Halophiles Secondary article Shiladitya DasSarma, University of Massachusetts, Amherst, Massachusetts, USA Article Contents Priya Arora, University of Massachusetts, Amherst, Massachusetts, USA . Introduction . Halophilicity and Osmotic Protection Halophiles are salt-loving organisms that inhabit hypersaline environments. They include . Hypersaline Environments mainly prokaryotic and eukaryotic microorganisms with the capacity to balance the . Eukaryotic Halophiles osmotic pressure of the environment and resist the denaturing effects of salts. Prokaryotic Halophiles . Biotechnology . Introduction Conclusions and Future Prospects Halophiles are salt-loving organisms that inhabit hypersa- line environments. They include mainly prokaryotic and organisms can grow both in high salinity and in the absence eukaryotic microorganisms with the capacity to balance of a high concentration of salts. Many halophiles and the osmotic pressure of the environment and resist the halotolerant microorganisms can grow over a wide range denaturing effects of salts. Among halophilic microorgan- of salt concentrations with requirement or tolerance for isms are a variety of heterotrophic and methanogenic salts sometimes depending on environmental and nutri- archaea; photosynthetic, lithotrophic, and heterotrophic tional factors. bacteria; and photosynthetic and heterotrophic eukar- High osmolarity in hypersaline conditions can be yotes. Examples of well-adapted and widely distributed deleterious to cells since water is lost to the external extremely halophilic microorganisms include archaeal medium until osmotic equilibrium is achieved. To prevent Halobacterium species, cyanobacteria such as Aphanothece loss of cellular water under these circumstances, halophiles halophytica, and the green alga Dunaliella salina. Among generally accumulate high solute concentrations within the multicellular eukaryotes, species of brine shrimp and brine cytoplasm (Galinski, 1993). When an isoosmotic balance flies are commonly found in hypersaline environments. with the medium is achieved, cell volume is maintained. Halophiles can be loosely classified as slightly, moderately The compatible solutes or osmolytes that accumulate in or extremely halophilic, depending on their requirement halophiles are usually amino acids and polyols, e.g. glycine for NaCl. The extremely halophilic archaea, in particular, betaine, ectoine, sucrose, trehalose and glycerol, which do are well adapted to saturating NaCl concentrations and not disrupt metabolic processes and have no net charge at have a number of novel molecular characteristics, such as physiological pH. A major exception is for the halobacteria enzymes that function in saturated salts, purple membrane and some other extreme halophiles, which accumulate KCl that allows phototrophic growth, sensory rhodopsins that equal to the external concentration of NaCl. Halotolerant mediate the phototactic response, and gas vesicles that yeasts and green algae accumulate polyols, while many promote cell flotation. Halophiles are found distributed all halophilic and halotolerant bacteria accumulate glycine over the world in hypersaline environments, many in betaine and ectoine. Compatible solute accumulation may natural hypersaline brines in arid, coastal, and even deep- occur by biosynthesis, de novo or from storage material, or sea locations, as well as in artificial salterns used to mine by uptake from the medium. salts from the sea. Their novel characteristics and capacity for large-scale culturing make halophiles potentially valuable for biotechnology. Hypersaline Environments Though the oceans are, by far, the largest saline body of Halophilicity and Osmotic Protection water, hypersaline environments are generally defined as those containing salt concentrations in excess of sea water Although salts are required for all life forms, halophiles are (3.5% total dissolved salts). Many hypersaline bodies distinguished by their requirement of hypersaline condi- derive from the evaporation of sea water and are called tions for growth. They may be classified according to their thalassic. A great diversity of microbial life is observed in salt requirement: slight halophiles grow optimally at 0.2– thalassic brine from marine salinity up to about 3– 0.85 mol L 2 1 (2–5%) NaCl; moderate halophiles grow 3.5 mol L 2 1 NaCl, at which point only a few extreme optimally at 0.85–3.4 mol L 2 1 (5–20%) NaCl; and ex- halophiles can grow, e.g. Halobacterium, Dunaliella, and a treme halophiles grow optimally above 3.4–5.1 mol L 2 1 few bacterial species. Athalassic waters are those in which (20–30%) NaCl. In contrast, nonhalophiles grow opti- the salts are of nonmarine proportion, found for example mally at less than 0.2 mol L 2 1 NaCl. Halotolerant after the concentration of sea water leads to precipitation ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1 Halophiles of NaCl, leaving a high concentration of potassium and California coast, Lake Sivash near the Black Sea, and magnesium salts. This point marks the upper limit of Sharks Bay in western Australia. Hypersaline evaporation resistance of all biological forms. ponds have also been found in Antarctica (e.g. Deep Lake, The two largest and best-studied hypersaline lakes are Organic Lake and Lake Suribati), several of which are the Great Salt Lake, in the western United States, and the stratified with respect to salinity. Dead Sea, in the Middle East. The Great Salt Lake is larger A number of alkaline hypersaline soda brines also exist, (3900 km2) and shallower (10 m), and contains salts that including the Wadi Natrun lakes of Egypt, Lake Magadi in are close in relative proportion to sea water. The Dead Sea Kenya, and the Great Basin lakes of the western United is smaller (800 km2) and deeper (340 m), and contains a States (Mono Lake, Owens lake, Searles Lake and Big very high concentration of magnesium salts. Both of these Soda Lake), several of which are intermittently dry. Soda lakes are close to neutral pH, although the Great Salt Lake brines are lacking in magnesium and calcium divalent is slightly alkaline while the Dead Sea is slightly acidic. cations because of their low solubility at alkaline pH. Compared to smaller hypersaline ponds, the compositions Many smaller hypersaline pools represent especially of these lakes are fairly constant as a result of their size, dynamic environments, experiencing significant seasonal although recent human activities have had significant variations in size, salinity and temperature. effects on the chemistry and biology of both. For example, In addition to natural hypersaline lakes, numerous a railroad causeway built in 1959 divided the Great Salt artificial solar salterns have been constructed for the Lake into northern and southern sections, leading to production of sea salts (Figure 1). These usually consist of a dilution of the southern section, which receives the greatest series of shallow evaporation ponds connected by pipes inflow of fresh water from streams, and the concentration and canals. As evaporation occurs, brine is directed into of the northern section to nearly saturating salinity. ponds with progressively greater salinities until sequential Diversion of incoming freshwater streams for irrigation precipitation of calcium carbonate, calcium sulfate (gyp- in the Dead Sea basin in recent years has also had sum) and NaCl (halite) occurs. After NaCl precipitation, significant impact on its size and salinity (Watzman, 1997). the concentrated potassium and magnesium chloride and Many small evaporation ponds or sabkhas are found sulfate brines (‘bitterns’) that remain are usually returned near coastal areas, where sea water penetrates through to the sea. seepage or via narrow inlets from the sea. Notable among Hypersaline environments also occur in subterranean these are Solar Lake, Gavish Sabkha and Ras Muhammad evaporite deposits and deep-sea basins created by the Pool near the Red Sea coast, Guerrero Negro on the Baja evaporation and flooding of ancient seas. Deep-sea brines Figure 1 Halobacterial bloom in a solar saltern. Dense growth of halophilic microorganisms in hypersaline environments, like this saltern near San Francisco, leads to reddening of the brine. Cell densities of 107 ml 2 1 species Halobacterium are not uncommon. Courtesy of Drs Nicole Tandeau de Marsac and Germaine Stanier, Pasteur Institute, Paris. 2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Halophiles PR-6 Fs Ds Ah H Eukaryotic Halophiles 100 Multicellular eukaryotes Few such organisms can tolerate hypersaline conditions and the highest salinity at which any vertebrates have been observed (e.g. Tilapia species) is about 1 mol L 2 1 NaCl. A variety of obligate and facultative halophytic plants, e.g. Growth rate (%) Atriplex halimus and Mesembryanthemum crystallinum, Hatch range can survive in moderately high saline soils. There are also a surprising number of invertebrates that can survive in Sea water 0 hypersaline environments. Some examples are rotifers such Salinity (%) 0 100 200 300 –1 Saturation as Brachionus angularis and Keratella quadrata, tubellarian NaCl (molL )10 2 34 5 worms such as Macrostomum species, copepods such as Figure 2 Salt-tolerance of halophilic organisms. Relative growth rate is Nitocra lacustris and Robertsonia salsa, ostracods such as plotted against both percentage salinity and NaCl concentration. The five Cypridis torosa, Paracyprideinae spp., Diacypris compacta, microorganisms are Agmenellum quadraplicatum