An extreme acidophile has a pH optimum for growth at or below pH 3.0. This definition excludes microorganisms that are tolerant to pH below 3, but that have pH optima closer to neutrality, including many fungi, yeast, and bacteria (e.g., the ulcer- and gastric-cancer causing gut bacterium Helicobacter pylori).
Extremely acidic environments occur naturally and artificially. The hyperthermophilic extreme acido-philes Sulfolobus, Sulfurococcus, Desulfurolobus, and Acidianus produce sulfuric acid in solfataras in Yellowstone National Park from the oxidation of elemental sulfur or sulfidic ores. Other members of the Archaea found in these hot environments include species of Metallosphaera, which oxidize sulfidic ores, and Stygiolobus spp., which reduce elemental sulfur. The novel, cell-wall-less archaeon, Thermoplasma volcanium, which grows optimally at pH 2 and 55 °C, has also been isolated from sulfotaric fields around the world. The bacterium Thiobacillus caldus has been isolated in hot acidic soils. Bacillus acidocaldarius, Acidimicrobium ferrooxidans, and Sulfobacillus spp. have also been isolated from warm springs and hot spring runoff.
The most extreme acidophiles known are species of Archaea, Picrophilus oshimae and Picrophilus torridus, which were isolated from two solfataric locations in northern Japan. One of the locations, which contained both organisms, is a dry soil, heated by solfataric gases to 55 °C and with a pH of less than 0.5. These remarkable species have aerobic heterotrophic growth with a temperature optimum of 60 °C and a pH optimum of 0.7 (i.e., growth in 1.2 M sulfuric acid).
In addition to Archaea, the phototrophic red alga Galdieria sulphuraria (Cynadium caldarium), isolated in cooler streams and springs in Yellowstone National
Park, has optimum growth at pH of 2-3 and 45 °C, and is able to grow at pH values around 0. The green algae Dunaliella acidophila is also adapted to a narrow pH range from 0 to 3.
The majority of extremely acid environments are associated with the mining of metals and coal. The microbial processes that produce the environments are a result of dissimilatory oxidation of sulfide minerals, including iron, copper, lead, and zinc sulfides. This process can be written as Me2+S2~ (insoluble metal complex) ^ Me2+ + SO|~; where Me represents a cationic metal. As a result of the extremely low pH in these environments, and due to the geochemistry of the mining sites, cationic metals (e.g., Fe2+, Zn2+, Cu2+, and Al2+) and metaloid elements (e.g., arsenic) are solubilized; this process is referred to as microbial ore leaching.
Most mining sites tend to have low levels of organic compounds, and as a result chemolithoautotrophs, such as the bacteria Thiobacillus ferrooxidans, Thiobacillus thiooxidans, Leptospirillum ferrooxidans, and Leptospirillum thermoferrooxidans, are prolific. In addition, mixo-trophic Thiobacillus cuprinus and heterotrophic Acidiphilium spp. have been isolated from acidic coal refuse and mine drainage. Thermophilic acidophilic bacterial species include Thiobacillus caldus from coal refuse, Acidimicrobium ferrooxidans from copper-leaching dumps, and Sulfobacillus spp. from coal refuse and mine water. The archaeal microorganism Thermoplasma acidophilum is frequently isolated from coal refuse piles. Coal refuse contains coal, pyrite (an iron sulfide), and organic material extracted from coal. As a result of spontaneous combustion, the refuse piles are self-heating and provide the thermophilic environment necessary for sustaining Thermoplasma and other thermophilic microorganisms.
In illuminated regions (e.g., mining outflows and tailings dams), phototrophic algae, including Euglena, Chlorella, Chlamydomonas, Ulothrix, and Klebsormidium species, have been isolated. Other eukaryotes include species of yeast (Rhodotorula, Candida, and Cryptococcus), filamentous fungi (Acontium, Trichosporon, and Caphalosporium) and protozoa (Eutreptial, Bodo, Cinetochilium, and Vahlkampfia).
Acidophiles (and alkaliphiles; see Section VII) keep their internal pH close to neutral. Most extreme acidophiles maintain an intracellular pH above 6, and even Picrophilus maintains an internal pH of 4.6 when the outside pH is 0.5-4. As a result, extreme aci-dophiles have a large chemical proton gradient across the membrane. Proton movement into the cell is minimized by an intracellular net positive charge and as a result cells have a positive inside-membrane potential. This is caused by amino acid side chains of proteins and phosphorylated groups of nucleic acids and metabolic intermediates, acting as titratable groups. In effect, the low intracellular pH leads to protonation of titratable groups and produces a net intracellular positive charge. In addition to this passive effect, some acidophiles (e.g., Bacillus coagulans) produce an active proton-diffusion potential that is sensitive to agents that disrupt the membrane potential, such as ionophores.
The ability of lipids from the archaeon Picrophilus oshimae to form vesicles is lost when the pH is neutral, thus indicating that the membrane lipids are adapted for activity at low pH to minimize proton permeability. In Dunaliella acidophila, the surface charge and inside membrane potential are both positive, which is expected to reduce influx of protons into the cell. In addition, it overexpresses a potent cytoplasmic membrane H+-ATPase to facilitate proton efflux from the cell.
The pH to which a protein is exposed affects the dissociation of functional groups in the protein and may be affected by salt and solute concentrations. Few periplasmic surface-exposed or -secreted proteins from extreme acidophiles have been studied to identify the structural features important for activity and stability. In Thiobacillus ferrooxidans, the acid stability of rusticyanin (acid-stable electron carrier) has been attributed to a high degree of inherent secondary structure and the hydrophobic environment in which it is located in the cell. A relatively low number of positive charges have been linked to the acid stability of secreted proteins (thermopsin, a protease from Sulfolobus acidocaldarius, and an a-amylase from Alicyclobacillus acidocaldarius), by minimizing electrostatic repulsion and protein unfolding.
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