Halotolerant or halophilic microorganisms, able to live in saline environments, offer a multitude of actual or potential applications in various fields of biotechnology. The technical applications of bacteriorhodopsin comprise holography, spatial light modulators, optical computing, and optical memories. Compatible solutes are useful as stabilizers of biomolecules and whole cells, salt antagonists, or stress-protective agents. Biopolymers, such as biosurfactants and exopolysaccharides, are of interest for microbially enhanced oil recovery. Other useful biosubstances are enzymes, such as new isomerases and hydrolases, that are active and stable at high salt contents. Halotolerant microorganisms play an essential role in food biotechnology for the production of fermented food and food supplements. The degradation or transformation of a range of organic pollutants and the production of alternative energy are other fields of applications of these groups of extremophiles.

Many hydrocarbon-contaminated environments are characterized by low or elevated temperatures, acidic or alkaline pH, high salt concentrations, or high pressure, Hydrocarbon-degrading microorganisms, adapted to grow and thrive in these environments, play an important role in the biological treatment of polluted extreme habitats. The biodegradation (transformation or mineralization) of a wide range of hydrocarbons, including aliphatic, aromatic, halogenated and nitrated compounds, has been shown to occur in various extreme habitats. The biodegradation of many components of petroleum hydrocarbons has been reported in a variety of terrestrial and marine cold ecosystems. Cold-adapted hydrocarbon degraders are also useful for wastewater treatment. The use of thermophiles for biodegradation of hydrocarbons with low water solubility is of interest, as solubility and thus bioavailability, are enhanced at elevated temperatures. Thermophiles, predominantly bacilli, possess a substantial potential for the degradation of environmental pollutants, including all major classes. Indigenous thermophilic hydrocarbon degraders are of special significance for the bioremediation of oil-polluted desert soil. Some studies have investigated composting as a bioremediation process. Hydrocarbon biodegradation in the presence of high salt concentrations is of interest for the bioremediation of oil-polluted salt marshes and industrial wastewaters, contaminated with aromatic hydrocarbons or with chlorinated hydrocarbons. Our knowledge of the biodegradation potential of acidophilic, alkaliphilic, or barophilic microorganisms is limited.

Biodegradation of petroleum hydrocarbons in cold environments, including Alpine soils, is a result of indigenous cold-adapted microorganisms able to degrade these contaminants. In the present study, the prevalence of seven genotypes involved in the degradation of n-alkanes (Pseudomonas putida GPo1 alkB; Acinetobacter spp. alkM; Rhodococcus spp. alkB1, and Rhodococcus spp. alkB2), aromatic hydrocarbons (P. putida xylE), and polycyclic aromatic hydrocarbons (P. putida ndoB and Mycobacterium sp. strain PYR-1 nidA) was determined in 12 oil-contaminated (428 to 30,644 mg of total petroleum hydrocarbons [TPH]/kg of soil) and 8 pristine Alpine soils from Tyrol (Austria) by PCR hybridization analyses of total soil community DNA, using oligonucleotide primers and DNA probes specific for each genotype. The soils investigated were also analyzed for various physical, chemical, and microbiological parameters, and statistical correlations between all parameters were determined. Genotypes containing genes from gram-negative bacteria (P. putida alkB, xylE, and ndoB and Acinetobacter alkM) were detected to a significantly higher percentage in the contaminated (50 to 75%) than in the pristine (0 to 12.5%) soils, indicating that these organisms had been enriched in soils following contamination. There was a highly significant positive correlation (P < 0.001) between the level of contamination and the number of genotypes containing genes from P. putida and Acinetobacter sp. but no significant correlation between the TPH content and the number of genotypes containing genes from grampositive bacteria (Rhodococcus alkB1 and alkB2 and Mycobacterium nidA). These genotypes were detected at a high frequency in both contaminated (41.7 to 75%) and pristine (37.5 to 50%) soils, indicating that they are already present in substantial numbers before a contamination event. No correlation was found between the prevalence of hydrocarbon-degradative genotypes and biological activities (respiration, fluorescein diacetate hydrolysis, lipase activity) or numbers of culturable hydrocarbon-degrading soil microorganisms; there also was no correlation between the numbers of hydrocarbon degraders and the contamination level. The measured biological activities showed significant positive correlation with each other, with the organic matter content, and partially with the TPH content and a significant negative correlation with the soil dry-mass content (P < 0.05 to 0.001).

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