Methane, the same natural gas that burns beneath the pot on your kitchen stove, issues forth from the seabed off the coast of Oregon. In other places, for example on land in the Arctic where the temperature is low enough, this same gas, vented cold, can be trapped within an ice cage of water to form "methane hydrates". Many of these structures on the sea floor are seen as dirty lumps of ice or ridges dotted with stone chimneys.
Methane hydrates, when they melt above 6C, release not only methane gas but hydrogen sulphide and ammonia as well.
In wedges of mud replete with methane hydrates dwell - so far as is known - only one kind of animal: the ice worm ( Hesiocaeca methanicola ). This strange polychaete, an annelid that belongs to the same phylum as the common earthworm, probably invaded this strange habitat from much more temperate marine surroundings. Like other polychaetes, H. methanicola creates a current of water around itself... but here it wears away its icy cage to form a burrow inside of which it traps and feeds on methane, sulphide and ammonia-tolerant bacteria.
Tales of animals in unlikely and, for us, extremely uncomfortable habitats make up most of the narrative of this book by David Wharton. So how extreme are the "extreme conditions" under which some of our planet-mates thrive? What kind of "extremophilic" life (that is, life that persists and grows under conditions extreme for people) has evolution by natural selection generated in the past 3.5 trillion years? The general answer is that life of some kind survives to grow at temperatures below freezing and up to 113C; in oxygen at undetectably low concentrations; at battery acid (pH below 1) and soda lake alkalinities (pH above 11); in total darkness; at radiation levels typical of the cooling water of nuclear reactors; and at hydrostatic pressures up to 1,100 atmospheres ("imagine five elephants standing on the tip of your finger").
No organism, however, can tolerate the total absence of water. But many organisms of all kinds can and do survive complete desiccation, although they do not grow. Much of this book is dedicated to the description of how they stay alive without any water in their environment, many to revive within minutes as water returns. The water waiting game is played by brine shrimp, midges, nematodes, mites and springtails as well as, of course, by fungal and bacterial spores, protist cysts, seeds of desert plants and tardigrade tuns. Wharton, who calls the phenomenon of "suspended animation" due to desiccation "anhydrobiosis", reviews the literature well; he shows the importance of the sugar trehalose and of the sugar alcohol glycerol in retaining intracellular water.
He has collected several nice animal stories of survival of water stress.
Take the Bactrian (two-humped) and Arabian (dromedary, one-humped) camels.
Wharton calls them two species but claims that they are closely related. He insists that the suggestion of Pliny the Elder ( c. AD 32-79) is incorrect: no anatomical, physiological or other evidence exists that supports the belief that these camels store water in their humpsI or anywhere else in their bodies. Rather they have splendid talents for conserving water. Most of the rest of us mammals sweat, pant and move our blood vessels to the skin as we control tightly our body temperature. But not the camels. They reduce water loss by permitting their temperatures to fluctuateI by as much as 6C between a cool morning and hot afternoon on a single day. In doing so, these camels are estimated to save as many as 5 litres of water a day.
In addition, their light-coloured skin and thin woolly coats shield their bodies, and the fat in their humps insulates their exposed backs as they face the sun. Apparently: "Their tall body shape and long legs allow heat to be lost from the hairless, poorly insulated lower half of the body and raises the body above the ground to where temperatures are cooler." Unlike us, camels can lose 30 per cent of their total body water. Even at 30-35C, camels survive without drinking any water for as long as 15 days. A dehydrated camel drinks a bathtub full of water (130 litres) in only a few minutes.
Life thrives, as we all can attest, in the warmth of the sunlight on the seashore, the stream bank or the lakeside. Serious excursions from our holiday ideal - August on the beach - we deem extreme. None of us can lose more than 15 per cent of our body's water and survive. What Wharton shows is that we humans are not the measure of all things and that many organisms, from the smallest bacteria to the seeds of large plants, live in environments that threaten even the toughest men.
Most books and review articles that describe the lives of extremophils focus on the best-known examples, by far the most of which are micro-organisms. Fungal spores germinate after years of desiccation. A week of continuous boiling is nothing to many Bacillus spores. In 30 seconds, they revive and grow. These spores also tolerate months, even years, in the total absence of oxygen.
The unique aspect of Wharton's book is his collection of stories, and in some cases even data, of extreme tolerance of major environmental restrictions in plants, protists, fungi and especially animals. His book presents the challenges of environmental extremes in an accessible style.
However, it has deficiencies: no table or figure lists, and a poor index makes locating specific facts difficult. But my most serious criticism is the failure, common to most scientists trained in zoology, to understand the real range of metabolism of life on earth. All organisms require specific sources of carbon (and other elements), energy and electrons. This lack of comprehension of fundamental microbiological principles leads Wharton to make erroneous statements about the independence of life at the deep-sea vents and the nature of food and energy sources in rocky substrates and reduces the reliability of his statements about extraterrestrial life.
Lynn Margulis is professor in the department of geosciences, University of Massachusetts, Amherst, US.
Life at the Limits: Organisms in Extreme Environments
Author - David A. Wharton
ISBN - 0521782120
Publisher - Cambridge University Press
Price - £18.95
Pages - 307