All living organisms contribute to the biogeochemical cycles, but microorganisms, due to their high abundance, their tremendous metabolic capacities and adaptation potential, play a key role in the functioning and the evolution of biogeochemical cycles. Consequently, they are keyplayers in adaptation, resistance and resilience of ecosystems. The role of microorganisms in the main biogeochemical cycles (carbon, nitrogen, sulfur, phosphorus, silicon, metals), in soils, freshwater and marine ecosystems is presented. Microbial processes involved in the turnover of biogeochemical cycles are discussed from gene to ecosystem (natural and anthropogenic ecosystems), at global, regional and local scales, as well as in targeted microenvironments (such as particles or microniches). The biodiversity of microorganisms is highlighted and their metabolic pathways on which are based exchanges and biotransformations of organic and mineral components within ecosystems are described in details. The impacts of human activities on the microbial actors and processes of biogeochemical cycles, and the cascading ecological effects (greenhouse gas emissions, acid rains, dystrophic crises, etc.), are also discussed.