The Sulfur Cycle

Emissions of the gases sulfur dioxide (SO2) and hydrogen sulfide (H2S) from volcanoes and fumaroles (vents for hot gases) are the only significant natural nonbiological fluxes of sulfur. These emissions release, on average, between 10 and 20 percent of the total natural flux of sulfur to the atmosphere, but they vary greatly in time and space. Large volcanic eruptions spread great quantities of sulfur over broad areas, but they are rare events. Terrestrial and marine organisms also emit compounds of sulfur. Certain marine algae produce large amounts of dimethyl sulfide (CH3SCH3), which accounts for about half the biotic component of the sulfur cycle. Sulfur is apparently always abundant enough to meet the needs of living organisms.

Sulfur plays an important role in global climate. Even if air is moist, clouds do not form readily unless there are small particles around which water can condense. Dimethyl sulfide is the major component of such particles. Therefore, increases or decreases in atmospheric sulfur levels can change cloud cover and hence climate.

Humans have altered the sulfur cycle, as well as the nitrogen cycle, by the burning of fossil fuels. An important regional effect of these alterations is acid precipitation—rain or snow whose pH is lowered by the presence of sulfuric acid (H2SO4)

pH class

58.11 Acidification of Lakes Exterminates Fish Species The number of fish species found in lakes in the Adirondack region of New York State is inversely correlated with pH. (Recall from Chapter 2 that lower numbers indicate greater acidity, and a pH of 7 is neutral.The numbers above the bars indicate the number of lakes in each pH category.)

and nitric acid (HNO3), derived in large part from the burning of fossil fuels. These acids enter the atmosphere and may travel hundreds of kilometers before they settle to Earth in precipitation or as dry particles.

Acid precipitation now characterizes all major industrial countries and is particularly widespread in eastern North America and Europe. The normal pH of precipitation in New England is about 5.6, but precipitation there now averages about pH 4.4, and there are occasional storms with a precipitation pH as low as 3.0. Precipitation with a pH of about 3.5 or lower causes direct damage to the leaves of plants and reduces photosynthetic rates. Acidification of lakes in the Adirondack region of New York has reduced fish species richness by causing the extinction of acid-sensitive species (Figure 58.11). Fortunately, as a result of the establishment of a flexible regulatory system under the 1990 Clean Air Act Amendments, precipitation in much of the eastern United States is less acid today than it was 18 years ago, primarily because of reductions in sulfur emissions (Figure 58.12).

Ecologists in Canada studied the effects of acid precipitation on small lakes by adding enough H2SO4 to two lakes to reduce their pH from about 6.6 to 5.2. In both lakes, nitrifying bacteria failed to adapt to these moderately acidic conditions. As a result, the nitrogen cycle was blocked, and ammonium accumulated in the water. When the ecologists stopped adding acid to one of the lakes, its pH increased to 5.4, and nitrification resumed. After about 1 year, the pH of the lake returned to its original value. These experiments show that lakes are very sensitive to acidification, but pH can return rapidly to normal values because water in lakes is exchanged at a rapid rate.

Average pH of precipitation

Because of prevailing winds, acid rains affect areas far from the pollution sources.

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Essentials of Human Physiology

Essentials of Human Physiology

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