Acidophiles
The acid test
The acidity of water is expressed by a measure called pH that has a scale that typically ranges from 0 to 14. Pure water is neutral pH, that is, pH 7. More acidic water has a pH lower than 7. The pH scale is set up so that a value one unit lower means it is ten times more acidic. Some common foods are somewhat acidic. For example, white vinegar is pH 2.4 and the popular drink kombucha is pH 2.8-3.2. Foods like these are produced by beneficial bacteria that ferment sugars in plant materials or milk and produce acids. The low pH of these foods helps to prevent the growth of harmful bacteria because most bacteria cannot grow in such acidic conditions. Some environments are much more acidic than these fermented foods. The lowest recorded environmental pH is -3.6 in underground waters in the Richard Mine at Iron Mountain, California. The record for growth at low pH is held by the archaeon Picrophilus torridus, isolated from soil near a hot spring in Hokkaido, Japan. It grows at about pH 0 and at 60°C (140°F). Moderately acidic environments are found where stagnant water is heavy with decaying plant material. Acid is produced by microbial decomposition of this material, Kombucha (L) and kefir (R) cultures contain communities of microbes that ferment sugars in chiefly by bacteria. Marshes tea (kombucha) and milk (kefir) and produce contain reeds and grasses while acids. The microbes clump together forming a large mass or small granules. swamps support trees. Their waters may flow and living plants and moving water keeps the water oxygenated. Consequently, the pH of water in these environments stays neutral or even a bit basic. Bogs, by contrast, have heavy amounts of decaying plants, called peat, stagnant water, and poor underlying soils. The waters become anoxic (containing virtually no oxygen), allowing anaerobic (oxygen-hating) bacteria to grow and produce acids. Bog water typically has a pH of 3.3-5.5. The most acidic environments are found in some hot springs and
streams flowing out of metal or coal mine sites. They become acidic from exposure of sulfurous gases emanating from the Earth or sulfurcontaining minerals to oxygen. Oxygen converts hydrogen sulfide and sulfur dioxide gases to sulfuric acid, a strong acid. Sulfuric acid is also produced when oxygen comes in contact with the iron and sulfur-containing mineral Bog in Upper Peninsula Michigan, USA. pyrite found in iron mines and the rock debris piled nearby. Similar sulfuric minerals of copper and other metals contribute to acid production in waters draining from mines and areas near mines where waste rock is deposited. Some acid-loving bacteria, called acidophiles, combine oxygen with these sulfurous minerals to produce sulfuric acid. Some grow to pH as low as 2. Acidic waters draining away from mines is sometimes very red because Turbulent Pool in the Mud Volcano Group, Yellowstone National Park, Wyoming, USA. It has a temperature of 138°F (59°C). The water is yellow because insoluble yellow particles of sulfur have formed through the chemical and microbial transformation of hydrogen sulfide gas coming up out of the Earth into sulfur. Some microbes further transform the sulfur into sulfuric acid, so the pool is pH 2.
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Outflow from an acidic hot spring at Central Basin at West Thumb Geyser Basin, Yellowstone National Park, Wyoming, USA. The acid water prevents the growth of grass.
he iron that is dissolved in the water is in the form of iron like that found in rust, called ferric iron. Ferric iron is produced by combining the kind found in metal-containing rocks, called
Acid mine drainage at the Ohio Valley Mushroom Farm in North Lima, Ohio, USA. This drainage results from surface mining conducted in the 1980's and an abandoned underground mine from the late 1800's. The red color is caused by dissolved iron being converted to insoluble iron (like rust). The water is about pH 4.
ferrous iron, with oxygen from the air either by a chemical process or by the action of bacteria, some of which are acidophiles. The Rio Tinto river (Spanish for “red river”) in southwest Spain runs through an area mined for iron, copper, silver and gold for thousands of years. The river is pH 2-2.5 in most areas as a result of chemical and microbiological production of sulfuric acid.
Acid mine drainage is a sever problem in coal mining areas. The acidic waters flowing away from the mines do not support plant or animal life in the water and kill nearby vegetation. When these streams enter rivers, they can seriously affect life in and along the river. Acidophiles have been used to extract metals from waste minerals left over from metal mines. If the price of the metal is high, it becomes economical to try to extract the small amount of metal left in the waste. Water is passed over over the waste rock and the acidophiles acidify the water by converting the sulfurous minerals to sulfuric acid. They also remove the metals from the rock and transform them to a form soluble in the acidified water. The metals can then be harvested from the run-off water.
Treatment of minerals with microbes to extract metals that would otherwise be difficult to remove from the rock. Photo from Biomin Technologies Limited.
Photo credits: Cover, Rio Tinto river in Spain. The red color is caused by suspended iron in the water. The river has a pH of about 2. https://commons.wikimedia.org/wiki/File:Rio_Tinto_25.jpg Kombucha fermentation https://commons.wikimedia.org/wiki/File:Kombucha.jpg Kefir fermentation http://phickle.com/getting-started-and-faq/milk-kefir/ Bog https://www.nps.gov/piro/learn/nature/wetlands.htm Acidic hot spring outflow https://www.flickr.com/photos/daveynin/4782592959 Turbulent pool https://www.flickr.com/photos/jsjgeology/14517428096 Acid mine drainage at Ohio Valley Mushroom Farm https://commons.wikimedia.org/wiki/ File:Ohio_Valley_Mushroom_Farm,_AcidMine_Drainage_(AMD)_(13670918103).jpg Metal recovery https://www.chemistryworld.com/features/microbial-miners/7879.article
Online resources: Microbial Life Educational Resources https://serc.carleton.edu/microbelife/extreme/acidic/index.html Center for Water, Earth Science and Technology – Acid mine drainage https://www.colorado.edu/cwest/research/research-themes/water-andenvironmental-quality/acid-minerock-drainage American Society for MIcrobiology – Microbiology of fermented foods https://www.asm.org/index.php/general-science-blog/item/466-food-forthought-microbes-and-fermented-food