Acidophiles in acid mine drainage

[4] Genera such as Acidithiobacillus and Leptospirillum bacteria, and Thermoplasmatales archaea, are present in syntrophic relationships in the more mundane environments of concrete sewer pipes[5][6] and implicated in the heavy-metal-containing, sulfurous waters of rivers such as the Rheidol.

A.ferrooxidans is a chemolithoautotrophic bacteria, due to the oligotrophic nature (low dissolved organic carbon concentration) of acidic environments, and their lack of illumination for phototrophy.

However, favourable geochemical conditions quickly develop with an acidic interface between the bacteria and the mineral surface, and pH is lowered to a level closer to acidophilic optimum.

[7] Though Pseudomonadota (formerly proteobacteria) display impressive acid tolerance, most retain a circumneutral cytoplasm to avoid denaturation of their acid-labile cell constituents.

[1] 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.

[14] 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.

To elaborate upon the figure, the bacteria also use membrane proton blocking to maintain a high cytoplasmic pH, which is a passive system as even non-respiring A.ferrooxidans exhibit it.

[1] Acidophiles harness the strong proton motive force (PMF), caused by the pH gradient across their cell membrane, for ATP production.

There are a number of methods for dealing with AMD, some crude (such as raising pH through liming, removing water, binding iron with organic wastes) and some less so (application of bactericides, biocontrol with other bacteria/archaea, offsite wetland creation, use of metal-immobilising bacteria, galvanic suppression).

A number of other neutralising agents are available (pulverised fuel ash-based grouts, cattle manure, whey, brewer's yeast) many which solve a waste disposal problem from another industry.

Though slower than conventional methods, the microorganisms (which can also include fungi) enable the exploitation of extremely low grade ores with minimum expense.

AMD continues to be important in the River Rheidol, and in the near future further treatment will be needed in the area around Aberystwyth, which contains 38 of the 50 worst polluting metal mines in Wales.

The fast and efficient protein and DNA repair systems show promise for human medical uses, particularly with regard to cancer and ageing.

' Methods of pH homeostasis and energy generation in acidophiles' (with reference to Baker-Austin & Dopson, 2007 [ 1 ] and Apel, Dugan, & Tuttle, 1980): [ 2 ] (1) Direction of transmembrane electrochemical gradient (pH) and blocking of H+ by the cell membrane; (2) Reversed membrane potential through potassium transport, a modification towards maintaining a stable Donnan potential; (3) Secondary transporter protein; the H+ and Na+ gradient is harnessed to drive transport of nutrients and solutes; (4) Proton pump actively removes H+, balancing the energy gained from the H+ entry to the cytoplasm. (5) Vesicles containing protons avoid acidification of the cytoplasm, but still generate ATP from the electrochemical gradient (in A.ferrooxidans ); (6) Uncouplers (uncharged compounds), such as organic acids, permeate the membrane and release their H+, leading to acidification of the cytoplasm; (7) To avoid this, heterotrophic acidophiles may degrade the uncouplers; (8) Alternatively, cytoplasmic enzymes or chemicals may bind or sequester the protons.