18.1 Studying Viruses and Prokaryotes

Recall from Chapter 13 that nitrogen fixation is the processof converting atmospheric nitrogen into ammonia (NH 3 ) andother nitrogen compounds that plants can then use. Prokaryotessupply usable nitrogen to ecosystems ranging fromgrasslands and forests to the arctic tundra.Some types of nitrogen-fixing bacteria are free-living, whileothers live along with other organisms. Legumes, a group ofplants including peas, beans, alfalfa, and clover, have a mutualisticrelationship with nitrogen-fixing bacteria. These bacterialive in the plant’s nodules, small rounded lumps that form theroots, as shown in FIGURE 18.16. The bacteria provide usablenitrogen to the plant by capturing nitrogen gas from airtrapped in the soil. They combine the nitrogen with hydrogento produce ammonia. In return, the plant supplies food andshelter to the bacteria.Scientists have found many ways to use prokaryotes to benefit industry andthe environment. One important use of prokaryotes is in bioremediation(by-oh-rih-mee-dee-AY-shuhn), a process that uses microbes and other livingthings to break down pollutants. For example, some types of bacteria candigest oil, which is helpful for cleaning up oil spills and other industrialaccidents. Workers spray oil-polluted beaches with a fertilizer that helps thebacteria grow.Bacteria can digest almost any product that humans can make, includingpoisons. Therefore, they play an important role in recycling and composting.When you hear the term biodegradable, it often refers to the ability of bacteriato break down a material. Some of the only materials made by humans thatcannot be biodegraded are certain types of plastics.Apply When there is a toxic chemical spill, sometimes workers will spray bacteriaover the contaminated area. Why might they do this?FIGURE 18.16 Root nodules ofthis white clover contain nitrogenfixingbacteria. The symbiotic bacteriaconvert nitrogen from theatmosphere (N 2 ) into a formusable by the clover. In return, theplant produces carbohydratesthrough photosynthesis that thebacteria can consume. (inset coloredSEM; magnification 90)18.5 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEAS1. Describe two ways bacteria providenutrients to humans.2. What are two roles prokaryotes playin the cycling of elements in anecosytem?CRITICAL THINKING3. Connect Think of an examplein which the use of bioremedia-tion either has improved the environmentor has the potential todo so.4. Synthesize How do prokaryoteslend stability to an ecosystem?Connecting CONCEPTS5. Ecology Prokaryotes in cowintestines produce moremethane if the cow is fed adiet high in grains rather thangrass. Some scientists proposethat overfeeding grain to cowscontributes to global warming.How did these scientists arriveat this hypothesis, and howcould it be tested?Chapter 18: Viruses and Prokaryotes 561