Cryptobiosis

“A physiological state in which metabolic activity is reduced to an undetectable level without disappearing altogether.” – the definition of cryptobiosis from Oxford Languages.

 

Several weeks ago, Current Biology published a paper on the revival of 24,000-year-old microscopic animals called rotifers from the Siberian permafrost. This seeming resurrection from the dead was first observed by the famed microsopist Van Leeuwenhoek, way back in 1701. His sample of rotifers came from reddish gutter water from his house. Intriguingly, the organisms shrank in size as they dried out. Gutter water turned to dust in the dry and hot summer. Water was added and behold – the rotifers came back to life!

 

The rotifer vignette is one of several stories of organisms perched near the fuzzy boundary between life and death in Carl Zimmer’s new book, appropriately named Life’s Edge. Forty years after Van Leeuwenhoek’s discovery, nematode worms were found by famed naturalist Needham to survive several years in dormancy. Then came tardigrades, sometimes known as water bears, well-experimented on by modern day scientists and known to survive the vacuum of outer space, temperatures close to zero kelvin, and being hit by speeding bullets.

 

Water resurrects these creatures in the twilight zone.

 

How do they do it? According to Zimmer, some species produce a sugar, which “thanks to its chemical structure, trehalose can help proteins keep their proper shape, much like water does.” Others “make a new batch of proteins that link together to form a kind of biological glass. It entombs the cell’s DNA and other molecules in their three-dimensional form, so that they’re ready to revive when water returns.”

 

While larger creatures have not been classified as cryptobiots – we can detect their unusually low metabolic levels – many of those in colder climates use a hibernation strategy when winter arrives. Bears do it. Bats do it. Bees do it. Even one species of bird. (Most others migrate.) Trees do it too. Could humans do it? Maybe. If we tipped into another ice age. And if you believe the situation in Early Riser is possible.

 

In his short Origins of Life book, Freeman Dyson proposes a simple toy model to model the transition between the living and the dead. The two states are ‘attractors’ in complexity theory parlance, and the hilltop between the two is an unstable transition state. With simple assumptions and parameters, Dyson calculates that a population of 2,000-20,000 containing 6-8 monomeric species (that also serve as catalysts), with a discriminative factor of 60-100 (modern enzymes are a hundred-fold more discriminatory) allows for moving from being dead to alive.

 

The caveat? In Dyson’s words: “The basic reason for the success of the model is its ability to tolerate high [reproductive] error rates. The model overcomes the catastrophe by abandoning exact replication. It neither needs nor achieves precise control of its molecular structure. It is this lack of precision that allows… [a] jump into an ordered state without invoking a miracle.” This also means it’s easy to die without invoking a miracle. By changing the parameters in line with evolutionary processes improving the fidelity of replication, Dyson’s toy model shows that life is more easily preserved, but resurrection becomes much, much more difficult.

 

As someone focusing on the chemical origins of metabolism, I find Dyson’s model intriguing. Coincidentally I’ve been considering numbers not too different from his (based on thermodynamic and kinetic data), at least in my small and restricted model although much bigger than Dyson’s and requiring orders-of-magnitude more computer time. The idea of treating such a chemical system as cryptobiotic is a possible framework. And water may play a key role, as we’re seeing in recent wet-dry cycle origin-of-life experiments. Maybe I’m into the business of resurrection after all – or perhaps just resuscitation.