The answer is I, II and III. These microbes can be funds even in hydrothermal vents at the bottom of ocean trenches where they convert chemicals to organic matter. Most are critical in the many nutrient cycles such as the nitrogen and carbon cycles. These microbes form the base of the food chain since they support higher organisms such as paramecium which are in turn eaten by higher organism such as amoeba. The amoebas are then fed by tadpoles, for example, and the tadpoles are eaten by fish…and etcetera.