Two fundamentally different types of cells exist, Prokaryotic cells having a simpler morphology and lack a true membrane de-limited nucleus. All bacteria are prokaryotic.
• Eukaryotic cells have a membrane-bound nucleus; are more complex morphologically and larger than prokaryotes. Algae, fungi, protozoa, higher plants, and animals are eukaryotes
• For many years biologists have divided organisms into five kingdoms; Monera, Protists, Fungi, Animalia and Plantae.
• In the last few decades great progress in three areas has been made that affect microbial classification.
• First – detailed structure of microbial cells has been studied using EM
• Second – microbiologists have determined the biochemical and physiological characteristics of many different microorganisms
• Third – sequences of nucleic acids and proteins from a wide variety of organisms have been compared.
• It is now clear that there are two quite different groups of prokaryotic organisms; Bacteria and Archaea.
• The differences between bacteria, archaea and eukaryotes seem so great that many microbiologists proposed that organisms should be divided among three domains; Bacteria (the true bacteria or eubacteria), Archaea and Eucarya (all eukaryotic organisms).
Members of microbial world
There are five major members of microorganisms, Archaea, Bacteria, Algae, Protozoa , and Fungi . The Archaea and Bacteria are prokaryotic cells. Unicellular algae and protozoa and fungi are eukaryotic cells.
Archaea
The Archaea are a group of single-celled microorganisms. They have no cell nucleus or any other membrane-bound organelles within their cells. Archaea and bacteria are quite similar in size and shape, although a few archaea have very unusual shapes, such as the flat and square-shaped cells. Similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation. The archaea biochemistry are unique, such as presence of ether lipids in their cell membranes. Archaea use a much greater variety of sources of energy than eukaryotes: ranging from familiar organic compounds such as sugars, to ammonia, metal ions or even hydrogen gas. Archaea reproduce asexually by binary fission, fragmentation, or budding; unlike bacteria and eukaryotes, no known species form spores. Initially archaea were seen as extremophiles that lived in harsh environments, such as hot springs and salt lakes, but they are now found in a broad range of habitats, including soils, oceans, marshland. Archaea play roles in both the carbon cycle and the nitrogen cycle. No archaea pathogens or parasites are known, but they are often mutualists or commensals. Methanogens are used in biogas production and sewge treatmen, and enzymes from extremophile archaea that can endure high temperatures and organic solvents are exploited in biotechnology.