Humans from time to time have thirsted and drunk salty waters. But real salty water (e.g. sea or ocean water) is not tolerated for ingestion and provides no real nourishment for humans – only more thirst. Drinking sea water soon leads to illness and death of mammals. Yet, despite the inability of almost all higher forms of plant and animal life to tolerate hyperosmotic conditions, there are bacteria – extremophilic bacteria termed "halophiles" – that survive and thrive in very salty or hyperosmotic conditions.
Where Are Halophiles Found – Salty Waters, Soils and Foods
"Halophilism" is an unusual life existence of salty lakes, soils and some salted foods. Microbiologists and other scientists who investigate the Dead Sea, Utah's Great Salt Lake and related salty water regions have reported that certain types of bacteria – primitive and the lowest of the free-living life forms – live and thrive in very salty waters. Halophiles can be isolated from salty places all over the planet – saline water and soils and salted foods.
The Great Salt Lake is divided by a 30-foot railway causeway into a Northern and a Southern region. The South Arm of the Great Salt Lake in Utah is about 13% salinity and the salt concentration fluctuates broadly based on fresh water conditions. Fresh water comes to the South Arm from rain, groundwater springs and three freshwater rivers: Bear, Weber and Jordan. The North arm does not receive large amounts of freshwater and is 2X saltier (26-28%) than the South Arm. This concentration of salt causes salt crystals to develop and precipitate out.
Halophiles Halobacterium and Halococcus live and multiply within these highly salty waters of the Great Salt Lake and similar sites and they impart a purple to red coloration to the waters there (see photos 2 and 3 below. Click to enlarge). These halophiles can multiply and grow into dense populations of as many as1,000,000 (1 million) to 100,000,000 (100 million) bacteria per milliliter!
Halophiles, special bacteria, are members of the ancient bacterial kingdom named "Archaea". Dr. Thomas D. Brock who was a pioneer in thermophilic bacterial research helped to define microbial life in weird, strange and difficult environments – the world of extremophiles.
How do Halophilic Bacteria Survive High Salt and Hyperosmotic Conditions
In the early history of biology scientists noted that unless the water conditions or osmolarity was right many organisms do not live. Thus, salt water is too salty for many fresh-water creatures and fresh-water is too salty for many salt-water creatures. Halophiles love salt water and high osmolarity. Consider these terms that are important to understanding solutions and osmotic conditions and cells:
- solvent – a liquid into which compounds or chemicals can dissolve. Water is the "universal solvent" that dissolves salts and sugars.
- solute - a chemical compound or substance that dissolves in water.
- solution – a combination of dissolved solute and solvent. Examples: salt water and sugar water.
- isoosmotic (isotonic) – a condition where the salt or dissolved chemicals are balanced correctly on each side of a selectively-permeable membrane. Iso-osmolarity or iso-osmolar conditions means that the water flow rates are equal from one side of a selectively-permeable membrane to the other.
- hypoosmotic (hypotonic) – a condition where there is less salt and hypoosmotic water flow is toward the more saline "hyper-" side of the membrane.
- hyperosmotic (hypertonic) - condition where there is more salt in a solution than typical and water flows from the hypoosmotic side to the hyperosmotic solution. Hypertonic or hyperosmotic solutions draw water into them.
- proteins are part of the structure of all living membranes. Some proteins are important catalysts termed enzymes that enable biochemical reactions. When cell membranes proteins, enzymes or other cell protein structures are damaged, in one or more severe or critical ways, cells die. Therefore, it was suspected that halophiles have unique proteins, genes and natural chemicals that protect and facilitate halophilic life in one or more ways. This has proven true.
Halophiles and Halophilic Gene and Protein Characterization – How Halophiles Survive
Biochemists and microbiologists have uncovered some secrets of the halophiles. In 2008, Paul et al. from India compared the genomes (genes) and proteomes (protein patterns) of halophiles and non-halophiles. Proteins of halophilic species have:
- low hydrophobicity, i.e. they are more hydrophilic than hydrophobic
- lots of acidic amino acids – especially Asp (aspartic acid); under-representation of Cys (cystine)
- limited helix formation and higher occurrence of coiled structures
At the DNA level, the dinucleotide abundance profiles of halophilic genomes have some distinctive and common characteristics, i.e., specific DNA salt-adaptation signatures.
Thus, halophiles have special membrane proteins and internal proteins that enable them to survive salty environments. Halophiles exist in less free water environments that any human ever thought possible.
Rossi et al. note some important ideas about "extremozymes" – the enzymes that are isolated from extremophiles. Extremozymes are heat-stabilized by
- "ion pair formation, hydrogen bonds, and hydration, rather than hydrophobic interactions"
- "salt bridges and ion binding" These findings were based on a "...comparison of malate dehydrogenases from extremophiles (halophiles, thermophiles, and psychrophiles) and mesophiles by neutron scattering experiments."
Simply remember that halophiles have special membrane and internal proteins that enable their survival in salty environments.
Halophiles and Halophilic Osmoprotectant Characterization – How Halophiles Survive
Osmoprotectans (osmoprotectants) have been discovered in moderate halophiles. Osmoprotectans such as glycine betaine, ectoine and hydroxyectoine in halophiles occur in high cytoplasmic concentrations. These fairly simple, low-molecular-weight organic compounds overcome cellular osmotic stresses and maintain positive, cellular osmotic pressure. Both ectoine and hydroxyectoine also protect certain enzymes from freeze-thaw damage – another very useful protectant effect. Moderate halophiles serve as a resource for both osmotic and freeze-thaw stress protectants for industrial and research applications.
In summary, halophiles have special, osmoprotective biochemicals that enable them to survive the osmotic stress of high salt.
Unique Halophiles are Common in Salty Environments
Arsenophiles isolated from Mono Lake in California substitute arsenic for phosphorous in critical energy transfer molecules. Mono Lake is also very alkaline and has a high salt concentration. Extremophiles are very resilient microbes – they are unlike any other life known on the planet. Halophiles interest scientists, especially outer space scientists, who believe that such forms – which are unlike any other life known on this planet – may thrive on other planets somewhere in this universe.
Sources
Baldasseroni F and S. Pascarella. 2009. Abstract. "Subunit interfaces of oligomeric hyperthermophilic enzymes display enhanced compactness". Int J Biol Macromol. 44(4):353-60. Accessed 14 December, 2010 @ ncbi.nlm.nih.gov/pubmed
Fergus, C. 2007. "Discovering a New Life Form in the Hot Springs of Yellowstone." Astrobiology. Accessed 8 December, 2010 @ astrobiology.nasa.gov
Paul, S. et al. 2008. "Molecular signature of hypersaline adaptation: insights from genome and proteome composition of halophilic prokaryotes." Genome Biol. 9(4):R70. Accessed 22 December, 2010 @ ncbi.nlm.nih.gov/pubmed
Rossi, M. et al., 2003. "Extremophiles 2002." J Bacteriol. 2003 July; 185(13): 3683–3689. Accessed 8 December, 2010 @ jb.asm.org
Ventosa, A. et al., 1998. "Biology of Moderately Halophilic Aerobic Bacteria." Microbiol Mol Biol Rev. 62(2): 504–544. Accessed 22 December, 2010 @ ncbi.nlm.nih.gov
Donald Reinhardt - Think, read, write & live well always. DJR has a PhD in Biology/Microbiology & is a Fellow & Diplomate, ASM Amer Acad Micro.
Join the Conversation