ABSTRACT
Ever since life appeared on the Earth 3.5-4 billion years ago, with the first lower unicellular organisms, ancestors of actual bacteria, microorganisms (or protists) have colonised and grown in all the super-ficial terrestrial environment (soils, continental waters, oceans, sediments, atmosphere...). They have established themselves by adopting nutritive and energetic strategies, as shown in Figure 6.1, which are highly diverse and enable them to exist by using organic compounds (micro-organisms known as chemo-organotrophs or heterotrophs) as well as mineral compounds (bacteria known as chemolithotrophs or autotrophs). They grow and reproduce in the presence or absence of oxygen (aerobic or anaerobic respiration and fermentation).
