Tripping on Acid: Life as an Acidophile

What are Extremophiles?

An extremophile is an organism that can both live and thrive in an environment that has extreme conditions unsuited to most life on earth. For these specially adapted organisms, such conditions are not “extreme”, but perfectly normal. We can find extremophiles in the most unlikely places, experiencing scorching heat or freezing cold (thermophiles), crushing pressures (barophiles) and even highly corrosive acidic environments (acidophiles). This blog will explore more about this group of acid lovers.

Rio Tinto, Southwest Spain, in the mountains of Andalusia. The river has become this brilliant colour from the huge numbers of acidophiles oxidising metals, creating very acidic iron-polluted water.
Photograph of Rio Tinto, Southwest Spain, in the mountains of Andalusia. The river has become this brilliant colour from the huge numbers of acidophiles oxidising metals, creating very acidic iron-polluted water. – Antonio

So what are Acidophiles?

File:2713 pH Scale-01.jpg
pH scale with corresponding household items- Openstax College

They are micro-organisms that have optimal growth in highly acidic environments, at or below pH 3. Some of the best adapted species are able to not only tolerate but survive at pH 0, which is the equivalent strength of battery acid. Picrophilus torridus is considered one of the most acidophilic organisms on earth, growing at even pH 0 with the added ability to survive in temperatures of up to 60˚C. Acidophiles have been found from each domain of life on earth, including bacteria, archaea and even some multicellular eukaryotes.

Normally cells would be destroyed in acidic conditions, but acidophiles are adapted to live in these environments without harm. To survive they have a variety of useful adaptations to maintain internal pH homeostasis, despite their acidic surroundings. Three ways of this are:

  • Creating a barrier membrane which separates the inside cell contents from hostile exterior environment
  • Actively pumping excess protons out of the intracellular space
  • Secreting ‘buffer molecules’ that make the contents less acidic, neutralising the effects of protons that enter

As a result of reducing protons within the intracellular space, acidophiles are able to keep their cytoplasm around neutral pH. This maintains the required pH gradient for living in its surroundings.

Where can we find them?

Grand Prismatic Host Spring, Yellowstone National Park (75 by 91 m). The orange colouration is from thermo-acidphillic algae, bacteria and archaea –Jim Peaco

So where exactly can we find these acidophiles in the real world? Well, the early earth environment is widely thought to have been a much hotter and more acidic climate than what we live in today. As oxygen concentrations in the atmosphere rose, organisms were better able to perform oxidative processes which allowed acidophiles to diversify. This type of environment probably gave rise to these organisms which we consider as extremophiles today.

Acidophiles inhabit both man-made and naturally occurring environments, as wide ranging as our stomachs and concrete sewage pipes. Still more exotic locations include Yellowstone National Park, geysers and underwater hydrothermal vents. They are also commonly found in the metal-rich waters of abandoned mines.

So why do we care?

Despite how previously stated, a few species of acidophiles, including Acetobacter aceti, don’t keep a neutral pH and have an acidified cytoplasm. To achieve this their  proteins and enzymes have to become acid-stable. Research is beginning  to show how they retain integrity under such conditions and early findings related to acid-stable enzymes have already begun being commercially used in food processing and pharmaceutical industries.

Environmental Concerns

Acidophiles do have a major impact on the environment. Their unique ability to live in mineage runoff can become harmful. During mining, pools or even lakes of metal-rich waters often develop in which acidophiles thrive. In deriving their energy from oxidising iron and sulphur rich minerals, metals and sulphur are ultimately released into the water, lowering the pH. This process is called Acid Mine Drainage (AMD) and is a hazard to the surrounding environment. Acid Mine Drainage persists as one of the largest pollution problems of surface water contamination in the world.  In 1969 heavy metal AMD runoff were thought to have contaminated more than 10,000 miles of surface streams in the Northern US.

A serious pollution situation also developed far closer to home at the former copper mine of Mynydd Parys in Amlwch, Angelsey. This was due to the acidophile Acidithiobacillus ferrooxiadans, which is perfectly suited to the flooded mine shaft conditions, acquiring its energy from breaking down the surrounding metal-rich minerals. The result was 50,000 cubic meters of acidic water, laden with sulphuric acid solution and heavy metals, as a lingering source of pollution. Already, the Afon Goch ‘red river’, a contaminated runoff river of the mine, was measured as the largest source of copper inflowing the Irish Sea. Finally, after discovering a crack in the concrete dam within which the polluted lake was being contained, the Environment Agency were provided with funds to drain the 274,000 m³ of pH2 water enclosed there.

There are over 200 other mines across England and Wales and the potential pollution from any of these abandoned mines remain a threat to plant, river and the marine systems they drain into. The cost of tackling mining clean up in the UK is estimated around £372 million. Worldwide, this threat becomes even larger and more expensive, especially in developing countries where mining laws are more rudimentary or operations are much larger in scale.

Biomining

Don’t go blaming acidophiles just yet. We made the mining pits in which they have proliferated and they even have a solution. The same biological process by which acidophiles cause acid mine drainage, can also be used to extract valuable metals, including copper, nickel and gold. The advantage of using acidophiles for ‘biomining’ is the ability to extract metals from low-grade ores, which previously would have been discarded in other mining methods. These benefits are multiplied considering metal resources are depleting, but demand continues to increase. Biomining can also be used to clean up pollution caused by previous mining operations. A.ferrooxidans can even help recover metals including Cadmium from used batteries. These ‘greener’ extraction methods, reduce the carbon footprint and high energy costs compared to traditional mining. Further benefits are reduced air pollution and reduced disturbance to the surrounding area. Energy conscious greener mining will hopefully help towards mitigating the effects of climate change.

Biomining is at present much slower and less well understood than traditional mining methods. However, research has been put into refining this technology and early research has begun into recovering metals from acidic surface or groundwater lakes, such as those produced in Acid Mine Drainage. A further application recently demonstrated the use of biofuel cells innoculated with acidophillic bacteria. These act as a catalyst in the process of  oxidising sulphur-rich mine waste water and can generate electricity in the process.

Overall, it is important to consider that actually acidophiles are, and will remain, a big part of our lives. They are part of our bodies, and are both a threat and solution to our current climate and pollution problems.

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