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The spring-tail, one of the most abundant ‘micro-creature’ on the planet | (Source: http://www.dailymail.co.uk/sciencetech/)

Soil, air, water… Is there any space that doesn’t have a touch of life..? Is it possible for ‘life’ to get squeezed with in a drop of water or in a pinch of soil…? Yes, there are trillions of microorganisms, which have occupied every space around you. But you will never get to see those tiny life forms teeming in a single drop of pond water or a cluster of tiny nematodes wiggle rapidly together in a pinch of wet soil, unless there wasn’t microscopy. These tiny life forms which can only identify under a microscope are called microorganisms. Microorganisms live in all parts of the biosphere where there is water, soil, hot springs, on the high in the atmosphere and deep inside rocks within the Earth’s crust. Most microorganisms are unicellular (single-celled), but there are some multicellular organisms which are too small to be seen by the naked eye. So they also can only be viewed under the microscope.

History  
Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3–4 billion years ago. The smallest object that can be seen with the unaided human eye ranges from about 1/40 to 1 mm, but “microscopic” often refers to any animal smaller than 1 mm in width, especially smaller than 1/10 mm in width. A scientist called Robert Hooke was the first to use a microscope to observe living things.He observed the fleas under the microscope and was able to observe the tiny hairs on the fleas’ bodies. There are several types of microorganisms. Single-celled ones include planktons, protozoans, viruses and bacteria, while mites, copepods, rotifers and nematodes are been multicellular microorganisms.

Lifestyles
Bacteria are the smallest micro-organisms, ranging from between 0.0001 and 0.001 mm in size. Phytoplankton and protozoa range from about 0.001 mm to about 0.25 mm. The largest phytoplankton and protozoa can be seen with the naked eye, but most can only been seen under a microscope. Even though it’s difficult to see these organisms, they are present in huge numbers. Bacteria have been found everywhere life can exist. They occur in the depths of the ocean and in the ice sheets at the poles. Some have been recovered and germinated from ice and snow which was deposited in the Antarctic centuries ago. The most common types of bacteria vary in size from 0.0005 mm to 0.005 mm. This means that a row of 100 of the largest bacteria would reach across the full stop at the end of this sentence. Protozoans occur in a great variety of habitats including the sea, freshwater, soil and the bodies of other organisms. In fact, generally wherever there is moisture, protozoans occur. These minute animals have all the functions of larger creatures: they take in food, excrete wastes, reproduce and communicate. They feed directly on phytoplankton, bacteria and other protozoa.Phytoplanktons are plants that derive their energy for growth from sunlight during the process of photosynthesis.
The various species are distinguished by their shape and size, or by their photosynthetic pigments. A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria.

Playing a big role  
Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. As some microorganisms can fix nitrogen, they are a vital part of the nitrogen cycle. And recent studies indicate that airborne microbes may play a role in precipitation and weather, While Phytoplankton can be regarded as the pasture of the seas and other aquatic ecosystems, as they provide the food and energy for the food web. When the weight and number of these organisms are multiplied together, micro-organisms are the greatest biomass on earth. What they lack in size they make up for in number.

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Daphnia, a common zoo-plankton in water | Source: biology.ucsd.edu
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A typical free-living, bacterial-feeding nematode, less than 1mm in length | Source: www.extension.org
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This rod-shaped Mycobacterium tuberculosis Causes the disease called Tuberculosis | Source: www2.le.ac.uk/research/
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Beauty of phytoplankton | Source: www.pnas.org
A protozoan paramecium Pic by: Rogelio Moreno | (Source: www.scientificcomputing.com)
A protozoan paramecium
Pic by: Rogelio Moreno | (Source: www.scientificcomputing.com)