In the past called archaebacteria, Archaea is a group of microorganisms similar to bacteria but often much smaller and with a distinct evolutionary past and chemical composition. Some species that can survive acidic environments include members of the genera Ferroplasma and Picrophilaceae, which can also grow in the presence of hydrochloric acid and negative pH values. However, the species Thermoplasma acidophilum can only tolerate 1.5 to 2 pH values.
Some species of microscopic algae are also acidophiles. The green algae Dunaliella acidophila and Chlamydomonas acidophila, as well as the red alga Cyanidium caldarium, can survive when in contact with hydrochloric acid and other substances with pH below 1. The optimum pH value for the growth of red algae is about 4 whereas green algae thrives in environments with pH values not higher than 1.
Microscopic fungi species also show resistance to low pH environments. Acontium cylatium, Cephalosporium and Trichosporon cerebriae are some species that can grow near pH 0. However, most yeasts cannot tolerate the presence of hydrochloric acid or other substances with pH values lower than 3 or 4. The genus Cephalosporium includes plant pathogens that parasitizes wheat and other grasses.
Most bacteria species do not survive in extremely low pH levels. However, some species of the genuses Alicyclobacillus, Acidithiobacillus and Acidithiobacillales can survive in the presence of hydrochloric acid at very low concentrations, with pH values not lower than 1.5. Alicyclobacillus can also survive in extremely hot temperatures, often applied during pasteurization procedures.