Organisms are sensitive to the acidity of their environment
Acidity and its converse, alkalinity, are measures of the ability of a solution to behave as an acid or a base, respectively. Acids are compounds that give up protons (H+) to the water they are dissolved in.
Bases take up protons or give up hydroxide ions (OH-). Examples of common acids include the citric, tannic, and ascorbic acids found in fruits. Examples of common bases include sodium bicarbonate (baking soda) and other carbonate minerals in rock. Acidity and alkalinity are measured as pH, which is equal to the negative of the logarithm (- log10) of the concentration of H+. Thus, one pH unit represents a tenfold change in the concentration of H+. Pure water has a neutral pH of 7.0. Solutions with pH values higher than 7.0 are alkaline (basic), and solutions with pH values lower than 7.0 are acidic.The pH values of water have important effects on organismal function. Changes in pH values can directly affect metabolic activity. The pH values of water also determine the chemistry and availability of nutrients, as we will see in Concept 22.4. Organisms have a limited range of pH values that they can tolerate. Natural levels of alkalinity (when the pH of the environment exceeds 7) tend not to be as important as levels of acidity as a constraint on organismal function and distributions.
Under natural conditions the pH of ocean water does not vary appreciably, because the chemistry of seawater buffers changes in pH—that is, the salts in seawater bind free protons and thereby minimize changes in pH. Thus, pH tends to be more variable in terrestrial and freshwater ecosystems than in the ocean. However, increases in atmospheric CO2 concentrations due to human activities are increasing the acidity of the oceans, with negative effects on marine ecosystems. Marine animals that build shells using calcium carbonate are less able to construct and maintain their shells under more acidic conditions (Orr et al.
2005). We will discuss this phenomenon more thoroughly in Concept 25.1.On land, the pH of surface waters and soils varies naturally. What causes this variation? Water can become more acidic over time through the input of acidic compounds derived from several sources, most associated with soil development (which is covered in more detail in Concept 22.1). Two of the main components of soil are mineral particles from the breakdown of rocks, and organic matter from the decomposition of dead plants and other organisms. Some rock types, such as granites, generate acidic salts, while other rock types, such as limestones, generate basic salts. Soils become more acidic as they age because the basic salts leach away more easily and because decomposition and leaching of plant matter adds organic acids to the soil. The emission of acidic pollutants into the atmosphere by the burning of fossil fuels, as well as overuse of agricultural fertilizers, can increase the acidity of soil and water. We will cover these sources of acidity in more detail in Concept 25.3.