Microorganisms in the Environment

At various points in this book we have referred to the different environments in which particular microorganisms are to be found. Like other living organisms, they live as part of ecosystems, and we shall consider the three main types of ecosystem - terrestrial, freshwater and marine -later in this chapter.

First, however, we must turn our attention again to the subject of energy relations in living things. In Chapter 4, we looked at the different ways in which microorganisms can derive and utilise energy from various sources. We now need to put these processes into a global perspective. All organisms may be placed into one of three categories with respect to their part in the global flow of energy:

• (Primary) Producers: autotrophs that obtain energy from the sun or chemical sources (e.g. green plants, photosynthetic bacteria, chemolithotrophic bacteria). They use the energy to synthesise organic material from carbon dioxide and water.

• Consumers: heterotrophs that derive energy through the consumption of other organisms (producers or other consumers). They may serve as a link between the primary producers and the decomposers.

• Decomposers: organisms that break down the remains and waste products of producers and consumers, obtaining energy and releasing nutrients, including CO2, that can be reused by the producers.

Natural systems exist in a balance; carbon and all the other elements that make up living things are subject to repeated recycling, so that they are available to different organisms in different forms. Think back to Chapter 6, in which we discussed how algae, green plants and certain bacteria capture light energy, then use it to synthesise organic carbon compounds from carbon dioxide and water. What happens to all this organic carbon? It does not just accumulate, but is recycled by other living things, which convert it back to carbon dioxide by respiration. This can be seen in its simplest form in Figure 16.1. Many other elements such as sulphur, nitrogen, and iron are similarly changed from one form to another in this way, by a cyclic series of reactions. Microorganisms are responsible for most of these reactions, oxidising and reducing the elements according

Living organisms, together with their physical surroundings, make up an ecosystem.

^^ Autotrophs

Organic carbon CO2

Organic carbon CO2

Figure 16.1 The carbon cycle. Autotrophs fix CO2 as an organic compound, which het-erotrophs convert back to CO2. The recycling of carbon satisfies the requirements of both nutritional types to their metabolic needs. The continuation of life on Earth is dependent on the cycling of finite resources in this way.

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