Acidophiles in acid mine drainage

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'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.

The outflow of acidic liquids and other pollutants from mines is often

microorganisms
; these are the acidophiles in acid mine drainage.

Rheidol.[7]

Such microorganisms are responsible for the phenomenon of acid mine drainage (AMD) and thus are important both economically and from a conservation perspective.[8] Control of these acidophiles and their harnessing for industrial biotechnology shows their effect need not be entirely negative.[1]

The use of acidophilic organisms in mining is a new technique for extracting trace metals through bioleaching, and offers solutions for acid mine drainage in mining spoils.

Introduction

Upon exposure to

buffering capacity of the surrounding rocks (‘calcium carbonate equivalent’ or ‘acid neutralising capacity’), the surrounding area may become acidic, as well as contaminated with high levels of heavy metals.[9][10] Though acidophiles have an important place in the iron and sulfur biogeochemical cycles, strongly acidic environments are overwhelmingly anthropogenic in cause, primarily created at the cessation of mining operations where sulfide minerals, such as pyrite (iron disulfide or FeS2), are present.[8]

Acid mine drainage may occur in the mine itself, the spoil heap (particularly

colliery spoils from coal mining), or through some other activity that exposes metal sulfides at a high concentration, such as at major construction sites.[11] Banks et al.[7]
provide a basic summary of the processes that occur:

2FeS2 Pyrite + 2H2O water + 7O2 oxygen ⇌ 2Fe2+ferrous iron + 4SO42− sulfate + 4H+ acid

Bacterial influences on acid mine drainage

The oxidation of metal sulfide (by oxygen) is slow without colonization by acidophiles, particularly

catalyst
 :

Fe2+ + 1/4O2 + H+ → Fe3+ + 1/2H2O

Under the above acidic conditions, ferric iron (Fe3+) is a more potent oxidant than oxygen, resulting in faster pyrite oxidation rates.

A.ferrooxidans is a

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.[13]

The process proceeds through A.ferrooxidans exhibiting a quorum level for the trigger of acid mine drainage (AMD). At first colonisation of metal sulfides there is no AMD, and as the bacteria grow into microcolonies, AMD remains absent, then at a certain colony size, the population begins to produce a measurable change in water chemistry, and AMD escalates.[13] This means pH is not a clear measure of a mine's liability to AMD; culturing A.ferrooxidans (or others) gives a definite indication of a future AMD issue.[13]

Other bacteria also implicated in AMD include

Leptospirillum ferrooxidans, Acidithiobacillus thiooxidans and Sulfobacillus thermosulfidooxidans.[7]

Archaean acidophiles

Though

phospholipid bilayer.[17]

It is possible that the family Ferroplasmaceae may in fact be more important in AMD than the current paradigm, Acidithiobacillaceae.[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.[7]

The isolation from

liposomes, which could be present in AMD acidophiles.[20]

Interactions in the mine community

Tentatively, there may be examples of syntrophy between acidophilic species, and even cross-domain cooperation between archaea and bacteria. One mutualistic example is the rotation of iron between species; ferrous-oxidising chemolithotrophs use iron as an

heterotrophs use iron as an electron-acceptor.[8]

Another more

synergistic behaviour is the faster oxidation of ferrous iron when A.ferrooxidans and Sulfobacillus thermosulfidooxidans are combined in low-CO2 culture.[21] S.thermosulfidooxidans is a more efficient iron-oxidiser, but this is usually inhibited by low-CO2 uptake. A.ferrooxidans has a higher affinity
for the gas, but a lower iron oxidation speed, and so can supply S.thermosulfidooxidans for mutual benefit.

The community possesses diversity beyond the bacteria and archaea however; the approximately constant pH present during acid mine drainage make for a reasonably stable environment, with a community that spans a number of

yeasts, algae and protozoa.[8]

Physiology and biochemistry

Acidophiles display a great range of adaptations to not just tolerating, but thriving in an extreme pH environment (the definition of an acidophile being an organism that has a pH optimum below pH 3). Principal in these is the necessity of maintaining a large pH gradient, to ensure a circumneutral cytoplasm (normally, however not in Picrophilus species). The archaeans have already been discussed above, and further information on their and bacterial adaptations are in basic form in the Figure. 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.[2] Acidophiles are also able to extrude protons against the pH gradient with unique transport proteins, a process more difficult for moderate- and hyper-thermophiles; a higher temperature causes cell membranes to become more permeable to protons, necessarily leading to increased H+ influx, in the absence of other membrane alterations.[20]

Proton motive force

To grow at low pH, acidophiles must maintain a pH gradient of several pH units across the cellular membrane.

proton motive force (PMF), caused by the pH gradient across their cell membrane, for ATP production. A large amount of energy is available to the acidophile through proton movement across the membrane, but with it comes cytoplasmic acidity.[1] Instead ions such as sodium can be used as a substitute energy transducer to avoid this pH increase (ATPases are often Na+ linked, rather than H+ linked).[20]

Expelling H+ containing vesicles

Alternatively bacteria can use H+ containing vesicles to avoid cytoplasmic acidity (see Figure), but most require that any H+ taken in must be extruded after use in the electron transport chain (ETC).[1] On the subject of the ETC, an adaptation to living in the mine environment is in the use of different ETC

electron acceptors to neutralophiles; sulfur, arsenic, selenium, uranium, iron, and manganese in solid form[22]
rather than O2 (most commonly Fe in dissimilatory iron reduction, frequent in AMD).

Genomic adaptations

Genomic adaptations are also present, but not without complications in organisms like Thermoplasmatales archaea, which is both

pyramidine codons in long open reading frames for protection from acid-stress.[1] More generally, and presumably to reduce the chances of an acid-hydrolysis mutation, all obligate hyperacidophiles have truncated genomes when compared to neutralophile microorganisms. Picrophilus torridus, for instance, has the highest coding density of any non-parasitic aerobic microorganism living on organic substrates.[23]

Improved repair

Acidophiles also benefit from improved DNA and protein repair systems such as chaperones involved in protein refolding.[1] The P.torridus genome just mentioned contains a large numbers of genes concerned with repair proteins.

Biotechnology applications

brewer's yeast) many which solve a waste disposal problem from another industry.[7]

As supplies of some metals dwindle, other methods of extraction are being explored, including the use of acidophiles, in a process known as bioleaching. Though slower than conventional methods, the microorganisms (which can also include fungi) enable the exploitation of extremely low grade ores with minimum expense.[24] Projects include nickel extraction with A.ferrooxidans and Aspergillus sp. fungi[24] and sulfur removal from coal with Acidithiobacillus sp..[25] The extraction can occur at the mine site, from waste water streams (or the main watercourse if the contamination has reached that far), in bioreactors, or at a power station (for instance to remove sulfur from coal before combustion to avoid sulfuric acid rain).

Future of the technique

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.[26][27]

In 2007, the UK government endorsed a return to coal as an energy source

Ffos-y-fran, Merthyr Tydfil
). Much preventative work will be required to avoid the AMD associated with the last generation of coal mines.

The fast and efficient protein and DNA repair systems show promise for human medical uses, particularly with regard to cancer and ageing. However further research is required to determine whether these systems really are qualitatively different, and how that can be applied from microorganisms to humans.

As discussed above, acidophiles can have the option to use electron acceptors other than oxygen. Johnson (1998)[8] points out that facultative anaerobism of acidophiles, previously dismissed, could have major implications for AMD control. Further research is needed to determine how far current methods to block oxygen will working, in light of the fact that the reaction may be able to continue anaerobically.

See also

References

  1. ^
    PMID 17331729
    .
  2. ^
    PMID 7372573
    .
  3. S2CID 11241922
    .
  4. S2CID 85892765
    .
  5. S2CID 23327298
    .
  6. PMID 16957253
    .
  7. ^
    S2CID 59465457
    .
  8. ^
    S2CID 86091190
    .
  9. ^ Mills, Chris. "The Role of Microorganisms in Acid Mine Drainage". TechnoMine. Retrieved 11 March 2013.
  10. JSTOR 2402377
    .
  11. doi:10.1016/j.jclepro.2004.09.006
    .
  12. PMID 10758854
    .
  13. ^
    S2CID 129323041
    .
  14. ^
    PMID 16104851
    .
  15. PMID 10843038
    .
  16. PMID 10966867
    .
  17. S2CID 23846350
    .
  18. PMID 8522509
    .
  19. S2CID 42049238
    .
  20. ^
    doi:10.1016/0168-6445(96)00007-1
    .
  21. PMID 33725781
    .
  22. doi:10.1016/j.gca.2005.08.020
    .
  23. PMID 15184674
    .
  24. ^
    doi:10.1016/j.hydromet.2006.07.001
    .
  25. S2CID 95926939
    .
  26. ^ Environment Agency, 2002, in Pearce, N.J.G., Hartley, S., Perkins, W.T., Dinelli, E., Edyvean, R.G.J., Priestman, G., Bachmann, R. & Sandlands, L. (2007) Dealginated seaweed for the bioremediation of mine waters in mid-wales: Results of field trials from the "BIOMAN" EU life environment project. IMWA Symposium 2007: Water in Mining Environments, Cagliari, Italy, 27-31 May.
  27. S2CID 3609487
    .
  28. ^ Department of Trade and Industry (2007) Meeting the Energy Challenge: a white paper on energy. pp.111-112. Accessed 27/02/08 Archived March 11, 2009, at the Wayback Machine
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