Overview:
• Monerans – The Good, The Bad & The Ugly
• Protistans – The Food-Chain & Some Parasites
• Fungi - Food, Drugs, Sex & Money
...... Introduction – The three Kingdoms described below represent most of the living organisms on earth. Life on this planet as we know it began with the evolution & development of the relatively simple bacterial cell; today scientific research is able to harness the synthetic and reproductive powers of bacteria to make medically-important proteins and drugs.
...... On the other hand, the protistans took the bacterial cells a step further and developed structurally-complex cellular features which have been keys to their survival over the millennia; today the unicellular protists continue to occupy a key link in the world's food-chain.
...... Lastly, the fungi developed multi-cellular structures including the capacity for sexual reproduction; as we'll see later, the sexual reproductive process yielding genetically diverse gametes (i.e., eggs and sperm) has ensured genetic diversity within the fungal Kingdom (a pattern which will be continued and amplified within the Plant and Animal Kingdoms).
...... Case Study Question - In your field trip, were you able to observe evidence of bacteria, protests, or fungi?
KINGDOM - MONERA
...... Most of the Kingdom Monera is made up by single-celled organisms called bacteria; photosynthetic blue-green algae are also included in the Kingdom. Bacteria by far are the most common life-form on earth; in fact there are likely more bacteria on the head of a pin than there are people living in New York City. Bacteria along with fungi make up the `decomposers' of the world – organisms needed to recycle dead organic matter back into the food chain for eventual use by higher organisms. This can be dramatized by the fact that if a forest were to be `sterilized' (killing off all bacteria within), the forest would soon die off; plants like trees and bushes would be unable to obtain needed nutrients from the soil which are made available to them through the decomposing activity of bacteria and fungi. Since we humans are directly or indirectly tied to plants for food, we are naturally dependent upon bacteria doing their job in freeing up nutrients for crops.
...... For the most part, bacteria are `chemotrophic' feeders meaning that they exist mainly on simple carbon compounds as sources of energy; their nutritional needs are relatively simple. Blue-green algae on the other hand derive their energy from the sun via photosynthetic processes. Bacteria and blue-green algae are referred to as `prokaryotic' cells; compared to the highly-organized `eukaryotic' cells which make up the other 4 Kingdoms, bacterial cells are structurally simple containing genetic material made up of a single, circular chromosome made up of DNA. Eukaryotic cells on the other hand have a highly-structured nucleus which contains varying numbers of DNA-containing chromosomes.
......
...... Classifications of Bacteria - Bacteria are conveniently classified according to their shape, as well as their metabolic or ecological characteristics. With regard to shape, rod-shaped bacteria are called bacillus (plural-bacilli); spherical-shaped bacteria are called coccus (plural-cocci); and those bacteria which are spiral or curved are called spirillum (plural-spirilla). Moreover, many bacteria grow in chains and are referred to as `strepto-bacillus' or `streptococcus'.
...... On the other hand there are ways of classifying bacteria according to their metabolic requirements: aerobic (need oxygen); anaerobic (do not need oxygen); photosynthetic (need sunlight); thiobacteria (need sulfur like that found in hot springs); methanobacteria (need methane gas as found in soil and human GI tract); acidophilic bacteria (need acid conditions like in processes to make vinegar, cheese and yoghurt); nitrogen-fixing bacteria (found associated with legumous plant roots; iron-oxidizing bacteria (need iron like those bacterial masses growing on the sides of the sunken H.M.S.Titanic); and thermophilic (need very high temperatures like that found in hot springs). With regard to the thermobacteria, a new, distinct Kingdom composed of bacteria-like cells have recently been identified growing near hot spring locations like in Yellowstone National Park. Based upon DNA evidence plus other characteristics different from the Monerans, the new Kingdom has been called `Archaea', and may in fact represent ancestors to the oldest forms of life on earth.
...... Also bacteria can be sub-divided according to how they can be stained in the laboratory; for example, Gram-positive vs Gram-negative stained bacteria. Furthermore, many bacteria are classified according to their susceptibility to various antibiotics; for example, streptomycin-sensitive vs streptomycin-positive.
Links:
For more information on bacterial classification and properties see: "http://www.bact.wisc.edu/Bact330/Bact330Homepage"
www.bact.wisc.edu/Bact330/Bact330Homepage and http://www.wsu.edu:8080/~hurlburt/pages/Chap6.html http://www.wsu.edu:8080/~hurlburt/pages/Chap6.html, and http://www.ultranet.com/~jkimball/BiologyPages/E/Eubacteria.html www.ultranet.com/~jkimball/BiologyPages/E/Eubacteria.html
...... Generalized Structure - If we examine the widely studied E.coli bacteria, we find a bacillus which has a plasma membrane boundary made up of phospholipids and proteins like that found in eukaryotic cells; this membrane is accompanied by a cell wall layer made up of glycoproteins (carbohydrates + protein) which is antigenic to our immune system (i.e., the foreign proteins activate our immune system to make antibodies against the bacteria). Attached to the cell wall are hair-like projections called pili which allows the bacteria to fasten itself to a surface. Some bacteria like the spirillum also have a flagellum which acts as a tail to provide mobility.
...... Within the cell, there are free ribosomes where proteins are made; this is in contrast to eukaryotic cells where ribosomes are confined to a membranous complex called the endoplasmic reticulum. The genetic material (called the `genome') is confined to a circular chromosome (double-stranded DNA) which is distributed within the cytoplasm (where the human DNA genome is stretched out would extend 5-6 feet, the E.coli genome is one millimeter in length). While not true for all bacteria, E.coli and others have plasmids distributed within the cytoplasm; as we'll see later plasmids are small loops of DNA which can isolated and used commercially to make DNA recombinant proteins such as the human form of insulin called `humulin'.
...... Growth and Reproduction - Bacteria reproduce by first replicating their circular DNA chromosome forming two big loops within the cell; subsequently the bacterial cell splits in two by a fission process with each daughter chromosome being distributed to each new cell (this is in contrast to the complex mitotic process of eukaryotic cells). The time between divisions (or generation time) varies between species, and is dependent upon the growth conditions. For example, under optimal conditions of temperature, pH, nutrient and salt concentrations, and oxygen tension, the E.coli is capable of dividing every 20 minutes! To give you some idea of the amazing reproductive capacity of bacteria, if you started with a single E.coli cell growing under optimal conditions, in 24 hours you will be left with 64 billion cells! If conditions are not optimal which is normally the case in nature, the generational times will be proportionately extended. However, it should be stated here that some bacteria have much longer, genetically-determined doubling times; for example, the bacteria that causes leprosy has an optimal generation time measured in days rather than minutes like E.coli.
...... It should also be pointed out here that `antibiotics' is general act upon the growth and/or reproduction of bacteria. For example, the pioneering drug penicillin inhibits cell wall synthesis as its mode of action (see, www.pfizer.com/history/1928.htm) , while other antibiotics inhibit the abilities of bacteria to reproduce its DNA, or to make protein; the net effect of this class of drugs is not to directly kill bacteria but to significantly impact its growth and reproduction. Hopefully, the body's immune system will respond and finish what the antibiotics started.
“The Good Guys”
...... As mentioned earlier, life as we know it could not exist on this planet without bacteria (and fungi) recycling nutrients back into the food-chain; whether it is the pristine forests of our national parks, the seven seas, corn fields of Iowa, or your backyard garden, bacteria have the capacity of breaking down dead, complex organic matter into simple forms which subsequently are made available to support the nutrient needs of the remaining 4 Kingdoms (Protists, Fungi, Plants, & Animals).
...... Several agricultural plants such as the legumes (peas, beans, soy, alfalfa) develop a symbiotic relationship with nitrogen-fixing bacteria; colonies of these soil bacteria grow associated with the root systems and consequently are able to convert atmospheric nitrogen gas into nitrate and nitrite salts which are soluble forms of nitrogen needed by plants to make protein.
...... Many commercial products are derived from the metabolic activities of bacteria (as well as certain fungi); for example milk and cream sources can be converted into cheeses and yoghurts, while grain and fruits can be fermented into a variety of spirits such as wines, beers, and vinegars.
...... Selected strains of bacteria have been shown to be able to digest raw, unrefined crude oil; as a result, these bacteria have been commercially produced and made available to those involved in fighting oil spills.
...... Through applied research studies, certain bacteria have been found to control insect pests; consequently these bacteria are now commercially available to fight garden pests as well as on a larger scale in farms.
Recombinant DNA Products
...... Only 20 years ago or so, the public was greatly frightened over the prospect of `monsters' being grown in research labs; it was at that time that scientists began to take their knowledge of molecular genetics and `do something constructive with it'. As a prominent example, the common intestinal bacteria E.coli was being genetically transformed for the first time. At first there was great concern that the `transformed' E.coli would cause social havoc and wide-spread disease; however as experimentation took place within `high security' labs, it was quickly learned that recombinant DNA technology had a fantastic future. The worst fears of society (and more than a few scientists) were quickly put to rest. A case in point was the development `Humulin' by Eli Lilly Pharmaceuticals (http://www.eillilly.com/health/products/index.html); this drug made by E.coli is in actuality identical to the insulin made by the human pancreas.
...... How did Eli Lilly do it? The company scientists used a widely-used molecular genetic procedure called `recombination DNA technology' which involves the following steps: 1. plasmids made of circular strands of DNA were extracted from E.coli; 2. the DNA plasmids were cut open by special scissors called `restriction enzymes'; 3. pieces of human DNA containing the coding regions (genes) for insulin were joined to the cut ends of the plasmids (this why it is called `Recombinant DNA Technology'); 4. plasmid ends were rejoined so loops were formed again; 5. the recombined plasmids were inserted back into the E.coli (actually an electric current is applied to the bacteria which causes pores to form allowing the plasmids to enter the cells); 6. E.coli is grown in large batches within optimum growing conditions – making lots of bacteria as well as making lots of bacterial proteins including THE HUMAN PROTEIN – HUMULIN !; 7. Humulin is then harvested from the batches and later purified; and 8.Ely Lilly puts the potential drug through its pre-clinical and clinical testing program according to the FDA Guidelines for Safety and Effectiveness.
...... The rest is history – Humulin has turned out to be an extremely important drug in the successful treatment of diabetes. Today there is a growing list of commercially-produced recombinant DNA proteins (derived from bacteria) available to treat many human illnesses; the future is now!
...... After observing the human condition back in the 1800s, the American poet Ralph Waldo Emerson commented – “As soon as there is life, there is danger”. The major causes of death at the turn of the 20th century were from rampant viral and bacterial infections; most people died much earlier back then due to infections such as pneumonia and the flu. For the most part many of these infections have been successfully dealt with through better understanding of personal and public hygiene, medical applications of the `germ theory', vaccinations, and the advent of antibiotics. While some of the figures involved in the above advances will be reviewed later in the `Historical Notes' section, some bacterial infections current today are briefly listed below.
...... Enteric Pathogens - There are several bacteria which cause GI illnesses; most of these bacteria make toxins which are released within the gastro-intestinal tract causing diarrhea, cramps, and if left untreated may have lethal consequences. Salmonella and specific strains of E.coli are among the enteric pathogens. In addition, cholera and campylobacter cause serious uncontrolled diarrhea which if not adequately treated can lead to dangerous drops in blood pressure, even death. Interestingly, these bacteria release a toxin which in fact is a salt channel; this channel protein binds to the lining of the GI tract causing salt to leave the cells of the lining, which consequently through osmotic processes causes the loss of water (hence, copious watery diarrhea). Furthermore, not too long ago it was discovered that most intestinal ulcers are caused by a specific bacterial infection; ulcers were long thought to be caused by a combination of stress and dietary factors. Moreover, current research has shown that some bacterial infections may be linked with various heart ailments.
...... Skin Pathogens - Many bacteria common to our every-day environment have the potential of infecting skin causing pimple, boils, and ulcerations.
...... Lung Pathogens - In addition to viral causes there are particular bacteria which can lead to pneumonia and tuberculosis.
...... Neural Pathogens – Among the bacteria that selectively infect components of the nervous system are leprosy bacterium (they can target peripheral sensory neurons), meningitis bacterium (target protective covering of the brain and spinal cord – the meninges), and syphilis spirochete (targets the brain in the late stages of the infection).
...... Oral Pathogens – Several bacteria are capable of causing bad breath (halitosis) and tooth decay – if allowed to remain within the oral cavity, bacteria can grow and reproduce, and in the process secrete acids which can erode dental enamel causing small pits, and decay sites.
...... Sexually Transmitted Diseases (STD) – There are several STDs which are caused by bacteria; the most common examples are: syphilis, gonorrhea, and chlamydia.
Historical Notes & Issues
...... Example of On-Going Evolutionary Process – As you know from newspaper stories there is a rise in antibiotic-resistant bacteria. Since bacteria mutate frequently, and since antibiotics are overly-prescribed in many settings (and countries), it should come as no surprise that many bacteria have evolved into several antibiotic-resistant strains. In these cases, antibiotics act as a `selection pressure' favoring those bacteria that are antibiotic-resistant, and killing off those that are not.
...... Birth of Antibiotics – The German scientist Ehrlich first coined the term `antibiotic' when using an arsenic compound (Salvarsan) to successfully treat syphilis.
...... Sulfa Drugs – The drug prontosil was found to be effective in treating bacterial infections in animals, but was not effective in killing bacteria grown in cultures. Why? It was later found that prontosil is converted to an active sulfa drug called sulfanilamide within the body; it is the sulfanilamide which interferes with bacterial growth (hence, it acts as an effective antibiotic). Interestingly, it was a contaminated supply of sulfa drugs which initiated the formation of the US Food & Drug Administration (FDA). After a public outcry from deaths attributed to the tainted sulfa drugs, the US Congress enacted legislation which formed the Food And Drug Administration whose responsibility was to safeguard the public from unsafe and ineffective drugs and food additives. Moreover, sulfa drugs (and penicillin) had major roles in combating battlefield infections during WWII.
...... Germs and Disease (Development of `Koch's Postulates)– Robert Koch in 1876 first studied Bacillus anthracis as the cause of `anthrax' in cattle and humans. Through his careful experimentation with anthrax and other diseases such as cowpox, Koch developed his famous `Postulates' which are still used today to determine the cause and transmission route of disease caused by bacteria (as well as other microbes). Briefly, Koch's Postulates can be summarized as follows: 1. samples such as blood are removed from a diseased animal suspected of being infected with a pathogen (such as bacteria or virus; 2. the suspected pathogen is then grown in pure cultures and subsequently isolated; 3. the pathogen is injected into a healthy animal to see if disease symptoms develop; 4. if disease symptoms do develop, steps 1,2 and 3 are repeated. Using this methodology, scientists (even today) a pathogen causing a disorder can be identified, as well as disease model system in which to test the potential efficacy of therapies.
...... Case Study Questions – 1. If you had a pet dog at home which you thought might be ill from some infection, how would your vet apply `Koch's Postulates' to determine the cause of the infection?; 2. Assume you are a scientist interested in the cause and treatment of AIDS (`Acquired Immune-Deficiency Syndrome'; how would you apply `Koch's Postulates' towards learning the cause and treatment of AIDS?
...... Where Does Life Come From? – Since the days of Aristotle (384-322 BC), and up until the mid 19th century, the popular belief was that living things can `spontaneously generate'. For example, maggots feasting on raw meat or infesting a dead fish on a beach `just appeared' out of thin air. Diseases such as malaria were attributed to organisms that spontaneously arose from the warm, damp air found in tropical swamps; hence the `bad air' or `mal aire' connotations. However, it was Louis Pasteur (1822-1895) who put the argument to rest through his classical set of experiments. In his work, Pasteur used a custom-made glass beaker which was connected at the top to a `S' shaped tube. From his experiments he concluded: a. no bacterial growth was found in the beaker which had broth previously boiled (hence the birth of the `pasteurization' process to sterilize milk and dairy products); b. there was growth in the beaker when there was free access of air through the tube from the outside air (i.e., source of bacteria); c. there was no growth in the beaker in which there was a cotton plug in the `S' shaped tube; and d. there was growth in the beaker in which the cotton plug was allowed to fall from the tube into the beaker (hence, cotton plug was source of the bacteria).
...... Joseph Lister and Aseptic Surgical Procedures – Prior to WWII, the primary cause of death in most wars were bacterial infections arising from battlefield injuries as well as from surgical attempts to treat the injuries (and the infections themselves). The Scottish surgeon Joseph Lister (1827-1912) had closely followed the debate on `spontaneous generation' and the pioneering studies of Pasteur; as a result Lister reasoned that surgical infection (or sepsis) might be the result of bacterial infection of tissues exposed to the air during surgery. Hence, Lister was able to significantly cut back surgical sepsis through the sterilization of instruments, disinfected dressings, and a fine spray of phenol disinfectant during the actual surgery.
...... The Microscope Opened The Doors To Biology (& Bacteria) – Early in the 16th century, a revolution in biological thinking began with the Dutchman Antoine VanLeeuwenhoek's construction of a rudimentary magnifying device (aka, microscope); this device permitted him to visualize for the first time living organisms he called `animalicules'. In a drop of pond water he saw a wide array of organisms – tiny algal cells, protozoans like amoebae and paramecium, small invertebrates like rotifers, insect larvae, and all sorts of bacteria. For the very first time, 99% of the world's organisms (including bacteria) were revealed with VanLeeuwenhoek's discovery. It would take another 200 years and the advent of the high-power electron microscope before scientists would be able to visualize virus particles (as well as large cellular molecules such as DNA and protein).
KINGDOM - PROTISTA
...... Introduction: What are they?- Generally, most people do not easily recognize this Kingdom; however, when we mention a number of species which belong to this group, then the Kingdom usually gets quick recognition. For example, the amoeba belongs to this group which those who know some biology can easily recognize. The algae that quickly builds up in a pond or swimming pool during the summer belong to this Kingdom. On the other hand if you hear the illness 'malaria' being uttered, we quickly identify the mosquito as being the culprit; however in fact it is one of the parasitic protists which causes malaria (the mosquito is only the `vector' in transmitting the parasite to the human blood stream).
...... Characteristics of the Protists:
...... Contrary to the prokaryotic Moneran Kingdom, the Protist Kingdom is comprised of organisms made up of eukaryotic cells; i.e., these cells are highly organized and complex containing a membrane-bound nucleus (with its genetic blueprint contained in the chromosomal DNA), mitochondria (with enzyme-driven pathways to breakdown sugar and fat into ATP energy), chloroplasts (with light-sensitive pigments necessary to convert light energy into chemical energy needed to make sugars, fats and proteins), and endoplasmic reticulum (with its membranes and ribosomes necessary to synthesize new proteins under the direction of codes embedded within DNA).
...... On an evolutionary scale, the Protists can be considered to be the `experimental models' of biology; i.e., over millions of years of evolution approximately 60,000 Protist species became extinct, only to be replaced by about 60,000 species living today. Many of our prominent landforms (e.g., limestone rock layers of the Niagara River gorge, and Grand Canyon) are due to calcium deposits made over geologic time from countless Protists once living within the oceans of the world.
...... The Protist Kingdom can be loosely divided into 3 sub-groups: a. protozoa; b. algae; and c. slime molds. Most of these organisms are unicellular, while many are colonial. While most are free-living, some of the Protists are parasitic (i.e., depend on a living host cell or tissue in order to complete its life-cycle).
...... Most Protists reproduce by the asexual `mitotic' process; i.e., diploid set of chromosomes (genome composed of chromosome pairs) are duplicated in prophase, aligned at equatorial plate in metaphase, and diploid set is subsequently divided between the daughter cells. In rare cases, sexual reproduction is possible in some species.
...... The protozoans obtain energy either through direct ingestion of food particles (including bacteria), while algae gain energy through photosynthesis. On the other hand, parasitic forms (e.g., plasmodium) obtain nutrients from host organisms (e.g., red blood cells).
...... Ecological Food Chain – According to the Laws of Thermodynamics, energy within the biosphere is constant, and energy can be transformed from one kind to another. Where do the Protists fit in? If you picture the ecological food chain as a pyramid, the bacteria and Protists layers of organisms would be on the bottom; that is, these organisms obtain their nutirients from very simple sources (sunlight, gases, mineral salts, and bits and pieces of organic molecules) and use them to synthesize their own cellular components. In turn, bacteria and Protists provide food material for the next rung on the pyramid (e.g., lower animals like juvenile insects and fish); and consequently, the lower animals provide nutrients higher up the food chain. All the while this trophic process is going on, photosynthetic organisms like blue-green algae, protistan algae, and plants draw energy from the sun to synthesize new cellular tissue which subsequently becomes the energy sources for the herbivore organisms. To complete the food chain, dead tissue needs to be recycled back into the biosphere; this where the fungi and bacteria come in as the `decomposers' of the pyramid. Both fungi and bacteria can acquire their needed energy from decomposing matter from dead organisms. Hence, Protists play integral roles within the world's food chain; for example, the oceans are alive with life as you know, however it is the `zooplankton' (protozoans) and `phytoplankton' (algae) which provide the key energy sources for all of us humans to enjoy (viz., tunafish sandwhich, whale watching, grilled swordfish for dinner,, and snorkeling coral reefs).What evidence of a food chain were you able to establish on your field trip?
...... Case Study Questions? – Applying what you know how protests reproduce (viz., mitosis), what strategy would you apply to your swimming pool which is over run by algal growth? What chemicals might you apply? What specific effect might these chemicals have on the ability of an algal cell to divide? Is there a parallel in your strategy to how cancer chemotherapy works?
Some Examples of Protozoans:
...... Sarcodina – this group contains the amoeba which lives in damp soil, and watery habitats; it is able to move by pseudopodia (false feet) which are retracting extensions of its cytoplasm (there is a parasitic form within this group which causes `amoebic dysentery).
...... Ciliophora – these ciliated organisms like the paramecium propel themselves through water by cilia (hair-like extensions from the cell membrane).
...... Mastigophora – these organisms contain a flagella-like tail which wiggles and propels them through water (one of the flagellates, Trypanosoma, is a parasite which causes African Sleeping Sickness; the Tse Tse fly is the vector of this disease parasite).
...... Sporozoa – this group contains parasitic forms like Plasmodium which causes malaria; bites from an infected mosquito causes the parasite to enter the blood stream, and eventually destroy host red blood cells.
Some Examples of Algae:
...... The algae represent tremendous diversity – organisms which exhibit ranges in color (containing red, yellow, orange, green photosynthetic pigments), shapes (leafy to spherical shapes), and sizes (from single-celled to colonial forms). Some prominent examples:
...... Euglenophyta – these are commonly called euglena; are protozoan-like with a flagella for movement, and if in a sunny habitat, are capable of photosynthesis.
...... Chlorophyta – group contains over 7500 species, including the `duck weed' form called Chlorella; the colonial, spherical form called Volvox, and a host of other forms large and small (e.g., various seaweeds like kelp and sea cabbage, filamentous spirogyra, and the unicellular Chlamydomonas).
...... Pyrrhophyta – these are the dinoflagellates which contain red pigment, often causing `red tides' detrimental to fish and shellfish.
...... Phaeophyta – brown algal forms accounting for many of the cold water kelps.
...... Rhodophyta – another red pigment-containing form often found in deep, warm waters.
...... Chrysophyta – a large group prominently represented by the `diatoms'; these forms come in a range of geometric shapes (e.g., circles, ovals, diamonds, trapezoids), and possess calcium-rich shells with pores for nutrient exchange (one of the major components of the `phytoplankton' of the oceans, as well as a prominent source of limestone/sedimentary rock formations).
Links:
For more information on the Protists (including the slime molds not reviewed here) see: www.ultranet.com/~jkimball/BiologyPages/P/Protists.html
...... Background - When we think of fungi, we usually think of mushrooms. However, the fungi include a broad category of organisms represented by approximately 70,000 known species (with possibly millions more yet to be studied and understood). By studying fungi, we can learn about life and importance of the food-chain, learn about fungi as a valuable food resource, learn how fungi are responsible for growing array of medically-important drugs, learn how drug companies earn vast sums of money from a growing list of fungal products, and we'll learn how fungi evolved and refined the sexual reproduction process and in turn the genetic basis of biodiversity.
...... General Classifications -There are several divisions within the fungal kingdom: Mastigomycota which include forms which possess flagella for movement (especially the gametes); Ascomycetes commonly referred to as the `sac fungi' (includes types like `baker's yeast, Saccharomycetes, and Neurospora which has been used historically in classical genetics studies); Basidiomycetes which includes the common mushrooms we are all familiar with; and Deuteromycetes which contains non-sexual forms like Penicillium and Aspergillus, which as you might know are sources of many important antibiotics.
...... Properties – While diverse organisms, all fungi are composed of eukaryotic cells (like the protists, plants and animals); in addition to complex intracellular organelles like the nucleus and mitochondria, fungal cells have a cell wall composed of chitin and glucans. Some groups of fungi are unicellular like yeast, but most are multicellular exhibiting structural complexity (e.g., like the structural components seen in the common mushroom).
...... Most fungi are `saprophagic' feeders which indicates that they are nourished by living off dead organic tissues. In effect, fungi are able to secrete digestive enzymes into the surrounding medium (e.g., soil, compost heap), thereby breaking down complex organic matter into simple molecules (e.g., sugar, fatty acids, amino acids) making them accessible for absorption by the fungus. This ability to break down dead organic matter is referred to as `composting'.
...... As was included in the Week 2 set of notes, the famous American writer Walt Whitman once wrote a poem entitled “This Compost” where he elegantly describes the fear he has in walking through the woods in which he knows the dead and gone, buried in the ground.Within this poem, many lessons of biology are there for all to read. Most importantly, the poem illustrates the critical role of fungi (and bacteria) in decomposing dead organic matter, and recycling simple molecules back into the food-chain.
...... Case Study Question – Did you observe a decaying log in your trip to a natural setting? Upon close examination, did you uncover evidence of fungi in the decomposition process?
...... Life-Cycle of the Mushroom:
...... Asexual Reproduction - Many fungi reproduce asexually by simple mitotic division; for example, the unicellular yeast cell reproduces by creating two daughter cells through mitosis.
...... Sexual reproduction - On the other hand, most fungi like the common mushroom reproduces both asexually as well as sexually. The easily seen mushroom cap is actually the fruiting body of the fungus where male (+) and female (-) gametes are made through a cell division process called meiosis. In this process within the fruiting body of the cap, diploid cells [i.e., carrying pairs of chromosomes (2N) rather than the haploid set of single chromosomes (N)] go through a two part process; in meiosis I, the diploid cell pairs up the chromosomes, and then replicates them so in effect there a temporary tetraploid state (4N). While these chromosomes are replicated and aligned, they undergo genetic `cross-over' where pieces of DNA (genes) are exchanged; in effect the genetic playing cards are shuffled.
...... This is a very important aspect to sexual reproduction in that there is genetic mixing which insure biodiversity within the next generation (if the cells were to simply divide by mitosis, there would be simple cloning of cells, and no genetic mixing). Once crossing-over has occurred, the chromosome pairs are equally divided between the two daughter cells, forming two 2N cells again (but are genetically different from the parent 2N cell because of the genetic crossing-over). In the second stage of meiosis called meiosis II, the 2N cells undergo another division, except in this process each chromosome from each pair is `segregated' or distributed to each daughter cell; in this case haploid spore cells (N) are produced.
...... Each mushroom is capable of releasing millions of spores into the local environment; each spore being genetically different from one another. Hence through sexual reproduction process biodiversity is ensured. Assuming that environmental conditions are tolerable, the spore is now free to reproduce asexually within the soil. Typically, spores germinate (reproduce) by forming long multicellular processes called `hyphae'; eventually a hyphal network is formed (called a mycelium) and this constitutes the `haploid' generation of the mushroom (i.e., all cells are haploid).
...... Usually a mushroom will remain within the hyphal haploid state until there is some change in the environment. For example, the change may consist of alterations in temperature (increase or decrease), moisture content (increase or decrease), light (increase or decrease), or nutrient levels (nitrogen, phosphate, potassium,etc). Faced with environmental changes which may affect its ability to survive as a species, the fungus can respond in two ways: in spite of less than optimal conditions, it can try to continue to grow vegetatively through asexual means; or it can adapt to the new conditions by reproducing anew through sexual means. If you recall from meiosis discussion above, `genetic crossing-over' yields a tremendous number of new offspring (haploid spores); because of genetic variability some spores will better adapt to the new set of environmental conditions. Hence, most fungi have developed sexual means of reproduction, which has insured genetic variability of its offspring, as well as a better means toward adaptability and survival.
...... Mycorrhiza – Fungi growing in tandem with plant root systems are referred to as mycorrhiza. This is an advantageous situation for plants, because soil hyphae are active in breaking down organic matter into simple molecules which can be readily absorbed by the plant root system (another good reason to `compost' your garden soils with peat moss or other organic additives).
...... Lichens – These are symbiotic associations of fungi and algae living together in which both types of organisms benefit. Lichens are often found in harsh environmental conditions like the tundra in the northern hemispheres.
...... Commercial Uses of Fungi:
...... As you know already from earlier lectures, fungi have been the sources of invaluable antibiotics (penicillin, streptomycin, chloromycetin) and drugs to inhibit cholesterol synthesis (mevacor, and other proprietary analogues). However, it should be kept in mind that mushrooms also serve other important commercial functions:
...... Fermentation products – The distillation process to make vinegars, beers, ales and wines (plus other spirits) depend on fungal species to metabolize fruit and grain carbohydrates in the relative absence of oxygen (anaerobic); because oxygen is not there to act as the final hydrogen-acceptor (which under aerobic conditions results in the formation of water), other hydrogenated by-products are formed like alcohol, acids and esters (which taken together give taste and alcohol content to a host of recreational beverages cited above). Furthermore, many cheeses and yoghurts are produced through fermentation processes of fungi.
...... Food Sources – Edible mushrooms the world over are used as a good source of food protein, as well as a tasty adjunct to meals. The morels are especially prized and quite costly. However, there are a very large number of mushrooms which are poisonous and should be avoided; for example, the deadly Aminita makes a toxin which is a powerful inhibitor of DNA-dependent RNA polymerase (viz., inhibits mRNA formation). Interestingly, this chemical is routinely used by molecular biologists and biochemists in situations where they purposely wish to inhibit mRNA formation (and in effect inhibit protein synthesis).
Fungal Diseases:
...... There is an immense list of fungal infestations which affect nearly every aspect of life. Some examples:
...... Famine - The course of Irish history changed forever when the fungus Phytophthora infestans caused the infamous `Irish potato famine' in the 1800s.
...... Wine-Making - Plasmopara viticola is a fungus which causes downy mildew in the grape plant; destruction of vineyards has far-reaching effects on the commercial wine business.
...... Grain Industry - Several fungal species cause wilting and rotting of grain plants; ergot fungus in particular is dangerous because of the toxins it makes.
...... Garden Pests - Most garden flowers and plants are susceptible to a variety of fungal infestations causing rusts, blights, and mildews.
...... Wooden Structures - Many homes, barns, marine docking facilities, etc are susceptible to `dry rot' caused by fungal deterioration of wooden frames, structures, and decking; as you know recent improvements in `pressure treated' wood products have gone a long way toward preservation.
...... Animals - Many fish, and amphibians (as well as dogs and cats) are susceptible to various fungal infections.
...... Human Ailments - There are several fungal infestations which affect humans, such as athlete's foot and ringworm.
Links:
For more complete information on fungi go to the following sources: www.wisc.edu/botany/fungi/volkmyco.html
and, http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Fungi.html
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