Microorganisms. Higher Plants. Control of Pathogens

Microorganisms (or microbes) are microscopic organisms. Bioscientists use the term microorganisms for life-forms in diverse biological phyla (major divisions of the animal kingdom). Basically there are "lower" microorganisms, such as viruses, bacteria, and cyanobacteria (blue-green algae), and "higher" microorganisms, such as algae, fungi, and protozoa.

Microorganisms are systematized basically by cell wall structures, cell nucleus structures (if any), and their cell organelles (specialized cellular parts that are analogous to organs). Microorganisms are observed, analyzed, and classified by microbiologists, although applied sciences such as medicine add substantially to knowledge about them.

Microorganisms live in all media on Earth, mainly soil, freshwater, and saltwater, but specialized forms are adapted to live under extreme pH, extreme temperatures, and high pressures, such as volcanic environments, polar ice zones, and the deep sea. Some microorganisms, suchas viruses, chlamydiae (intracellular bacteria), and mycoplasmas (nonmotile microorganisms without cell walls), can survive only in other microbes or even in higher organisms.

The spores of microorganisms, which are durable instars (forms existing between molts) produced to allow microorganisms to survive suboptimal living conditions, are dispersed by air and can be carried to the highest mountains and other remote places and even across continents. Of course, microorganisms are also spread by animals, air raiding seeds (such as in dandelions), and human activities, such as migration and trade.

The first microorganisms, procaryotes, developed probably 3.5 billion years ago. They lived on organic compounds and were anaerobic (living without oxygen) because the atmosphere contained methane instead of oxygen. The first microorganisms to produce oxygen were cyanobacteria (about 3.4-2.5 billion years ago).

The oxygen was slowly enriched by cyanobacteria activities in the oceans and afterward in the air. This development must have led to the extermination of most anaerobic life-forms, causing the first environmental disaster on Earth by biological processes. Anaerobic microorganisms still exist and are considered to be the oldest life-forms on Earth.

Most microorganisms today depend on the degradation of organic material. However, a few are autotrophic—they can turn sunlight energy and carbon dioxide into complex, energy-rich organic compounds and oxygen, contributing to the basis of the global food web and benefiting oxygen-based life-forms.

All life-forms, especially the higher ones, depend on the influence of microorganisms. Also, the spread of plants, animals, and humans is limited, between other determinants, by the microbial burden in given areas. Microorganisms acquired early in phylogeny (the evolutionary history of an organism) of higher cells became cell organelles, which are included in all cells of higher life-forms. Two of these organelles are mitochondria, which provide energy within higher cells, and chloroplasts, which produce carbohydrates and oxygen by using carbon dioxide and sunlight energy in green plants.

Microorganisms help to digest nutrition in the intestines of higher animals, some of which, for example cattle, live off of microorganisms in their digestive tracts rather than from their actual nutrient uptake. The fodder that the cattle eat serves the microorganisms and allows them to propagate at sufficient rates for the cattle to consume them and subsist on the nutrients they provide.

Higher Plants. Higher plants depend on microorganisms that live around and even inside their roots. Such microorganisms provide salts of minerals, sulfur, iron, and nitrogen, which are essential to plants.

Microorganisms are concentrated in topsoils. Estimates of numbers of microorganisms (per gram) in A_r-horizon soil (nutrient-rich topsoil found from the surface to approximately 10 centimeters of depth) are: aerobic bacteria, 7.8 million; anaerobic bacteria, 1.9 million; actinomycetes, 2.1 million; fungi, 119,000; and algae, 25,000. Numbers decrease with depth, reducing microorganisms to almost none in the B₂-horizon (approximately 30 to 60 centimeters from the surface).

However, the amount of microorganisms in the upper soil strata makes topsoils the medium on Earth with the highest biomass (the amount of living matter). Microorganisms help break up the lithosphere (the outer part of the Earth composed of rock) and decompose organic matter, turning it into inorganic compounds and thus recycling biomolecules to the inorganic sphere.

Life-forms can be endangered by microorganisms because many of them are parasites, both on other microorganisms and on higher life-forms. Pathogenic (disease-causing) microorganisms produce toxins harmful to host organisms or affect the genetic material of host organisms. Microorganisms reach their hosts by uptake (with nutrition), by microbial burden (concentration of microbes) of the surrounding medium (via air or water), by aerosols, or by contact between infected and noninfected organisms.

Plants and animals have innate capabilities (immune system) to cope with pathogens. The acquisition of an immune system is a result of a co-evolution process between microorganisms and hosts. As humans began to domesticate animals, which brought both into closer contact, opportunistic pathogens from animals were acquired by humans and became human pathogens, causing specific diseases (for example, tuberculosis).

The absence of specific pathogens in the environment of indigenous populations meant an absence of specific genetic-based immune responses, allowing epidemics after contact with organisms from other biomes (major ecological community types), such as continents. An example is the decimation of Native Americans by disease after European contact; similarly, Europeans contracted syphilis as a venereal disease new to them, as well as the bubonic plague and influenza.

The spread of pathogens depends on the density of host organisms, some of which can persist only in large populations with close individual contact (e.g., groups of wild animals and plant communities dominated by few species), in densely populated areas such as towns, in domesticated herds of animals, and in plants under cultivation (monocultures).

However, contact between continents leads to new anthropogenic licenses (ecological niches made by humans), allowing pathogens to affect human interests, as in the case of the mosquito-transmitted West Nile virus, which entered the United States from the Old World recently. Environmentally important are recent attempts to incorporate immunity-providing DNA sequences into organisms that have lost immunity during their domestication. Other attempts use genetic engineering of microbe-transmitting vermin (e.g., mosquitoes) to reduce disease risks.

Control of Pathogens. Human culture depends in many ways on the control of pathogens. Control strategies are developed in epidemiology and follow basic principles of hygiene, including clean drinking water, clean food, proper sewage and refuse disposal, inoculation, and separation (quarantine) or extermination of diseased individuals. Minimal differences in hygiene standards must have been decisive for political and economic success in history. For example, many wars were more influenced by the health of the soldiers than by war technology.

The exploitation of microorganisms has influenced human history profoundly. Storage of food is possible only if microbial deterioration of staples can be avoided. All storage techniques are based on two processes. The first is to dehydrate the stored food because microorganisms depend on water. Dehydration is accomplished by drying (including freeze drying around the frost border in the Andes Mountains) or by adding salt or sugar (honey).

The second is to introduce antimicrobial substances by smoking (e.g., meat) or using chemicals. Both processes must have been used early in human history. After the development of durable containers people could control microorganisms for food production, refinement, and conservation. Another important development was the controlled treatment of raw milk with microorganisms (fermentation), thus introducing cow-, sheep-, and goat-based cultures even in those human populations that are genetically not able to digest raw milk.

The discovery of fermentation of alcohol also was important because beer and wine consumption was probably less dangerous than uptake of drinking water, at least in dryer areas of the world, for thousands of years because of the antimicrobial nature of alcoholic solutions. Storage of vegetables became possible by using lactic acid fermentation, as, for example, is used to pickle cabbage, which became the nutritional foundation of early European ship travel because it prevented scurvy. Lactic acid fermentation is also used in preparing sausages.

A recent breakthrough in molecular biology uses an enzyme extracted from thermophile (growing at high temperatures) microorganisms to amplify DNA in vitro (outside the living body). The invention of the so-called polymerase chain reaction (PCR) allows molecular scientists to specify the genetic code of any organism, which is the prerequisite for altering organisms genetically.