Halophilic extremophiles, or simply halophiles, are a group of microorganisms that can grow and often thrive in areas of high salt (NaCl) concentration. These hypersaline areas can range from the salinity equivalent to that of the ocean (~3-5%), up to ten times that, such as in the Dead Sea (31.5% average 3). Halophiles have been found belonging to each domain of life but primarily consist of archaea.14 They are metabolically diverse, ranging from simple fermenters to iron reducers and sulfide oxidizers.
Naturally-Occurring Hypersaline Environments
Hypersaline environments are present on each continent and are primarily found in “arid and semi-arid regions.”17 Some are commonly known, like Utah’s Great Salt Lake, or the Dead Sea between Israel and Jordan. Others are less well known, such as Antarctica’s Deep Lake or Papua New Guinea’s undersea geothermal vents.6 19
Constructed Hypersaline Environments
Anthropogenic hypersaline environments are commonly created by the salt industry. Salterns are large ponds that are filled with saltwater from the ocean or another source that are then evaporated away. In this process, the salinity of the water gradually increases as water evaporates until it reaches saturation (~26% at 20°C). The salt then precipitates out and is harvested. Halophiles take advantage of this environment and often their presence becomes visible due to pigments they produce.
|Slight Halophile||Moderate Halophile||Extreme Halophile|
|Percent Salt||2 - 5%||5 - 20%||20 - 30%|
|Molarity||0.34 - 0.85 M||0.85 - 3.4 M||3.4 - 5.1 M|
Often in hypersaline environments, salts other than NaCl are also present. A salt profile for the Dead Sea is shown below. However, because halophiles are defined in relation to NaCl concentration, other salt content is not considered for halophilic classification.
|Salt Profile of the Dead Sea 3|
|Percent Total Salts||51.26%||28.19%||13.64%||4.57%||2.17%||0.10%||0.03%|
|Salinity and Temperature Measurements for Saline Environments.3 6|
|Pacific Ocean||Deep Lake, Antarctica||Great Salt Lake, USA||Dead Sea, Israel/Jordan|
|NaCl Percent (g/L)||3.4 - 3.7%||21 - 28%||12 - 33%||31.5 - 34.2%|
|Temperature (C)||1.4 - 30 C||0 - 11.5 C||-5 - 35 C||21 - 36 C|
Often in hypersaline environments, the salinity is just one extreme microbes must overcome. Hypersalinity often co-occurs with extreme temperature conditions, both hot and cold.
In the case of Deep Lake, Antarctica, extreme cold and high salinity meet. Deep Lake has salinity levels ranging from 21-28%, putting the halophiles present in the extreme halophile classification. Psychrophiles are “organisms having an optimal temperature for growth at about 15°C or lower, a maximal temperature for growth at about 20°C, and a minimal temperature for growth at 0°C or below”.11 In the case of Deep Lake, eight months of the year are spent below 0°C, with a yearly maximum temperature of only 11.5°C.6 Organisms that survive and thrive here are not only halophiles, but also psychrophiles. Similar overlaps of cold and hypersaline environments have been found in other places as well.
In the case of the black smokers off the coast of Papua New Guinea, a hypersaline environment was created by a hydrothermal vent. This hydrothermal vent releases a hypersaline effluent that was measured at greater than 250°C. Samples taken from the internal walls of the smoker contained Halomona and Haloarcula species.19 This classifies Halomona and Haloarcula as both thermophiles and halophiles. Such overlapping conditions of hypersalinity and extreme heat are also present in some hot springs and other hydrothermal vents.
Some hypersaline environments have been found that overlap with other extremes, such as low and high pH, and dry, desiccating conditions. Organisms in such conditions would be considered haloacidophiles, haloalkaliphiles, and haloxerophiles, respectively.
The majority of extreme halophiles are archaea 14. A thorough study of salt-crystallizing ponds from several places around the world by Oren in 2002 showed consistent communities between saltern ponds. The genera isolated were primarily Haloferax, Halogeometricum, Halococcus, Haloterrigena, Halorubrum, Haloarcula, Halobacterium, and Natronococcus. At lower degrees of salinity, diversity of genera present increases.
Bacteria belonging to the genus Salinibacter have been found to be “no less salt-dependent and salt-tolerant than the most halophilic among the Archaea…” 14 As such, within specific extreme environments, some bacteria “may coexist with the halophilic archaeal community .”14 As with archaea, bacterial diversity also increases as salinity decreases.
At the extreme end of hypersalinity, eukarya are absent. However, there are a few moderately or slightly halophilic eukaryotes that can contribute to the halophilic communities. There are also types of halophilic fungi, such as Debaryomyces hansenii and Hortaea werneckii .47
According to Oren in 2002, “The metabolic diversity of halophiles is great as well: they include oxygenic and anoxygenic phototrophs, aerobic heterotrophs, fermenters, denitrifiers, sulfate reducers, and methanogens.” Additionally, halophilic sulfide oxidizers, iron-reducers, and acetogens have also been discovered.202123 Halophilic bacteria have been found to perform fermentation, acetogenesis, sulfate reduction, phototrophy, and methanogenesis.13
Mechanisms for Saline Resistance
One mechanism halophiles use to survive in high concentrations of salt is the synthesis of osmoprotectants, which are also known as compatible solutes. These work by balancing the internal osmotic pressure with the external osmotic pressure, making the two solutions isotonic, or close to it. Compatible solutes are small-molecular weight molecules. “Some are widespread in microorganisms, namely trehalose, glycine betaine and α-glutamate, while others are restricted to a few organisms.”16 The use of compatible solutes is the most common mechanism of salt resistance among halophiles.
A second, less common mechanism of defense against salt is through controlling potassium levels. This mechanism is performed by pumping in large amounts of K+ ions into the cytoplasm. To deal with the increased solute level, and to increase the solubility of the proteins within the cytoplasm, many proteins have evolved to become more acidic.16
Diversity and Systematics Research
A large number of research projects have been focused on finding and classifying novel halophiles.
A paper by Ndwigah in 2016 isolated a halophilic fungus, determining its optimal growth conditions and its metabolites.12
McDuff in 2016 characterized several CO-oxidizing halophiles. This paper also provided a novel species description of Haloferax namakaokahaiae.10
Amoozegar et al. provided a novel species description of a halophilic bacterium, Oceanobacillus halophilus. This bacterium was isolated from a brine lake in Iran.1
Because of the unique conditions to which the proteins of halophiles are subjected, their proteins are heavily studied, especially for their use in industry.
Varun in 2009 looked at the possibility of using a specific halophile to generate electricity. This species under its normal growth conditions fermented glycerol to ethanol, and produced electricity through iron reduction. According to the paper, “Glycerol is a major byproduct of biodiesel industry and therefore bacterial fermentation of this glycerol to ethanol would help to manage the waste as well as produce a value-added product.” This species could be potentially used in microbial fuel cells.21
A paper by Calegari-Santos in 2016 also looked at using halophiles in industry to produce carotenoids. These carotenoids have numerous “potential uses in Biotechnology and Biomedicine.”5
The ubiquity of viruses does not stop when the environment gets salty. Viral infection in hypersaline environments is also being actively investigated.
Atanasova in 2016 investigated how haloviruses and their hosts interact on an ecology level, and gave general lifestyle descriptions of numerous haloviruses. 2
Pietila in 2016 worked on the classification of haloviruses, proposing a new family for “archaeal pleomorphic viruses with single-stranded or double-stranded DNA genomes.”15
1. Amoozegar, M. A., M. Bagheri, A. Makhdoumi, M. M. Nikou, S. A S Fazeli, P. Schumann, C. Sproder, C. Sanchez-Porro, and A. Ventosa. "Oeanobacilllus Halophilus Sp. Nov., a Novel Moderately Halophilic Bacterium from a Hypersaline Lake." International Journal of Systematic and Evolutionary Microbiology 66 (2016): 1317-322. Microbiology Society Online. Web. 28 Mar. 2016.1
2. Atanasova, Nina S., Dennis H. Bamford, and Hanna M. Oksanen. "Virus-Host Interplay in High Salt Environments." Wiley-Blackwell and Society for Applied Microbiology (2016): n. pag. Wiley Online Library. Web. 28 Mar. 2016.2
3. Bentor, Yaacov K. "Some Geochemical Aspects of the Dead Sea and the Question of Its Age." Geochemica Et Cosmochimica 25 (1961): 239-60. Elsevier. Web. 28 Mar. 2016.3
4. Butinar, Lorena, Silva Sonjak, Polona Zalar, Ana Plemenitas, and Nina Gunde-Cimerman. "Melanized Halophilic Fungi Are Eukaryotic Members of Microbial Communities in Hypersaline Waters of Solar Salterns." Botanica Marina 48 (2005): 73-79. Degruyter. Web. 28 Mar. 2016.4
5. Calegari-Santos, Rossana, Ricardo Alexandre Diogo, Jose Domingos Fontana, and Tania Maria Bordin Bonfim. "Carotenoid Production by Halophilic Archaea under Different Tissue Culture Conditions." Current Microbiology (2016): 1-11. Springer. Web. 28 Mar. 2016. 5
6. Franzmann, P. D., E. Stackebrandt, K. Sanderson, J. K. Volkman, D. E. Cameron, P. L. Stevenson, T. A. Mcmeekin, and H. R. Burton. "Halobacterium Lacusprofundi Sp. Nov., a Halophilic Bacterium Isolated from Deep Lake, Antarctica." Systematic and Applied Microbiology 11.1 (1988): 20-27.Elsevier. Web. 27 Mar. 2016.6
7. Gunde-Cimerman, Nina, Jose Ramos, and Ana Plemenitas. "Halotolerant and Halophilic Fungi." Mycological Research 113.11 (2009): 1231-241.Elsevier. Web. 24 Apr. 2016.7
8. Hochstein, Lawrence I., and Frank Lang. "Purification and Properties of a Dissimilatory Nitrate Reductase from Haloferax Denitrificans." Archives of Biochemistry and Biophysics 288.2 (1991): 380-85. Elsevier. Web. 28 Mar. 2016.8
9. Khanafari, A., D. Khavarinejad, and A. Mashinchian. "Solar Salt Lake as Natural Environmental Source for Extraction Halophilic Pigments."Iranian Journal of Microbiology 2.2 (2010): 103-09. NCBI. Web. 25 Apr. 2016.9
10. McDuff, S., G. M. King, S. Neupane, and M. R. Meyers. "Isolation and Characterization of Extremely Halophilic CO-oxidizing Euryarchaeota from Hypersaline Cinders, Sediments and Soils Description of a Novel CO Oxidizer, Haloferax Namakaokahaiae Mke2.3, Sp. Nov." Microbiology Ecology(2016): n. pag. FEMS Microbiology Ecology. Web. 28 Mar. 2016. 10
11. Morita, Richard Y. "Psychrophilic Bacteria." Bacteriological Reviews 39.2 (1975): 144-67. National Center for Biotechnology Information. Web. 28 Mar. 2016.11
12. Ndwigah, F. I., I. H. Boga, W. Wanyoike, and R. Kachiuri. "An Aspergillus Isolate and Its Secondary Metabolites from Lake Elmentaita in Kenya." Journal of Agriculture, Science and Technology 17.1 (2016): 28-41. Journal of Agriculture, Science and Technology. Web. 28 Mar. 2016. 12
13. Olliveier, Bernard, Pierre Caumette, Jean-Louis Garcia, and Robert A. Mah. "Anaerobic Bacteria from Hypersaline Environments." Microbiological Reviews 58.1 (1994): 27-38. American Society for Microbiology. Web. 28 Mar. 2016.13
14. Oren, Aharon. "Molecular Ecology of Extremely Halophilic Archaea and Bacteria." FEMS Microbiology Ecology 39 (2002): 1-7. Elsevier. Web. 27 Mar. 2016.14
15. Pietila, Maija K., Elina Roine, Ana Sencilo, Dennis H. Bamford, and Hanna M. Oksanen. "Pleolipoviridae, a Newly Proposed Family Comprising Archaeal Pleomorphic Viruses with Single-stranded or Double-stranded DNA Genomes." Archives of Virology 161.1 (2016): 249-56. Springer. Web. 28 Mar. 2016.15
16. Santos, Helena, and Milton S. Da Costa. "Compatible Solutes of Organisms That Live in Hot Saline Environments." Environmental Microbiology 4.9 (2002): 501-09. Wiley Online Library. Web. 28 Mar. 2016.16
17. Shadrin, N. V. "The Crimean Hypersaline Lakes: Towards Development of Scientific Basis of Integrated Sustainable Management." Proceedings of the 13th World Lake Conference (2009): n. pag. Web. 27 Mar. 2016.17
18. Stan-Lotter, Helga, and Sergiu Fendrihan. "Halophilic Archaea: Life with Desiccation, Radiation and Oligotrophy over Geological Times." Life 5.3 (2015): 1487-496. MDPI. Web. 28 Mar. 2016.18
19. Takai, Ken, Tetsushi Komatsu, Fumio Inagaki, and Koki Horikoshi. "Distribution of Archaea in a Black Smoker Chimney Structure." Applied and Environmental Microbiology 67.8 (2001): 3618-629. American Society for Microbiology. Web. 27 Mar. 2016.19
20. Vannini, Claudia, Guilio Munz, Gualtiero Mori, Claudio Lubello, Franco Verni, and Guilio Petroni. "Sulphide Oxidation to Elemental Sulphur in a Membrane Bioreactor: Performance and Characterization of the Selected Microbial Sulphur-oxidizing Community." Systematic and Applied Microbiology 31.1 (2008): 461-73. Elsevier. Web. 28 Mar. 2016.20
21. Varun, Paul. Electricity Generation and Ethanol Production Using Iron-reducing, Haloalkaliphilic Bacteria. Thesis. Missiouri University of Science and Technology, 2009. N.p.: Missouri U of Science and Technology, 2009.21
22. Zhilina, Tatjana N., and Margarete A. Pusheva. "Halophilic Acetogenic Bacteria." Acetogenesis. By George A. Zavarzin. N.p.: Springer, n.d. 432-44. Acetogenesis. Springer. Web. 28 Mar. 2016.22
Edited by Dallas Dominguez, a student of Mary Beth Leigh at the University of Alaska Fairbanks. Template adapted from one used by Angela Kent University of Illinois at Urbana-Champaign.