How important are microorganisms in the Earth's ecosystems? They are vital. Without microorganisms, the ecosystems of the world would collapse and die. First, they would be swamped in dead bodies or "knee-deep in dead elephants" as one witty microbiologist once told me, when emphasising the importance of decomposers in the ecosystem. Most decomposers are microorganisms and they are vital for removing the dead and recycling nutrients. Otherwise, those nutrients would be locked away and lost. So microorganisms play a vital role as decomposers and in this role also make soil. Plants cannot grow without healthy soils: without microorganisms there would be no healthy soils.
The second area in which microorganisms are vital to the continuation of life and earth's ecosystems is oxygen production. It has been said that rainforests are the lungs of the world. However higher plants probably only contribute half the oxygen to the world's ecosystems. The other half is produced by autotrophic microorganisms: algae and cyanobacteria (blue green algae). These single celled plants make up the phytoplankton of the oceans plus are found in fresh water lakes and ponds. The oxygen they produce is vital for the survival of the animals in all ecosystems.
Of course these plants, being autotrophs, produce food as well as oxygen. Almost all food chains begin with autotrophs. On land this is mainly the role of higher plants but in the oceans, the phytoplankton are the basis of all but a few marine food chains. Phytoplankton produce sugars and are consumed by the zooplankton which are consumed in turn by fish and whales. All higher marine animals depend on the food chains that begin with phytoplankton. Even benthic communities, which feed mainly on detritus filtering down from surface waters, are indirectly dependent on phytoplankton. The few communities that are independent of these microscopic plants, depend instead on other microorganisms such as bacteria. If the phytoplankton and zooplankton were to die out, the rest of the marine ecosystems would collapse and die too.
There are two other vital roles that microorganisms play in Earth's ecosystems. Many microorganisms are parasites on higher organisms and cause disease. This may seem to be a negative effect and yet it is vital for the evolution of species and therefore ecosystems. Parasites and predators remove weak individuals from the system, thus leaving enough food for the survivors to grow and reproduce. In systems where diseases or predators have been artificially controlled, populations increase beyond the carrying capacity of the ecosystem and the system collapses. So although parasites and disease-causing organisms cause a lot of pain and suffering, they are also necessary parts of natural systems and help to maintain the overall health of the ecosystem by removing weak individuals.
The last and least obvious area in which microorganisms are vitally important for ecosystems is as symbiotic partners with higher organisms. Cows could not digest grass without microbes. Termites could not digest wood without their gut flora. In fact most higher animals, including ourselves, are dependent on our gut flora to help us digest our food.
Plants too need their symbiotic microorganisms. Many higher plants have fungi that live in their roots and help with nutrient uptake. All lichens are a symbiotic relationship between fungi and single celled algae. Root nodule bacteria live in the roots of legumes and help them capture nitrogen which is vital for soil health.
In marine ecosystems, microorganisms form symbiotic relationships that are vital for the survival of those ecosystems. Coral reefs, the most diverse of marine ecosystems, can only form because of the single celled algae living in the tissues of coral animals and providing them with food. Without those algae, the coral reef ecosystems would collapse and die.
We think of ourselves and higher animals as being the most important beings on the planet but we are dependent on the other components of ecosystems. This includes those tiny organisms that we cannot even see, the microorganisms, which are vital in many different ways for the survival of Earth's ecosystems.