Many of New Zealand’s hot springs and volcanic craters are very acidic. There are many acid-loving extremophiles that thrive there, such as the alga Cyanidium caldarium. Unlike other algae, it is able to survive in water with pH values down to zero – close to the level of battery acid. Cyanidium is also a moderate thermophile and can grow in temperatures up to 56°C. Growths of Cyanidium are present in New Zealand geothermal areas where acid waters are found, including Rotokawa and White Island.
Scientists use the pH scale, from 0 to 14, to measure how acid or alkaline a solution is. Pure water is neutral, with a pH of 7. Values below 7 are acid, and those above are alkaline. For comparison, lemon juice has a pH of 2; sea-water is 8.2; and a concentrated ammonia solution is about 12.
Despite being able to survive extremely acid conditions, these organisms cannot tolerate such acidity inside the cell because essential molecules such as DNA become unstable. Acidophiles have evolved mechanisms to pump acids out of the cell in order to maintain weak to neutral acid conditions (pH 5–7). Many acidophiles also excrete protective, acid-resistant polysaccharides on their cell membranes.
Acidophiles and sulfur
Most acidophilic types of bacteria and archaea grow where sulfur compounds are present. This is not surprising given that the origin of very acid conditions is usually related to the chemical transformation of sulfur.
Examples of common acidophiles are Alicyclobacillus acidocaldarius (a moderately thermophilic, acidophilic bacterium) and the extremely thermophilic Sulfolobus acido caldarius, a member of the archaea domain.