Extending the life box

Extremophiles thrive in extreme environments, while cryptobiotes can survive extreme conditions until more moderate conditions return. There are, however, other responses to extreme conditions. Organisms may avoid the extreme conditions by migrating to more favourable ones. Snow geese avoid the cold of the Arctic winter by migrating south to more moderate conditions. Desert insects avoid desiccation and heat during the day by burrowing into the sand. Some organisms can modify their external or internal environment to make conditions more normal and less extreme. In the case of temperature, this type of response is mainly found in birds and mammals.

Most organisms are ectotherms - they are at the same temperature as that of their environment. Birds and mammals, however, are endo-therms. They can generate their own heat and maintain a higher temperature within their bodies than that in their surrounding environment. This is achieved firstly by burning fuel (food) to generate heat through metabolism and secondly by mechanisms to reduce the loss of heat to the environment, such as through insulation (fur,

Table 1.1 Some extreme terminology

Stress

Required

Tolerated and/

Metabolism

for growth

or slow growth

ceases

philic

tolerant

biosis

Cold

Psychrophilic

Psychrotolerant

Cryobiosis

psychro

Cold tolerant

cryo

Heat

Thermophilic

Thermotolerant

Thermobiosis

thermo

Pressure

Piezophilic

Piezotolerant

baro

Barophilic

Barotolerant

piezo

Osmotic

Halophilic

Halotolerant

Osmobiosis

halo

Osmophilic

Osmotolerant

osmo

Acid/alkali

Acidophilic

Acidotolerant

acido

Alkaliphilic

Alkalitolerant

alkali

No oxygen

Obligate

Facultative

Anoxybiosis

anaero

anaerobes

anaerobes

anoxy

Desiccation

Xerophilic

Xerotolerant

Anhydrobiosis

anhydro

xero

feathers and fat beneath the skin). These mechanisms enable birds and mammals to live in some very cold places.

Humans show the final response to extreme conditions. We modify our external environment to bring the conditions within the range we can survive. Other animals do this to a limited extent, by building nests and burrows, but our ability to modify our environment has enabled humans to colonise, or at least survive in, almost any place on Earth. If we get cold, we put on extra clothing or turn on a heater. We carry water with us into the desert and can even desalinate saltwater to provide freshwater for drinking or irrigation where it is otherwise in short supply. Where there is no oxygen, we carry it with us. Our ability to modify our environment has enabled us to survive at the tops of the highest mountains, in the depths of the sea, in the driest, hottest deserts, in the barren waste of the Antarctic polar plateau and even in space.

resistance and capacity adaptations Organisms may have two types of responses to adverse conditions. Resistance adaptations enable the organism to avoid or survive the stress until conditions become favourable again; capacity adaptations enable the organism to grow and reproduce under the harsh conditions. Although these terms were originally proposed for adaptations to temperature stress (in 1958 by Precht, a German physiologist), they could be applied to other stresses such as lack of oxygen, osmotic stress and desiccation. Precht considered capacity adaptations to be those that operated within the normal range of temperatures experienced by an organism, while resistance adaptations operated at extreme temperatures. I have extended Precht's meaning to make the terms apply to the response of organisms to extreme environments. For an organism displaying capacity adaptation to an extreme habitat, the extreme conditions become the normal range of conditions. The enzymes, membranes and other systems of the organism are optimised to operate at extremes (such as at very high or very low temperatures). If the conditions for the optimum growth of an organism which lives in a harsh environment are different from those of organisms that live in more benign environments, this would provide evidence for capacity adaptation: for example, if the optimum growth temperature of an Antarctic organism was lower than those of its relatives from warmer regions.

For an organism displaying resistance adaptation to extreme conditions, its normal range of conditions is the same as those of most other organisms. When faced with extreme conditions, outside the range where it can normally operate, it can survive until conditions become

'normal' again, whereas other organisms may die. Resistance adaptations often involve some sort of dormancy, such as hibernation or the production of a resting stage like a cyst, spore or seed. In response to conditions becoming extreme, or in response to changes which indicate that extreme conditions are on their way (such as the onset of winter), the organism becomes inactive. This involves a lowering of the metabolic rate of the organism, reaching its ultimate expression in cryptobiosis, in which there is no measurable metabolism. The lowering of metabolic rate may in itself provide some resistance to the adverse conditions, by reducing the rate at which food reserves are consumed. There may also be more specific mechanisms which provide protection against the stress - for example, the synthesis of cryoprotec-tants in response to low temperatures.

Extremophiles thus show capacity adaptation to extreme environments, while cryptobiotes show resistance adaptation. The difference between the two has been illustrated in Figures 1.2 and 1.3. The two types of adaptation are not mutually exclusive and many organisms show both capacity and resistance adaptations to extreme conditions. The extreme thermophilic bacterium Pyrococcus furiosus, an inhabitant of deep-sea hydrothermal vents, has an optimum growth temperature of 100 °C and the temperatures in which it can grow range from 70 °C to 105 °C. At temperatures below 70 °C, it becomes dormant. This bacterium thus shows capacity adaptation to high temperatures, but resistance adaptation when temperatures become too low for it.

some basics

For most of the rest of this chapter, I will introduce some physical and biological concepts that are important for understanding the ideas covered later in the book. Card-carrying biologists might wish to skim through this.

Normal and extreme conditions

What are normal and extreme conditions for life? As we have seen, temperature has important influences on organisms and both high and low temperatures may be considered extreme. Many terrestrial organisms maintain some sort of normal activity over the rather narrow range of 10°C to 48°C (a range of 38 °C). The lowest natural temperature recorded on Earth is — 89.2 °C, at Vostok, Antarctica (temperatures within a fraction of a degree of absolute zero, — 273 °C, have been achieved in laboratories). The highest natural temperatures at the Earth's surface occur associated with volcanoes. In the absence of geo-thermal activity, the highest reported shade temperature is 58 °C, recorded at Al'Aziziyah, Libya. This gives a range of recorded temperatures (in the absence of geothermal activity) of about 147°C. The largest natural recorded temperature range at one location is 105 °C (at Verkhoyansk, Siberia). High temperatures (above 48 °C), low temperatures (below 0°C) and large temperature ranges, particularly if they occur within a short period of time, may be considered extreme.

What are normal temperatures? The Earth is, on average, a cold place. More than two-thirds of the Earth's surface is covered by ocean and the temperature of most of the ocean stays close to 2°C. Including the ocean depths, the polar ice caps and the land, four-fifths of the planet is below 5 °C all the time. What we might think of as 'normal' temperatures, say 10°C to 30°C, are really not normal at all but occur only in restricted parts of the world. Abundant life is associated with the warmer parts of the Earth (but not too warm!).

Organisms on land experience the weight of the air pressing down on them. We are so used to this that we are barely conscious of it unless it suddenly changes, as, for example, in a plane where our ears may pop as ascent or descent results in different pressures on either side of our eardrums. At sea level, the pressure is 1 kilogram per square centimetre or 1 atmosphere. Changes in this pressure may be experienced as a stress by an organism. Most organisms do not experience low pressure naturally. Even high mountains will not produce a lowering in pressure sufficient to stress an organism (although there may be a problem with a lack of oxygen). Some bacteria, nevertheless, can happily grow at low pressures, and even in a vacuum, which is an issue for the spoilage of vacuum-packed food.

Terrestrial organisms also rarely experience high pressures. Even the deepest mine does not produce a significant increase in pressure (at a depth of 1500 metres, the pressure is only about one-sixth higher than at the surface). In the ocean, however, changes in pressure with depth are significant. The hydrostatic pressure (the weight of water pressing on the organism) increases by 1 atmosphere for every 10 metres' depth. Even at the bottom of shallow coastal waters or a lake, the pressure is several times greater than that experienced at the surface. In the deepest parts of the ocean, some 11 kilometres deep, the pressure is 1100 atmospheres (that is, 1100 times that found at the surface). This represents a considerable stress for any organism that might live there (1100 kilograms of pressure per square centimetre - imagine five elephants standing on the tip of your finger!).

When some substances dissolve in water, they result in the splitting of water molecules (H2O) to generate hydrogen (H+) or hydroxyl (OH-) ions. A high concentration of H+ ions (and a low concentration of OH~ ions) produces an acid solution, whereas a low concentration of H+ ions (and a high concentration of OH~ ions) produces an alkaline solution. The degree of acidity or alkalinity is indicated by the pH of the solution. This is a measure of the concentration of H+ ions in the solution (on a logarithmic scale). Water is pH 7.0, which is considered neutral (neither acid nor alkaline), solutions with pH values lower than 7.0 are acidic and those with pH values higher then 7.0 are alkaline.

What is a normal pH? A neutral pH of 7.0 is usually thought of as normal, but, in fact, many natural sources of water (seas, lakes, rivers, soil water) are slightly acidic with a pH of 5.6. This is because carbon dioxide, from the atmosphere, dissolves in the water to produce a weak acid (carbonic acid). Despite living in slightly acidic conditions, the pH inside most cells is 7.7. Living cells control their internal pH at this level because it is optimum for the functioning of their enzymes - the biological catalysts which control the chemical reactions of organisms and enable them to grow, reproduce and maintain their organisation. Strong acids (like sulphuric acid) and alkalis (such as caustic soda) not only prevent the efficient functioning of enzymes but can also destroy the proteins, membranes and other structures which make up the body of the organism.

When substances dissolve in water, they impose other stresses on organisms, in addition to the effect that some of them have on the pH. When common salt (sodium chloride) is added to water, it dissolves; if more is added, more will dissolve, until no more can dissolve and the solution is saturated. As more salt is added, the concentration of salt in the solution increases, but, conversely, the concentration of water in the solution decreases (the amount of water in the container remains the same but the amount of salt is increasing and so the concentration of water is decreasing). When an organism is placed in a salty solution, if the concentration of water outside the organism is lower than that inside, water will leave the organism (unless it is able to prevent it from doing so) to try and restore the balance (a process called diffusion in which molecules, such as water, move from a region of high concentration to one of low concentration). The alternative is that the salt would diffuse into the organism's cells. However, the membranes of cells allow some substances to pass through them but not others (they are permeable to some substances but impermeable to others, and are referred to as being semi-permeable). The cell membrane is permeable to water but impermeable to salts (or at least more permeable to water than it is to salts). The diffusion of water across a semi-permeable membrane is called osmosis and a condition in the external environment that results in such movement is referred to as an osmotic stress.

When the salt and water concentrations inside and outside cells are equal, there will be no osmotic stress and we might perhaps think of this as being the normal (or, at least, unstressed) condition. This is true of many marine organisms whose internal fluids have the same osmotic concentration as the surrounding seawater. The cells of the organism will be under an osmotic stress if the internal and external conditions are not equal. There are two types of osmotic stress. A hyperosmotic stress is when the concentration of water outside the cell is lower than that inside (due to a higher salt concentration) and water leaves the cell. This dehydrates the cell and causes it to shrink. A

hyposmotic stress is when the concentration of water outside the cell is higher than that inside (due to a lower salt concentration outside the cell) and water enters the cell. The entry of water due to a hyposmotic stress causes the cell to swell and, if the cell doesn't remove the excess water, it is in danger of bursting. Most organisms regulate the internal salt concentration (or osmotic concentration) of their cells so that it is slightly higher than that of the external environment. This creates a slight positive pressure within the cell (like a gently inflated balloon) which keeps the cell firm (turgid).

Organisms that live in seawater (in which the salt concentration is approximately equivalent to a 0.85 per cent solution of sodium chloride) will experience an hyposmotic stress if they are transferred into freshwater, and most will die. The pressure generated by the osmotic stress can be considerable. For a cell whose internal osmotic concentration equals that of seawater, the entry of water upon immersion in freshwater creates a pressure of 22.4 atmospheres (22.4 times normal atmospheric pressure, about the same pressure experienced by a diver at a depth of half a kilometre). Conversely, freshwater organisms may die if exposed to hyperosmotic stresses by immersion in seawater. Extreme hyperosmotic stresses can develop in lakes and ponds where salts dissolved from the surrounding rocks become concentrated by the evaporation of water. The Dead Sea is one such place and has a salt concentration of 28 per cent.

As well as regulating their water contents, organisms also regulate the substances that are dissolved in their internal water. When sodium chloride dissolves in water, it splits into a sodium ion (Na+) and a chloride ion (Cl-). These, together with potassium ions (K+), are the main inorganic ions found in the fluids of organisms. Lower concentrations of calcium (Ca2+), magnesium (Mg2+), sulphate (SO4-), phosphate (PO3-) and bicarbonate (HCO-) ions are also found. High or low concentrations of these various ions may stress the organism.

Most organisms release energy from sugars such as glucose by utilising oxygen from the atmosphere to oxidise them (they 'burn' them via aerobic respiration to release the energy stored within their chemical structure). For these aerobic organisms, low oxygen concentrations in their environment may be stressful. Low levels of oxygen are found in environments where the oxygen has been used up by biological activity (such as in the centre of a compost heap or in the mud of an estuary) or in sites where there is limited access to oxygen (such as in the centre of the intestine of a cow). Some organisms, however, can survive temporary or permanent exposure to conditions where oxygen is absent, or at low concentrations, by respiring anaerobically (in the absence of oxygen). Most processes of anaerobic respiration use food much less efficiently than aerobic respiration. An anaerobic organism thus generally gets much less energy from a given amount of food than does an aerobic organism. This may limit the rate at which they can grow and reproduce.

Therefore, the main physical stresses on organisms are temperature extremes, pressure, desiccation, acidity or alkalinity, osmotic and ionic stress and low oxygen levels. Others include toxic chemicals and radiation (particularly, ultraviolet radiation).

The necessities of life

There are only three essentials for sustaining life - a source of energy, water and a range of conditions that the organism can tolerate (i.e. conditions that lie within the life box of the organism).

Living organisms must burn fuel for metabolism which drives growth, reproduction and the maintenance of the structure and integrity of their bodies. Animals and some microbes gain this fuel from the consumption or decomposition of other organisms (they are hetero-trophs, 'feeding on others'). Plants and some other microbes are autotrophs ('self-feeders'), which means they can utilise energy to convert inorganic materials (such as water and carbon dioxide) to organic materials (such as those that make up the bodies of living organisms). There are two main sources of energy. Plants and plant-like microbes utilise the energy of sunlight via the process of photosynthesis (phototrophs, 'light-feeders'). Photosynthesis converts water and carbon dioxide, from the atmosphere, into glucose, an organic sugar. The sugar then acts as the fuel for the cell's metabolism. This is achieved most efficiently by breaking down the sugar in the presence, and with the involvement, of oxygen (oxidising or 'burning' the sugar). Some microbes can derive their energy from the oxidation of chemicals (chemotrophs, 'chemical feeders'). Sulphur bacteria get their energy by oxidising sulphur to sulphur dioxide, which then dissolves in water to form sulphurous acid and then sulphuric acid. Other bacteria perform similar tricks with iron, ammonia or hydrogen.

The second essential for life is water, or at least periodic access to water. Water is part of the structure of many of the molecules that make up living organisms. It also provides the medium in which the chemical reactions of living organisms take place. Without water, there is no metabolism and hence no life. However, as we will see, cryp-tobiotes can suspend metabolism, and perhaps life, and survive periods of complete water loss. Where there is liquid water and a source of energy which an organism can utilise, there will be life.

The forms of life

Let us look at some of the different types of organisms which we will encounter in this book. The forms of life are tremendously varied. Scientists have attempted to make sense of this variety by devising various schemes for arranging organisms into groups on the basis of their similarities and differences. This process, known as classification, has a long history. Linnaeus (1707-1778), a Swedish naturalist, was the first to attempt such a classification, dividing living things into animals and plants. Linnaeus was, however, limited to organisms he could see with his naked eye. The invention of the microscope and the discovery of the great variety of microorganisms made things much more complicated. Later schemes used various lines of evidence (such as morphology, embryology, geographical distribution, physiology and the fossil record) to develop classifications that attempted to express the evolutionary relationships between organisms.

One of the most important divisions of life is into organisms which consist of a single cell (unicellular) and those which consist of many cells (multicellular). Most microorganisms are unicellular. Plants, animals, fungi, slime moulds and some algae are multicellular. Another important division is based on the types of cells that make up the organism. Eukaryotic cells have their genetic material contained within a nucleus which is bounded by a nuclear membrane. The remaining part of the cell consists of the cytoplasm, which contains membrane-bound organelles of various types and functions. Prokaryotic cells are much simpler. The genetic material of a prokary-otic cell is concentrated into a particular region, but there is no membrane separating this from the rest of the cell and hence no nucleus. There are also no organelles in prokaryotic cells. The unicellular pro-tists and yeasts are eukaryotes, as are most multicellular organisms. The remaining unicellular microorganisms are prokaryotes.

These classification schemes, which were based largely on the criteria of morphology and lifestyle, culminated in the five-kingdom model (proposed by Robert Whittaker of Cornell University in 1969) which divided life into five major groups or kingdoms (Figure 1.5): animals, plants, fungi, protists and bacteria. Animals are multicellular eukary-otes which rely on other organisms as a source of organic molecules (usually by eating them). There are 30 or so different phyla of animals (representing different body plans), ranging in complexity from sponges, jellyfish and worms through to birds, mammals and humans. You'll find descriptions of the main groups mentioned in the text in the glossary at the end of this book. Plants are multicellular eukaryotes which produce their own organic materials via the process of photosynthesis using energy from sunlight and which live mainly on land. The simplest plants (such as liverworts and bryophytes or mosses) lack roots or well-developed vascular tissues (tubes which transport water and nutrients). The simplest vascular plants (such as ferns and horsetails) do not produce seeds but reproduce via spores. Gymnosperms (conifers and cycads) produce naked seeds, unprotected by a seed coat, whereas angiosperms (flowering plants) have protected seeds.

Fungi are eukaryotes which do not photosynthesise but which do not ingest food either, as do animals, but absorb it after secreting

Fungi

Monera (bacteria)

ugure 1.5 The five-kingdom classification of life.

enzymes to the outside of their bodies. They are mainly multicellular, but include the unicellular yeasts. The remaining unicellular eukary-otes are classified as protists along with some multicellular eukaryotes which do not fit in with the animals, plants or fungi. Protists are thus a bit of a grab bag of organisms (with some 30 to 40 phyla included) which are perhaps not that closely related. The protists include animal-like (protozoa), plant-like (algae) and fungus-like organisms (the multicellular slime moulds). Seaweeds are multicellular algae.

In the five-kingdom model, all prokaryotic organisms were grouped together as bacteria (Monera). More recent studies have indicated that there is at least as much diversity, and probably more, among prokary-otes as there is among eukaryotes and that there are several kingdoms of prokaryotes. Bacteria include organisms with a great variety of lifestyles including heterotrophs (which feed on organic molecules) and autotrophs (which make their own). The autotrophs include photo-autotrophs (which photosynthesise, such as cyanobacteria) and chemoautotrophs (which obtain energy by oxidising various inorganic substances). Most are unicellular, although some form colonies or

22 LIFE AT THE LIMITS

aggregations and some even show a primitive multicellular organisation in which there is a division of labour between two or more specialised types of cell (some cyanobacteria).

The five-kingdom model clearly has trouble classifying all the organisms that we can observe on Earth. This has led to additional kingdoms being proposed and to groups of organisms being transferred from one kingdom to another (such as including some multicellular algae along with the plants). Some organisms, however, don't seem to fit in with these schemes. Viruses consist of genetic material (DNA or RNA) enclosed within a protein coat, and are not cells at all. They can only reproduce by infecting the cell of another organism and using the host cell's machinery to produce copies of the virus. Perhaps viruses cannot even be considered to be alive, since they cannot reproduce without the aid of a host cell. Lichens can be commonly observed growing on the surface of rocks, walls and tombstones. They turn out to be not a separate type of organism but a close association (symbiosis) between two different types of organisms: a fungus and an alga or cya-nobacterium.

The main problem with traditional models of classification is that the criteria they use are, at least to some extent, subjective. The growth in the methods of molecular biology has allowed the development of molecular techniques that produce classifications based on more objective criteria. These techniques compare the sequences of sub-units (amino acids or nucleotides) that make up the structure of particular proteins and nucleic acids which are found throughout the organisms to be classified. Mutations produce changes in the sequences of these molecules which accumulate over time. Two organisms that have similar sequences are thus closely related and those with different sequences are distantly related, with the degree of difference associated with how far back in evolutionary time they parted company. These techniques have revealed a very different picture from that based on morphological and physiological criteria (Figure 1.6). The diversity of prokaryotes has been unveiled. The bacteria have been shown to consist of two domains (each containing a number of king-

Bacteria

Archaea

Eukarya

Methanogens

Methanogens

figure 1.6 A simplified tree of life based on studies which use molecular systematic data. Life is divided into three domains: Bacteria, Archaea and Eukarya. The branches represent kingdoms within these domains. Only kingdoms of organisms mentioned in this book are labelled. The protistans, which were grouped together in the five-kingdom classification, are recognised as separate kingdoms. The branches of the Archaea represent functional, rather than phylogenetic, groupings.

figure 1.6 A simplified tree of life based on studies which use molecular systematic data. Life is divided into three domains: Bacteria, Archaea and Eukarya. The branches represent kingdoms within these domains. Only kingdoms of organisms mentioned in this book are labelled. The protistans, which were grouped together in the five-kingdom classification, are recognised as separate kingdoms. The branches of the Archaea represent functional, rather than phylogenetic, groupings.

doms): the bacteria and the archaea. These are more different from one another than is a toadstool from a whale. The third domain, the eukarya, includes all the eukaryotic organisms. The archaea are associated particularly with extreme environments. They include three main functional groups: the methanogens (which produce methane gas as an end product of their metabolism), extreme halophiles (which live in very salty conditions) and extreme thermophiles (which live in high temperatures, such as hot springs and deep-sea hydrothermal vents). The archaea were once considered to be a form of bacteria (archaeabac-teria) but they are now recognised as a distinct group. I may, however, in places still refer to them as bacteria - for which may the archaeal biologists forgive me!

Another way of grouping organisms is on the basis of their mode of life. Primary producers manufacture sugars and other organic compounds using the energy from sunlight (plants, algae, cyanobacteria and other photosynthetic bacteria) or from the oxidation of various inorganic substances (some groups of bacteria). Consumers (animals, fungi, some bacteria and protists) satisfy their requirements for organic molecules and energy by eating (or otherwise utilising the products of) the primary producers (or other consumers). Some consumers are decomposers (fungi, bacteria, some animals), deriving their food from dead organisms or organic waste such as faeces and fallen leaves, and some are parasites (or other types of symbionts), living in close association (within or on) other organisms.

Size matters

The size of an organism to a large extent determines how much of a problem it has with an environmental stress. Imagine: you are late for dinner. This is not the first time, your husband or wife is annoyed with you and, instead of putting your dinner back in the oven, has left it out on the table. If you're only 10 minutes late, the potatoes may still be hot but the peas have gone cold. Ten more minutes and the potatoes have gone cold too. How is it that the peas cool quicker than the potatoes? The answer is, of course, that size matters. Small things lose (or gain) heat faster than large things. This is because objects lose heat through their surface and the rate at which heat is lost from an object depends on the area of its surface compared with its volume. A pea has a larger surface area relative to its volume than does a potato and therefore loses its heat faster. A small object also has less heat to lose than a large object. Small organisms thus have more problems in regulating their temperature than do large organisms.

What is true for heat is also true for substances that pass through the surface to enter or leave an organism. The rate at which oxygen can pass (diffuse) through the surface of a small organism may be sufficient to supply it with the oxygen it needs. For large animals, such as humans, diffusion across the surface is not sufficient to supply their requirements. This problem is solved in humans, and most other terrestrial vertebrates, by the development of lungs. The lungs consist of a branching network of air passages (tracheae, bronchi, bronchioles)

which end in air pockets (alveoli). This provides a large surface area across which the uptake of oxygen can occur. The total surface area of the lungs of humans is 100 square metres, while that of the skin is two square metres. The area available for oxygen uptake through the lungs is thus 50 times greater than would be available if oxygen uptake occurred via the skin.

While small organisms have much less of a problem with oxygen uptake than do large organisms, the reverse is true when it comes to desiccation. When exposed to desiccation, a small organism that loses water through its surface will dehydrate at a much faster rate than a large organism, because of its greater surface area relative to its volume. Desiccation is thus likely to be much more of a problem for a small organism than for a large organism. The organism cannot prevent desiccation by becoming completely impermeable to water - if the organism was impermeable to water, it would also not be able to breathe. There are no biological structures that are impermeable to water but permeable to oxygen. If water loss from the body is reduced by an impermeable skin or cuticle, there need to be openings, such as our noses or the pores in an insect's cuticle (spiracles) or in the surface of a plant (stomata), which let the organism breathe.

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