loving characteristic of these species. == Metabolism ==

Acidithiobacillus ferrooxidans is classified as a chemolithoautotroph because it uses sulfur and iron as its primary energy sources. The bacteria contains two electron transport chains to help move electrons from sulfur or iron to generate energy in the form of ATP and NADH. == Genome ==

Within Acidithiobacillus ferrooxidans is a circular chromosome, known as ATCC 23270, that contains around 3 million base pairs and over 3,000 protein-coding genes, more than half of which are composed of cytosine and guanine. As observed in Slowik et al. (2024), strains of A. ferrooxidans that do possess plasmid DNA have been found to express multiple hydrogenases and type c cytochromes which enhance their survivability by using various energy sources. Hydrogenases allow the organism to have an alternative source of energy, in addition to iron and sulfur, through the oxidation of hydrogen. Due to the role of type c cytochromes in the bacterium's chemolithoautotrophic metabolism and aerobic respiration, an increased number of these enzymes enhances electron transfer efficiency and provides redundancy, ensuring continued function even if some electron carriers are damaged. == Ecology ==

Acidithiobacillus ferrooxidans is found within acidic and metal-rich environments, such as mining drainage systems, volcanic areas, and sulfur springs that other organisms would struggle to survive in. One study by Tonietti et al. (2024) highlights the “bioleaching versatility” of A. ferroxidans which describes its ability to extract a wide variety of metals out of ores using biological processes as opposed to high-temperature smelting. The industrial use of this organism within mining operations is further being discovered. == Significance ==

Due to its unique bioleaching ability, Acidithiobacillus ferrooxidans can leverage its specialized metabolism to assist in the recycling of electronic waste, waste treatment, and more effective material gain from mining processes. The bacterium is able to perform its chemical processes at low temperatures and does not require the addition of large‑scale quantities of concentrated acids, bases, or other harsh reagents that are typical of normal metal leaching processes. The organism can also be utilized to recycle materials from electronic waste, such as used batteries. Studies have shown up to 98% extraction of silver from silver oxide-zinc battery cells with the use of A. ferrooxidans. Beyond the extracting value, A. ferrooxidans also aids in remediation of the environment and waste treatment by cleaning or neutralizing toxic metals in soil and water, which significantly reduces the costs of standard mining practices and global electronic waste. The organism is also considered a model for astrobiology studies due to its chemolithoautotrophic metabolism, which could sustain life in environments such as ancient Mars. == References ==