Industrial and environmental applications of halophilic microorganisms
Department of Plant and Environmental Sciences, The Institute of Life Sciences, and the Moshe Shilo Minerva Center for Marine Biogeochemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
Taylor and Francis
(Received 6August 2009; Accepted 28 September 2009 ) In comparison with the thermophilic and the alkaliphilic extremophiles, halophilic microorganisms have as yet found relatively few biotechnological applications. Halophiles are involved in centuries-old processes such as the manufacturing of solar salt from seawater and the production of traditional fermented foods. Two biotechnological processes involvinghalophiles are highly successful: the production of β-carotene by the green alga Dunaliella and the production of ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid), used as a stabilizer for enzymes and now also applied in cosmetic products, from moderately halophilic bacteria. The potential use of bacteriorhodopsin, the retinal protein proton pump of Halobacterium, in optoelectronicdevices and photochemical processes is being explored, and may well lead to commercial applications in the near future. Demand for salt-tolerant enzymes in current manufacturing or related processes is limited. Other possible uses of halophilic microorganisms such as treatment of saline and hypersaline wastewaters, and the production of exopolysaccharides, poly- βhydroxyalkanoate bioplastics andbiofuel are being investigated, but no large-scale applications have yet been reported. Keywords: halophilic; β-carotene; ectoine; bacteriorhodopsin; enzymes
Introduction In comparison to other groups of extremophiles, and especially the thermophilic and the alkaliphilic prokaryotes that are extensively used for the production of valuable enzymes, the extremely andmoderately halophilic microorganisms are to some extent a neglected group when considering the number of their biotechnological applications. This is even more true when considering the great diversity of halophiles: they are found in all three domains of life, Archaea, Bacteria and Eucarya, and they contain representatives of many different physiological types, adapted to a wide range of saltconcentrations up to salt saturation. Earlier reviews that discussed the possible applications of halophiles in biotechnological and environmental processes [1–7] provided long lists of potential uses that have been and are still being explored. However, the number of applications of halophiles that are currently exploited is very small indeed. The true success stories of halophile biotechnology arethe commercial production of β-carotene by strains of the unicellular green alga Dunaliella and the production of ectoine, synthesized by many moderately halophilic bacteria to provide osmotic balance to the cells, and its application as a stabilizing agent for sensitive
ISSN 0959-3330 print/ISSN 1479-487X online © 2010 Taylor & Francis DOI: 10.1080/09593330903370026http://www.informaworld.com
enzymes and in cosmetic preparations. Many other products synthesized by halophiles or processes performed by them may have potential applications, but these have not yet led to commercially viable operations. Applications (current and potential) of halophilic microorganisms can be divided into a number of categories: (1) Centuries-old processes such as themanufacturing of solar salt from seawater and the production of traditional fermented foods. Such processes existed long before the nature of the microorganisms involved became known, and little if anything is done to control these microorganisms to improve the production processes. (2) Utilization of the salt tolerance of halophilic microorganisms and of enzymes produced by them to catalyze processes in...