Prokaryotic of Lifestyles - Prokaryotes: The First Single-Celled Creatures - The Evolution and Diversity of Life

THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

16. Prokaryotes: The First Single-Celled Creatures

16.6. Prokaryotic of Lifestyles

Archaea

Many of the archaea that survive today are methanogens, prokaryotes that use hydrogen (H₂) gas to reduce carbon dioxide (CO₂) to methane (CH₄). Methanogens are strict anaerobes, poisoned by oxygen gas. They live in swamps and marshes, where other microbes have consumed all the oxygen. The methane that they produce bubbles up as “marsh gas.” Methanogens also live in the guts of cows and other herbivores that live on a diet of cellulose, converting the CO₂ produced by its digestion to methane gas. The best understood archaea are extremophiles, which live in unusually harsh environments, such as the very salty Dead Sea and the Great Salt Lake (over 10 times saltier than seawater). Thermoacidophiles favor hot, acidic springs such as the sulfur springs of Yellowstone National Park (figure 16.7), where the water is nearly 80°C, with an acidic pH of 2 or 3.

Figure 16.7. Thermoacidophiles live in hot springs.

These archaea growing in Sulfide Spring, Yellowstone National Park, Wyoming, are able to tolerate high acid levels and very hot temperatures.

Bacteria

Almost all prokaryotes that have been described by scientists are members of the kingdom Bacteria. Many are heterotrophs that power their lives by consuming organic molecules, whereas others are photosynthetic, gaining their energy from the sun. Cyanobacteria are among the most prominent of the photo synthetic bacteria. We have already mentioned the critical role that the members of this ancient phylum played in the history of the earth by generating the oxygen in our atmosphere. Cyanobacteria are filamentous bacteria, like the Anabaena pictured in figure 16.8. Nitrogen fixation occurs in almost all cyanobacteria, within specialized cells called heterocysts (the enlarged cells that appear along the filament of Anabaena and in many of the other filamentous members of this phylum). In nitrogen fixation, atmospheric nitrogen is converted to a form that can be used by living organisms.

Figure 16.8. The cyanobacterium Anabaena.

Individual cells adhere in filaments. The larger cells (areas on the filament that seem to be bulging) are heterocysts, specialized cells in which nitrogen fixation occurs. These organisms exhibit one of the closest approaches to multicellularity among the bacteria.

There are numerous phyla of nonphotosynthetic bacteria. Some are chemoautotrophs, but most are heterotrophs. Some of these heterotrophs are decomposers, breaking down organic material. Bacteria and fungi play the leading role in breaking down organic molecules formed by biological processes, thereby making the nutrients in these molecules available once more for recycling. Decomposition is just as indispensable to the continuation of life on earth as is photosynthesis.

Although bacteria are unicellular organisms, they sometimes form associations, as in Anabaena; also layers of bacterial cells, called biofilms, can form on the surface of a substrate. By forming biofilms, the bacteria create a microenvironment that facilitates their growth. Biofilms are found in nature but also impact humans because they can form on teeth and on medical equipment such as catheters and contact lenses. Biofilms can protect the bacteria from disinfectants.

Bacteria cause many diseases in humans (table 16.2), including cholera, diphtheria, and leprosy. Among the most serious of bacterial diseases is tuberculosis (TB), a disease of the respiratory tract caused by the bacterium Mycobacterium tuberculosis. TB is a leading cause of death throughout the world. Spread through the air, tuberculosis is quite infectious. TB was a major health risk in the United States until the discovery of effective drugs to suppress it in the 1950s. The appearance of drug-resistant strains in the 1990s has raised serious concern within the medical community, and the search is on for new types of anti-TB drugs.

Key Learning Outcome 16.6. Most commonly encountered prokaryotes are bacteria; some cause significant diseases in humans.

TABLE 16.2. IMPORTANT HUMAN BACTERIAL DISEASES

Disease	Pathogen	Vector/Reservoir	Symptoms and Mode of Transmission
Anthrax	Bacillus anthracis	Farm animals	Bacterial infection that can be transmitted through inhaled endospores, by contact, or ingestion. Rare except in sporadic outbreaks. Pulmonary (inhaled) anthrax is often fatal, while cutaneous anthrax (infection through cuts) is readily treated with antibiotics. Anthrax endospores have been used as a biological weapon.
Botulism	Clostridium botulinum	Improperly prepared food	Contracted through ingestion of contaminated food; endospores can sometimes persist in cans and bottles if the containers have not been heated at a high enough temperature to kill the spores. Produces acutely toxic poison; can be fatal.
Chlamydia	Chlamydia trachomatis	Humans (STD)	Urogenital infections with possible spread to eyes and respiratory tract. Occurs worldwide; increasingly common over past 20 years.
Cholera	Vibrio cholerae	Humans (feces), plankton	Causes severe diarrhea that can lead to death by dehydration; 50% peak mortality if the disease goes untreated. A major killer in times of crowding and poor sanitation; over 100,000 died in Rwanda in 1994 during a cholera outbreak.
Dental caries	Streptococcus	Humans	A dense collection of this bacteria on the surface of teeth leads to secretion of acids that destroy minerals in tooth enamel—sugar will not cause caries but bacteria feeding on it will.
Diphtheria	Corynebacterium diphtheriae	Humans	Acute inflammation and lesions of mucous membranes. Spread through contact with infected individual. Vaccine available.
Gonorrhea	Neisseria gonorrhoeae	Humans only	STD, on the increase worldwide. Usually not fatal.
Hansen's disease (leprosy)	Mycobacterium leprae	Humans, feral armadillos	Chronic infection of the skin; worldwide incidence about 10 to 12 million, especially in Southeast Asia. Spread through contact with infected individuals.
Lyme disease	Borrelia burgdorferi	Ticks, deer, small rodents	Spread through bite of infected tick. Lesion followed by malaise, fever, fatigue, pain, stiff neck, and headache.
Peptic ulcers	Helicobacter pylori	Humans	Originally thought to be caused by stress or diet, most peptic ulcers now appear to be caused by this bacterium; good news for ulcer sufferers as it can be treated with antibiotics.
Plague	Yersinia pestis	Fleas of wild rodents: rats and squirrels	Killed one-fourth of the population of Europe in the 14th century; endemic in wild rodent populations of the western United States in the 1990s.
Pneumonia	Streptococcus, Mycoplasma, Chlamydia, Klebsiella	Humans	Acute infection of the lungs, often fatal if not treated.
Tuberculosis	Mycobacterium tuberculosis	Humans	An acute bacterial infection of the lungs, lymph, and meninges. Its incidence is on the rise, complicated by the emergence of new strains of the bacteria that are resistant to antibiotics.
Typhoid fever	Salmonella typhi	Humans	A systemic bacterial disease of worldwide incidence. Less than 500 cases a year are reported in the United States. The disease is spread through contaminated water or foods (such as improperly washed fruits and vegetables). Vaccines are available for travelers.
Typhus	Rickettsia	Lice, rat fleas, humans	Historically, a major killer in times of crowding and poor sanitation; transmitted from human to human through the bite of infected lice and fleas. Typhus has a peak untreated mortality rate of 70%.