I.

Introduction

Prokaryote, 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.