Life at the Limits by David Wharton, Cambridge, 拢18.95, ISBN
0521782120
WHAT is an extreme environment? In the deepest parts of the ocean, pressure
can reach 1100 atmospheres, enough to crush not just us, but all the fish
familiar to us as well. These benthic waters are also utterly dark, and
temperatures a chilly 2 and 3 掳C. Surely this qualifies as an extreme
environment? Well, perhaps not. David Wharton points out that the deep sea
represents 75 per cent of the biosphere. This sunless, frigid place is therefore
鈥渘ormal鈥� by many definitions and far from empty of life. Extreme depends on
where you stand, says Wharton, and it is a dynamic, evolving concept.
People became more interested in organisms living in extreme environments
after Kary Mullis invented the polymerase chain reaction (PCR), became a
multimillionaire and won a Nobel prize. Mullis made use of an enzyme from
Thermus aquaticus, a humble bacterium discovered in hot springs in
Yellowstone National Park in Wyoming. T. aquaticus thrives at 70
掳C, and its Taq enzyme鈥攚hich works at high temperatures 鈥攊s the
key to PCR, the basis of DNA amplification and the biotechnology industry.
Advertisement
But hot water is just one of the many extreme environments that Wharton, a
zoologist at the University of Otago in New Zealand, describes in Life at
the Limits. This is a thorough and thoughtful survey of life under
conditions that we humans鈥攐xygen-breathing land animals that prefer
tropical or temperate climes鈥攖end to see as extreme.
Wharton devotes most of the book to explaining how microorganisms, plants and
animals survive in places that are deep under water, or without water, very hot
or very cold. He explains why and how each stress is a problem, and then how
certain organisms solve the problem.
Liquid water is necessary to life, but in many places, such as deserts, there
isn鈥檛 very much to be had. The Namib desert beetle cleverly extracts moisture
from the air, but this behavioural adaptation is a minor thing compared to the
nematode worms which practice 鈥渁nhydrobiosis鈥�, or life without water. Some
species of these tiny worms can lose up to 99 per cent of their water and still
survive. The nematodes Wharton mentions live in Antarctica. We may not think of
it as dry, yet its 鈥渨ater-locked鈥� dry valleys put hot deserts to shame when it
comes to the absence of water.
The most interesting adaptations to heat turn out to be deep under water,
where mid-ocean vents that spout mineral-laden water at temperatures of 300
掳C into the frigid deep also support a large community of animals. When it
comes to cold, Wharton takes us back to the oceans for a look at the antifreeze
chemicals that enable fish to swim right up against the Arctic and Antarctic
ice. He adds shorter sections on other forms of stress, such as the osmotic
stress of salt: the solution in salt lakes is so concentrated it will suck the
water out of all but the hardiest organisms.
A rarer problem is radioactivity. Pseudomonas radiodurans bacteria
have evolved the ability to survive in the cooling water of nuclear reactors.
The radiation constantly damages their DNA, but the bacteria have evolved
extraordinary repair mechanisms that can continually resynthesise new DNA and
splice their genes together.
Which brings us back to the question of who really lives in an extreme
environment? The anaerobic bacteria in our guts and in soil seem exotic to us
oxygen-breathers. But Earth鈥檚 atmosphere began almost devoid of oxygen. When
plants began to produce it in large amounts, the 鈥渢oxic鈥� gas created an extreme
environment most life forms could not endure.
Wharton deserves some credit for his thorough survey of all the varieties of
extreme habitats, and his clear explanations of the science involved. But his
prose can鈥檛 quite bring the various plants and animals to life.