Extreme environments affect the responses of organisms at many levels, from the structure of their proteins and membranes, to the ways in which they carry out their lives, their interactions with their environment and other organisms, and their life history, behaviour and evolution. While it would be an unhelpful truism to say that organisms are adapted to live in the places in which they live, there are some particular features of the biology of organisms in extreme environments. The extreme conditions which they face affect all aspects of their biology – theirs is an extreme biology which may give us some important insights into the nature of life in general. In this chapter, I will explore whether we can see any general patterns for life in extreme environments. But, first, let us revisit what we mean by an extreme organism.

WHAT IS EXTREME, REVISITED

In Chapter 1, I proposed a definition of extreme organisms by reference to the life boxes of organisms: the range of physical conditions under which they exist (the non-biological features of their ecological niche). I also proposed that we could recognise two groups of extreme organisms. Extremophiles have life boxes which are beyond the limits of those of the majority of organisms. Cryptobiotes have life boxes which are the same as, or substantially overlap, those of the majority of organisms, but, when the conditions of their environment deteriorate beyond those under which they can maintain their life processes (metabolism), rather than dying (as do most organisms), they cease metabolism and await the return of favourable conditions.

As a child, I lived in the Midlands in the centre of England. Family holidays meant a long car journey and I can still remember the excitement of that first rare glimpse of the sea. We feel a similar sense of elation on discovering a waterfall or a lake during a walk in the country or on hearing the patter of rain after a long dry period. Perhaps the emotional response we feel to water is a recognition of how important it is to us and to life in general. For we all know that water is essential for life. However, although it is true that life cannot exist without at least periodic access to water, organisms can live in some very dry places and some can survive the almost total loss of water from their bodies.

Why is water so important? Water comprises by far the largest proportion of the chemical make-up of the bodies of most organisms, usually from 60 to 90 per cent. Indeed, as JBS Haldane once remarked, ‘Even the Pope is 70 per cent water’. Living organisms need metabolic processes which maintain their structure and enable them to move, grow and reproduce. Metabolism involves chemical reactions and these reactions need a medium in which they can take place in a controlled manner. Consider, for example, how organisms get their energy. Energy comes mainly from the oxidation or burning of sugars.

… there's no place like home (in the words of the poem by John Howard Payne). Although the environments inhabited by some organisms seem humble to us, to these organisms they are ‘home’. Extreme habitats abound on Earth. They seem extreme to us because of high temperatures (hot springs, hydrothermal vents, hot deserts), low temperatures (polar regions, alpine environments, winter temperate environments, cold deserts), lack of water (deserts), high pressures (ocean depths), acidic or alkaline conditions (acid mine waste, the stomach, soda lakes), high salt concentrations (salt lakes) and lack of oxygen (decomposing organic material, estuarine muds, vertebrate intestine). Other extreme situations include toxic chemicals and exposure to ultraviolet radiation. Many extreme habitats challenge the organisms that live there with some combination of these stresses. Let us look at the characteristics of some of these extreme habitats and the adaptations which enable organisms to call them home.

DESERTS

Not all deserts are hot and sandy. A desert is defined by its lack of water rather than its temperature. This does not mean that there is no water but that rainfalls, and other inputs of water, are irregular and infrequent. Deserts cover one-third of the Earth's land surface (Figure 2.1). This includes semi-arid areas (annual precipitation of less than 600 millimetres), arid areas (less than 200 millimetres) and hyper-arid areas (less than 25 millimetres). Deserts form in the leeward side of mountain ranges, in inland areas which are remote from oceans and where dry stable air masses form, resisting convective currents which would bring rain.

So far, we have looked at organisms that can survive extreme temperatures (hot and cold) and desiccation (anhydrobiosis). There are, however, a number of other extreme environmental stresses that offer challenges and opportunities to living organisms. I will consider some of these in this chapter.

UNDER PRESSURE

There are few situations where organisms are naturally exposed to low pressure, but high pressure is a rather more common hazard than we might expect. Organisms that inhabit rocks and sediments beneath the surface of the Earth are likely to be under pressure (see Chapter 2 in the section ‘The underworld’). The study of these organisms is in its infancy. We know rather more, however, about those of the other main high-pressure environment, the deep sea (see Chapter 2, ‘The cold deep sea’). The deep sea is considered to be that volume of the oceans which is below the depth of 1000 metres. The oceans cover 71 per cent of the Earth's surface and are an average of 3800 metres deep. In volume, the deep sea comprises 75 per cent of the biosphere. This makes it the largest environment, or rather group of environments, on Earth, but it is one of the least understood. Since hydrostatic pressure increases by 1 atmosphere for every 10 metres in depth, organisms inhabiting the deep sea have to cope with the crushing forces of very high pressures – reaching up to 1100 atmospheres in its deepest parts. However, pressure is not the only problem organisms face.

As this quote from Mr Spock of the original Star Trek TV series highlights, science fiction writers (and scientists) have imagined many different forms of extraterrestrial life. These range from life consisting of ‘pure energy’, to life in the thermonuclear furnace of a star, to life within a few degrees of absolute zero which uses superconductivity to provide its energy. It may be rather hard to gain concrete evidence for the existence of such life, at least given our present abilities, even if such things do exist. Perhaps we would be better sticking to life as we know it. In this chapter, I will focus on what our knowledge of organisms in extreme environments might tell us about what is perhaps the greatest unanswered question in science: is there life elsewhere in the universe?

Organisms are found in a wide range of conditions on Earth. There are those that can live at temperatures above 100°C, at high or low pH, or at high salinities, those that can survive temperatures below 0°C, those that can dry out completely but survive, those that live at high pressures in the ocean's depths and those that can tolerate high levels of radiation exposure. The discovery of the communities of organisms associated with deep-sea hydrothermal vents in the 1970s (see Chapter 2) came as a complete surprise. Here was an ecosystem that did not derive its energy from sunlight but, instead, its primary producers, which produced the organic material, were chemoautotrophic bacteria that obtained their energy from chemical oxidations.

In 1989, I was lucky enough to visit the Antarctic for the first time, as part of the New Zealand Antarctic Research Programme. I was looking for nematodes, a group of worm-like invertebrate animals that live associated with the algae and moss that grow in the meltwater from snow and glaciers, and around the edges of lakes and small ponds. I visited various sites around the McMurdo Sound area of Antarctica, including the Dry Valleys which form the largest area of ice-free land on the continent. Parts of the Dry Valleys are called ‘oases’, because they support some visible signs of life. If you were expecting palm trees, however, you would be disappointed. Small patches of moss are as good as it gets in this part of Antarctica. The organisms that live here face some of the most extreme conditions experienced on Earth.

We all have ideas as to what might be normal and extreme environmental conditions. We might like our normal environment to be lying on the back lawn in the dappled sunlight with a gin and tonic. The heat of the desert (without the gin and tonic!) or the cold and wind of the Antarctic or Arctic might, in contrast, seem somewhat extreme. For other organisms (and even other humans), however, these places are home. In this book, I aim to explore the adaptations that have enabled organisms (including plants, animals and microbes) to live in situations that we might consider extreme and to try to develop a framework for thinking about organisms and extreme environments.

TEMPERATURE AND LIFE

Changes in temperature have profound effects on biological processes. In general, the rate at which life processes (such as metabolism, respiration and growth) proceed declines as the temperature decreases and elevates as it increases. This is because high temperatures supply more kinetic energy to reactions than do low temperatures, allowing the reacting molecules to come into contact and to interact more frequently. If the temperature continues to increase, however, it starts to have destructive effects on organisms. The rate of metabolism, and other life processes, declines again. There is thus an optimum temperature at which metabolism is at its greatest rate and, if the temperature decreases or increases from this optimum, the metabolic rate declines (see Figure 1.1). As was pointed out in Chapter 1, the decline in metabolic rate at low temperatures (below the optimum) is due to very different causes from those producing a decline at high temperatures (above the optimum). The rate declines at low temperatures mainly because there is less kinetic energy driving the metabolic reactions, but the effect is largely reversible (unless freezing or some other event occurs which the organism cannot survive; see Chapter 5). The decline in metabolic rate at high temperatures is, however, due to destructive effects on the proteins, and the other components, of the organism. Some of these destructive effects may be reversible if the temperatures are not too high. At higher temperatures, irreversible changes occur or there is too much damage to repair and the organism will die.

I hope you will excuse me starting by talking about myself but it might help to explain some of my background and how I came to write this book. When I was doing my bachelor's degree in Zoology at the University of Bristol in England, one of my final year projects was on the structure of the eggshell, and the hatching mechanisms of the eggs, of a nematode parasite of cockroaches that rejoices in the splendid name of Hammerschmidtiella diesingi (named after a German nematologist, Hammerschmidt). Nematodes are a group of worm-like invertebrate animals. The eggshell turned out to have complex systems of pores and spaces in its outer layers. I stayed on at Bristol to do a PhD that followed up this work and looked at the eggshells of a variety of parasitic nematodes. The nematode eggshell is one of the most resistant of all biological structures. In some cases, it can even survive immersion in concentrated sulphuric acid. The eggshell has a layer of lipid, which restricts the exchange of materials between the egg and its environment. The eggs lose water very slowly when exposed to desiccation, enabling the larvae enclosed within them to survive the total loss of water from the egg. The pores in the eggshell of H. diesingi are involved in this process.

The free-living stages of some parasitic nematodes (those developing after the hatching of the egg), and of some free-living nematodes, show extraordinary abilities to survive extreme environmental stresses. They will tolerate freezing, complete desiccation and exposure to chemicals that are fatal to most other organisms (such as the fixatives used to kill and stabilise specimens for microscopy).