There are many substances that are toxic to organisms, but, in most cases, it depends on their levels of exposure. We can cope (quite happily!) with a moderate intake of alcohol (ethanol), but large quantities are likely to make us feel quite ill or even kill us. Even too much water can be fatal. Some substances, however, are toxic in low concentrations. Toxins are produced by some organisms themselves as protection against being eaten or to paralyse their prey. The arrow-poison frogs (Dendrobates and Phyllobates) of Central and South America, for example, have glands in their skin which produce a variety of toxins to discourage predators.
Mineral deposits can generate a toxic environment of non-biological origin. Natural accumulations of the element sulphur occur in association with volcanic activity and in sedimentary deposits. Some sulphur is essential for life; it forms part of the structure of many proteins. However, some of its compounds are extremely toxic. Toxic sulphur compounds associated with natural deposits of sulphur include hydrogen sulphide (the 'bad eggs' gas) and sulphuric acid. Most organisms do not tolerate these. There are, however, some bacteria which actually use the oxidation of sulphur compounds as an energy source (sulphur bacteria). Thiobacillus, for example, oxidises thiosul-phate and elemental sulphur to sulphate. The bacteria that are associated with deep-sea hydrothermal vents, and which are in symbiotic associations with some of the animals that live there (see Chapter 2), gain their energy mainly from the metabolism of hydrogen sulphide.
High levels of heavy metals and some other elements (such as lead, copper, arsenic and antimony) are toxic to most organisms. There are microorganisms which can tolerate these elements and may even use them in their metabolism. These can be useful for cleaning up polluted areas. There are also microorganisms that can utilise any form of naturally occurring organic compounds which derive from biological activity. Microbes which degrade petroleum, and other types of hydrocarbon deposits, may be a nuisance under some circumstances, but are useful for cleaning up spills. Not all organic compounds are metabolised by microorganisms. Some of human origin (such as plastics, detergents and pesticides) are not degraded and thus accumulate in the environment, often with harmful effects.
Although this chapter, and the three which preceded it, have considered environmental stresses largely in isolation from each other, organisms are usually faced with a combination of physical and biological challenges. In Chapter 2, the stresses faced by organisms living in some of the extreme environments found on Earth were outlined. Desert organisms, for example, face problems of water availability, high temperatures, high solar radiation and a scarcity of nutrients. Terrestrial Antarctic organisms not only must survive low temperatures but also face problems with restricted and periodic access to liquid water. They may also be exposed to high levels of UV radiation during the summer and have to cope with transient and unstable substrates. There are common features between the effects of different environmental stresses on organisms and the response of organisms to them. Freezing, desiccation and osmotic stress, for example, all produce problems with the water content of cells. It is thus not surprising that the mechanisms which enable organisms to survive these three stresses have some features in common. I will look at this in more detail in Chapter 8.
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