Do you think microbes can also be used as sources of energy? If yes, how?

Explanation

Yes. There are certain bacteria which grow anaerobically on cellulosic material and produce large amount of methane along with `CO_(2)` and Hr. These bacteria are called methanogens.
The excreta of cattle (dung) commonly called 'gobar' is rich in these bacteria and undigested cellulose. Therefore, dung can be used to generate a gaseous fuel called biogas or gobar gas.