Characteristics of Archaebacteria

All living things are classified into six 'kingdoms'. Archaebacteria are the oldest organisms living on the earth. They are prokaryocytes and unicellular and belong to a kingdom, Archae. They were first discovered in 1977 and classified as bacteria. Most archaebacteria appear like bacteria, when observed under the microscope. However, they are quite different from bacteria and eukaryotic organisms. Archaebacteria are found in very harsh conditions such as in the volcanic vents or at the bottom of the sea. They survive in the environments such as sea vents releasing sulfide-rich gases, hot springs or boiling mud around volcanoes. All forms of archaebacteria are non-pathogenic.

Groups of Archaebacteria

There are three phyla or groups of archaebacteria, such as methanogens, halophiles and thermoacidophiles. Methanogens harvest energy by converting H2 and CO2 into methane gas. They are found in the intestinal tracts of humans and some animals such as cows and in the marshes. Halophiles survive in a high salt atmosphere. Hence, they are found in the Great Salt Lake, Dead Sea and other areas with a high salt concentration. Thermoacidophiles are found in the areas with a very high temperature and extremely acidic conditions. They can be found in hydrothermal vents and volcanic vents.

Characteristics of Archaebacteria

Archaebacteria are obligate anaerobes and they survive only in oxygen-free environment. They are known as extremophiles, as they are able to live in a variety of environments. Some species can live in the temperatures above boiling point at 100 degree Celsius or 212 degree Fahrenheit. They can also survive in acidic, alkaline or saline aquatic environments. Some can withstand the pressures of more than 200 atmospheres.

The size of archaebacteria ranges from 1/10th of a micrometer to more than 15 micrometers. Some of archaebacteria have flagella. Like all prokaryotes, archaebacteria don’t possess the membrane-bound organelles. They don’t have nuclei, endoplasmic reticula, Golgi complexes, mitochondria, chloroplasts or lysosomes. The cells consist of a thick cytoplasm that contains all the compounds and molecules required for metabolism and nutrition. Their cell wall doesn’t contain peptidoglycan. The rigid cell wall supports the cell and allows archaebacterium to maintain its shape. It also protects the cell from bursting when present in a hypotonic environment. Archaebacteria have lipids in their cell membranes. They are composed of branched hydrocarbon chains, connected to glycerol by ether linkages.

Since these organisms don’t have a nucleus, the genetic material floats freely in the cytoplasm. They consist of ribosomal RNA (rRNA). DNA contains a single, circular molecule, which is compact and tightly wound. No protein is associated with DNA. The archaebacterial cell may contain plasmids, which are small, circular pieces of DNA. They can duplicate independent of larger, genomic DNA circle. Plasmids often code for antibiotic resistance or particular enzymes.

Archaebacteria reproduce by an asexual process known as binary fission. During this process, the bacterial DNA replicates. The cell wall pinches off in the center, due to which the organism is divided into two new cells. Each cell consists of a copy of circular DNA. It is quite rapid process. Some species divide every 20 minutes. Although genetic material can be exchanged between the cells by three various processes, sexual reproduction is not seen in archaebacteria.

During transformation, DNA fragments released by one bacterium are taken up by another bacterium. In the process of transduction, a bacterial phage (a virus infecting bacterial cells) transfers genetic material from one organism to another. In the process of conjugation, genetic material is exchanged between two bacteria. These mechanisms lead to genetic recombination, causing the continued evolution of archaebacteria.