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Halostagnicola larsenii is a non-motile, gram-negative, rod shaped archaeon. It is a halophilic, neutrophilic, chemo-organotroph and was isolated from samples taken from a saline lake in China. H. larsenii has been implicated in oil recovery and is being studied for its potential to produce electrical energy from light energy.

Discovery
In September 2003, researchers from the University of Seville, Spain, obtained samples of sediment from a lake in Inner Mongolia, China. Lake Xilinholt is an extremely saline lake, thus providing the optimum growth conditions for Halostagnicola larsenii . The samples were cultivated in a 20% saline solution. Nutrient agar plates were used to cultivate the samples. The media contained sodium chloride and was optimized at a pH of 7.5. H. larsenii grows optimally at 15% NaCl, 37 ° Celsius and pH 7-8. It is unable to grow at temperatures above 50 ° Celsius. Further characterization of the species was conducted and it was proposed by Castillo et al, that strain XH-48 be identified as a new species within the Halostagnicola phylum.

Characterization
The etymology of the name comes from Halos Greek for salt, stagnum Latin for standing water, cola Latin for inhabitant or dweller, and Larsenii named after the Norwegian microbiologist, Helge Larsen, who was a pioneer in research regarding halophiles.

Morphology
Morphology was determined using phase contrast optics by a Olympus BX41 microscope. Cells of Halostagnicola larsenii XH48 are 0.5-1.0 micrometers wide and 1.0-3.0 micrometers long. Cells of H. larsenii are pleomorphic, and display rod, square or disc shaped cells. This reflects the strain's ability to change size and shape in response to changes in the environment, such as salinity. The colony morphology of H. larsenii is circular, smooth, opaque and pink in color. Polar ether lipids found in its membrane include phosphatidylglycerol and phosphatidylglyceromethylphosphate. These lipids were extracted with chloroform and methanol. Tests revealed this organism is oxidase positive and catalase negative.

Metabolism
Halostagnicola larsenii is a halophilic, neutrophilic, chemo-organotroph and uses oxygen as its terminal electron acceptor. H. larsenii can utilize a variety of carbohydrates such as fructose, glycerol, lactose, glucose, arabinose, acetate, ribose, starch, maltose, galactose, ribose, xylose, glutamate, and propionate as substrates for growth. Growth substrates were determined through the use of the isolation medium, which contained the substrate being tested along with yeast extract. Additionally, H. larsenii undergoes assimilatory nitrate reduction to nitrite to ammonia. This process differs from nitrate reduction because it occurs aerobically and uses ferrodoxin as an electron donor.

Antibiotic Resistance
H. larsenii is resistant to the following antibiotics: ampicillin, chloramphenicol, erythromycin, gentamicin, nalidixic acid, neomycin, penicillin G, rifampicin, streptomycin, and tetracycline. The organism is sensitive to bacitracin and novobiocin. Antibiotic sensitivity and resistance was determined using the agar diffusion test in which paper discs saturated with antibiotics were placed on agar plates.

Ecology
Halostagnicola larsenii was originally discovered in a saline lake in Inner Mongolia, China. It has also been isolated from rock pit sea water in the West Coast of Maharashtra, India. Typically, haloarchaea such as H. larsenii require high salinity environments for growth and can be found in the sediment of aquatic environments such as freshwater lakes.

Genomics
In 2014, the complete genome of H. larsenii was sequenced using Ilumina HiSeq 2000 by Iain Anderson as part of the Archaeal Tree of Life Project supported by the Joint Genome Institute. The genome consists of 2.79 Mega-bases on a circular chromosome with four circular plasmids. The genome includes 4,246 genes of which 4,171 are protein coding genes, 19 are pseudogenes, 6 rRNA genes, and 49 tRNA genes. The GC-content of the genome is 61%.

In a 2008 study by Castillo, et al, chromosomal DNA was ioslated using the Marmur methods of simple cell disruption by detergent lysis, nucleic extraction by an organic solvent, and DNA recovery by ethanol precipitation were used. The 16S ribosomal RNA gene sequence of H. larsenii was studied using ARB software. The neighbor-joining method was used to conduct 16s rRNA gene sequence analysis and determine phylogenetic relationships. The closest neighboring species Natrialba aegypitaca and Natrialba asiatica had a 94.5% and 93.3% genome similarity, respectively. The key difference from Natrialba is that H. larsenii lacks the key bases 403G and 560G.

Industrial Use
Halophilic archaea such as species in the genus Halostagnicola can be applied in oil recovery, due to some halophiles containing exopolysaccharide, an extracellular polymeric substance or EPS. This forms a biofilm which absorbs ions and has protective nutrients. The EPS, which has the ability to break down complex carbohydrates in water, is useful in the recovery of oil.

Haloarchaea have been proposed as a type of life that could be sustained in Martian conditions, due to their ability to tolerate all kinds of environmental stressors. It is also proposed that these haloarchaea could potentially be able to travel from planet to planet and survive radiation. Halostagnicola larsenii is also being studied for its potential to produce bacteriorhodopsin, a membrane protein which can convert light energy into electrical energy. Bacteriorhodopsin is an enzyme that acts as a proton pump and can play a major role in energy metabolism in archaea that live in aquatic environments. It uses the energy obtained from pumping protons against their electrochemical gradient to convert light energy into electrical energy. A strain of H. larsenii isolated from India was tested and found to perform this same process, a trait which could have implications in understanding how to produce different kinds of energy.