Metabolism

Archaeal metabolism varies greatly between the members of different groups. They are just as diverse physiologically. They can be aerobic, facultatively anaerobic, or strictly anaerobic. Nutritionally they range from chemolithoautotrophs to organotrophs. Archaeal carbohydrate metabolism is best understood. The enzyme 6-phosphofructokinase has not been found in archaea, and they do not appear to degrade glucose by way of the Embden-Meyerhof pathway. Extreme halophiles and thermophiles catabolize glucose using a modified form of the Entner-Doudoroff pathway in which the initial intermediates are not phosphorylated. In contrast with glucose degradation, gluconeogenesis proceeds by a reversal of the Embden-Meyerhof pathway in halophiles and methanogens. All archaea that have been studied can oxidize pyruvate to acetyl-CoA. They lack the pyruvate dehydrogenase complex present in eucaryotes and respiratory bacteria and use the enzyme pyruvate oxidoreductase for this purpose. Halophiles and the extreme thermophile Thermoplasma do seem to have a functional tricarboxylic acid cycle.

Archaeal Taxonomy

Based on the rRNA data, it has been divided into two major phylum namely Euryarchaeota, Crenarchaeota. On the basis of morphology and physiology, it has been divided into five different major groups. (1. Methanogenic archaea, 2.Archaea sulfate reducers, 3.Extremely halophilic archaea, 4. Cell wall–less archaea, 5.Extremely thermophilic S⁰ -metabolizers).

1. Phylum- Euryarchaeota (Eures: wide)

The euryarcaeota are given this name because they occupy many diverse ecological niches and have a variety of metabolic patterns. It consists of seven classes (Methanobacteria, Methanococci, Halobacteria, Thermoplasmata, Thermococci, Archaeglobi, and Methanopyri), nine orders and 15 families.

Methanogens

They are strict anaerobes that obtain energy through the conversion of CO₂ and H₂ into methane. This is the largest group of archaea. There are five orders (Methanobacteriales, Methanococcales, Methanomicrobiales, Methanosarcinales, and Methanopyrales) and 26 generas. Methanogens thrive in anaerobic environments rich in organic matter: the rumen and intestinal system of animals, freshwater and marine sediments, swamps and marshes, hot springs, anaerobic sludge digesters, and even within anaerobic protozoa. Rumen methanogens are so active that a cow can belch 200 to 400 liters of methane a day.

Halobacteria

They are aerobic heterotrophs with respiratory metabolism and require complex nutrients, usually proteins and amino acids, for growth. Species are either nonmotile or motile by lophotrichous flagella. The extreme halophiles, class Halobacteri a, are another major group of archaea, currently with 15 genera in one family, the Halobacteriaceae . The most obvious distinguishing trait of this family is its absolute dependence on a high concentration of NaCl. These procaryotes require at least 3 to 4 M NaCl (17 to 23%) for their growth optimum. Halobacterium produces energy by trapping light and synthesize ATP with the help of rhodopsin.