ABSTRACT
Introduction Natural environments are usually inhabited by a diverse population of micro-organisms. These can encompass a wide range of physiological and nutritional types, from autotrophs to heterotrophs, from psychrophiles to hyperthermophiles and from obligate aerobes through micro-aerophilic bacteria to oxygen-sensitive anaerobes. There are exceptions to this general rule, usually where the environment has been manipulated by man or where the extreme physicochemical nature limits the types able to grow. Examples of the latter include highly alkaline and saline lakes and highly acidic hot springs. Even under these extreme conditions, however, diversity can be much greater than was thought possible in earlier years – in recent years many other concepts in environmental microbiology have been replaced. It was previously considered, for example, that micro-organisms dwelt in discrete compartments for each physiological and nutritional type but it is now recognized that most microbial ecosystems are heterogeneous. More recently still, advances in methodology, in the framework of challenges to earlier orthodoxies, have gone a considerable way to elucidate the development and function of such diverse populations.
