Acidophiles in Acid Mine Drainage - Archaean Acidophiles

Archaean Acidophiles

Though proteobacteria display impressive acid tolerance, most retain a circumneutral cytoplasm to avoid denaturation of their acid-labile cell constituents. Archaea such as Ferroplasma acidiphilum, which oxidises ferrous iron, have a number of intracellular enzymes with an optimum similar to that of their external acidic environment. This may explain their ability to survive pH as low as 1.3. The differing cell membranes in archaea compared to the bacteria may hold part of the explanation; ether lipids that link isoprene, compared to proteobacteria’s di-ester linkage, are central to the difference. Though lacking a cell wall, F. acidiphilum cell membranes contain caldarchaetidylglycerol tetraether lipids, which effectively block almost all proton access, Thermoplasma acidophilum also uses these bulky isoprenoid cores in its phospholipid bilayer.

It is possible that the family Ferroplasmaceae may in fact be more important in AMD than the current paradigm, Acidithiobacillaceae. From a practical viewpoint this changes little, as despite the myriad physiological differences between archaea and bacteria, treatments would remain the same; if pH is kept high, and water and oxygen are prohibited from the pyrite, the reaction will be negligible.

The isolation from solfataric soils of two Picrophilus species of archaea P.oshimae and P.torridus are of note for their record low of survival at pH 0, indicating that further AMD microorganisms may remain to be found which operate at an even lower pH. Though the genus Picrophilus is not known to be involved in AMD, its extreme acidophily is of interest, for instance its proton-resistant liposomes, which could be present in AMD acidophiles.