Microorganisms are the foundation of the cycles that take the essential elements of life from the environment to any organism and back again. Tiny as they are, microorganisms are the engines that keep the wheel of life turning.
The Carbon Cycle
This cycle takes carbon dioxide from the air and incorporates it into the life processes of microorganisms, plants, and animals. Then it returns it to the atmosphere again. After water, carbon is the most abundant element in living things. Microorganisms make the carbon cycle possible.
In the oceans and on land, microorganisms such as cyanobacteria and photosynthetic algae draw carbon dioxide from the air through photosynthesis. They combine this atmospheric gas with water and, using the energy of the sun, produce carbohydrates.
For this reason, photosynthesizing organisms are called autotrophs, or primary producers. They make the food that is the basis of all life. In the process, they liberate oxygen, which is a waste product to them, but essential for all animals to breathe.
Microorganisms produce far more oxygen than higher plants do. In habitats without oxygen, like the sediments of ocean bottoms, carbon must be converted to an organic form a different way, through chemosynthesis or through fermentation, in slower processes that work without oxygen.
The organic carbon from primary producers is consumed by the heterotrophs, those organisms that cannot make their own food. They are mostly animals and plants that are not green. They return this carbon to the atmosphere when they breathe out, and, along with the autotrophs, when they decompose.
Beneath layers of sediment, the earth holds layers of organic material that did not decompose. In fact, it is estimated that 99% of earth's carbon is locked in organic sediments, also called fossil fuels, composed mostly of organic remains processed by anaerobic bacteria.
Probably, plants processed by anaerobic bacteria formed coal, while prehistoric zooplankton and algae were heated and compressed into petroleum. It is the burning of these fuels which some believe to add heat-trapping carbon dioxide to earth's atmosphere.
The Nitrogen Cycle
All living things need nitrogen, the most common element in the air. However, atmospheric nitrogen is in the wrong form for most organisms to use. It must be fixed. The process of fixing nitrogen for use, and then using it, is the nitrogen cycle. It is an essential part of the ecosystem, and run by microorganisms.
Nitrogen can be fixed, converted into usable form, by bacteria in the soil. Nitrogen fixers include Rhizobium, which lives in a symbiotic relationship with legumes, and other bacteria which live free in soil. Actually, a whole chain of microorganisms participate in the nitrogen cycle.
In rice paddies, cyanobacteria fix nitrogen. Once it is fixed, nitrogen is in a biologically accessible form that can be taken up by plants. Plants draw up nitrates through their roots, to use in essential molecules like enzymes, chlorophyll, and DNA.
Once the plants have taken up the nitrogen, it may be returned to the soil when the plant decays, or it may be eaten by a grazing animal (the bacteria and other microbes in its specialized stomach will break down the plant fibers), which may be eaten by an omnivore or carnivore.
Bacteria in a predator's gut help it digest its food. The nitrogen will eventually return to the soil in any case, either in an animal's dung or in its ultimate decomposition. In the low oxygen conditions found in wetlands and sediments, anaerobic bacteria do the decomposition.
Producing rot is one of the major roles of microorganisms. It makes compounds available for new growth by recycling nutrients. It frees up space in the biome, the regional ecosystem, for new life to use.
The detritus of dead plants and animals is broken down by different organisms at different stages. Lignin, a tough wood compound, can only be broken down by certain fungi. Decomposition by bacteria and fungi releases carbon back into the atmosphere, making its reuse possible.
Microorganisms create human food, and are also found within us, helping digest it. The human intestines have about ten times as many microorganisms as the body has cells. Human intestines contain some fungi and protozoa, but mostly bacteria. The bacteria help train the immune system to recognize invaders. They also manufacture vitamin K and biotin. Some of them break down carbohydrates into a form that is efficiently absorbed by the body.
Microorganisms that we categorize as pathogens may sometimes serve a useful purpose too. They keep biomes in balance, helping to ensure that no one organism will take over.
When there are too many organisms of one kind, often some disease decimates them, knocking them back to a smaller role. The pathogen is often one that has existed in the population for some time, only becoming a problem when conditions become too dense and stressful.
Microorganisms produce oxygen, recycle carbon dioxide into food, and decompose dead material. They maintain the world's balance. Life on earth would be impossible without them.