Thursday, January 12, 2012

Cryptobiosis: a convenient "death"

Water is life. Nothing on earth can live in its total absence, and few organisms can go without it for very long. A human can survive only a week or so without drinking. A camel, with its water- and food-storing hump, can go much longer, but will still die of thirst when its stores run out. Some cacti can survive for a year on the water stored in their stems, replenishing it only in the desert's short rainy season.

There's more than one way to survive dryness, though. Camels and cacti use about the same amount of water all the time; when there's none to be had outside, they fall back on their internal stores. A spikemoss species called Selaginella lepidophylla has found another way.

When it's well-hydrated, S. lepidophylla is green and perky, with tiny leaves in scale-like patterns that conjure images of steamy Jurassic jungles. When there's no water around, though, it does what most ferns would do: it dehydrates and shrivels, its branches curling into a sad little desiccated clump. You'd think it truly dead, right until the rains return and it promptly earns its common name -- the resurrection fern.

That lifeless ball of brittle twigs sucks up the water with desperate greed, and then it unfurls. The brown leaves soften and go green again. In a few short hours, the resurrection fern comes right back to life. It will live like this, soaking up the water and the sun, for as long as the water lasts; then, just as it did before, it will dry out and "die." It can do this over and over, and it can survive in dry state for months or years on end.

The video below is a time-lapse of a store-bought resurrection fern rehydrating, over the course of about five hours. Watch and enjoy.

The phenomenon -- an organism tamping down its metabolism to wait for more hospitable conditions -- is called cryptobiosis. The most famous cryptobiotic organisms are all microscopic: bacteria in cryptobiotic cysts can hibernate for centuries in unlikely places like permafrost and bedrock, and encysted tardigrades (water bears) have been exposed to excessively lethal doses of radiation, and even to the vacuum of space, with no ill effects.

Compared to that, Selaginella lepidophylla may not seem impressive, but keep in mind that it's many orders of magnitude bigger than those champions. It's resurrection on a familiar scale, and it happens over and over again. Beat that, Lazarus.

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