The ancient Greeks divided all life into two kingdoms: plants (Plantae) and animals (Animalia). In 1674 the microscope revealed tiny living cells that, in 1866, became the third kingdom, the Protista. In 1925 it was noticed that some of those cells had a nucleus (Eukaryota) and some didn’t (Prokaryota) leading, in 1938, to a fourth kingdom, the Monera (the Prokaryota) while the Protista became the kingdom of the Eukaryotic protozoans. In 1969 the fungi were recognized as different enough from everybody else that they formed a fifth kingdom, the Fungi. Finally, in 1977, the prokaryota (Monera) were subdivided into the two kingdoms of Bacteria and Archaea (originally Archaebacteria) to give us today’s six kingdoms. To add to the confusion, modern biologists also subdivide all life into three domains which cut across the six kingdoms: Eukaryota, Bacteria, and Archaea which could be reduced to two: Eukaryota and Prokaryota (Bacteria + Archaea).
Kingdoms and Domains are at the top of the classification system of life, the most general divisions. The classification hierarchy begins with kingdom and proceeds through ever finer divisions to end at species: Kingdom, Phylum, Class, Order, Family, Genus, and Species; in some instances there can be intermediate divisions like a superorder or a subfamily. When referring to a specific organism, the genus and species are normally listed. It is the custom to list the genus first, then the species. The genus (plural is genera) is capitalized, the species is not; both are in italics; higher order divisions are not italicized. As an example, we are Homo sapiens. It sometimes happens, especially when talking about bacteria, that there may be a genus like Crenothrix that has many species, all of which are doing pretty much the same thing (in this case, reducing iron) or producing the same disease. In such cases the species is not listed as you would otherwise have to make a long list of the various species and all you really care about is the collective action of the genus. Or, you might encounter something like Crenothrix spp. which means Crenothrix species.
There are two frequently-used terms: microorganism and microbe. Both include any life that can’t be seen without a microscope such as fungi, protozoans, bacteria, and archaea. The difference is that ‘microbe’ often suggests something that is harmful whereas ‘microorganism’ is more neutral. The term, ‘germs,’ is the non-scientific equivalent of microbes. Finally there is the virus, smallest of all, so small that most filters can’t stop it. Biologists are still arguing whether viruses are alive or not since it cannot replicate outside of another living cell. Another mystery is its origin: is it a degenerate bacterium (singular form) or an archaeum (singular form) or did part of a disrupted cell manage to survive as what became known as a virus? Whatever it is, it tends to sit outside the neat classifications of kingdoms and domains. Life is inherently messy.
So, what organisms create problems in water? The Archaea don’t seem to create any major and common problems but just about everybody else can. Algal blooms can be toxic in lakes and the oceans. A parasitic worm, Schistosoma, found in tropical waters, can cause Schistosomiasis (aka Bilharzia or snail disease) in humans. Standing water can bolster the mosquito population which is notorious for transmitting a host of diseases. However, restricting the discussion to drinking water, the most common problem-causers are bacteria, protozoa (Protista), parasites, and viruses.
Bacteria come in three basic shapes (there are a few exceptions) and their descriptive classification is based on those shapes.
Some examples of the various bacteria types and diseases they might cause (not all of them cause disease in humans nor are necessarily in water) are shown in the table below.
The bacteria in water that commonly cause disease are shown in the table below.
There are other bacteria that do not cause disease but which can affect the water quality, one good example being Crenothrix. Crenothrix is one of a group of iron-reducing bacteria which get energy by converting iron compounds in the ground when there is very little oxygen present (an anaerobic or reducing environment) into a form of iron (ferrous iron, Fe++) which is soluble in water, in the process producing hydrogen sulfide gas (H2S). The result is that the groundwater becomes saturated with dissolved iron (iron water) and smells like rotten eggs (the hydrogen sulfide gas – sulfur water). When pumped out of a well and exposed to the oxygen of the air (an aerobic or oxidizing environment), the ferrous iron quickly oxidizes to the much less soluble ferric iron (Fe+++) which promptly precipitates, creating bright reddish-orange stains (ferric hydroxide, Fe(OH)3 all over your plumbing and, if you were washing laundry, all over your clothes. Crenothrix and its colleagues like to make a protective slime (biofilm) that can block the movement of water into a well, thereby affecting not only the water quality but its quantity too.
Elements and compounds which enter the body, including through drinking water, have a biological half-life which is analogous to the radiological half-life of a radioactive isotope. If it is an element, the body may eliminate it by exhalation, through sweat, or by urination or defecation. Some of it may become stored in various body organs and then slowly released back into circulation. A good example of this is lead which in the blood has a biological half-life of 28 to 36 days, either being eliminated from the body entirely or stored in the bones. Lead in the bones has a biological half-life of about ten years. Water itself has a biological half-life of from one to two weeks in the normal human body.
If it is a compound, the body may break the compound down, turning it into something else which may have its own biological half-life. A good example of this is nicotine which has a biological half-life of about two hours, some of it being eliminated from the body and some of it being broken down into cotinine which has a biological half-life of 15 hours. Other examples include caffeine with a biological half-life of ~ 5 hrs, cocaine at 1 to 1.5 hrs, and lisinopril (a blood pressure medication) at 12 hrs.
The radioactive iodine-131 isotope has both a biological half-life (~ 66 days) and a radiological half-life (8 days) so you lose it for two different reasons. Combining the effects of these two half-lives generates an effective half-life which, in the case of I-131, is about 5.5 days.
Iodine is an essential micronutrient which is concentrated in the thyroid gland. If it is not available in sufficient quantities in the diet, the thyroid will greatly expand to produce a medical condition known as goiter in an attempt to increase its efficiency at extracting what little iodine might be available in the diet. In order to prevent iodine deficiency, iodine is added to ordinary dietary salt (NaCl) in the form of NaI.
It could happen that Iodine-131 could be released into the environment as a result of a nuclear power plant failure. Because of this, nuclear power plants may have a supply of non-radioactive iodine (NaI) pills to distribute to the surrounding population. The idea is to flood the body with the non-radioactive iodine so that the thyroid would not absorb the iodine-131. In effect, this would temporarily greatly reduce the biologic half-life of iodine.