Nutritional categories

Microorganisms can be categorised according to how they obtain their carbon and energy. As we have seen, carbon is the most abundant component of the microbial cell, and most microorganisms obtain their carbon in the form of organic molecules, derived directly or indirectly from other organisms. This mode of nutrition is the one that is familiar to us as humans (and all other animals); all the food we eat is derived as complex organic molecules from plants and other animals (and even some representatives of the microbial world such as mushrooms!). Microorganisms which obtain their carbon in

A cofactor is a nonprotein component of an enzyme (often a metal ion) essential for its normal functioning.

A heterotroph must use one or more organic compounds as its source of carbon.

An autotroph can derive its carbon from carbon dioxide.

A chemotroph obtains its energy from chemical compounds. A pho-totroph uses light as its source of energy.

this way are described as heterotrophs, and include all the fungi and protozoans as well as most types of bacteria. Microorganisms as a group are able to incorporate the carbon from an incredibly wide range of organic compounds into cellular material. In fact there is hardly any such compound occurring in nature that cannot be metabolised by some microorganism or other, explaining in part why microbial life is to be found thriving in the most unlikely habitats. Many synthetic materials can also serve as carbon sources for some microorganisms, which can have considerable economic significance.

A significant number of bacteria and all of the algae do not, however, take up their carbon preformed as organic molecules in this way, but derive it instead from carbon dioxide. These organisms are called autotrophs, and again we can draw a parallel with higher organisms, where all members of the plant kingdom obtain their carbon in a similar fashion.

We can also categorise microorganisms nutritionally by the way they derive the energy they require to carry out essential cellular reactions. Autotrophs thus fall into two categories. Chemoautotrophs obtain their energy as well as their carbon from inorganic sources; they do this by the oxidation of inorganic molecules such as sulphur or nitrite. Photoautotrophs have photosyn-thetic pigments enabling them to convert light energy into chemical energy. The mechanisms by which this is achieved will be discussed in Chapter 6.

The great majority of heterotrophs obtain energy as well as carbon from the same organic source. Such organisms release energy by the chemical oxidation of organic nutrient molecules, and are therefore termed chemoheterotrophs. Those few het-erotrophs which do not follow this mode of nutrition include the green and purple non-sulphur bacteria. These are able to carry out photosynthesis and are known as photoheterotrophs.

There is one final subdivision of nutritional categories in microorganisms! Whether organisms are chemotrophs or phototrophs, they need a molecule to act as a source of electrons (reducing power) to drive their energy-generating systems (see Chapter 6). Those able to use an inorganic electron donor such as H2O, H2S or ammonia are called lithotrophs, while those requiring an organic molecule to fulfil the role are organotrophs. Most (but not all) microorganisms are either photolithotrophic au-totrophs (algae, blue-greens) or chemo-organotrophic heterotrophs (most bacteria). For the latter category, a single organic compound can often act as the provider of carbon, energy and reducing power. The substance used by chemotrophs as an energy source may be organic (chemoorganotrophs) or inorganic (chemolithotrophs).

A lithotroph is an organism that uses inorganic molecules as a source of electrons. An organ-otroph uses organic molecules for the same purpose.

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