An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system These biotic and abiotic components are linked together through nutrient cycles and energy flows. Ecosystems are controlled by external and internal factors. External factors such as climate, parent material which forms the soil and topography, control the overall structure of an ecosystem but are not themselves influenced by the ecosystem. Physicochemical factors (physical and chemical conditions) are abiotic factors that affect the environment for geomicrobes.