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Soil
Soil is the base camp of microorganisms， plenty of bacteria， fungus and protozoon live here. There are millions of bacteria， hundreds of thousands of fungal spores and thousands of protozoon in per gram of agricultural soil. Microorganisms are not arranged equally in the soil， most of them live in the top soil within 10 centimeters deep. The deeper the soil is ， the less microorganisms are. As plants may secrete some nutriments， the soil close to the plant roots breeds more abundant microorganisms， whose quantity is several times and even a dozen times more than those in surrounding areas.
Air
There are also microorganisms in the air. They are mostly in the dust particles blown out by wind， small water drop， casts of human and animal bodies， as well as the sprays of respiratory tracts. Generally speaking， as there is little food， and terrible ultraviolet rays exists， air is not the ideal living environment of organisms. The pollution level of air directly influences the number of microorganisms， for example， there is no microorganism in the sky of the north pole， and there are only several microorganisms in per cubic meter of air above the ocean， while the muber of microorganisms of the same volume in the cities can be several thousands， there might be millions of microorganisms in the air of livestock farms.
Water
Microorganisms also live in the water as ponds， lakes， brooks， rivers and oceans. Generally speaking， the cleaner the water is， the fewer the microorganisms are. There are always more microorganisms in the water containing lots of organic matters or polluted by sanitary sewage and industrial wastewater， for the polluted water brings more food for the microorganisms. Different conditions of illumination and oxygen cause the microorganisms living at the different depth of water， for instance， many photosynthetic microorganisms live near to the water surface， while the deep water live the microorganisms who don’t like oxygen.
Living Body
Some microorganisms cannot live independently but have to live in the hosts’ bodies. If they just greedily suck the nutrition of the living body and bring great pain to the hosts， they are parasitic organisms； if they gain nutrition while benefiting the hosts， cooperating and stick together with the hosts， and even forming unique organization structures， then we call this relationship mutualistic symbiosis. The nodule bacteria and leguminous plants （such as soybean， pea and peanuts） could form root nodules， that is a typical case of mutualistic symbiosis. The bacteria can fix nitrogen in the root nodules， and transform nitrogen gas into the nitrogen that can be used by plants， while plants provide the bacteria food.
Extreme Environment
Microorganisms are the most widespread living things on the earth. Except for the area where other plants and animals could live， many microorganisms， especially the archabacteria live in different kinds of extreme environments， they determine the territory of life. Nowadays， people have been able to separate the lives of microorganisms from the acid mine water with a acidity of pH0 （ the acidity is at least 100 thousand times of the living environment for normal cells）， the seabed with hundreds of atmospheric pressures， the underground at a depth of several thousand meters， the ice layer at a depth of several thousand meters， the salt lakes of saturated concentration and the seabed craters at hundreds of degrees Celsius.
Soil is the base camp of microorganisms， plenty of bacteria， fungus and protozoon live here. There are millions of bacteria， hundreds of thousands of fungal spores and thousands of protozoon in per gram of agricultural soil. Microorganisms are not arranged equally in the soil， most of them live in the top soil within 10 centimeters deep. The deeper the soil is ， the less microorganisms are. As plants may secrete some nutriments， the soil close to the plant roots breeds more abundant microorganisms， whose quantity is several times and even a dozen times more than those in surrounding areas.
Air
There are also microorganisms in the air. They are mostly in the dust particles blown out by wind， small water drop， casts of human and animal bodies， as well as the sprays of respiratory tracts. Generally speaking， as there is little food， and terrible ultraviolet rays exists， air is not the ideal living environment of organisms. The pollution level of air directly influences the number of microorganisms， for example， there is no microorganism in the sky of the north pole， and there are only several microorganisms in per cubic meter of air above the ocean， while the muber of microorganisms of the same volume in the cities can be several thousands， there might be millions of microorganisms in the air of livestock farms.
Water
Microorganisms also live in the water as ponds， lakes， brooks， rivers and oceans. Generally speaking， the cleaner the water is， the fewer the microorganisms are. There are always more microorganisms in the water containing lots of organic matters or polluted by sanitary sewage and industrial wastewater， for the polluted water brings more food for the microorganisms. Different conditions of illumination and oxygen cause the microorganisms living at the different depth of water， for instance， many photosynthetic microorganisms live near to the water surface， while the deep water live the microorganisms who don’t like oxygen.
Living Body
Some microorganisms cannot live independently but have to live in the hosts’ bodies. If they just greedily suck the nutrition of the living body and bring great pain to the hosts， they are parasitic organisms； if they gain nutrition while benefiting the hosts， cooperating and stick together with the hosts， and even forming unique organization structures， then we call this relationship mutualistic symbiosis. The nodule bacteria and leguminous plants （such as soybean， pea and peanuts） could form root nodules， that is a typical case of mutualistic symbiosis. The bacteria can fix nitrogen in the root nodules， and transform nitrogen gas into the nitrogen that can be used by plants， while plants provide the bacteria food.
Extreme Environment
Microorganisms are the most widespread living things on the earth. Except for the area where other plants and animals could live， many microorganisms， especially the archabacteria live in different kinds of extreme environments， they determine the territory of life. Nowadays， people have been able to separate the lives of microorganisms from the acid mine water with a acidity of pH0 （ the acidity is at least 100 thousand times of the living environment for normal cells）， the seabed with hundreds of atmospheric pressures， the underground at a depth of several thousand meters， the ice layer at a depth of several thousand meters， the salt lakes of saturated concentration and the seabed craters at hundreds of degrees Celsius.