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  Picture of acridine orange stained cells of a new Haloquadratum isolate from an australian salt lake.



First discovered by Tony Walsby in 1980 in water from a sabkha (salt pool) near the Red Sea. They were soon recognised as being common in salt lakes around the world. They are commonly the dominant microbial group in salt-saturated waters, being 40-90% of the total cell count. Despite many attempts, they had never been cultivated in the laboratory until the publication by our group (see below) in FEMS Micro Lett (online 21 Aug, in print Sept. 2004). This received considerable publicity, both locally (uninews, abc-science) and internationally (Nature news, Faculty of 1000).

The cells are very thin and flat (like tiles), and form 2D arrays of 10 or more cells (picture at left). They are fragile, and their gas vesicles are easily collapsed by gentle pressure. When in sheets, cell-cell contacts are easily broken (as in the picture by Kessel and Cohen, bottom of page).

square grp
Square team: Peter Janssen, David Burns, Mike Dyall-Smith, Helen Camakaris (2004)

The isolate, given the strain designation of C23, has now been formally described, and is the type strain of a new genus, Haloquadratum. The species name, walsbyi, acknowledges Tony Walsby's original observation (Burns et al., 2007). The 16S rRNA gene sequence matches very closely to the sequence of square haloarchaea (e.g. X84084) reported by Benlloch et al. (1995), and which was used to derive specific probes for to identify square cells by in situ hybridisation ( Anton et al.,1999, 2000). I am currently involved in determining the genome sequence of the C23 strain, and comparing its sequence to the only other reported strain (HBSQ001), isolated by Henk Bolhuis and colleagues from a spanish saltern,

There are many important questions that can now be addressed. Why do they appear to be genetically so closely related compared to other organisms in the same environment? (the16S genes of the australian and spanish isolates have only 2 nt differences!). Is there any selective advantage in forming square, sheets of cells? Why are they commonly the dominant organism in salt lakes? How do they interact (metabolically and genetically) with other organisms in this extreme environment? How do viruses influence their growth and evolution?


Anton, J., E. Llobet-Brossa, F. Rodriguez-Valera, and R. Amann. 1999. Fluorescence in situ hybridization analysis of the prokaryotic community inhabiting crystallizer ponds. Environ Microbiol 1:517-23.

Anton, J., R. Rossello-Mora, F. Rodriguez-Valera, and R. Amann. 2000. Extremely halophilic bacteria in crystallizer ponds from solar salterns. Appl Environ Microbiol 66:3052-7.

Benlloch,S., Martinez-Murcia,A.J. and Rodriguez-Valera,F. Sequencing of Bactrerial and Archaeal 16S rRNA genes directly amplified from a Hypersaline Environment. Syst. Appl. Microbiol. 18, 574-581 (1995)

Burns, D.G., Camakaris, H.M., Janssen, P.H. and Dyall-Smith, M. (2004) Cultivation of Walsby’s square haloarchaeon. FEMS Microbiol. Lett. 238(2):469-473. [abstract] First available online, 21 August 2004; published Sept, 2004.

Burns, D. G., P. H. Janssen, T. Itoh, M. Kamekura, Z., Li, G. Jensen, F. E. Rodriguez-Valera, H. Bolhuis and M. L. Dyall-Smith. (2007) Haloquadratum walsbyi gen. nov., sp. nov., the square haloarchaeon of Walsby, isolated from saltern crystallizers in Australia and Spain. IJSEM 57: 387-392.

Kessel, M., and Y. Cohen. 1982. Ultrastructure of square bacteria from a brine pool in Southern Sinai. J Bacteriol 150:851-60. (picture below - click to enlarge).

EM Kessel

Walsby, A. E. 1980. A square bacterium. Nature (London) 283:69.








2008: Since leaving the University of Melbourne, I am now in the Oesterhelt laboratory, studying the genomics of the square haloarchaeon Haloquadratum walsbyi C23T.

May '08: Version 7 of the Halohandbook released.

Mar '08: Sadly, Dick Shand passed away after a long battle with cancer. Well known for his work on haloarchaea, particularly halocins. He will be greatly missed.

2007: The taxonomic description of the square haloarchaea of Walsby, Haloquadratum walsbyi, has been published in IJSEM. The type species, strain C23, was isolated in my laboratory.