Burns et al. 2007
Haloquadratum ("salt square") is a genus of the family Halobacteriaceae. The first species to be identified in this group, Haloquadratum walsbyi, is highly unusual since its cells are shaped like flat, square boxes.
First discovered in 1980 by A.E. Walsby in the Gavish Sabkha, a coastal brine pool on the Sinai Peninsula in Egypt, this archaeon was not cultured until 2004. While attempting to culture Haloquadratum walsbyi, researchers isolated Haloarcula quadrata, which has "predominantly square-shaped, somewhat pleomorphic, flat cells", but this organism is genetically quite different from Haloquadratum, belongs to a separate genus, and is not a dominant microbe in salt lakes.
Haloquadratum are remarkable for their regular square-shaped cells and relative abundance in halophilic environments.
The cells typically contain polyhydroxyalkanoate (PHA) granules as well as large numbers of refractile gas-filled vacuoles which provide buoyancy in a watery environment and may help to position the cells to maximise light-harvesting. The cells may join with others to form fragile sheets as extensive as 40 micrometres.
Haloquadratum walsbyi can be found anywhere in hypersaline waters. When sea water evaporates, high concentration and precipitation of calcium carbonate and calcium sulphate result, leading to a hypersaline sodium chloride-rich brine. Further evaporation results in the precipitation of sodium chloride or halite, and then to a concentrated magnesium chloride brine termed bittern. During the final stage of halite formation, before magnesium chloride concentration causes the brine to become sterile, Hqr. walsbyi flourishes and can make up 80% of the brine's biomass.
Description and significance
The archaeon Haloquadratum walsbyi ("salt squares of Walsby") was first discovered in 1980 by A.E. Walsby in the Gavish Sabkha, a coastal brine pool in the Sinai peninsula, Egypt, and formally described by Burns et al. in 2007. The "square haloarchaea of Walsby" were notable because of their extremely thin (around 0.15 μm), square-shaped structure. While this archaeon was discovered in 1980, it was not cultivated in the lab until 2004.
The mapping of Haloquadratum walsbyi's genome has been completed, giving a better understanding of the organism's genealogy and taxonomy, and the role it plays in the ecosystem. A genomic comparison of the Spanish and Australian isolates (strains HBSQ001 and C23T) strongly suggests a rapid global dispersion, as they are so similar and have retained gene order (synteny).
Cell structure and metabolism
Haloquadratum walsbyi has a unique cell shape – that of an extremely thin square, its best-known characteristic. The cells possess an abundance of intracellular refractile bodies known as gas vacuoles – vacuoles filled with gas which provide buoyancy – maintaining upper position in the water column. Individual square cells are joined with others to form large sheets, sometimes as large as 40 µm. These sheets are extremely fragile and the connections between the cells are easily broken.
Its mode of metabolism is not completely known; complete genetic information will give researchers necessary insight.
Haloquadratum walsbyi was first noticed and taken from saline pools in Egypt, but it can be found in hypersaline bodies of water all over the world. Evaporation of water in these pools leaves high concentrations of salt, making for Haloquadratum walsbyi's optimal growth environment. According to Bolhuis et al., "In this sense, they are the most hyperhalophilic organisms known, as further concentration of the magnesium salts (bitterns) leads to sterility of the brines." Their precise role in the ecosystem is not known for sure, but because of its unique morphology learning more about it will surely provide some information on the evolution and morphological adaptation of archaeans.
- See the NCBI webpage on Haloquadratum. Data extracted from the "NCBI taxonomy resources". National Center for Biotechnology Information. Retrieved 2007-03-19.
- Stoeckenius W (1 October 1981). "Walsby's square bacterium: fine structure of an orthogonal procaryote". J. Bacteriol. 148 (1): 352–60. PMC 216199. PMID 7287626.
- Oren, A.; Ventosa, A.; Gutierrez, M. C.; Kamekura, M. (1999). "Haloarcula quadrata sp. nov., a square, motile archaeon isolated from a brine pool in Sinai (Egypt)". International Journal of Systematic Bacteriology 49 (3): 1149. doi:10.1099/00207713-49-3-1149.
- Oren A, Pri-El N, Shapiro O, Siboni N (2006). "Buoyancy studies in natural communities of square gas-vacuolate archaea in saltern crystallizer ponds". Saline Systems 2: 4. doi:10.1186/1746-1448-2-4. PMC 1459177. PMID 16613609.
- Burns, D. G.; Janssen, P. H.; Itoh, T.; Kamekura, M.; Li, Z.; Jensen, G.; Rodriguez-Valera, F.; Bolhuis, H.; Dyall-Smith, M. L. (2007). "Haloquadratum walsbyi gen. nov., sp. nov., the square haloarchaeon of Walsby, isolated from saltern crystallizers in Australia and Spain". International Journal of Systematic and Evolutionary Microbiology 57 (2): 387. doi:10.1099/ijs.0.64690-0.
- J. Craig Venter Institute
- Dyall-Smith, M. L.; Pfeiffer, F.; Klee, K.; Palm, P.; Gross, K.; Schuster, S. C.; Rampp, M.; Oesterhelt, D. (2011). "Haloquadratum walsbyi : Limited Diversity in a Global Pond". In Lopez-Garcia, Purification. PLoS ONE 6 (6): e20968. doi:10.1371/journal.pone.0020968. PMC 3119063. PMID 21701686.
- Burns DG, Camakaris HM, Janssen PH, Dyall-Smith ML (2004). "Cultivation of Walsby's square haloarchaeon". FEMS Microbiol. Lett. 238 (2): 469–73. doi:10.1016/j.femsle.2004.08.016. PMID 15358434.
- Oren A, Ventosa A (2000). "International Committee on Systematic Bacteriology Subcommittee on the taxonomy of Halobacteriaceae. Minutes of the meetings, 16 August 1999, Sydney, Australia". Int. J. Syst. Evol. Microbiol. 50: 1405–7. PMID 10843089.
- Gibbons, NE (1974). "Family V. Halobacteriaceae fam. nov.". In RE Buchanan and NE Gibbons, eds. Bergey's Manual of Determinative Bacteriology (8th ed.). Baltimore: The Williams & Wilkins Co. ISBN 0-683-01117-0.