THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
16. Prokaryotes: The First Single-Celled Creatures
16.5. Importance of Prokaryotes
Prokaryotes and the Environment
Prokaryotes were largely responsible for creating the properties of the atmosphere and the soil for over 2 billion years. They are metabolically much more diverse than eukaryotes, which is why they are able to exist in such a wide range of habitats. The many autotrophic bacteria—either photoautotrophic or chemoautotrophic—make major contributions to the world carbon balance in terrestrial, freshwater, and marine habitats. Other heterotrophic bacteria play a key role in world ecology by breaking down organic compounds. The carbon, nitrogen, phosphorus, sulfur, and other atoms that make up living organisms come from the environment, and when organisms die and decay, they all return to the environment. The prokaryotes, and other organisms such as fungi, that carry out the breakdown portion of this cycle are called decomposers. Another key role of prokaryotes in the world ecosystem relates to the fact that only a few genera of bacteria—and no other organisms—have the ability to fix atmospheric nitrogen and thus make it available for use by other organisms.
Bacteria and Gene Engineering
Applying genetic engineering methods to produce improved strains of bacteria for commercial use holds enormous promise for the future. Bacteria are under intense investigation, for example, as nonpolluting insect control agents. Bacillus thuringiensis produces a protein that is toxic when ingested by certain insects, and improved, highly specific strains of B. thuringiensis have greatly increased its usefulness as a biological control agent. Genetically modified bacteria have also been extraordinarily useful in producing insulin and other therapeutic proteins. Gene modified bacteria are also playing a part in removing environmental pollutants. Oil-degrading bacteria were used to clean up the Exxon Valdez oil spill off the coast of Alaska. The rocks on the left in figure 16.6 below show the oil contamination, while the rocks on the right show an area that was cleaned up using bacteria.
Figure 16.6. Using bacteria to clean up oil spills.
Bacteria can be used to remove environmental pollutants, such as the hydrocarbons released into the Gulf of Mexico in the 2010 oil spill. In areas contaminated by the Exxon Valdez oil spill (rocks on the left), oil-degrading bacteria produced dramatic results (rocks on the right).
Bacteria, Disease, and Bioterrorism
Some bacteria cause major diseases in plants and animals, including humans. Among important human bacterial diseases that can be lethal are anthrax, cholera, plague, pneumonia, tuberculosis (TB), and typhus. Many pathogenic (disease-causing) bacteria like cholera are dispersed in food and water. Some like typhus and plague, spread among rodents and humans by fleas. Others, like TB, are spread through the air in water droplets (from a cough or sneeze), infecting those who inhale the droplets. Among these inhalation pathogens is anthrax, a disease associated with livestock that rarely kills humans. Most human infections are cutaneous, infecting a cut in the skin, but if a significant number of anthrax endospores are inhaled, the pulmonary (lung) infection is often fatal. Biological warfare programs in the United States and the former Soviet Union focused on anthrax as a near-ideal biological weapon, although it has never been used in war. Bioterrorists struck at the United States with anthrax endospores in 2001.
Key Learning Outcome 16.5. Prokaryotes make many important contributions to the world ecosystem, including occupying key roles in cycling carbon and nitrogen.