15 November 2006

Be halophilic or be square!

The halophilic (salt-loving) archaeon Haloquadratum walsbyi is, of course, both. This organism thrives in habitats where you and I and just about every other organism on earth would shrivel up into a lifeless bag of desiccated stuff.

The natural habitat of H. walsbyi is NaCl-saturated pools that also have very high MgCl2 concentrations and are subject to high solar irradiance. In such places, the low water concentration, decreased solubility of oxygen, high solar radiation and Mg2+ ions, which reduce the availability of some nutrients by forming complexes with them, turn survival into a big challenge.

A recent paper by Bolhuis et al.1 discusses the genomic characteristics of H. walsbyi and explains some of the unique adaptive traits H. walsbyi has evolved to survive in its extreme habitat.

Two of those traits attracted my attention. One is a gene that encodes a huge 9,159-amino acid long protein called halomucin. The authors note that halomucin "is similar in amino acid sequence and domain organization...to animal mucins, which play an important role in protecting various tissues against desiccation (e.g. in bronchial epithelium and eyes) or harsh chemical conditions (e.g. in epithelia along the digestive tract)... In function, mediating a specific adaptation to desiccation stress, halomucin resembles the mucous cocoon of lungfish that can escape dehydration for several years outside the water." This is either an example of convergent evolution or the genes encoding for halomucin and animal mucins are the descendants of some very old ancestral gene that encoded a similar protein.

According to the authors, the extreme flatness of H. walsbyi (0.1 - 0.5 micrometer) gives it the honor of being the microbe with the highest surface to volume ratio. Its high surface to volume ratio makes it easier for H. walsbyi to obtain enough nutrients from its environment.

The other interesting finding is that the genome of H. walsbyi encodes 2 bacteriorhodopsin proteins. These proteins help H. walsbyi turn solar energy into chemical energy: "Similar to solar panels, the ultra thin cells of H. walsbyi
collect light as alternative energy source making optimally use of both sides of the membrane."

But they still don't know how H. walsbyi maintains its square shape.

1. Henk Bolhuis, Peter Palm, Andy Wende, Michaela Falb, Markus Rampp, Francisco Rodriguez-Valera, Friedhelm Pfeiffer, Dieter Oesterhelt. The genome of the square archaeon Haloquadratum walsbyi : life at the limits of water activity. BMC Genomics 7:169 (4 July 2006). pdf

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