29 July 2009

Survival at extreme temperatures: what is it good for?


JOHAN MERTENS, LYNDA BELADJAL, ANGELICA ALCANTARA, LIESJE FOUGNIES, DOMINIQUE VAN DER STRAETEN and JAMES S. CLEGG (2008). Survival of dried eukaryotes (anhydrobiotes) after exposure to very high temperatures Biological Journal of the Linnean Society, 93, 15-22 pdf

Ever since Antonie van Leeuwenhoek demonstrated the appearance of animalcules* in cultures made with previously dry dust more than 300 years ago, biologists have been getting kicks out of pushing the limits of the survivability of dried animals and plant seeds. This is one of those studies the object of which seems to have been to demonstrate how high a temperature certain organisms can survive—regardless of whether or not the results had any biological significance.

Mertens et al. showed that the seeds or spores of various plants (Cardamine, Taraxacum, Adiantum, etc.), the cysts of the crustaceans Branchipus schaefferi (fairy shrimp) and Artemia franciscana (brine shrimp) as well as adult bdelloid rotifers and tardigrades survived exposures to temperatures as high as 130 °C for as long as 10 min.

The trick was to desiccate the seeds, spores and the animals first (for 3 days over silica gel) before heating them slowly at a rate of 4 °C per minute. However, the bdelloid Philodina did survive exposure to 120 °C even when it was heated rapidly (100 °C/min).

This is all very interesting, but what exactly do these results mean considering that these organisms are unlikely to experience such high temperatures in their habitats except perhaps during wild fires? The authors try hard to come up with a speculative scheme to explain how the ability to tolerate high temperatures could have arisen during the Devonian or earlier when the earth was supposedly a warmer place—but not that warm. But this doesn’t explain how adaptations to survive temperature extremes unlikely to be encountered in the organisms’ present or Devonian habitats arose to begin with.

Such extreme over-adaptations are likely to be the unintended byproducts of moderate over-adaptations to environmental stresses likely to be encountered occasionally. If desiccated rotifers and tardigrades or the seeds and spores of various plants are routinely exposed to temperatures around 40 °C, then those individuals that have evolved to incorporate a safety factor into their adaptation that makes them survive slightly higher temperatures, say 50 °C, will fare better in the long run when and if the habitat temperature happens to rise that high. It is likely that the mechanism that confers protection at 50 °C also protects at much higher temperatures that the animals will never experience as long as the cost of maintaining such a mechanism for occasional use is not too expensive.

*Leeuwenhoek’s animalcules were probably bdelloid rotifers, see Tunnacliffe, A. and J. Lapinski. 2003. Philosophical Transactions: Biological Sciences 358:1755-1771.

1 comment:

Harlan Ratcliff said...

Maybe it is just a result of the dessication. The only thing magic about 100 degrees C is that is the temperature that boils water. Without water, the high temperature must directly destroy the remaining chemicals in order to damage the organism.