Microbial growth and survival are also influenced by the pH of the habitat. The pH was defined in chapter 2 as the degree of acidity or alkalinity of a solution. It is expressed by the pH scale, a series of numbers ranging from 0 to 14, pH 7 being neither acidic nor alkaline. As the pH value decreases toward 0, the acidity increases, and as the pH increases toward 14, the alkalinity increases. The majority of organisms live or grow in habitats between pH 6 and pH 8 because stronger acids and bases can be highly damaging to enzymes and other cellular substances.
Although most microorganisms living in soil, freshwater, or the bodies of plants and animals are neutrophiles, living within the range of pH 5.5 to 8, some can manage exposure to pH extremes. Highly acidic or alkaline habitats, such as acidic bogs and streams or alkaline soils and ponds, can provide numerous habitats for specialized microbial communities. Obligate acidophiles include Euglena mutabilis, an alga that grows in acid pools between 0 and 1.0 pH, and Thermoplasma, an archaea that lives in hot coal piles at a pH of 1 to 2 and will lyse if exposed to pH 7. A few species of algae, archaea, and bacteria can survive at a pH near that of concentrated hydrochloric acid near a pH of 0. Not only do they require such a low pH for growth, but a few species of bacteria actually help maintain the low pH by releasing strong acid. Because many molds and yeasts tolerate moderate acidity, they are the most common spoilage agents of acid-pickled foods.
Alkalinophiles live in hot pools and soils that contain high levels of basic minerals. Probably the upward limit of tolerance is shown by proteobacteria in California’s Mono Lake that have adapted to pH 12. Bacteria that decompose urine create alkaline conditions, because ammonium (NH4 +) can be produced when urea (a component of urine) is digested. Metabolism of urea is one way that Proteus spp. can neutralize the acidity of the urine to colonize and infect the urinary system.
