Haloquadratum walsbyi gen. nov., sp. nov., the square haloarchaeon of Walsby, isolated from saltern crystallizers in Australia and Spain.

Strains C23T and HBSQ001 were isolated from solar salterns and are novel square-shaped, aerobic, extremely halophilic members of the domain Archaea and family Halobacteriaceae. Cells stained Gram-negative and grew optimally in media containing 18 % salts at around neutral pH. Mg2+ is not required. The DNA G+C content of both isolates was 46.9 mol% and DNA-DNA cross-hybridization showed a relatedness of 80 %. Their 16S rRNA gene sequences showed only 2 nucleotide differences (99.9 % identity) and phylogenetic tree reconstructions with other recognized members of the Halobacteriaceae indicated that they formed a distinct clade, with the closest relative being Halogeometricum borinquense PR 3T (91.2 % sequence identity). The major polar glycolipid of both isolates was the sulfated diglycosyl diether lipid S-DGD-1. Electron cryomicrosopy of whole cells revealed similar internal structures, such as gas vesicles and polyhydroxyalkanoate granules, but the cell wall of isolate HBSQ001 displayed a more complex S-layer compared with that of isolate C23T. The phenotypic characterization and phylogenetic data support the placement of isolates C23T and HBSQ001 in a novel species in a new genus within the Halobacteriaceae, for which we propose the name Haloquadratum walsbyi gen. nov., sp. nov. The type strain of Haloquadratum walsbyi is C23T (=JCM 12705T=DSM 16854T).