Apart from plants and animals, microbes form a permanent

Apart from plants and animals, microbes form a permanent biotic component in an ecosystem.While plants have been referred to as autotrophs and animals as heterotrophs, what are microbes referred to as?
How do these microbes fulfill their energy requirements?

Microbes are referred to as heterotrophs and saprotrophs. They fulfil their energy requirement by feeding on dead remains of plants and animals through the process of decomposition.