Life on the Edge: Amazing Creatures Thriving in Extreme Environments
- Michael Gross
Extremists of all kinds tend to challenge firmly held concepts and ideas. This is particularly true with the discovery in the past two or three decades of microorganisms that inhabit conditions previously thought to be totally incompatible with life. The first such organisms were isolated from hot springs in Yellowstone National Park by Thomas Brock. The surprise was not just that they live at temperatures of up to 95 °C, but that they cannot be cultured at lower temperatures; they have become thermal specialists par excellence, unable to cope with change.
This finding encouraged microbiologists to expand their travel budgets and explore a wide range of very uncomfortable environments, including boiling mud-holes, volcanoes, solfatara fields and even ‘black smokers’ on the ocean floor, all of which are home to microorganisms. Microbial communities have since been found in the extreme cold of the Antarctic, in the hypersaline Dead Sea in Israel, under the very high hydrostatic pressures of ocean trenches, and even in rock drillings from 1,500 metres beneath the US state of Washington. These astonishing discoveries extended the microbiological lexicon to include such terms as extremophile, hyperthermophile and chemolithoautotroph. Michael Gross has written a highly readable account of these organisms and the excitement surrounding their discovery. His overview is masterfully broad, linking their discovery with a number of recent developments in molecular biology, biotechnology, pollution control, biogeochemistry and the origin of life.
Perhaps the most fascinating aspects of these microorganisms is how they thrive in conditions that would rapidly kill more familiar organisms. The proteins of hyperthermophiles must be particularly resistant to denaturing at high temperatures, although Gross shows that there is no simple or consistent set of structural features that account for this. He describes heat-shock proteins and their role in refolding heat-damaged proteins, although their specific role in extremophiles is uncertain because they are found in virtually all living organisms, including those inhabiting polar habitats. Similarly, he discusses the antifreeze glycoproteins of polar fish, even though they have no role in microbial adaptation.
Yet Gross virtually ignores the issue of membrane adaptation through the modification of lipid structures, an important feature of adaptation to extremes of both temperature and pressure in a wide variety of animals, plants and microbes. Hyperthermophiles in particular possess very unusual membrane lipids, some of which appear to suppress the excessive molecular motion that high temperatures might otherwise cause.
Gross is on safer ground in showing that extremophiles are more than just a scientific curiosity. Their discovery has underpinned the emerging recognition of a new branch of life, the Archaea; it has led to the discovery of submarine and cave-dwelling biotopes that are entirely independent of sunlight; and it has provided insights into the processes of planetary geochemical change. Moreover, extremophiles have potentially important implications for biotechnology and medicine, which are given extensive treatment. Hyperthermophiles are the source of that most valuable biotechnological product, thermostable DNA polymerase, and seem likely to deliver other innovative products and biotechnological processes.
Gross concludes by considering the possibilities for the existence of life in the Universe beyond Earth. The discovery of extremophiles has moved the goalposts in extending the range of conditions and environments considered compatible with life. Nevertheless, the technical problem remains of defining exactly how life, even of a simple microbial form, can be detected by remote probes; the most likely method is the discovery of unexpected gas compositions in planetary atmospheres.
Gross has a writing style that is both informal and entertaining. The text is enlivened by simple line diagrams and a series of vignettes describing a range of topics, including short anecdotal biographies of the principal researchers. The result is a tale of modern biology linking the improved understanding of microbial biodiversity to an impressive range of scientific, medical and commercial advances. It is written at a level that would suit non-specialists, even non-biologists, and therein lies its value: bringing a complex and developing tale of modern biology to a new audience.
Even uncomfortably extreme conditions of temperature, pressure and salinity can be optimal for some species. Perhaps the most challenging problems result not so much from survival under extreme but constant conditions, but from routine exposure to normal but more variable conditions.
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Cossins, A. Some like it hot. Nature 393, 227–228 (1998). https://doi.org/10.1038/30391