Web Bit 2-2: Life at Low pH
By Allan Tobin & Jennie Dusheck
Organisms that live in water usually require a pH near 7. For thousands of years, people have killed unwanted organisms in their food by pickling—by soaking meats or vegetables in vinegar, a weak acid. Some microorganisms, however, thrive in acidic environments. As lactobacilli bacteria grow in milk, they reduce the pH of their environment. This acidity not only prevents the growth of most other organisms, it also induces the coagulation of the milk into buttermilk, sour cream, cheese, or yogurt (depending on the species of lactobacilli). The characteristic flavors of these milk products all develop from the action of the bacteria.
Other microorganisms can tolerate still lower pH. Bacteria growing at pH below 2 have been isolated from the acid hot springs of Yellowstone National Park, from the refuse piles of coal mines, and from rivers and ponds polluted with industrial wastes. These bacteria appear to have special adaptations that maintain their internal pH near 7 in spite of their acidic external environment.
The organisms that can withstand extremes of pH are mostly small. This means that when mining or industrial pollution reduces the pH of a lake or river, only microorganisms survive. The fish, the frogs, and the multicellular plants begin to die off.
Less extreme acidity also affects organisms. When soil water becomes acidic, plants and animals are less able to survive. The land areas surrounding acid-polluted waters—like the waters themselves—have reduced vegetation and animal life.
Figure 2.2: Acid Rain. Trees along the Blue Ridge Parkway in North Carolina are dying as a result of acid rain.
As illustrated in Figure 2.2, acidity in soils, lakes, and rivers has become increasingly widespread in the last 50 years. The burning of sulfur-containing coal and of nitrogen-containing gasoline releases sulfur and nitrogen oxides into the atmosphere. Winds can carry these oxides hundreds or even thousands of miles, where they can dissolve in rain water and produce strong acids—sulfuric, sulfurous, nitric, and nitrous. The resulting "acid rain" reduces the pH of lakes, ponds, rivers, and soils.
Pollution by acid rain is both a national and an international problem. The industrial wastes of England and Germany have changed the pH of the lakes and rivers of Sweden, and the factories of the American midwest have a direct effect on the wildlife and agriculture of the northeastern United States and Canada.