Halobacterium NRC-1

Cell structure and metabolism

Halobacterium sp.NRC-1 is a rod shaped aerobic chemoorganotroph which uses organic molecules as its source of energy, carbon and electrons. This halophile possesses facultative anaerobic and phototrophic capabilities as well. Research has shown that it is unable to metabolize sugars, but instead rely on amino acids that are eventually catabolized by the citric acid cycle during aerobic respiration. It survives on the remains of less halophilic organisms that have lysed due to the overwhelming amounts of salt in the environment. Outside of their natural environment, cells are cultured best in a complex medium. A minimal medium described for Halobacterium includes all but 5 of the 20 amino acids for growth.

This microorganism has been studied extensively and has been shown to contain some of the usual features found in halophilic archaea, for example, an S-layer glycoprotein, ether-linked lipids, and purple membrane. This purple membrane consists of the light-driven ion transporters bacteriorhodopsin and halorhodopsin, and the phototaxis receptors, sensory rhodopsins I and II. In order to survive in low oxygen environments, Halobacterium sp. NRC-1 increases its production of Bacteriorhodopsin, which is a unique protein that can use light as an energy source, much like chlorophyll can in cyanobacteria and phototrophic eukaryotes. When the retinal in Bacteriorhodopsin absorbs light, it results in a series of conformational changes that translocates protons through the cell's membrane into the periplasmic space. This light driven proton pumping generates an electrochemical proton gradient which is then used to power the synthesis of ATP. This phototrophic capability is particularly useful to Halobacterium sp. NRC-1 as oxygen is not very soluble in concentrated salt solutions. In addition to its ability to use light as an energy source, it is also capable of anaerobic respiration using dimethyl sulfoxide (DMSO) and trimethylamine-N-oxide (TMAO) as terminal electron acceptors. Arginine fermentation can also be used for anaerobic energy production as well.

Halobacterium sp. NRC-1 is also classified as an obligate halophilic microorganism which has adapted to be able to grow in conditions of extremely high salinity, up to 10 times that of seawater. In order to survive under these conditions it maintains a very high concentration of salts internally in the form of KCl to enable it to remain isotonic to its preferred environment. Halorhodopsin plays a very important role in helping to maintain the osmotic balance within the cell. This membrane protein acts as a light driven pump by transporting chloride and potassium ions into the cell. Halorhodopsin saves the organism a large amount of metabolic energy by using the energy of the yellow light that it captures to fuel the movement of these ions.