Wednesday, September 19, 2007
A microorganism (also spelt as microrganism) or microbe is an organism that is microscopic (too small to be seen by the human eye). The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea or protists, but not viruses and prions, which are generally classified as non-living. Micro-organisms are generally single-celled, or unicellular organisms; however, there are exceptions as some unicellular protists are visible to the average human, and some multicellular species are microscopic.
Microorganisms live almost everywhere on earth where there is liquid water, including hot springs on the ocean floor and deep inside rocks within the 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 every year.
Further information: Timeline of evolution
Single-celled microorganisms were the first forms of life to develop on earth, approximately 3—4 billion years ago.
Prior to Anton van Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery as to why grapes could be turned into wine, milk into cheese, or why food would spoil. Leeuwenhoek did not make the connection between these processes and microorganisms, but he did establish that there were forms of life that were not visible to the naked eye.
Microorganisms can be found almost anywhere in the taxonomic organization of life on the planet. Bacteria and archaea are almost always microscopic, whilst a number of eukaryotes are also microscopic, including most protists and a number of fungi. Viruses are generally regarded as not living and therefore are not microbes, although the field of microbiology also encompasses the study of viruses.
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.