on the



Michael Gross:
Life on the Edge. Amazing Creatures Thriving in Extreme Environments
Hardback: Plenum, May 1998;
reprinted by Perseus Books August 1999, ISBN 0-306-45786-5, $ 25.95, 200 + xiii pp.
Paperback (with a new afterword): Perseus Books January 2001, ISBN 0-738-20445-5, $ 15.00, 210 + xiii pp.


Introduction: Life and its limits

Things you need for a Living
What Do We Mean by "Normal", after All?
The Limits of Life on Earth

(Manuscript version, i.e. pre-copy-editing!)


Life is a paradoxical phenomenon. It is enormously varied -- with creatures ranging from a thousandth of a millimeter to dozens of meters in length, lifespans from hours to thousands of years, and species which spread over the whole of the globe while others stay in their tiny ecological niche. And still, life is extremely uniform. All cells are built according to the same principles from the same molecular building blocks, no matter whether they are freeliving microbes or a tiny part of a huge organism.

Life is almost everywhere, as long as we stay on the Earth's surface. However, if we consider the whole planet from its centre to the outer reaches of its atmosphere, the biosphere will appear as a wafer-thin layer between the boiling lava and the freezing stratosphere. And if we were to "ask a learned astronomer" where to find life, the answer would be: almost nowhere.

Seeing that life is ubiquitous on the Earth's surface, but extremely rare in space, we conclude that the conditions under which life can persist fall in a very limited range. Presumably this is even more true for the conditions under which life can arise. What makes our planet such a hospitable place is the fact that the environmental conditions are rather unspectacular compared with the rest of the universe, and amazingly constant over time. In fact, the observation that the average temperature of the Earth's biosphere has barely changed during the three and a half billion years since the origin of life, although the energy intake from the sun was 30 percent less than it is now, led James Lovelock to formulate the much-disputed "Gaia hypothesis". His theory, named after the ancient Greek Earth goddess, states that the entire planet, including the biosphere as well as the lifeless geo- and atmosphere, is a self-regulating cybernetic system, like a hyper- organism. We will have a closer look at Gaia in chapter 6.

In spite of the surprisingly moderate and constant climate, conditions in many areas of our planet are extreme by biological standards. In many places, microorganisms can only survive with the help of adaptive mechanisms against high or low temperatures, pressure or chemical stress that evolved over millions of years. The number of different species in such extreme biotopes is typically much lower than in moderate habitats and multicellular organisms are often absent. Exploring how the adaptation of microorganisms works and where it encounters its limits, we will also learn something about life, its fundamental principles, its origin billions of years ago, its diversity and its limits.

And it is not just naturalists who are interested in microbes resisting extreme conditions. Engineers will appreciate that extremophiles may operate under high temperature and/or high pressure conditions which are quite common in chemical plants but would be fatal for normal bacteria. Enzymes from hyperthermophilic bacteria, for instance, could be used in biotechnological processes at 100 C or beyond. Technological applications of these possibilities are only just beginning to be explored. In chapter 3 we will discuss some of the most promising current approaches.

However, before we deal with the extremists, with their habitats and limits, their potential usefulness and danger for humans, we should recapitulate the basic facts of ordinary life: the things we need for a living, what we mean by normal, after all, and which environmental conditions may limit the spreading of life.


Things you need for a living

A living organism is something extremely improbable. Not only because you hardly find any in the universe at large. Mainly it is improbable because it represents a highly complicated ordered structure, while the fundamental laws of thermodynamics (the science of energy conversions) tell us that disorder of the universe (or any other closed system) must increase with time . The secret of living beings -- why they can exist at least for a given lifespan despite the universal trend towards disorder -- lies in their energy consumption. Living beings use up energy, the production of which creates an amount of disorder elsewhere which outweighs the order created by the organism. This is why life is so dependent on energy flow. Even a very energy-efficient organism, which does not move and only replicates once in a hundred years will permanently need energy to defend its ordered state against the natural tendency for disorder.

Life's energy is normally (we will discuss the exceptions) obtained directly or indirectly from the sun. Those living beings which are able to transform the energy of sunlight into chemical energy and store it, namely the green plants and certain bacteria, are called the primary producers of the food chain. They use solar power to convert carbon dioxide from the air and water to carbohydrates and oxygen . Other organisms use this chemically stored energy when they eat plants and digest the carbohydrates or when they use the oxygen produced by plants to "burn" their food in energy-providing reactions. Further up the food chain, other animals eat the plant eaters, and so on. Even bacteria degrading crude oil are using the chemical energy that plants stored millions of years ago. Similarly, if we burn fossil fuels, such as coal, oil or natural gas, we reverse the photosynthesis reaction that plants carried out long ago. We burn the carbon which the plants built into organic molecules to form carbon dioxide and use up the oxygen produced by plants.

Next to the energy provision, the availability of liquid water is the most important requirement for life. As we will see in chapter 2, the limits of life are often defined by the boiling and freezing points of water, which under some circumstances (salty water, pressure) may be significantly different from the standard temperatures of 100 °C and 0 °C. The inability of other planets to support life can largely be ascribed to the fact that they lack oceans.

It must be said that water is not just any liquid. Most probably there is no other small molecule that combines so many special properties as the simple H2O. First of all, if the molecule behaved according to the periodic table of the elements, the substance should not be liquid at all -- it should evaporate at --70 °C and freeze at --100 °C. It should be more volatile than the heavier analogue, hydrogen sulphide (H2S), which is well known to be a gas at ambient temperature. Behind this apparently paradoxical behaviour hides a phenomenon that is immensely important for the molecular architecture of all living beings -- the hydrogen bond. This weak interaction links the molecules in liquid water to form an extensive network . , implying that the chemical formula H2O is an oversimplification. To appreciate the impact of these weak interactions, just remember that they increase the boiling point of water by 170 degrees above what it should be if water was just a collection of H2O molecules.

It's not only that this liquid should lawfully be a gas, it also exhibits further peculiarities in its behaviour. When water cools, its density increases -- as with most other liquids. At lowerr temperatures, molecules move more slowly and therefore need less space. However, if water is cooled to 4 °C, the effect is reversed: further cooling will decrease the density, and freezing will add to this. Both effects have important biological consequences. If for instance a lake cools down in winter (losing most of its heat through the surface), cold water from the surface will sink and ensure thorough mixing and uniform cooling as long as the temperature is above 4 °C. Below this threshold, however, cold water is lighter than the rest and will stay on top. Similarly, when the lake starts freezing over, the ice is lighter than the water and floats. Thus, the bottom of the lake can preserve the temperature of 4 °C during weeks of frost, isolated by the top layers of ice and cold water. The anomalous behaviour of water thus facilitates the survival of life in the lake even in severe winter conditions.

Moreover, the expansion of water upon freezing makes porous stones crack when they are soaked and then frozen. This effect, which we tend to regard as highly undesirable when it affects roads and bridges, is immensely useful in nature. It facilitates the weathering of rocks and thus speeds up the formation of soils suitable for plant growth. The scarfaced looks of the moon and our neighbour planet Mars, which retain marks of meteorite impacts suffered billions of years ago, remind us that the relatively rapid turnover of the surface is a very special feature of our planet.

All living organisms consist mainly of water, and need water to survive. The molecular building blocks of their cells need water to assume their correct functional architecture. Life is not only endangered when water freezes or evaporates. Chemicals such as salt ions can compete with cells or biomolecules for the proximity of water molecules and thus exert a chemical stress that only a very few microbes can cope with (chapter 2).

A further special and life-enhancing property of our planet is the presence of a variety of more than 80 chemical elements. Among the 15 most common elements of the earth's crust we find a wide range of chemical properties opening a plethora of possibilities. Except for the rare noble gases, all eight groups of the periodic table are represented, and in addition we find three transition group elements (titanium, manganese, iron). In contrast, the universe at large is dominated by the two lightest kinds of atoms . Roughly 90 percent of all atoms are hydrogen, another 9 percent helium. With these two alone, one could not even form a chemical compound, let alone anything resembling life.

Plants and animals mainly require ten chemical elements: carbon (C), oxygen (O), nitrogen (N), hydrogen (H), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), phosphorous (P) and sulphur (S). All of them are among the top 15, but it is still interesting to note those of the most common elements for which evolution did not have any use. For instance, the second most common element, silicon, was completely ignored by evolution, while the lighter analogue, carbon, was assigned the key role for the construction of biomolecules. This can be explained by the fact that it is easy to form long chain molecules with carbon, while silicon tends to form three-dimensional networks, as it does, for instance, in clay minerals. Furthermore, the oxides of carbon are volatile and therefore easily accessed for chemical reactions, while the silicon oxide (SiO2) as found in glass, quartz and sand, is an inert solid. Similarly, aluminium has not found biological use in spite of its high abundance in the earth's crust.

Of course the choice of the essential elements by early evolution was a consequence of supply and demand. If potassium had been absent when life originated, sodium could have done the job as well. Evolution could have used manganese instead of magnesium, or strontium rather than calcium, and could still have built cells that would have looked like the ones we know and would have populated the globe quite as efficiently. The essential elements, therefore, are not essential for life as such, but have become essential for life on our planet during its evolution.

Still, it is difficult to conceive of a diverse biosphere without carbon or nitrogen. Although researchers have speculated that the first genetic information was propagated in clay minerals before cellular life arose (chapter 6), such forms of "inorganic life" would have very limited potential to spread and adapt. If they ever existed, the "genetic takeover" by the more versatile organic molecules and cellular life has clearly marked their limitations.

In addition to the ten universally essential elements, evolution has recruited many more for services in certain families of organisms. Some bacteria require copper-containing enzymes, others incorporate selenium into their proteins, and there are many more examples of elements with special biological tasks. For the organisms concerned, these elements are of course as essential as the ones quoted above. However, these remain special cases which cannot be generalized.

Finally, life needs living space, preferably with a reasonably stable climate. The diversity of the biosphere owes a lot to the diversity of the biotopes and with the opportunities for life forms to invade new habitats and adapt to the requirements, until they differ from those who stayed behind. The slow geological processes which continuously turn over the earth's crust, like continental drift, or the formation of mountain ranges, have an important role to play in this, as they separate areas that were adjacent, create new biotopes, and change environmental conditions slowly enough to give organisms time to adapt. Once again, we are reminded or the inter-relationships of bio- and geosphere, which are the focus of the Gaia theory (chapter 6).

With the diversity of biotopes, we have almost arrived at the topic of this book, but before we set out to explore the extreme biotopes of our planet, we have to face a question.


What do we mean by "normal", after all?

The survival of an organism, a population, or a species may depend on numerous factors of widely different nature. First of all, there are biotic as well as abiotic factors. The former, such as predators, parasites and competitors, are the subject of the research field of ecology, which I will not attempt to present in any detail. The latter arise from the non-living part of the environment, from physical and chemical conditions that are directly related to the forms of stress that this book is about. In turn, they can be classified into physical and chemical factors.

Let us start with the simple things. Some physical parameters are practically constant on the whole surface of our planet. The atmospheric pressure at sea level is approximately one atmosphere (1.013 bar), wherever you are. The variations in atmospheric pressure which we experience as the changes in weather only amount to ca. ((xx)) percent of this constant. Earth's gravitational force is the same everywhere. Each falling body is accelerated by 9.81 meters per second every second of its fall, if we ignore friction for a moment.

The chemical composition of the atmosphere and the oceans is rather uniform as well. The freezing air over the poles as well as the steaming air of the jungle contains 21 percent (by volume) of oxygen, 78 percent nitrogen, with carbon dioxide and noble gases sharing the remaining one percent -- provided one removes the variable content of water vapour. Seawater always contains about 3.0 percent salt, the rivers and lakes much less (except for lakes with no outflow, like the Dead Sea).

So much for the global constants. The most striking variability is observed in temperatures. They range from --80 °C in the antarctic winter to 250 °C in the plumes of deep sea hydrothermal vents. As most organisms don't enjoy the comfort of an inbuilt central heating system as we mammals do (to say nothing about insulation standards), the "body temperature" is, for most cells, exclusively governed by the environmental temperature.

Which temperature you regard as normal depends strongly on whether you were born and bred an Inuit or a Californian, a polar bear or a gut bacterium. The latter, if they dwell in human intestines, would regard 37 °C as the most normal temperature in the world. This meets the approval of all medically or zoologically oriented biochemists and microbiologists, who define physiological conditions as a lukewarm salt brine. Researchers who favour practical considerations would define normality by the standards of the common or preferred room temperature, for instance 20 or 25 °C depending on the altitude of their habitat.

However, there is no such thing as a norm, or a physiological standard temperature for all living beings. At best, one could define a range of moderate temperatures, as opposed to extreme temperatures. Thus, a range from 20 to 40 °C would be a normality which could be accepted in a global opinion poll. Organisms that prefer warmer temperatures than these will henceforth be called "thermophilic", those that prefer cooler temperatures are labelled "psychrophilic". . But we will realize that for many living beings much higher or lower temperatures are perfectly normal living conditions.

In the oceans, hydrostatic pressure (which increases linearly with the depth under the surface) joins temperature as another physical variable, from which the cells cannot isolate themselves and to which they must therefore adapt. As we will see, our human view that one atmosphere should be called the normal pressure is factually incorrect for more than half of the biosphere. Three quarters of the total volume of the oceans, corresponding to 62 percent of the biosphere are subjected to hydrostatic pressures more than one hundred fold higher than our beloved atmospheric pressure.

With respect to the chemical variables, the average, most common values are normally regarded as standards. Neutral pH (neither acidic nor alkaline), moderate salt concentrations, availability of oxygen and nutrients -- these are typical conditions found in the more hospitable parts of the earth's surface. . But there are quite a lot of less hospitable places with less "physiological" conditions as well. These execeptions from the rules will teach us something about life's capacity for adaptation and the limits of life.


The limits of life on Earth

Even though life on earth has, in the course of evolution over billions of years, adapted to various extreme conditions in amazing ways as we will discover below, sometimes the laws of physics put a halt to adaptation and define an absolute limit, beyond which no life can exist. With respect to heating and cooling, the temperature as such is much less limiting than the availability of liquid water. In environments, where salt content and/or hydrostatic pressure increase the boiling point and decrease the freezing point of water, extremophilic microorganisms can thrive at 110 °C or at --5 °C, as long as the water remains liquid. Of course, the range cannot be expanded indefinitely. Research into the stability of biomolecules suggests that an upper temperature limit exists at around 120 °C, beyond which the molecules of the cell would decompose more rapidly than the cell can replace them.

As for the hydrostatic pressure in the deep sea, an upper limit of pressure tolerance of cells certainly exists, but the oceans are not deep enough to reach it. Even at the highest pressures occurring in the deepest trenches of the pacific (1,100 bar at 11 km below surface), bacteria can thrive. . (A schematic representation of the greatest heights and depths of the earth with the corresponding temperatures and pressures is shown in . ) The fundamental limit set by physics may lie somewhere between twice and three times the highest pressure found in the oceans. Of course no-one knows the limiting pressure to which organisms could have adapted, if there had been deeper oceans to populate.

Chemical stress, such as high salt concentration, or acidity, does not set firm limits to life. As we will see below, in the case of acidity this is partly due to the fact, that this stress factor can be excluded from the interior of the cell, requiring adaptation only for those parts that deal with the outside world.

In summary, we should acknowledge that our home planet is quite a nice place to live. The relatively mild and constant conditions have encouraged the evolution of a biosphere of enormous diversity. We will get an impression of how diverse life on earth is, when we visit its fringes, those extreme biotopes, which evolution may have had to conquer during millions of years of adaptation to extreme conditions. .

Order Life on the Edge from:




prose and passion

last update: