Halophiles By Aubrey McDonald

Overview What is a halophile? Diversity of Halophilic Organisms Osmoregulation “Compatible Solute” Strategy “Salt-in” Strategy Interesting Facts and Applications Summary and References

What is a halophile? The word halophile means “salt loving”. A halophile is an organism that can grow in higher salt concentrations than the norm. Based on optimal saline environments halophilic organisms can be grouped into three categories: extreme halophiles, moderate halophiles, and slightly halophilic or halotolerant organisms. Some extreme halophiles can live in solutions of 35 % salt. This is extreme compared to seawater which is only 3% salt.

Diversity of Halophilic Organisms Halophiles are a broad group that can be found in all three domains of life. They are found in salt marshes, subterranean salt deposits, dry soils, salted meats, hypersaline seas, and salt evaporation pools.

Unusual Habitats The bacterium pseudomonas was found living on a desert plant in the Negev Desert. The plant secretes salt through salt glands on its leaves. Bacillus was found in the nasal cavities of desert iguanas. These iguanas have salt glands in their nasal cavities that secrete KCl brine during osmotic stress.

Osmoregulation Living in high salinity poses a serious stress that halophiles have overcome through special processes or adaptations. The stress lies in the microbes ability to maintain an internal osmotic potential that equals their external environment. Osmosis is the process in which water moves from an area of high concentration to an area of low concentration.

Osmoregulation In order for cells to maintain their water they must have an osmotic potential equal to their external environment. As salinity increases in the environment its osmotic potential decreases. If you placed a non halophilic microbe in a solution with a high amount of dissolved salts the cell’s water will move into the solution causing the cell to plasmolyze.

Osmoregulation Halophiles have adapted to life at high salinity in many different ways. One way is through the modification of their external cell walls. They tend to have negatively charged proteins on the outside of their cell walls that stabilize it by binding to positively charged sodium ions in their external environments. If salt concentrations decline their cell walls may become unstable and break down.

“Compatible Solute” Strategy There are two strategies that halophiles have evolved to deal with high salt environments. In the “compatible solute” strategy cells maintain low concentrations of salt in their cytoplasm by balancing osmotic potential with organic, compatible solutes. They do this by the synthesis or uptake of compatible solutes.

“Compatible Solute” Strategy Compatible solutes include polyols such as glycerol, sugars and their derivatives, amino acids and their derivatives, and quaternary amines such as glycine betaine. Energetically this is an expensive process. Autotrophs use between 30 to 90 molecules of ATP to synthesize one molecule of the compatible solutes. Heterotrophs use between 23 to 79 ATP.

“ Compatible Solute” Strategy Energy is also expended in pumping out salts that dissolve into the cell. The uptake of available compatible solutes in the environment is an adaptation they have evolved to reduce the energy cost of living in high salt concentrations.

“Salt-in” Strategy Cells can have internal concentrations that are osmotically equivalent to their external environment. This “salt-in” strategy is primarily used by aerobic, extremely halophilic archaea and anaerobic bacteria. They maintain osmotically equivalent internal concentrations by accumulating high concentrations of potassium chloride.

“Salt-in” Strategy Potassium ions enter the cell passively via a uniport system. Sodium ions are pumped out. Chloride enters the cell against the membrane potential via cotransport with sodium ions. For every three molecules of potassium chloride accumulated, two ATP are hydrolyzed making this strategy more energy efficient than the “compatible solute” strategy.

“Salt-in” Strategy To use this strategy all enzymes and structural cell components must be adapted to high salt concentrations to ensure proper cell function.

Halobacterium: an example of an extreme halophile Halobacterium are members of the halophile group in the domain archaea. They are widely researched for their extreme halophilism and unique structure. They require salt concentrations between 15% to 35% sodium chloride to live. They use the “salt-in” strategy. They produce ATP by respiration or by bacteriorhodopsin.

Halobacterium They may also have halorhodopsin that pumps chloride into the cell instead of pumping protons out. The Red Sea was named after halobacterium that turns the water red during massive blooms.

Facts The term “red herring” comes from the foul smell of salted meats that were spoiled by halobacterium. There have been considerable problems with halophiles colonizing leather during the salt curing process.

Applications Current applications using halophiles include: the extraction of carotene from carotene rich halobacteria and halophilic algae that can then be used as food additives or as food-coloring agents. the use of halophilic organisms in the fermentation of soy sauce and Thai fish sauce.

Applications Many possible applications using halophiles are being explored such as: increasing crude oil extraction genetically engineering halophilic enzyme encoding DNA into crops to allow for salt tolerance treatment of waste water

Conclusions Halophiles are salt tolerant organisms. They are widespread and found in all three domains. The “salt-in” strategy uses less energy but requires intracellular adaptations. Only a few prokaryotes use it. All other halophiles use the “compatible solute” strategy that is energy expensive but does not require special adaptations.

Conclusions There are several current applications using halophiles and many that are being researched for future use.

References Oren, Aharon “Bioenergetic Aspects of Halophilism” Microbiology and Molecular Biology Reviews. Vol. 63 No. 2 pp 334-348. June 1999 Oren, Aharon. Nieto J, Joaquin. Ventosa, Antonio. “Biology of moderately Halophilic Aerobic Bacteria”. Microbiology Review and Molecular Biology Reviews. Vol. 62 No. 2. pp504-544. June 1998