1 trillion species on the planet, of which most would be microorganisms. currently, only one-thousandth of one percent of that total have been described. archael cells of some species aggregate and transfer dna from one cell to another through direct contact, particularly under stressful environmental conditions that cause dna damage. bacteria main article: bacteria staphylococcus aureus bacteria magnified about 10,000x bacteria like archaea are prokaryotic – unicellular, and having no cell nucleus or other membrane-bound organelle. bacteria are microscopic, with a few extremely rare exceptions, such as thiomargarita namibiensis. bacteria function and reproduce as individual cells, but they can often aggregate in multicellular colonies. some species such as myxobacteria can aggregate into complex swarming structures, operating as multicellular groups as part of their life cycle, or form clusters in bacterial colonies such as e.coli. their genome is usually a circular bacterial chromosome – a single loop of dna, although they can also harbor small pieces of dna called plasmids. these plasmids can be transferred between cells through bacterial conjugation. bacteria have an enclosing cell wall, which provides strength and rigidity to their cells. they reproduce by binary fission or sometimes by budding, but do not undergo meiotic sexual reproduction. however, many bacterial species can transfer dna between individual cells by a horizontal gene transfer process referred to as natural transformation. some species form extraordinarily resilient spores, but for bacteria this is a mechanism for survival, not reproduction. under optimal conditions bacteria can grow extremely rapidly and their numbers can double as quickly as every 20 minutes. eukaryotes most living things that are visible to the naked eye in their adult form are eukaryotes, including humans. however, many eukaryotes are also microorganisms. unlike bacteria and archaea, eukaryotes contain organelles such as the cell nucleus, the golgi apparatus and mitochondria in their cells. the nucleus is an organelle that houses the dna that makes up a cell's genome. dna (deoxyribonucleic acid) itself is arranged in complex chromosomes. mitochondria are organelles vital in metabolism as they are the site of the citric acid cycle and oxidative phosphorylation. they evolved from symbiotic bacteria and retain a remnant genome. like bacteria, plant cells have cell walls, and contain organelles such as chloroplasts in addition to the organelles in other eukaryotes. chloroplasts produce energy from light by photosynthesis, and were also originally symbiotic bacteria. unicellular eukaryotes consist of a single cell throughout their life cycle. this qualification is significant since most multicellular eukaryotes consist of a single cell called a zygote only at the beginning of their life cycles. microbial eukaryotes can be either haploid or diploid, and some organisms have multiple cell nuclei. unicellular eukaryotes usually reproduce asexually by mitosis under favorable conditions. however, under stressful conditions such as nutrient limitations and other conditions associated with dna damage, they tend to reproduce sexually by meiosis and syngamy. protists euglena mutabilis, a photosynthetic flagellate of eukaryotic groups, the protists are most commonly unicellular and microscopic. this is a highly diverse group of organisms that are not easy to classify. several algae species are multicellular protists, and slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms. the number of species of protists is unknown since only a small proportion has been identified. protist diversity is high in oceans, deep sea-vents, river sediment and an acidic river, suggesting that many eukaryotic microbial communities may yet be discovered. fungi main article: fungus the fungi have several unicellular species, such as baker's yeast (saccharomyces cerevisiae) and fission yeast (schizosaccharomyces pombe). some fungi, such as the pathogenic yeast candida albicans, can undergo phenotypic switching and grow as single cells in some environments, and filamentous hyphae in others. plants the green algae are a large group of photosynthetic eukaryotes that include many microscopic organisms. although some green algae are classified as protists, others such as charophyta are classified with embryophyte plants, which are the most familiar group of land plants. algae can grow as single cells, or in long chains of cells. the green algae include unicellular and colonial flagellates, usually but not always with two flagella per cell, as well as various colonial, coccoid, and filamentous forms. in the charales, which are the algae most closely related to higher plants, cells differentiate into several distinct tissues within the organism. there are about 6000 species of green algae. ecology main article: microbial ecology microorganisms are found in almost every habitat present in nature, including hostile environments such as the north and south poles, deserts, geysers, and rocks. they also include all the marine microorganisms of the oceans and deep sea. some types of microorganisms have adapted to extreme environments and sustained colonies; these organisms are known as extremophiles. extremophiles have been isolated from rocks as much as 7 kilometres below the earth's surface, and it has been suggested that the amount of organisms living below the earth's surface is comparable with the amount of life on or above the surface. extremophiles have been known to survive for a prolonged time in a vacuum, and can be highly resistant to radiation, which may even allow them to survive in space. many types of microorganisms have intimate symbiotic relationships with other larger organisms; some of which are mutually beneficial (mutualism), while others can be damaging to the host organism (parasitism). if microorganisms can cause disease in a host they are known as pathogens and then they are sometimes referred to as microbes. microorganisms play critical roles in earth's biogeochemical cycles as they are responsible for decomposition and nitrogen fixation. bacteria use regulatory networks that allow them to adapt to almost every environmental niche on earth. a network of interactions among diverse types of molecules including dna, rna, proteins and metabolites, is utilised by the bacteria to achieve regulation of gene expression. in bacteria, the principal function of regulatory networks is to control the response to environmental changes, for example nutritional status and environmental stress. a complex organization of networks permits the microorganism to coordinate and integrate multiple environmental signals. extremophiles main article: extremophile further information: list of microorganisms tested in outer space a tetrad of deinococcus radiodurans, a radioresistant extremophile bacterium extremophiles are microorganisms that have adapted so that they can survive and even thrive in extreme environments that are normally fatal to most life-forms. thermophiles and hyperthermophiles thrive in high temperatures. psychrophiles thrive in extremely low temperatures. – temperatures as high as 130 °c (266 °f), as low as −17 °c (1 °f) halophiles such as halobacterium salinarum (an archaean) thrive in high salt conditions, up to saturation. alkaliphiles thrive in an alkaline ph of about 8.5–11. acidophiles can thrive in a ph of 2.0 or less. piezophiles thrive at very high pressures: up to 1,000–2,000 atm, down to 0 atm as in a vacuum of space. a few extremophiles such as deinococcus radiodurans are radioresistant, resisting radiation exposure of up to 5k gy. extremophiles are significant in different ways. they extend terrestrial life into much of the earth's hydrosphere, crust and atmosphere, their specific evolutionary adaptation mechanisms to their extreme environment can be exploited in biotechnology, and their very existence under such extreme conditions increases the potential for extraterrestrial life. plants and soil the nitrogen cycle in soils depends on the fixation of atmospheric nitrogen. this is achieved by a number of diazotrophs. one way this can occur is in the root nodules of legumes that contain symbiotic bacteria of the genera rhizobium, mesorhizobium, sinorhizobium, bradyrhizobium, and azorhizobium. the roots of plants create a narrow region known as the rhizosphere that supports many microorganisms known as the root microbiome. these microorganisms in the root microbiome are able to interact with each other and surrounding plants through signals and cues. for example, mycorrhizal fungi are able to communicate with the root systems of many plants through chemical signals between both the plant and fungi. this results in a mutualistic symbiosis between the two. however, these signals can be eavesdropped by other microorganisms, such as the soil bacteria, myxococcus xanthus, which preys on other bacteria. eavesdropping, or the interception of signals from unintended receivers, such as plants and microorganisms, can lead to large-scale, evolutionary consequences. for example, signaler-receiver pairs, like plant-microorganism pairs, may lose the ability to communicate with neighboring populations because of variability in eavesdroppers. in adapting to avoid local eavesdroppers, signal divergence could occur and thus, lead to the isolation of plants and microorganisms from the inability to communicate with other populations. symbiosis the photosynthetic cyanobacterium hyella caespitosa (round shapes) with fungal hyphae (translucent threads) in the lichen pyrenocollema halodytes a lichen is a symbiosis of a macroscopic fungus with photosynthetic microbial algae or cyanobacteria. applications main article: human interactions with microbes microorganisms are useful in producing foods, treating waste water, creating biofuels and a wide range of chemicals and enzymes. they are invaluable in research as model organisms. they have been weaponised and sometimes used in warfare and bioterrorism. they are vital to agriculture through their roles in maintaining soil fertility and in decomposing organic matter. food production main articles: fermentation in food processing and food microbiology microorganisms are used in a fermentation process to make yoghurt, cheese, curd, kefir, ayran, xynogala, and other types of food. fermentation cultures provide flavour and aroma, and inhibit undesirable organisms. they are used to leaven bread, and to convert sugars to alcohol in wine and beer. microorganisms are used in brewing, wine making, baking, pickling and other food-making processes. some industrial uses of microorganisms: product contribution of microorganisms cheese growth of microorganisms contributes to ripening and flavor. the flavor and appearance of a particular cheese is due in large part to the microorganisms associated with it. lactobacillus bulgaricus is one of the microbes used in production of dairy products alcoholic beverages yeast is used to convert sugar, grape juice, or malt-treated grain into alcohol. other microorganisms may also be used; a mold converts starch into sugar to make the japanese rice wine, sake. acetobacter aceti a kind of bacterium is used in production of alcoholic beverages vinegar certain bacteria are used to convert alcohol into acetic acid, which gives vinegar its acid taste. acetobacter aceti is used on production of vinegar, which gives vinegar odor of alcohol and alcoholic taste citric acid certain fungi are used to make citric acid, a common ingredient of soft drinks and other foods. vitamins microorganisms are used to make vitamins, including c, b2 , b12. antibiotics with only a few exceptions, microorganisms are used to make antibiotics. penicillin, amoxicillin, tetracycline, and erythromycin water treatment further information: drinking water § water quality wastewater treatment plants rely largely on microorganisms to oxidise organic matter. these depend for their ability to clean up water contaminated with organic material on microorganisms that can respire dissolved substances. respiration may be aerobic, with a well-oxygenated filter bed such as a slow sand filter. anaerobic digestion by methanogens generate useful methane gas as a by-product. energy main articles: algae fuel, cellulosic ethanol, and ethanol fermentation microorganisms are used in fermentation to produce ethanol, and in biogas reactors to produce methane. scientists are researching the use of algae to produce liquid fuels, and bacteria to convert various forms of agricultural and urban waste into usable fuels. chemicals, enzymes microorganisms are used to produce many commercial and industrial chemicals, enzymes and other bioactive molecules. organic acids produced on a large industrial scale by microbial fermentation include acetic acid produced by acetic acid bacteria such as acetobacter aceti, butyric acid made by the bacterium clostridium butyricum, lactic acid made by lactobacillus and other lactic acid bacteria, and citric acid produced by the mould fungus aspergillus niger. microorganisms are used to prepare bioactive molecules such as streptokinase from the bacterium streptococcus, cyclosporin a from the ascomycete fungus tolypocladium inflatum, and statins produced by the yeast monascus purpureus. science