As you read the chapters that considered specific extreme environmental stresses (Chapters 3-6: desiccation, heat, cold, pH, osmotic stress etc.), you might have noticed that there were some similarities between the problems these caused to organisms. There are also some common physiological and biochemical solutions that organisms have developed to overcome these difficulties. Let us look at some of these problems and solutions.
Water is essential for the functioning of cells and any environmental stress that disrupts the cell's water balance is a serious problem for an organism. Exposure to desiccation is the most obvious cause of water loss, but water is also lost, or has the potential to be lost, during exposure to other types of environmental stress. Osmotic stress produces the movement of water, resulting in the loss of some water if the concentration of salts outside the cell is higher than that inside (since the concentration of water is lower outside than it is inside). Heat increases the rate of evaporation of organisms exposed to desiccation and this results in increased rates of water loss from the surface of terrestrial plants and animals. Mechanisms for cooling the organism (such as sweating, panting and transpiration) also produce increased rates of water loss. Water loss from cells is also a problem during freezing. Freezing of the liquid surrounding the cells raises its osmotic concentration. Salts are not incorporated into the ice and so, as water molecules join the growing ice crystals, the salts in the remaining unfrozen water become more concentrated. This results in an osmotic stress that draws the water out of cells (see Figure 5.1).
Problems with water may be reduced by restricting the rate at which it is lost and by measures that tend to retain it within the organism. The rate of water loss is reduced if the membranes of cells, or the skins or cuticles of plants or animals, have a low permeability to water. Many plants and insects, for example, have waxy cuticles and water loss is restricted to openings (stomata and spiracles) in their surface coverings. Water may be retained by animals by recovering it from the breath, urine and faeces before it is lost from the body. Desert organisms tend not to use the evaporation of water as a cooling mechanism, since they cannot withstand the loss of water that this involves. Instead, they either tolerate the high temperatures or they avoid them by retreating to refuges during the hottest parts of the day.
One way of reducing the problems with water caused by conditions which result in an osmotic stress is to accumulate substances within the organism or cell that reduce the difference between it and its surroundings. These substances are known as osmolytes since they take up space in a solution, thus reducing the osmotic concentration of water and balancing its levels inside and outside the cell. A variety of substances are used as compatible osmolytes (so called because they do not adversely affect the working of cells at the levels of concentrations to which they accumulate). Amino acids (e.g. proline, glycine, alanine and serine), substances derived from amino acids (e.g. glycine betaine and taurine), nitrogenous compounds (e.g. urea), sugars (e.g. trehalose, sucrose) and sugar alcohols (e.g. glycerol, sorbitol) are all used as compatible osmolytes by different organisms. Sodium, potassium and chloride ions are the most important solutes in cells. Changing the concentrations of these ions in response to osmotic stress gives a measure of control of water balance, but the extent to which this can be achieved is limited, since these ions are incompatible solutes and they are usually toxic to the cell in high concentrations.
Sugars, sugar alcohols and amino acids are also produced as antifreezes by freeze-avoiding insects and as cryoprotectants by some freezing-tolerant organisms (see Chapter 5). Although these substances play a number of roles in tolerating low temperatures, their ability to act as compatible osmolytes assists in dealing with the problems of water balance that result from exposure to subzero temperatures. Sugars are also produced by anhydrobiotic organisms in response to desiccation, protecting membranes and proteins against disruption as a result of water loss (See chapter 3). However, by raising the osmotic concentration of the body fluids, increased levels of osmolytes may also help slow down the rate of water loss from the organism. If the levels of osmolytes are high enough, the organism may even be able to absorb water from an otherwise desiccating environment.
Springtails (collembola) are common inhabitants of soil. They have very permeable cuticles which show little resistance to water loss. As the soil dries out, the springtail is at risk of losing water and of dying, since only a few springtails can survive anhydrobiotically. Folsomia candida is a common soil collembolan that can tolerate prolonged exposure to soils at 98.2 per cent relative humidity. The animal would be expected to lose water in soils below 100 per cent relative humidity. The springtail prevents this by producing high concentrations of glucose and myoinositol (a sugar alcohol). This raises its internal osmotic concentration sufficiently to arrest water loss to the environment. The springtail then starts to reabsorb water from the surrounding atmosphere. This enables it to remain active in the root zone of plants through a similar range of drought intensities tolerated by the plants themselves.
The accumulation of osmolytes is thus a common response to stress that involves problems with water balance. Trehalose, for example, is produced (by different organisms and under different circumstances) in response to desiccation, low temperatures, high temperatures and osmotic stress.
Maintaining the integrity of cell membranes is essential for the survival of an organism. The outer plasma membrane separates the cell from the environment and controls the exchange of materials between the inside and outside of the cell. Membrane systems within the cell (such as those of the endoplasmic reticulum, nucleus and mitochondria) play many important roles in the metabolism of cells. Cell membranes consist of lipids, proteins and carbohydrates. The lipid component, in particular, is important for maintaining the structure of the membrane and is susceptible to damage by changes in physical and chemical conditions.
Changes in temperature may produce a change in the state of the lipids and hence in the structure and properties of membranes. Membrane lipids melt at high temperatures. The membranes of thermophilic bacteria have a higher proportion of saturated fatty acids than do the membranes of bacteria that live at lower temperatures. This change in lipid composition makes their membranes more stable at high temperatures. The membranes of hyperthermophilic archaea are stable at even higher temperatures; their membranes have a different structure from those of other organisms, consisting of a monolayer rather than a bilayer (see Figure 4.4). This is responsible for their stability at extreme temperatures. Membrane lipids can also undergo a change in physical state and hence a loss of biological function as they solidify at low temperatures. The membranes of cold-tolerant organisms tend to have a higher proportion of unsaturated fatty acids than do those that live in higher temperatures (the reverse of the situation in thermophilic bacteria). Part of the cold-hardening process in plants involves an increase in the ratio of unsaturated to saturated fatty acids in their membranes (see Chapter 5).
The integrity and function of membranes is also threatened by desiccation. The structure of membranes is maintained by interactions between its lipids and water. As water is removed, the structure of the membrane changes and this can result in the fatal loss of cell contents once water returns. Trehalose (or sucrose in plants) is thought to replace water during desiccation, preventing changes in membrane structure and enabling organisms to survive anhydrobiotically (see Chapter 3). In plants, and perhaps other organisms, proteins (dehy-drins) may also be involved in stabilising membranes.
Both the structure and metabolism of cells depend on the functioning of proteins. In order to perform their roles in cells, proteins must maintain their correct folding or shape (conformation). A number of stresses disrupt the weak chemical bonds that hold the shape of proteins together, causing them to unravel, or otherwise change their shape, and lose their ability to function correctly (they denature). Denatured enzymes can no longer catalyse biological reactions and denatured structural proteins can no longer fulfil their roles in an organism. A stress which results in the permanent denaturation of a significant proportion of an organism's proteins brings about its death. Protein denaturation can result from high temperatures, low temperatures, desiccation, high pressures, high salt concentrations and acidic or alkaline conditions. Organisms have ways of preventing, or ameliorating the effects of, protein denaturation.
Molecular chaperones are a group of proteins that assist in the folding of other proteins, preventing them from sticking to each other and enabling them to attain a structure that allows them to function. These were first described as heat shock proteins, since they are produced in response to a mild heat stress (see Chapter 4). They are, however, triggered by every type of environmental stress that has been studied and they also function in unstressed cells. Protecting the conformation and function of proteins is a universal and ancient problem. Molecular chaperones are found in all organisms and must have arisen early in the evolution of life. It might be expected that molecular chaperones play a greater role in the biology of organisms from high stress/high variability environments than in those from low stress/low variability environments. As well as these molecular chaperones, other substances, such as trehalose, are involved in stabilising proteins and preventing their denaturation during exposure to stress.
Proteins that function as enzymes work best under a particular set of physical and chemical conditions. Where environmental conditions are outside the normal range, the enzymes must either be modified to function under these extreme conditions or the internal environments of cells must be modified to make them less extreme than those that surround them. Enzymes from organisms in cold, hot and high pressure environments are adapted to work under these conditions. These extremozymes are of interest for practical applications that require enzymes which function under extreme conditions.
Just like proteins, the nucleic acids that carry the genetic code (DNA) and those responsible for its translation (RNA) can be disrupted by exposure to various environmental stresses. Such changes are produced by, among other things, exposure to radiation, high temperatures, chemicals and oxygen radicals. Changes in DNA produce mutations that are usually harmful to the organism. Cells have a variety of mechanisms that limit or repair damage to nucleic acids and thus prevent the production of proteins that are malformed and useless. DNA molecules consist of two strands. If one of the strands is damaged, it will no longer pair up with the undamaged strand. The several different types of DNA repair mechanism rely on this lack of alignment between damaged and undamaged strands of the molecule. Enzymes called DNA repair nucleases recognise the altered portion of the molecule and separate it from adjacent unaltered regions on the same strand. A new copy of the damaged region is then made from the equivalent undamaged strand and inserted into the damaged strand to replace the altered region. These DNA repair mechanisms are particularly efficient in organisms that live in environments that are likely to induce high levels of DNA damage, such as in Pseudomonas radiodu-rans, a bacterium that survives in the cooling waters of nuclear reactors. As well as DNA repair, there are mechanisms that protect genes from damage and chromosomes contain proteins (histones) that bind to their DNA and help stabilise it.
Stress produces a greater rate of mutation, and of recombination (the process by which sexual reproduction produces new combinations of genes in the next generation by shuffling of genes from parents), which natural selection may act on to produce adaptation to stressful environments. Organisms exposed to stressful conditions thus tend to produce more variable offspring than those that are not exposed to stress. The increased variability may have a positive effect on the evolution of the organism in stressful conditions, since evolution depends on there being variability for the process of natural selection to have its effect. Stress may also, however, produce mutations that are harmful to the organism and the overall effects of stress are not necessarily beneficial in terms of the evolution of an organism. However, if some of the mutations induced by stress produce adaptations that help the organism survive the stressful conditions, these will be selected for and the organism will evolve to become better adapted to its environment.
A physiological definition of extreme?
Organisms that live in extreme environments have specific physiological and biochemical solutions to the problems which confront them. Perhaps we could recognise an environment as being extreme by the presence of such adaptations among the organisms that live there. There is, however, the danger of producing a circular argument - an extreme environment is considered extreme because the organisms that live there have adaptations to live in an extreme environment. All organisms have adaptations to the environment in which they live. An enzyme of a polar fish that works at —1.9 °C, or of a hot-spring bacterium which functions at high temperatures, no more defines that environment as being an extreme environment than does the presence of an enzyme that functions best at 20°C in a tropical fish. Maybe we could recognise an adaptation to an extreme environment as being one that was not possessed by the organism's ancestors, which inhabited less extreme conditions. I will look for an evolutionary solution to the problem of defining extreme environments later in this chapter.
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