a) Production of food: cheese, yogurt, vinegar, wine, sour cream, etc. synthesize drugs and chemicals. sometimes called symbionts. There are three main types of symbiosis, based upon the specific relationship between the species involved: mutualism, parasitism, and commensalism. Symbiosis that results in mutual benefit to the interdependent organisms is commonly known as mutualism. An example of mutualism is the coexistence of certain species of algae and fungi that together compose lichens. Their close association enables them to live in extreme environments, nourished only by light, air, and minerals. Living separately, the alga and fungus would not survive in such conditions. In parasitism, also known as antagonistic symbiosis, one organism receives no benefits and is often injured while supplying nutrients or shelter for the other organism. Parasites include viruses and bacteria that cause many diseases; certain protozoans that can infect plants and animals; tapeworms and flukes that infest the intestinal tracks and internal organs of animals. The type of symbiosis known as commensalism is a food-sharing association between two different kinds of nonparasitic animals, called commensals, that is harmless to both and in many cases is mutually advantageous. Many commensals are free to separate. Other commensals function together so completely that they cannot separate. They do not harm each other. An example is a polyp found in deep water off the coast of Newfoundland. It attaches itself to the shell of a certain species of hermit crab and, by budding, covers the entire shell with a colony that dissolves the original shell. Because the colony grows at the same rate as the crab, it furnishes continuous protection, and the crab does not shed its shell at periodic intervals as it normally would. The polyp, in turn, benefits by moving about with the crab, thereby obtaining a greater food supply than it would if attached to a stationary object. Uses of Bacteria in the Environment : Bacteria feed on dying material and convert it back into basic substances. This process of decomposition is as significant as photosynthesis, for without it food chains would cease, and fallen trees, leaves, and other refuse would simply pile up. Bacteria also strongly influence the movement of key elements, such as sulfur, iron, phosphorus, and carbon, around the globe. The weathering of rocks, which releases elements back into life systems for use, is substantially enhanced by the breakdown processes of bacteria. sewage treatment are a variety of specialized bacteria that convert, mostly through fermentation, the organic materials of sewage into carbon dioxide, methane, and hydrogen gases. There is a bacterial species involved with the production of nearly every familiar product. For example, vinegar, which is used as both a flavor enhancer and an important food preservative, results from the conversion of ethyl alcohol to acetic acid by acetic-acid bacteria. Specific enzymes extracted from bacteria are used in spot removers, meat tenderizers, laundry starches, and household detergents. Bacteria are now used throughout the growing biotechnology industry in the development of new products for medical treatment. Bacteria that can digest petroleum are even used in oil-spill cleanups. molecular atmospheric nitrogen is converted into a chemical compound that is essential for plant growth and is also used in industrial chemical production. The most widely used and most productive of the soil microorganisms capable of nitrogen fixation are symbiotic bacteria of the genus Rhizobium, which colonize and form nodules on the roots of leguminous plants such as clover, alfalfa, and peas. These bacteria obtain food from the legume, which in turn is supplied with abundant nitrogen compounds. Soils are sometimes inoculated with a particular species of Rhizobium to increase a legume crop, which is often planted to replenish the nitrogen depleted by other crops. Much smaller amounts of nitrogen are fixed in the soil by nonsymbiotic (free-living) bacteria such as the aerobes, which function in the presence of oxygen, and bacteria of the genera Klebsiella and Bacillus, which function without oxygen. Some forms of cyanobacteria (formerly known as blue-green algae) also fix nitrogen, such as the alga Anabaena, which, in symbiosis with the water fern Azolla pinnata, is said to markedly increase rice yields, as was the case in paddies in the Thái Bình region of northern Vietnam. The need for fixed nitrogen in agriculture today is far greater than can be supplied by natural biological processes, and the production of nitrogen compounds from atmospheric nitrogen is a major chemical industry.