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.

 

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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%.