Halobacterium NRC-1

Cell structure and metabolism

Halobacterium sp. NRC-1 is 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 it's preferred environment. Halorhodopsin plays a very energy efficient role in helping to maintain the osmotic balance within the cell by acting as a light driven pump to transport clorine ions into the cell. This microorganism has been extensively studied and shown to contain some of the classic features found in halophilic archaea, for example, an S-layer glycoprotein, ether-linked lipids, and purple membrane.

The 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 synthesizes 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 in Bacteriorhodopsin absorbs light, it results in a series of conformational changes that translocates the proton into the periplasmic space. This light driven proton pumping generates a pH gradient which is then used to power the synthesis of ATP by chemiosmosis. 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 phototrophic respiration capabilities, is also capable of anaerobic respiration using dimethyl sulfoxide (DMSO) and trimethylamine-N-oxide (TMAO).

• Halobacterium NRC-1 is an aerobic chemoorganotroph, growing on the degradation products of less halophilic organisms as the salinity reaches near saturation. In the laboratory, 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. Several amino acids may be used as a source of energy, including arginine and aspartate, which are passed to the citric acid cycle via 2-oxoglutarate and oxaloacetate, respectively. Under aerobic conditions, arginine is presumably converted to glutamate via the arginine deiminase pathway, and this amino acid then enters the cycle via glutamate dehydrogenase. The arginine deiminase pathway is coded by the arcRACB genes, which are found on pNRC200.