Micro-organisms are living organisms which are very small and not visible to the naked eye. They are various micro-organisms they include bacteria, fungi, archaea, and protists. Microbes live in a wide range of habitats: from hot springs, to the human body and the depths of the ocean, individual species are highly adapted to ecological niches. Bacteria and archaea are prokaryotes, they lack a cell nucleus and the other membrane bound organelles. They are almost always unicellular but bacteria which live in soil (myxobacteria) have large genomes, relative to other bacteria. They can aggregate into complex structures as part of their life cycle. Other bacteria can function and reproduce as individual cells, but often aggregate in multicellular colonies. Their genome is usually a single loop of DNA and they also have DNA inside the plasmids. Bacteria are surrounded by a cell wall, which provides strength and rigidity to their cells. They reproduce by binary fission and sometimes by budding, but they do not undergo sexual reproduction. Archaea are also single-celled organisms that lack nuclei. Archaea and bacteria are different because both their genetics and biochemistry are different. Protists are unicellular or multicellular without specialized tissues, they live in almost any environment that contains liquid water. Fungi are also eukaryotes; the difference between fungi and other eukaryotes is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants, which contain cellulose.
Pathogenic microbes are harmful, since they invade and grow within other organisms, causing diseases. However, most micro-organisms are important in an ecosystem because they act as decomposers and they play an important role in nutrient recycling. They are involved in major processes such as biogeochemical cycles, water purification and biotechnology.
There are four major biochemical cycle: sulphur, nitrogen, carbon and iron cycle. Every organism on earth needs carbon either for structure, energy or both. Carbon dioxide fixation is carried out by phototrophic (bacteria can be aerobic or anaerobic) or lithotrophic bacteria (bacteria that oxidize or reduce inorganic substrates), which reduce CO2 to form glucose and methanogens, reduce CO2 to form methane. The digestion of cellulose in ruminants occurs through microbial activity. Microbes which live inside ruminant release enzyme cellulase to hydrolyse cellulose to produce cellobiose and glucose. Glucose then undergoes bacterial fermentation and form volatile fatty acids, CO2 and methane. The relationship between the ruminant and microbes is symbiosis because both of them benefits from each other. Other microorganisms play a part in the carbon cycle. For example the sulphur bacteria are able to use the energy they gain from the degradation of a compound called hydrogen sulphide to degrade carbon compounds. Other bacteria such as Thiobacillus ferrooxidans uses the energy gained from the removal of an electron from iron-containing compounds to convert carbon. The anaerobic degradation of carbon is done only by microorganisms. This degradation is a combined effort of several bacteria. Examples of the bacteria include Bacteroides succinogenes, Clostridium butyricum, and Syntrophomonas sp. This bacterial relationship is responsible for the mass of the carbon dioxide and methane that is released to the atmosphere.
Nitrogen is a naturally occurring element that is essential for growth and reproduction in both plants and animals. It is found in amino acids that make up proteins, in nucleic acids, in DNA, and in many other organic and inorganic compounds. In addition, nitrogen comprises about 80% of the Earth's atmosphere. Azotobacter and Rhizobium are nitrogen fixation bacteria; they convert nitrogen from the air into ammonia. Nitrosomonas and Nitrobacter bacteria convert ammonia into nitrite followed by the oxidation of these nitrites into nitrates. Nitrifying bacteria work in aerobic condition. Denitrification is carried out by denitrifying bacteria, in this process nitrate is reduced to either nitrous oxide or nitrogen gas, which is then released into the atmosphere. Examples of denitrifying bacteria are Paracoccus denitrificans and various Pseudomonas. These bacteria only work in anaerobic conditions.
Sulphur is one of the constituents of many proteins, vitamins and hormones. Plants absorb sulphur when it is dissolved in water. Animals consume these plants, so that they take up enough sulphur to maintain their health. Most of the earth's sulphur is fixed in rocks and salts or buried deep in the ocean in oceanic sediments. Sulphur can also be found in the atmosphere. It reduces the amount of solar energy entering the earth, thereby contributing to a cooling of the planet. Both aerobes (Beggiatoa, Sulfolobus, Thiobacillus, Thiothrix) and anaerobes (Thiospirillum, Thiocapsa, Chromatium, Chlorobium, Prosthecocloris) oxidize sulphide to sulphur and then oxidize it to sulphate. Desulfovibrio reduce reduces SO42 to H2S. This is summarised below.
Figure 3: Sulphur cycle
http://www.atmosphere.mpg.de/enid/Nr_6_Feb__2__6_acid_rain/C__The_sulphur_cycle_5i9.html
Iron is the fourth most abundant element in the earth's crust. Microbial metal transformations are essential for the production of metallic ores. Iron is the key component of many enzymes, as well as being the main component of haemoglobin, in human blood. Iron oxidizing bacteria (Thiobacillus ferrooxidans, Gallionella), oxidisation of ferrous iron to ferric iron does not provide sufficient electrochemical potential to allow ATP generation or NADPH formation therefore, many iron bacteria also derive electrons from oxidation of hydrogen sulphide, sulphur and thiosulfate. Thiobacillus ferrooxidans uses the natural proton gradient in its low pH environment to generate ATP via ATP synthase. Iron reducing bacteria (Pseudomonas, Geobacter) reduce ferric iron to ferrous ion via anaerobic process.
Biotechnology uses microorganisms such as bacteria to perform industrial or manufacturing processes.
