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Prokaryotes include the archaebacteria and bacteria. Shown here are two types of bacteria. On the left is a cyanobacteria, called an autotroph, or self-feeder, because it carries out photosynthesis and produces its own food. On the right is a species of Salmonella, which must ingest organic compounds and so is called a heterotroph, or other feeder. The numerous flagella seen here enable Salmonella to move through the intestinal tracts of animals, where they can cause the food-borne illness salmonellosis.

Prokaryote, is a relatively simple unicellular organism, such as a bacterium, characterized by the absence of a nucleus and other specialized cell structures. Scientists distinguish prokaryotes from eukaryotes, which are more complex organisms with cells that contain a nucleus, such as plants and animals.

Scientists classify prokaryotes in different ways, depending on the classification system used. In 1938 American biologist Herbert Copeland proposed that unicellular organisms lacking nuclei be classified in their own kingdom called Kingdom Monera, now called Kingdom Prokaryotae. All bacteria were categorized in this new kingdom. This scheme was the first to establish separate kingdoms for prokaryotes (organisms without nuclei) and eukaryotes (organisms with nuclei). In the 1970s scientists determined that cyanobacteria, formerly known as blue-green algae, have physical features that make them more closely related to bacteria than to algae. Although the exact classification of cyanobacteria is still under debate, some scientists now classify cyanobacteria in the Kingdom Prokaryotae, while algae remains classified in the Kingdom Protista.

In 1990 American microbiologist Carl Woese proposed that bacteria be divided into two groups, archaebacteria and bacteria, based on their structural and physiological differences. Archaebacteria consist of a small group of primitive anaerobes (organisms that do not require oxygen). They are found in a narrow range of habitats—often in extreme environments such as hydrothermal vents on the deep ocean floor. In contrast, bacteria live in a wide range of environments with or without oxygen, at various temperatures, and at various levels of acidity. In some classification systems, the archaebacteria are considered prokaryotes; in other systems they are classified in a category known as the Domain Archaea.

Prokaryotic cells are relatively small, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. With the exception of a few species, prokaryotic cells are surrounded by a protective cell wall. The cell walls of archaebacteria and bacteria contain forms of peptidoglycan, a protein-sugar molecule not present in the cell walls of fungi, plants, and certain other eukaryotes. The archaebacteria cell wall has a more diverse chemical composition than the cell wall of bacteria.



Sea Vents

Life exists not just around vents, but inside them too. Unlike the life forms that crawl or swim around the vents, those inside are invisible. These microscopic bacteria (one-celled organisms) not only survive but even thrive in the dark and hot environment of the vent. In the absence of sunlight, specially adapted bacteria and similar organisms called Archaea convert the vent chemicals to usable bioenergy, in a process analogous to plants’ ability to use sunlight.

Yellowstone Hotsprings

Researchers have discovered a bizarre group of microbes that live inside rocks in the inhospitable geothermal environment at Wyoming’s Yellowstone National Park. One scientist describes the life–form, found in the pores of rocks in a highly acidic environment, as “pretty weird,” and resembling a lichen. Scientists believe similar kinds of geothermal environments may have once existed on Mars. The Yellowstone discovery may help steer the hunt for evidence of life on Mars.

Antarctica – Subglacial Lakes


In addition to the super-hot environment of sea vents and hot springs, bacterial life may also exist in the cold, dark environment beneath the Antarctic ice sheet. Scientists aren’t yet sure, but the suggestions are strong. Two separate research teams have drilled into Lake Vostok, a suspected body of water below the Antarctic ice sheet. (It is still “suspected,” and not proven, because scientists are reluctant to explore further until they know their actions will not contaminate a potentially unique environment.) Both teams found bacteria inside ice that is believed to be created from lake water. DNA analysis indicates that although the bacteria have been isolated for millions of years, they are biologically similar to known organisms.

Atacama Desert


Scientists now know that life exists not only in very hot and very cold liquid environments, but in a very dry environment as well. Environmental microbiologists have discovered evidence of microbial life about a foot below the rough terrain of Chile’s Atacama desert, one of the driest places on Earth. Their finding contradicts previous beliefs that the desert is too dry to support life, and may influence how scientists look for life in the similarly dry environment of Mars.


Europa ?

Some scientists speculate that if life does exist beyond Earth, it might be the form of vent bacteria. Because these microscopic life forms have already proven their ability to survive in the extreme environment of Earth’s hydrothermal vents, they might also survive in similar environments elsewhere – for example, on Europa. Europa is one of Jupiter’s moons, and is covered in ice. Scientists have recently uncovered strong evidence of liquid water beneath Europa’s ice, which may be due to hydrothermal vents, which may in turn host bacteria. Alternatively, scientists who have found evidence of bacteria living inside Antarctic ice speculate that they may also live inside Europa’s ice. The questions exceed the answers, but the clues are tantalizing.


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