Isolation and cultivation of Walsby's square archaeon

Abstract

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 MgCl(2) (>2 M) in the presence of 3.3 M NaCl. Fresh cultures contained extraordinary large cells (>40 x 40 microm) without any visible division structures, ranking them among the largest prokaryotes known to date. The genome was estimated to contain approximately three million basepairs.

Seasonal fluctuations in ionic concentrations drive microbial succession in a hypersaline lake community

“…Although no independent evidence is available suggesting that potassium or sulfate concentrations might influence microbial selection within the ranges observed, cultured isolates of Haloquadratum walsbyi have previously been shown to be much more tolerant of high magnesium than other halophilic Archaea (Bolhuis et al, 2004;Burns et al, 2004). These results support the hypothesis that observed changes in microbial community composition with ionic strength may be driven primarily by elevated magnesium concentrations, rather than other ionic species.…”

“…One the most successful microbes in extreme hypersaline waters, the square archaeaon Haloquadratum walsbyi, has a specialized cell morphology utilizing large intracellular gas vesicles to facilitate energy-efficient flotation of tightly packed, nearly twodimensional flat colonies (Walsby, 1980;Bolhuis et al, 2004). This unique morphology may be an adaptation enabling these organisms to take advantage of higher oxygen concentrations at the surface, especially in shallow ponds where currents and wave action are minimized.…”

The Selective Value of Bacterial Shape

“…The halotolerant archaea exhibit a curious range of unusual shapes, including triangular cells (144), square cells such as those of Haloarcula quadrata (242), and flat, wafer-shaped cells such as those of Walsby's square archeon (recently named Haloquadratum walsbyi) (26). First described by Walsby (350) and recently isolated and grown in pure culture (26,35), individual members of these square, flat cells are about 2 to 5 m wide and 0.1 to 0.5 m thick (26).…”

“…However, with the exception of the archaeal halobacteria (26,35,349,350), there are few really flat bacteria (63). The major reason may be that the surface area provided by flat cells is not significantly greater than that of thin filamentous cells (349), and a rod-shaped cell imparts an abundance of additional benefits (discussed below).…”

Inhibition of specific gene expressions by protein-mediated mRNA interference

“…We have identifi ed a new MazF homologue from Haloquadra walsbyi isolated from a hypersaline pool on the Sinai Peninsula 9 , which cleaves RNA at a specifi c sevenbase sequence. Th is archaeon is square-shaped but extremely thin; 2 -5 μ m in width and less than 0.2 μ m in thickness 9,10 , resembling other super halophilic microorganisms that require 3 -4 M NaCl for growth and are tolerant in 2 M magnesium 11 . H. walsbyi grows phototrophically using bacteriorhodopsin in the membrane 12 .…”