Haloquadratum walsbyi

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A Microbial Biorealm page on the genus Haloquadratum walsbyi


Higher order taxa

cellular organisms; Archaea; Euryarchaeota; Halobacteria; Halobacteriales; Halobacteriaceae; Haloquadratum; Haloquadratum walsbyi

(from NCBI



Description and significance

The square halophilic archaeon Haloquadratum walsbyi was first discovered by A.E. Walsby in 1980. First found in water from a salt pool near the Red Sea, Haloquadratum walsbyi was soon found in many salt lakes around the world, making it an archaeon of specific interest because of this abundance in hypersaline ecosystems, which suggests that is it plays an important ecological role. A.E. Walsby detected Haloquadratum walsbyi by "conventional microscopy in brine samples collected from a salt crust forming the surface of a hypersaline pool on the Sinai Peninsula" (Bolhius).

The cells of "Walsby's square archaeon," which are very thin and flat and measure between 2 to 5 micrometers, have a unique square-like shape. They form sheets of 2D arrays of 10 or more cells. The cells are very fragile and when in sheets, contacts between cells are easily broken. The gas vesicles within Haloquadratum walsbyi are easily collapsed even by gentle pressure. Its cells stained gram-negative and were found to grow optimally in media with 18% salts around neutral pH.

Despite their abundance in salt lakes around the world, cultivation of Haloquadratum walsbyi has been very difficult, and it has been marked an unculturable organism. In "Isolation and cultivation of Walsby's square archaon," the first cultivation of the organism is discussed.

It is important for this organism to be studied because of its halophilicity which suggests that Haloquadratum walsbyi plays an important ecological role in high-salt content habitats. The genome sequence gives insight in the molecular ecological & physiological properties of the organism, including its square shape and its ability to survive in the hypersaline environments it is found in. Additionally, the genome sequence allows us to understand its metabolic properties which allow Haloquadratum walsbyi to live in such an environment.

Genome structure

Haloquadratum walsbyi was found to be a member of a novel genus within the family of Halobateriaceae, based on its 16S RNA sequence. The 3.1 Mb large genome has been sequenced and is discussed in "The genome of the square archaeon Haloquadratum walsbyi: life at the limits of water activity" by Bolhius et al, published July 4 2006.

The genome of H. walsbyi strain HBSQ001, DSM 16790, is comprised of a 3,132,494 bp chromosome (which has 2738 open reading frames) and a 46,867 bp plasmid (which has 39 open reading frames). THe genome has a well conserved haloarchael region, which contains the origin of replication and the cell division control protein CDC6_1.

Its genome has a GC content of 47.9%, which is remarkably low compared to the generally high GC content of other haloarchaea (usually 60-70%). This GC content is constant throughout the genome. It is thought that the lowered GC content is compensated by the large number of photolyases which help it cope with the high levels of UV irradiation in the shallow coastal lagoons it inhabits.

Notable about H.walsbyi's genome include the following: its low coding density, its expression of a water enriched capsule, the fact that it encodes two bacteriorhodopsin proteins, as well as its utilization of a dihydroxyacetone via phophoenolpyruvate dependent phosphotransferase system.

TASKS FOR THIS SECTION: Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence? Does it have any plasmids? Are they important to the organism's lifestyle?

Cell structure and metabolism

Haloquadratum walsbyi have flat cells that have a unique square-like shape, unlike the spherical or cylindrical sahpe of many organisms. Easily recognizable for its perfect square morphology, Haloquadratum walsbyi has sharp edges and acute straight corners.

Another feature is the abundance of intracellular refractile bodies, which were identified as gas vesicles. These gas vesicles were also found to be easily collapsed by gentle pressure.

PHA storage granules have also been found.

NEED TO ADD: How it gains energy, What important molecules it produces


Describe any interactions with other organisms (included eukaryotes), contributions to the environment, effect on environment, etc.


How does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used?

Current Research

Enter summaries of the most recent research here--at least three required


Bolhuis, H., Poele E.M., and Rodríguez-Valera, F. (2004) "Isolation and cultivation of Walsby's square archaeon." Environ. Microbiol. 6, 1287-1291

Bolhuis, H., Palm, P., Wende, A., Falb, M., Rampp, M., Rodriguez-Valera, F., Pfeiffer, F., and Oesterhelt, D. "The genome of the square archaeon Haloquadratum walsbyi : life at the limits of water activity." BMC Genomics (2006) 7:169.

Burns DG, Janssen PH, Itoh T, Kamekura M, Li Z, Jensen G, Rodriguez-Valera F, Bolhuis H, Dyall-Smith ML. "Haloquadratum walsbyi gen. nov., sp. nov., the square haloarchaeon of Walsby, isolated from saltern crystallizers in Australia and Spain." Int J Syst Evol Microbiol. 2007 Feb;57(Pt 2):387-92.

Legault BA, Lopez-Lopez A, Alba-Casado JC, Doolittle WF, Bolhuis H, Rodriguez-Valera F, Papke RT. Environmental genomics of "Haloquadratum walsbyi" in a saltern crystallizer indicates a large pool of accessory genes in an otherwise coherent species. BMC Genomics. 2006 Jul 4;7:171

Edited by Heidi T. Tuason, student of Rachel Larsen and Kit Pogliano