Answer
The environmental impact of prokaryotes is immense on such a scale that it would be hard to imagine the planet without them. They are responsible for the processes of decomposition and nitrogen fixation and are the likely source of the mitochondria which allow animal cells to respire, for example.
Work Step by Step
There are numerous examples that can be given here to demonstrate the extensive role prokaryotes of all kinds play on the planet. Aside from the ones above, you could elaborate on the carbon cycle, processing of environmental wastes, original oxygenation of earth's atmosphere, etc. In short, prokaryotes supported the creation of life as we know it, and they continue to sustain it.
