A microorganism or microbe is an organism that is microscopic (too small to be seen by the human eye). Microorganisms include bacteria, fungi, archaea or protists, but not viruses and prions, which are generally classified as non-living. Most microorganisms are single-celled, or unicellular, but some are microscopic, and some unicellular protists are visible to the average human.
Microorganisms live almost everywhere on Earth where there is liquid water, including hot springs, on the ocean floor, and deep inside rocks within Earth's crust. Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. As some microorganisms can also fix nitrogen, they are an important part of the nitrogen cycle. However, pathogenic microbes can invade other organisms and cause diseases that kill millions of people and plants every year.
Microorganisms are vital to humans and the environment, as they participate in the Earth's element cycles such as the carbon cycle and nitrogen cycle, as well as fulfilling other vital roles in virtually all ecosystems, such as recycling other organisms' dead remains and waste products through decomposition. Microbes also have an important place in most higher-order multicellular organisms as symbionts.
There are 2 types of classifications, prokaryotes and eukaryotes.
Prokaryotes are organisms that lack a cell nucleus and the other organelles found in eukaryotes. Prokaryotes are almost always unicellular, although some such as myxobacteria can aggregate into complex structures as part of their life cycle. These organisms are divided into two groups, the archaea and the bacteria.
All living things which are individually visible to the naked eye are eukaryotes (with few exceptions, such as Thiomargarita namibiensis), including humans. However, a large number of eukaryotes are also microorganisms. Unlike prokaryotes, eukaryotes contain organelles such as the cell nucleus, the Golgi apparatus and mitochondria in their cells. Eukaryotes are further classified into Protists, Animals, Fungi, and Plants. For our purposes, we will be focusing on the relationship between the protists and fungi and plants.
Below is a list of the different soil microorganisms that we commonly see in gardening: