In the natural environment, sulphur is continually recycled between reservoirs of oxidized and reduced sulphur, notably the oceans, evaporites and shales. In the short term the cycle approximates a steady state but throughout geological time there have been episodic shifts from steady state that have led, for example, to large-scale deposition of mineral sulphides and elemental sulphur and to fluctuations in oceanic sulphur chemistry. Current evidence suggests that dissimilatory sulphate reduction played a major role in the geochemical sulphur cycle for the last 2-2.8 Ga. It can also be assumed that sulphur-oxidizing bacteria participated in the oxidative phase of the cycle, although their contribution cannot be evaluated quantitatively. Biogeochemical cycles are interdependent and the activities of sulphur bacteria have important implications with respect to the geochemistry of other elements such as carbon, iron, alkaline earths and oxygen.