A characteristic of microorganisms is their ability to grow and form a population of organisms. One of the results of microbial metabolism is an increase in the size of the cell. The many requirements for successful growth include those both chemical and physical. Chemical requirements. In order to grow successfully, microorganisms must have a supply of water as well as numerous other substances including mineral elements, growth factors, and gas, such as oxygen. Virtually all chemical substances in microorganisms contain carbon in some form, whether they be proteins, fats, carbohydrates, or lipids. Perhaps 50 percent of a bacterium's dry weight is carbon. Carbon can be obtained from organic materials in the environment, or it may be derived from carbon dioxide. Both chemoautotrophic and photoautotrophic microorganisms obtain their energy and produce their nutrients from simple inorganic compounds such as carbon dioxide. Chemoautotrophs do so through chemical reactions, whilephotoautotrophs use photosynthesis. Among the other elements required by microorganisms are nitrogen and phosphorous.Nitrogen is used for the synthesis of proteins, amino acids, DNA, and RNA. Bacteria that obtain nitrogen directly from the atmosphere are called nitrogen-fixing bacteria. They include species of Rhizobium and Azotobacter, both found in the soil. Phosphorusis an essential element for nucleic acid synthesis and for the construction of phospholipids. Oxygen is used by aerobic bacteria during the process of cellular respiration as a final electron acceptor. For aerobic organisms, oxygen is an absolute requirement for their energy-yielding properties. Certain microorganisms grow in oxygen-free environments and are described as anaerobic. Organisms such as these produce odoriferous gases in their metabolism, including hydrogen sulfide gas and methane. Certain pathogenic species, such as Clostridium species, are anaerobic. Certain species of microorganisms are said to be facultative. These species grow in either the presence or absence of oxygen. Some bacteria species are microaerophilic, meaning that they grow in low concentrations of oxygen. In some cases, these organisms must have an environment rich in carbon dioxide. Organisms such as these are said to be capnophilic. Other chemical requirements for microbial growth include such trace elements as iron, copper, and zinc. These elements often are used for the synthesis of enzymes. Organic growth factors such as vitamins may also be required by certain bacteria. Amino acids, purines, and pyrimidines should also be available. Physical requirements. Certain physical conditions affect the type and amount of microbial growth. For example, enzyme activity depends on the temperature of the environment, and microorganisms are classified in three groups according to their temperature preferences: psychrophilic organisms (psychrophiles) prefer cold temperatures of about 0°C to 20°C; mesophilic organisms (mesophiles) prefer temperatures at 20°C to 40°C; thermophilic organisms (thermophiles) prefer temperatures higher than 40°C (Figure 1 ). A minimum and a maximum growth temperature range exist for each species. The temperature at which best growth occurs is the optimum growth temperature. Figure 1 Three types of bacteria and the temperature environments in which they thrive. Another physical requirement is the extent of acidity or alkalinity, referred to as the pHof a solution. For most bacteria, the optimum pH is between 6.5 and 7.5. Since the pH of most human tissue is 7.0 to 7.2, these neutrophilic bacteria usually grow well in the body. Certain bacteria, such as those in sauerkraut and yogurt, prefer acidic environments of 6.0 or below. These bacteria are said to be acidophilic. Molds and yeasts are among other common acidophilic microorganisms. Microbial growth proceeds best when the osmotic pressure is ideal. Normally, the salt concentration of microbial cytoplasm is about 1 percent. When the external environment also has a 1 percent salt concentration, then the osmotic pressure is optimum. Should the external salt concentration rise, as when food is salted, water will flow out of the microbial cytoplasm by osmosis through the cell membrane into the environment, thereby causing the microorganisms to shrink and die. By comparison, if exterior water is free of salt, it will flow through the cell membrane into the cytoplasm of the cell, causing the organism to swell and burst. Microorganisms that live in marine environments can tolerate high salt concentrations. These organisms are said to be halophilic. They include diatoms and dinoflagellates, two types of unicellular algae that lie at the base of oceanic food chains. There are many other species of halophilic bacteria, fungi, protozoa, and algae.