Existing somewhere in every part of the biosphere, microorganisms serve many purposes. While, from the point of view of some other organisms, these might not all be positive, each and every one is important to the survival of life on Earth.
One of the primary roles played by microorganisms is decay. Without the elimination of dead biological material there is no way for the various cycles of life to continue. Microscopic bacteria, fungi and animals consume dead plant and animal material. In so doing they release the carbon and nutrients that are tied up in the structures of these dead organisms releasing them back to the soil and the atmosphere.
In the oceans, microscopic plants and animals, phytoplankton and zooplankton, gather nutrients in the waters and make up the base of the oceanic food chain. Consumed by the smallest and the largest of ocean life these microbes are the key to life on the planet.
Phytoplankton, microscopic plants, absorb nutrients and carbon dioxide dissolved in the oceans water. Through photosynthesis they combine the carbon dioxide with water to make sugars. This process is the same as that used by trees and grasses and all other plants.
Zooplankton, microscopic animals, consume phytoplankton as well as bits of dead life whether floating in the oceanic currents or fallen to the sea beds. As the next step in the food chain of the oceans, zooplankton are also essential to life. While most zooplankton is invisible to the naked eye, and therefore qualifies as being considered microorganisms, some macroscopic free floating shrimp, jellyfish and other animals are considered zooplankton as well.
An additional form of plankton, bacterioplankton, acts in a manner similar to both phytoplankton and zooplankton. Because of their cellular structure, bacterioplankton can’t be considered the same as either of those. However, some bacteroplankton have chloroplasts and can perform photosynthesis, like phytoplankton. Others can’t do that and serve in the same ecological niche as zooplankton.
Zooplankton consume phytoplankton and bacteroplankton, as well as one another. Also living on these microorganisms are the small filter feeders such as clams and coral. But, even the largest animals on the planet survive on plankton. Large baleen whales are filter feeders. Using structures in their mouth to bring in enormous quantities of ocean water, these giant animals separate the plankton from the water for sustenance. Even the largest fish in the oceans, whale sharks, oceanic sunfish and giant manta rays, survive by filtering plankton from the seas.
Symbiotic microorganisms are essential to many, if not all, forms of macroscopic life. Coral cannot survive without the symbiotic algae that they harbor and protect within their small bodies. Termites could not digest the cellulose that they consume from wood without bacteria that live within their guts. Even humans rely on symbiotic microbes to help digest their food.
Even those processes that we might consider to be negative serve a useful purpose to the ecosystem as a whole. Microorganisms are often responsible for disease and can cause death. To the creatures that are sick this can be a terrible thing. But, when looked at from the perspective of the ecosystem this can result in an overall benefit. Disease is more likely to eliminate those organisms that are weak. In so doing they make room for the stronger to survive and become healthier. Often a sad fact, in the long run this can make for a stronger environment.
The interactions of microorganisms on our planet are diverse and essential. Without this, all life on Earth would cease to exist.