Halophiles

Halophiles are salt‐loving organisms that flourish in saline environments and can be classified as slightly, moderately or extremely halophilic, depending on their requirement for sodium chloride. Although most marine organisms are slight halophiles, moderate and extreme halophiles are generally more specialised microbes, which inhabit hypersaline environments with salinity higher than in the sea. Hypersaline environments are found all over the world, in arid, coastal, and deep‐sea locations, underground salt mines, and artificial salterns. Halophilic microorganisms include a variety of heterotrophic, phototrophic, and methanogenic archaea, photosynthetic, lithotrophic, and heterotrophic bacteria, and photosynthetic and heterotrophic eukaryotes. Examples of well‐adapted and widely distributed extremely halophilic microorganisms include archaea for example, Halobacterium sp. NRC‐1, cyanobacteria such as Aphanothece halophytica, and the green alga Dunaliella salina. Multicellular halophilic eukaryotic organisms include brine shrimp and the larvae of brine flies. Halophilic organisms either accumulate internal organic compatible solutes to balance the osmotic stress of the environment or produce acidic proteins to increase solvation and improve function in high salinity.

Key Concepts:

  • Halophiles are salt‐loving organisms that inhabit saline and hypersaline environments and include prokaryotic (archaeal and bacterial) and eukaryotic organisms.
  • Many halophiles accumulate compatible solutes in cells to balance the osmotic stress in their environment.
  • Some halophiles produce acidic proteins that are able to function in high salinity by increasing solvation and preventing aggregation, precipitation and denaturation.

Keywords: archaea; Artemia; biotechnology; Dunaliella; Halobacterium; hypersaline environments; microbial diversity; microbial mat; osmotic protection; salt resistance

Figure 1. Bloom of halophilic microorganisms. Dense growth of halophilic microorganisms in hypersaline environments leads to reddening of the brine. Photo Dr. S. DasSarma.
Figure 2. Salt‐tolerance of halophilic organisms. Relative growth rate is plotted against both per cent salinity and NaCl concentration. The five microorganisms depicted are Synechococcus sp. PCC7002 (PR‐6), a slightly halotolerant cyanobacterium, Fabrea salina (Fs), a moderately halophilic protozoan, Dunaliella salina (Ds), a halophilic green algae, Aphanothece halophytica (Ah), an extremely halophilic cyanobacterium and Halobacterium sp. (H), an extremely halophilic archaeon. The salinity of seawater and the hatch range for brine shrimp are noted.
Figure 3. Structure of a hypersaline microbial mat. Adapted from Caumette (1993), with permission from Springer.
Figure 4. Integrated view of the biology of the extremely halophilic archaeon Halobacterium derived from its genome sequence. Many informational and operational processes revealed from the genome sequence are shown. Transporters in the membrane are highlighted, including light‐driven proton and chloride pumps, bacteriorhodopsin (BR) and halorhodopsin (HR), and the sodium/proton antiporter (NhaC), potassium uniporter (TrkAH and KdpABC), dipeptide and amino acid transporters and anion transporters (Ng et al., 2000. © 2000 National Academy of Sciences, USA.)
Figure 5. Extremely halophilic archaeal (a) and human (b) protein‐DNA complexes. Models of similar transcription initiation complexes, showing the protein surface charges (red for acidic or negative and blue for basic or positive), surrounding the DNA double helix. The haloarchaeal proteins are acidic whereas the human proteins are basic. Adapted from DasSarma et al. (2006) © S. DasSarma.
Figure 6. Structure of three common compatible solutes in halophiles. Zwitterionic forms of glycine betaine and ectoine, and the neutral glycerol are commonly found in halophilic microorganisms and help to balance the osmotic stress of the environment.
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 Further Reading
    book DasSarma S, Fleischmann EM, Robb FT et al. (eds) (1995) Archaea, A Laboratory Manual – Halophiles. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.
    book Gunde‐Cimerman N, Oren A and Plemenitaš A (eds) (2005) Adaptation to Life at High Salt Concentrations in Archaea, Bacteria, and Eukarya. Dordrecht, Netherlands: Springer.
    book Vreeland RH and Hochstein LI (eds) (1993) The Biology of Halophilic Bacteria. Boca Raton, FL: CRC Press, Inc.
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DasSarma, Shiladitya, and DasSarma, Priya(Mar 2012) Halophiles. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000394.pub3]