Halophiles

Abstract

Halophiles, salt‐loving organisms that flourish in saline environments, are classified as slight, moderate or extreme, depending on their requirement for sodium chloride. While most marine organisms are slight halophiles, moderate and extreme halophiles are generally more specialised microbes inhabiting hypersaline environments found all over the world in arid, coastal and deep‐sea locations, underground salt mines and artificial salterns. Halophilic microorganisms include heterotrophic, phototrophic and methanogenic archaea, photosynthetic, lithotrophic and heterotrophic bacteria and photosynthetic and heterotrophic eukaryotes. Examples of extremely halophilic microorganisms include Halobacterium sp. NRC‐1, an archaeon; Aphanothece halophytica, a cyanobacterium; and Dunaliella salina, a green alga. Common multicellular halophilic eukaryotes include brine shrimp and brine fly larvae that serve as an important food source for birds. In order to balance the osmotic stress of hypersaline environments, halophilic microorganisms either accumulate organic compatible solutes internally, produce acidic proteins to increase solvation and improve function in high salinity, or use a combination of strategies.

Key Concepts

  • Halophiles are salt‐loving organisms that inhabit saline and hypersaline environments and include prokaryotic (archaeal and bacterial) and eukaryotic organisms.
  • Halophiles may be classified as slight, moderate or extreme, and as obligate halophiles or halotolerant.
  • Many halophiles accumulate compatible solutes in cells to balance the osmotic stress in their environment.
  • Some halophiles produce acidic proteins that function in high salinity by increasing solvation and prevent protein aggregation, precipitation and denaturation.
  • Halophiles and their biomolecules are useful for applications in biotechnology, medicine and industry.

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

Figure 1. Laguna Colorada, Bolivia. Dense growth of halophilic microorganisms in hypersaline environments leads to reddening of the brine. Feeding birds are visible. Photo Dr Daniel Guzmán.
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 with permission from Caumette 1993 © Springer.
Figure 4. Common halophiles in hypersaline environments: (a) Aphanothece halophytica; (b) Dunaliella salina; (c) Halobacterium sp. NRC‐1; (d) brine shrimp and eggs. Scale bar: 30 µm for a–c, 10 mm for d. Photos Priya DasSarma (a–c) and Al Hartmann, Salt Lake Tribune (d).
Figure 5. Integrated view of the biology of the extremely halophilic archaeon Halobacterium sp. NRC‐1 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. Reproduced from Ng et al. 2000 © 2000 National Academy of Sciences, USA.
Figure 6. Extremely halophilic archaeal (a) and human (b) protein–DNA complexes. The models are for TBP–TFB 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 7. 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

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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(May 2017) Halophiles. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000394.pub4]