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Isolation and cultivation of Walsby's square archaeon

Authors

  • Henk Bolhuis,

    Corresponding author
    1. Department of Microbial Ecology, Centre of Ecological and Evolutionary Studies, University of Groningen, Kerklaan 30, 9751 NN Haren, the Netherlands.
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  • Evelien M. te Poele,

    1. Department of Microbial Ecology, Centre of Ecological and Evolutionary Studies, University of Groningen, Kerklaan 30, 9751 NN Haren, the Netherlands.
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  • Francisco Rodriguez-Valera

    1. Division de Microbiología and Evolutionary Genomics Group, Universidad Miguel Hernandez, Campus de San Juan, 03550 San Juan de Alicante, Spain.
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*E-mail h.bolhuis@biol.rug.nl; Tel. (+31) 5036 32162; Fax (+31) 5036 32154.

Summary

In 1980, A. E. Walsby described a square halophilic archaeon. This archaeon is of specific interest because of its unique shape and its abundance in hypersaline ecosystems, which suggests an important ecophysiological role. Ever since its discovery, the isolation and cultivation of ‘Walsby's square archaeon’ has been a holy grail for many microbiologists working on halophiles. Despite their abundance and easy recognition by microscopy, all cultivation attempts have failed up to now, marking the organism as one of the unculturables. Cultivation of the square archaeon is essential to understand their ecophysiological role and the nature of their unique morphologically features. Here, we report the isolation and cultivation of the enigmatic square archaeon that we propose to name Haloquadratum walsbyi. Pure cultures are easily maintained in simple artificial hypersaline media. Initial growth experiments revealed a tolerance to high concentrations of MgCl2 (>2 M) in the presence of 3.3 M NaCl. Fresh cultures contained extraordinary large cells (>40 × 40 µm) without any visible division structures, ranking them among the largest prokaryotes known to date. The genome was estimated to contain approximately three million basepairs.

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