Microbial Nutritional categories

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 inthis way are described as heterotrophs, and include allthe fungi and protozoans as well as most types of bacteria. Microorganisms as a group are able to incorpo-rate the carbon from an incredibly wide range of or-ganic 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 or-ganic 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 organ-isms, 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 energyinto chemical energy.

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 or-ganic 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 sourceof electrons (reducing power) to drive their energy-generating systems. 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 usedby chemotrophs as an energy source may be organic (chemoorganotrophs) or inorganic (chemolithotrophs).