Microbes play an important role in our ecosystems because they are responsible for the decomposition of plant and animal matter. They were also a precursor to Oxygenic Photosynthesis (That being Anoxygenic Photosynthesis). They have an extremely vast impact because they aid in the formation of soil via physical weathering, they also have complex relationships with plants and animals, in the latter they help to digest food matter such as carbohydrates and in the former the protect them from disease and assist them with nutrients and water.
Work Step by Step
Its easy to see how microbes are decomposers by just looking at a moldy piece of bread. It explains a lot of how they break down complex substances