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When one thinks of the 'pollution' of water we often think of chemical dumps or spills into water sources. However, more often the pollution of waters actually refers to the presence of microorganisms that originated from the intestinal tract of humans and other warm-blooded animals. Water pollution can also refer to the presence of compounds that promote the growth of microbes. The remediation of polluted water-the removal of the potentially harmful microorganisms or the reduction of their numbers to acceptable levels represents the purification of water. Microorganisms that reside in the intestinal tract find their way into fresh and marine water when feces contaminate the water. Examples of bacteria that can pollute water in this way are Escherichia coli, Salmonella, Shigella, and Vibrio cholera. Warm-blooded animals other than humans can also contribute protozoan parasites to the water via their feces.

The two prominent examples of health relevance to humans are Cryptosporidium parvum and Giardia lamblia. The latter two species are becoming more common. They are also resistant to chlorine, the most popular purification chemical. Normally, the intestinal bacteria do not survive long in the inhospitable world of the water. However, if they are ingested while still living, they can cause maladies, ranging from inconvenient intestinal upset to life-threatening infections. An example of the latter is Escherichia coli O157:H7. Pollution of the water with this strain can cause severe intestinal damage, life-long damage to organs such as the kidney, and especially in the young, elderly, and those whose immune systems are compromised-death. Microbes, also known as microorganisms, are defined solely on the basis of their size-they are too small to be seen by the naked eye.

Microorganisms can only be viewed if they are magnified in size, using an optical or electron microscope. Apart from their size, the major groups of microorganisms have little affinity in terms of their evolutionary history and systematics. Included among the microorganisms are viruses, bacteria, blue-green bacteria, some algae, some fungi, yeasts, and protozoans. Those nasty viruses. Although viruses are commonly considered to be microorganisms, they are actually "pseudo-organisms" because they do not display all of the characteristics of life. Viruses consist only of bits of nucleic acid (either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)) surrounded by a protein capsule (called a capsid). Viruses are not capable of independent reproduction, and they cannot perform other important metabolic functions.

To reproduce and grow viruses must invade and parasitize the living cells of other organisms, and appropriate the metabolic capabilities of their host. It is probable that viruses of various sorts infect all living cells, and in some cases, serious diseases are caused. Viral diseases of humans include colds, flu, and more deadly ailments such as smallpox, yellow fever, rabies, herpes, polio, and human immunodeficiency syndrome (HIV, also known as AIDS). Some viral diseases can be controlled by vaccination, a practice that involves infecting hosts with killed but intact virus bodies. These are non-virulent but nevertheless, cause the host to develop resistance to pathogenic forms of the strain. Vaccination made it possible to achieve the eradication of smallpox, a disease that had long been a scourge of humans.

Bacteria and Blue-Green Bacteria

Bacteria and blue-green bacteria are in the kingdom of life known as Monera. Most species in this group are bacteria; these have rigid or semi-rigid cell walls, propagate by binary division of the cell, and do not display mitosis during cell division. Blue-green bacteria or cyanobacteria (sometimes incorrectly referred to as blue-green algae) utilize chlorophyll dispersed within their cytoplasm as a pigment for capturing light energy during photosynthesis. The genetic material of monerans is organized as a single strand of DNA, and it is not contained within a membrane-bounded organelle called a nucleus, so these microorganisms are referred to as being prokaryotic. (All of the other kingdoms have a nucleus within their cells, and are known as eukaryotic.) In addition, prokaryotes do not display meiosis or mitosis, their reproduction is by asexual cellular division, and they do not have organelles such as chloroplasts, mitochondria, or flagella. Prokaryotes were the first organisms to evolve, about 3.5 million years ago. It was not until 2 million years later that the first eukaryotes evolved.

About 4,800 species of bacteria have been named, but microbiologists believe that there are many additional species that have not yet been discovered. Bacteria are capable of exploiting an astonishing range of ecological and metabolic opportunities. Some species can only function in the presence of oxygen, and others only under anaerobic conditions, although some species are able to opportunistically switch between these metabolic types. Some bacteria can tolerate very extreme environments, surviving in hot springs at temperatures as hot as 172°F (78°C), while other species have been found active in the frigid conditions that occur as deep as 436 yd (400 m) in glaciers. Most free-living bacteria are heterotrophic, and among the diversity of bacterial species are some that are capable of metabolizing virtually any organic substances as a source of nutrition.

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Other species of bacteria are photosynthetic (including blue-green bacteria), capable of capturing sunlight and using it to reduce carbon dioxide and water into simple sugars, which are used as a source of energy in more complex biochemical syntheses. Other bacteria are chemosynthetic, coupling their biosynthetic abilities to the energy released during the oxidation of certain inorganic compounds, as when pyritic sulfur or sulfide are oxidized to sulfate. Many species of bacteria are not free-living and instead live in a mutualistic symbiosis with more complex organisms, such as plants or invertebrates, or vertebrate animals. For example, numerous species of bacteria live as a microbial community within the rumen of cows and sheep, while others live in the gut of humans and other primates. These gut bacteria help with the digestion of complex organic foods, and they also synthesize vitamins and micronutrients that are useful to their host.

Other bacteria in the genus Rhizobium live in a mutualism with the roots of peas, clovers, and other leguminous plants, fixing atmospheric dinitrogen gas (N2) into ammonia (NH3), which after conversion to ammonium (NH4), it becomes a source of nitrogen that plants can utilize as a nutrient. Many bacteria are parasites of other organisms, and some cause important diseases. Significant diseases of humans caused by bacteria include various kinds of infections, bacterial pneumonia, cholera, diphtheria, gastric ulcers, gonorrhea, Legionnaire's disease, leprosy, scarlet fever, syphilis, tetanus, tooth decay, tuberculosis, whooping cough, most types of food poisoning, and the "flesh-eating disease," which is caused by a virulent strain of Streptococcus. Bacteria also cause diseases of other species, and this is sometimes used to the advantage of humans. For example, Bacillus thuringiensis is a pathogen of many species of moths, butterflies, and black flies, and strains of this bacterium have been used as a biological insecticide against certain insect pests in agriculture and forestry.

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