Unless otherwise noted, information contained in each edition of the Kansas School Naturalist reflects the knowledge of the subject as of the original date of publication.
Volume 36, Number 2 - December 1989
The Slime Organisms
ABOUT THIS ISSUE
Published by EMPORIA STATE UNIVERSITY
Prepared and Issued by THE DEPARTMENT OF BIOLOGICAL SCIENCES
Editor: Robert F. Clarke
Editorial Committee: David Edds, Tom Eddy, Gaylen Neufeld, Richard Schrock
ABOUT THE AUTHOR
My graduate work on slime molds was directed by a wonderful teacher, Prof. Constantine J. Alexopoulos, whose influence has bolstered my work and my life in subsequent years. I wish to dedicate this issue of the Naturalist to him--and to the public school biology teachers, whose conduct and contributions are too often not recognized nor commended. JEP
Dr. John E. Peterson is Emeritus Professor of Biology at ESU. He served as Dean of Liberal Arts and Sciences for 12 years before retiring. His personal publication, The Life of the Mind, has reached international proportions and has received praise from a wide variety of recipients. He is the author of several issues of the Naturalist.
Illustrations are by Dr. Robert F. Clarke.
The Slime Organisms
by John E. Peterson
The expression "slime ball" (as in "He is a slime ball!") is always used disparagingly. No one ever calls someone they like or admire a "slime
ball." Use of the word, slime, in any context often brings a "yuck," or a frown at the very least, from a listener. Why is that? What are the connotations of "slime" in most minds?
My dictionary gives three definitions for the word, slime. They are: (1) Soft, moist earth or clay; viscous mud; (2) Any dirty substance that is moist, soft, and adhesive; (3) The mucous or mucus-like secretion of the skin of slugs, land snails, etc. No wonder the word gets such bad press! One of my scientific dictionaries, however, does the word much more credit. It has only two simple definitions: (1) A wet, generally sticky, substance; (2) mucus. Now, that is more like it.
The outer cell layers of most organisms manufacture and exude materials which protect the organism from desiccation, from chemical and
physical damage, and which assist it in adapting to its environment. In so-called "lower organisms," the production of wet, viscous slimes is commonplace. Practically all organisms which exist in soil and decaying organic debris habitats do so. We are all familiar with the slime
traces left on sidewalks and other surfaces by slugs after their nightly forays out to eat our plants. We are all familiar with the slimy character of earthworms and insect larvae when we dig them out of the soil. Interestingly, some insect larvae in the soil and organic debris are quite slimy, whereas the adult stage of the same insect, which will spend its existence out in the atmopshere, is quite "dry."
Slime is a very common biological material. It varies chemically, but many cells and tissues produce it. Most of the microscopic organisms which we generally lump together as "microbes"- bacteria, protozoa, molds and other fungi, algae, tiny worms and other simple animals with which the soils and waters of the world literally teem--are invariably slime producers. They produce slime, in greater or lesser amounts, as a shield against their environment, as a layer which keeps them from drying out, from being damaged by temperature fluctuations, from being affected by various chemicals in their environment.
If one contemplates the situation a moment, it soon becomes clear, in view of the tremendous populations of these microbes in our world, that there must be a heck of a lot of slime produced. Indeed, we may say that we live in a slimy world. It is of special interest, then, that, even under such circumstances, there are three groups of organisms which are such outstanding slime producers that this characteristic
has become an integral part of their names.
The purpose of the following pages is to acquaint you--the teacher, the student, the general reader--with these three groups of very common, but generally unseen and unknown, absolutely fascinating organisms. They may be thought of collectively as "the slime organisms."
The Names and Sizes of the Groups of Slime Organisms
The common names for these three groups of slime organisms are the slime bacteria, the cellular slime molds, and the acellular, or true, slime molds.
The technical name for the slime bacteria is myxobacteria, or the Myxobacterales, to make it even more technical and give it its proper ending as an order of the bacteria. The "myxo" part of the name means "slime" in the Greek. Only about 30 species of myxobacteria are recognized, but they occur in great profusion in soils and decaying organic debris all over the world.
The technical name for the cellular slime molds is Acrasiae, Acrasiales, or various other designations, depending on the authority. That for the true slime molds is Myxomycetes, though the ending may vary, depending on the authority doing the classifying. Note the "Myxo" part, again meaning "slime." The "mycetes" portion means "fungus" or "mold" in the Greek, so the name means "slime mold," as we have said above.
About 40 species of the cellular slime molds have been described. They are generally found in soil, but they are also found in considerable profusion in and on decaying plant materials. There are about 500 species of true slime molds recognized.
Habitats and Modes of Existence
Though they all need moisture for growth and the completion of their life cycles, there are no truly aquatic members of the slime organisms, as they are viewed today. Nor are any of them found in decaying animal tissues. All of them are found in soils and in or on organic debris near the soil. Various of them prefer different types of soil, of course. For example, some species may most commonly be found in cultivated soils in great profusion, whereas others prefer undisturbed forest or field soils. A few prefer desert soils; even though they may be found in all soils, they can most readily be found in profusion in arid and semi-arid soils. They are better able to compete, no doubt, in those soil environments where there are fewer total organisms.
The "organic debris" in which they are found is predominately of plant origin, though animal dungs are a rich source for them. Rabbit dung pellets are such a source for slime bacteria and for some of the true slime molds, but other dungs are always worth the gathering.
A habitat which has proven to be very rich in many species of the slime bacteria and of the true slime molds is the bark of living trees. The cellular slime molds are not common here, but one can find one once in a while. I did my masters thesis on the myxomycetes on the bark of living trees and, while doing that, found so many myxobacteria there that I proceeded to do my doctoral research on the myxobacteria
on the bark of living trees. What is tree bark anyway? Non-living cells of plant origin in various stages of decomposition. Each piece of bark is a micro-environment in its own right.
What are the slime organisms doing in these soil and organic debris habitats? What is their mode of existence here? most of them are busily devouring true bacteria. They get their energy to grow, to move, to make their slime, to multiply, and to complete their life cycles from bacteria. They are, as we say, eubacteriolytic. The "eu" means "true" and the "lytic" means "dissolve," so the word simply says they "dissolve true bacteria." In doing so, they get the energy for all of their activities.
A few of the slime organisms are able to utilize large molecules such as cellulose, the main constituent of plant cell walls. Their source of energy, then--their mode of existence--would be from these large molecules.
Later, we will examine the characteristics of each of the three groups of the slime organisms in some detail. At this point, let us simply generalize on what they are like, on what are the parts of their life cycles.
They all have a vegetative stage, a sporulating stage, and a resistant stage. The vegetative stage is where the feeding and growing occurs. This stage is motile. The slime is produced here. Cell division occurs here.
In the slime bacteria, this vegetative stage is a single bacterial cell. It produces enzymes which dissolve the true bacteria near it and then it absorbs the nutrient "goodies" into itself. As the cell grows, it divides into two cells, both of which continue to feed and grow and, ultimately, divide. Consequently, there would soon be literally millions of these cells at work.
The vegetative stage in the cellular slime molds is a single amoeba. It engulfs bacterial cells inside itself in typical amoeboid fashion. The enzymatic dissolving of the bacteria--the "digestion"--then takes place internally. Of course, it grows and divides and produces populations of millions of amoebae like itself.
The vegetative stage of the true slime molds is a naked, multinucleate mass of protoplasm. It is like a multicellular organism, or tissue, of several thousand cells, but there are no walls or membranes dividing it into separate cells, nor is there a membrane around the entire mass. It is a naked, flowing mass of protoplasm. It feeds by engulfmg bacteria--many at a time--and, then, digesting them within itself.
this vegetative stage in the true slime molds is called a plasmodium.
To summarize the vegetative stages of the three slime organism groups: that of the slime bacteria is a bacterial cell, in the cellular slime molds, it is an amoeba, and the true slime mold vegetative stage is a mass of naked protoplasm called plasmodium. This stage is responsible for the feeding, growth, division, and slime production in all three cases.
When the food supply is depleted, when it becomes too dry, when it becomes too hot or too cold, or when the environment otherwise becomes unsatisfactory, the slime organism begins to form resistant spores. This is the sporulating, or spore-forming, or fruiting stage.
In both the slime bacteria and the cellular slime molds, several thousand--even several million--cells do this collectively. At a given signal, cells begin to move toward a center, they begin to pile up on each other to form intricate spore-bearing structures. Do contemplate this process! Thousands of individual cells with no connection to each other, at a given time, aggregate through an intricate maneuver which always ends up with a magnificent, though tiny, structure of exactly the same form for that particular species! What controls this process? It is still an unanswered question, though we have some ideas about how it works.
Since the plasmodium of the true slime molds is already a multinucleate structure of some size, the spore-bearing process is simply a matter of it clumping-up, raising itself above the substrate, and forming its distinctive spore-bearing structure. When this process is complete, it is in its resistant stage.
The resistant stage in all three groups of slime organisms is a single cell called a spore. Sometimes it is called a "myxospore" because the protective wall around it is produced from slime. Many of these spores are produced in a single "fruiting body" or "sporangium." The "angium" part of the term means "vessel," so a sporangium is a "spore vessel." The fruiting body produced by a given species is always the same for that species. Our taxonomy of the organisms is based on what the fruiting body looks like. They vary from simple dry-slime-encrusted clumps to tiny, intricate tree-like structures. We shall consider the various forms of them in more detail shortly.
The spores themselves are microscopic single-cells enclosed in a wall of dried slime to make them highly resistant to adverse conditions. We have records of some of them lasting for over 100 years in storage. When taken out of storage and provided with moisture and proper temperature, they go ahead and germinate into living, vegetative cells. After bringing the soil into the laboratory, I have been able to get them to grow out of soil which was so hot that I could not hold it in my hand when I collected it--185-190°F. And out of soils where there had been no rain for over a year. So, the spores are very resistant structures, indeed. They are the mechanism of the slime organisms by which they sit and rest until adverse conditions again become satisfactory for growth, feeding, and division.
THE SLIME BACTERIA
As we have said--and as the name implies--the vegetative structure of the myxobacteria is a bacterial cell. Bacteria are described as being prokaryotes (pro - before; karyo - nucleus) because they do not possess organized nuclei or other cell organelles. Consequently, though areas of lightness and darkness can be seen within the cell, few definitive characteristics can be seen under the microscope. There is little to say, therefore, about the structure of the cell itself.
There are, however, two distinctly different myxobacterial cells on the basis of shape. One is long, pointed at the ends, and flexuous. The other is shorter, rounded at the ends, and rigid. These cell types are shown in Figure 1.
Both types of cells glide across solid surfaces, such as that on an agar plate. The exact mode of locomotion is not known, but it is thought that they secrete slime off a rear quarter of the cell When that slime begins to gel, it pushes the cell forward. Though patience is required because they are slow, with the aid of a microscope one can easily enough watch these cells move. Often a lead cell will move off, leaving a slime trail which other cells readily glide along. The slime is a polysaccharide in nature.
The first purpose of this cell motility is feeding. Their movement, therefore, is in search of, and toward, a food supply.
As the cells feed and grow, they divide by pulling into two pieces. Under good conditions, division will occur every hour or so. Consequently, there will soon be many, many cells gliding about, feeding, dividing, and secreting slime. There will be so many, in fact, that the mass, which we will now call a colony, can be seen with the unaided eye. The colonies of most species are clear and without color, but a number of species produce pigments which color them various shades of yellow, orange, red, brown, and, even, black.
A second purpose of the cell motility is to permit them to move into appropriate position as they collectively produce the spore-bearing structure. This movement of thousands of cells toward a common goal is called aggregation.
The Fruiting Structure
The systematics of the thirty-some species, eight genera, and four families of the myxobacteria is based on the form of the fruiting body. It would serve no purpose to cover all the intricacies of that systematics here; rather, we will focus our attention on a few representative forms.
In the simplest fruiting structure, that found in species of the genus Myxococcus, the vegetative cells aggregate into a rounded clump. They
are continually embedded in slime. As the entire mass dries out, the long, flexuous cells all condense to spheres (the "coccus" part of the name) and produce a heavy wall of dried slime about themselves. They are called myxospores. The mature fruiting bodies, then, are dried-slime-enclosed globes filled with spherical myxospores. They will be found sitting on the surface of a piece of bark, animal dung, or other
plant debris (Figure 2).
In other genera, short stalks of slime are produced so that the globe of myxospores is raised up off the surface (Figure 3). In one group, the fruiting body is a coral-like, rather than globose, structure (Figure 4). in another, the interior of the mass is divided into a system of intestine-like tubules within which are contained the myxospores (Figure 5).
Those myxobacteria which possess the short, rounded-end, rigid vegetative cells produce the most complex fruiting bodies. These cells do not shorten appreciably, as did the long, flexuous ones. They do produce a slightly heavier wall around themselves; however, and they, too, are called myxospores.
The fruiting bodies produced by members of the genus Polyangium (poly - many; angium - vessel) are often clumps of sufficient size to be seen with the unaided eye. They are produced on various plant debris and dung surfaces. Those which decompose cellulose produce this type of fruiting body. They may be found on, and in, cellulose fibers. The clumps are bounded by a heavy sheath of slime within which are many smaller, circular slime-sheathed containers (the "many vessels") within which are myxospores (Figure 6). They come in a variety of colors from yellow to black. When mature, they are highly resistant and readily disseminated structures.
Members of the genus Chondromyces produce the most magnificent of fruiting bodies. They are like little trees, big enough to be spotted by the trained, but unaided, eye. And in color, too! (Figure 7)
Now, do turn your imagination to the coordinated effort that must be going here for thousands of individual cells--with no intimate contact between them, remember--to collectively build these structures of themselves and the slime they secrete. It is mind-boggling to envision such cooperative effort!
The entire purpose of these fruiting structures, of course, is to get those cells which end up as myxospores up in the air where they can be best preserved and best spread over the countryside. To that end, the vegetative cells aggregate into clumps, but the clumps continue to grow into pillars as cells swarm on top of cells and produce masses of slime while doing so. These pillars become the trunks of the tiny
trees. More cells glide into the process, the pillar is pushed higher and branches at the apex. And branches again. And again.
At the ends of the branches, masses of cells destined to be myxospores surround themselves with a common layer of slime. This layer is the wall of the cyst, or sporangium, in which the myxospores are contained. Not all cells make it into the cysts, by any means. The bulk of them give their all in the process and are left behind, embedded in the slime of the trunk and the branches, where they ultimately perish.
This production of cysts at the ends of the branches is distinctive for various species. In one, the cysts are sessile and cylindrical. Another, Chondromyces pediculatus, always produces a small stalk, or pedicel, under each cyst. In Chondromyces apicelatus, a pointed apex of dried slime is always left on the extremity of each cyst. And in Chondromyces catenulatus, the cysts are always produced in chains. How can such intricacy regularly be produced by a mass of unconnected, independent cells in a wallow of slime?
The Life Cycle
Now that we have become acquainted with the vegetative stages and the fruiting stages of the myxobacteria, it will be a relatively simple matter to summarize the life cycle of the group. The vegetative cells move about and feed, grow, produce slime, and divide. This continues as long as there are plenty of eubacteria on which to feed and as long as all conditions are favorable. Depletion of the food supply or the coming of unfavorable environmental conditions causes a shift toward the resistant state.
The cells aggregate, produce lots of slime, and begin to form clumps. In those species with the long, flexuous cells, the cells shorten to spheres and produce heavy walls around themselves to form myxospores. This happens within slime coverings in the clumps. The fruiting bodies are relatively simple. When the mass dries out, it tends to break apart and the myxospores are spread about. They, themselves, are the resistant structures.
In those forms with the short, rigid cells, the aggregation clumps tend to go further and produce fruiting bodies with many compartments within which the myxospores are encased. The compartments and cysts here tend to be the resistant units and the units which are spread about. Various of the many-vessel and tree-like fruiting bodies are produced by these species. When mature and dry, they tend to break apart so that they are more readily disseminated.
As myxospores--the resistant state--myxobacteria are able to tolerate long periods of extremely unfavorable conditions. They can sit for years without water (we kept some of them in dry soil for 10 years). They can tolerate freezing (we kept others frozen for 10 years). They can tolerate high temperatures (the desert soils too hot to hold in the hand, remember?).
When conditions again become appropriate--when temperatures are reasonable and moisture is present--the protoplasm of the myxospore becomes active, the dry slime coat softens and disappears, the cell becomes an active vegetative bacterial cell. One single cell will soon become an immense population ready to make the whole thing go around again.
Role in Nature and Importance to Man
Since most myxobacteria feed on various bacteria, they must, obviously, be important in regulating the numbers of such organisms in nature. They are. They have developed a range of enzymes which decompose bacteria and bacterial parts. Some of these have been studied and are useful in laboratory work, but none has been found to be directly useful to man in other ways.
Because of their habitats in soil and decaying debris and their feeding on various other bacteria, these myxobacteria must compete with many other organisms. They help keep their place in this busy scheme of things by having developed a system of antibiotics to keep other organisms--mostly other types of bacteria--from encroaching on, and overwhelming, them. These antibiotics, though of obvious benefit to the myxobacteria, have not been found to be of value to man.
Those few myxobacteria which get their sustenance by decomposing cellulose certainly play an important role in the scheme of things. Much carbon and energy is tied-up in the cellulose of plant cell walls. This carbon must be released to be used over and over again; relatively few organisms can do this. These few myxobacteria which can, therefore, are important in keeping the carbon cycle going. And particularly so in desert and arid situations. In such arid habitats, these myxobacteria must compete for cellulose and their ecological niche with fungi. Therefore, they have elaborated a range of antifungal antibiotics. Some of these do look promising for medicinal use.
The myxobacteria, then, play a role in the scheme of nature by assisting in the control of bacterial populations and by helping keep the carbon cycle going. They are of no direct use--or trouble either--to man. We must not forget, of course, that without the balance we find in nature and a functioning carbon cycle, man would be unable to survive. In this respect, the myxobacteria, like all of the components of the natural system, are quite important to man.
THE CELLULAR SLIME MOLDS
Many aspects of the cellular slime molds, the Arcrasiae, are similar to those of the myxobacteria. Now that we have some familiarity with the myxobacteria, it will be a simpler matter to discuss the cellular slime molds.
The Vegetative Structure
The vegetative structure in the cellular slime molds is a single cell, a single amoeba. As is the case with all other amoeba, this is a eukaryote (eu - true; karyote - nucleus). That means that each cell possesses a true, organized nucleus and various other organelles.
With usual nuclear-staining techniques, then, the nucleus in each cell can be seen with the aid of a microscope. And, at the right time, chromosomes can be seen, counted, and watched undergo the usual mitotic process. Other cell organelles-mitochondria, membranes, various vacuoles--are also present and can be seen. Food vacuoles, in particular, are prominent.
The amoebae of the cellular slime molds feed, primarily, on bacteria. Yeast cells and other particulate organic debris may also be engulfed and digested. Sometimes they appear to feed even on each other as cannibals. Bacteria constitute the bulk of the diet, however. The amoeba simply flows around a bacterium, produces a membrane around it and, thereby, incorporates it inside itself as a food vacuole. It is digested by a spectrum of enzymes produced by the protoplasm of the amoeba.
The amoebae flow about in typical amoeboid fashion, produce the slime which gives them their common name, grow, and divide. In a short period of time, large populations of them are produced when conditions are favorable. Depletion of the food supply or the coming of unfavorable environmental conditions causes aggregation of many amoebae into the fruiting structures.
The Fruiting Structure
Compared to the other two groups of slime organisms, the fruiting structures of the cellular slime molds are characteristically unshowy. Most of them consist of a simple structure of a mass of encysted cells, the myxospores, at the apex of a stalk of slime and amoebae. A few of them produce rudimentary branches and one genus produces chains of spores. Some are very tiny with only one spore on top of a slime stalk. Others are of considerable size with a mass of spores on the end of a stalk which is composed of both slime and cells. They are all quite simple, however. The best way to describe them is with a series of sketches (Figure 8).
The Life Cycle
As we have said, the vegetative cells, the amoebae, move about, produce their slime, engulf bacteria on which to feed, grow, and divide. Large populations are produced. When food supply becomes depleted or environmental conditions become unfavorable, the amoebae begin to aggregate.
Aggregation is a matter of many amoebae moving toward a central point. It is known that they move along pathways of chemical gradient to these centers. The chemical is even known and named. It is called acrasin, a name coined from the technical name for the group, the Acrasiae. One cell, apparently, begins to produce acrasin and this becomes the focal point to which other amoebae migrate. Any large population will, of course, produce several aggregation centers.
In some of the cellular slime molds, the aggregation results in a clump of one or more cells, which become the spores, and that is the end of it. In others, a stalk may be produced which will raise the developing spores up in the air. In several of them, however, aggregation is just the start of the fruiting process.
In these latter forms, the aggregated clump of many amoebae takes on an elongate form and begins to glide over the surface in a migration phase. It looks very much like a small garden slug and is sometimes called a "slug." More correctly, it is a pseudoplasmodium which means "false plasmodium." Figure 9 will give some idea of the form of this migrating pseudoplasmodium.
Much is known about the condition of cells during aggregation and during migration. It has been thought that this is a time of sexual reproduction and genetic recombination, but such has never been demonstrated to the satisfaction of most students of these organisms. Suffice it to say, for our purpose, that migration stops and a portion of the pseudoplasmodium is raised up on a stalk to become the spore mass. Those cells in the mass at the top become spherical and produce walls of slime around themselves. When dry and mature, they are the resistant structures, the spores, of the organism.
The spores are quite tolerant of adverse conditions--desiccation, freezing, etc. When conditions are again favorable, each becomes an active amoeba and the life cycle commences again.
Role in Nature and Importance to Man
As with the myxobacteria, the cellular slime molds play a role in the balance of nature. Their feeding on true bacteria certainly has something to do with keeping our world from being inundated by these rapidly-multiplying, voracious microbes. Past that, however, one is hard-pressed to assign them any other natural function.
Nor do they play a role in man's affairs. They cause no diseases of anything. They produce no products of value to us or our devices as far as we know at this time.
I ask you, again, to contemplate this situation where dozens of individual entities--the amoebae--begin to work together in a pre-orchestrated pattern of aggregation, migration, and production of the fruiting structure. There is never any connection between the individuals, yet it always goes as it should in a beautiful process of cooperation toward a common goal.
Think of the questions such a process raises about nature, about the nature of life, about how individuals can operate together. Certainly, there are lessons to be learned here about the basic meaning of existence. If we can unravel some of such secrets from this slime organism source, that, indeed, would be of immense importance to Homo sapiens, would it not?
THE TRUE SLIME MOLDS
Myxomycetes, the true slime molds, may be properly called the grandest of the three groups of slime organisms. They are much more commonly seen across the face of the world and they are much better known. They constitute a much 'larger group in that some 500 species have been described and generally agreed upon. Though clearly microorganisms, they nonetheless form structures, both in the vegetative and the fruiting states, which are big enough and showy enough to often be seen with the unaided eye. That, of course, is why they are more widely known throughout the world. Let us have a closer look at them.
The Vegetative Structure
As we have learned earlier, the vegetative structure of the myxomycetes is a multinucleate, naked mass of protoplasm called the plasmodium. Before the plasmodium comes into being, however, there are other structures which are of sufficiently vegetative nature for a portion of their existence to warrant their being discussed here. These are the swarm cells.
When the myxomycete spore germinates after lying about during periods of drought, cold, excessively high temperature, etc., it splits open and out come one-to-four swarm cells. Each possesses a single nucleus and each nucleus contains the haploid number of chromosomes. The spore contained a single diploid nucleus when it went into the resistant state. As it prepared to germinate, that diploid nucleus underwent meiosis.
Meiosis is often called reduction-division because the purpose is to produce four nuclei, each of the haploid, or reduced, number of chromosomes, from the original diploid nucleus. Once this has happened within the spore, the protoplasm divides into four portions, each of which surrounds a nucleus, the spore wall cracks open and out come the swarm cells. If all occurs as it should, there will be four, but it is not uncommon for something to go wrong and only one, two, or three swarm cells emerge.
The myxomycetes are eukaryotic. Each swarm cell has two flagella on one end. One of these is of such length that it can be seen readily under the light microscope. The other is so short that it must be seen under the electron microscope.
Since the swarm cells are flagellated, they readily swim about, engulf bacteria, grow, and, ultimately, divide by fission into two cells. The swarm cells may lose their flagella and glide about as amoebae; they are, then, called myxamoebae. The myxamoebae, also, engulf bacteria, grow, and divide by fission. Some of them may again form flagella if conditions are more appropriate for that form of locomotion. A swarm cell and a myxamoebae may be seen in Figure 10.
At some point, two myxamoebae or two swarm cells fuse to form a tiny protoplasmic mass with two haploid nuclei in it. This is certainly a sexual fusion, though the two cells are isogametes, which means "like gametes." Not all swarm cells or myxamoebae are compatible with each other; they are fussy about with whom they will mate. They will not, for example, fuse with another cell from the same spore or, even, from the same fruiting body. This, of course, makes sense because there will be better genetic recombination potential from such "unrelated" matings.
This fusion of two swarm cells or myxamoebae is the start of the plasmodium stage. The two haploid nuclei soon fuse, forming a diploid
nucleus. This nucleus will soon divide and the products will divide again and again. At the same time, the plasmodium will engulf bacteria which it digests for energy and growth. It manufactures more mitochondria and more vacuoles and builds more protoplasm which will be filled with the, nuclei. In a matter of hours, the mass will be of sufficient size to be seen with the unaided eye, if conditions for feeding and growth are at all satisfactory.
Since the plasmodium is a naked mass of protoplasm without even a membrane surrounding it, it has no definite form. One can stick a needle into it and withdraw it without disrupting anything; it is just like sticking a needle into a bowl of jelly. The plasmodium is more gelatinous--more slimy--than is water, so it does have some semblance of integrity, but it is impossible to describe its shape. The best that one can do is to say that it generally is somewhat fan-shaped as it moves forward like a small, slow tidal wave engulfing bacteria.
Bacteria are the plasmodium's main food source, but it will also engulf other particulate material, some of which it will utilize. It will also absorb some materials in solution. Those materials which are not digested and utilized will simply be left behind as waste as it flows forward. Consequently, one can actually see where a plasmodium has been as it moves across a surface.
The plasmodium of the myxomycetes is capable of producing an immediate resistant state in response to unfavorable environmental conditions. This structure is called a sclerotium. Sclerotia usually are dry and horny in consistency and appearance. They are usually full of irregularly-shaped compartments internally. In response to unfavorable conditions, the plasmodium simply clumps, condenses, produces something of a wall about itself, and dries out. When conditions are again sufficiently moist and appropriate, it becomes an active, moving, feeding plasmodium. Myxomycete sclerotia are known to remain viable for a good many months.
The Fruiting Structure
The approximately 500 species of myxomycetes are differentiated on the basis of the type of fruiting structures they produce. They are grand and diverse in form and color. Since their purpose is to produce spores, they are properly called sporangia, or "spore vessels."
Though all can be called sporangia, three basic forms of fruiting structures are recognized. These are plasmodiocarps, aethalia, and sporangia. Plasmodiocarps are so-named because they take the form of the plasmodium. The "carp" portion means "fruit," so it is simply a plasmodium-like fruit, which tends to be a system of tubes, or veins, directly on the surface. The plasmodium stopped flowing, humped-up a bit into elongated, tubular clumps, produced a wall about itself, and the protoplasm within divided into spores. In effect, the plasmodiocarp still has something of the shape of the plasmodium (Figure 11).
The aethalium, also, is without distinctive form, but it is more of a cushion, or clump, and higher than is the plasmodiocarp (Figure 12). It is simply a matter of a mass of protoplasm condensing into a rounded cushion, producing a wall about itself, with the protoplasm within dividing into spores.
Sporangia are generally stalked structures with the actual spore-containing portion on the end of the stalk. A few are sessile and, thereby,
rest directly on the substrate without being raised into the air. Far more species produce sporangia than any of the other types of fruiting structures. Since there are so many different types of sporangia, it is difficult to generalize about the forms which may be found. And they come in a variety of colors. Figure 13 illustrates some of the best-known sporangial shapes.
As has been indicated above, most myxomycete fruiting structures are stalked sporangia. A stalk, then, is a part of the majority of the fruiting structures. It is primarily composed of dried slime produced by the protoplasmic mass as it raises itself up, though some nuclei and cell organelles may be left trapped behind and, thus, become part of the stalk. Its purpose is to raise the spore up where they can be readily disseminated.
In addition to the stalk, there are three other parts of the fruiting structure worthy of mention. All of the fruiting structures contain round spores. Each spore contains a nucleus and a bit of protoplasm; each is bounded by a wall of dried slime. The spores are the actual tough, resistant structures.
Most fruiting structure possess a peridium, the name given to the dry, tough, protective wall of dried slime around the spore mass. In some, the peridium is so evanescent that it is gone by maturity, but it is quite persistent in most species.
A third structure found in a majority of the fruiting structures of the myxomycetes is known as the capillitium. The capillitium is a system of threads--produced from dried slime--intermingled throughout the structure. The capillitium is a part of the spore mass, but has no connection to the spores. Its purpose is to expand, something like a spring, thereby throwing the spores out into the surrounding atmosphere. The capillitium is a device to aid spore dissemination. Capillitial threads often are so distinctive that they can be used as characteristics for identifying the various species.
One further aspect of the characteristics of the fruiting structures should be pointed out. About half of them produce dark-colored spores; the others produce light-colored spores. Consequently, all the species can be readily divided into two groups on this basis of them metabolizing lime, calcium carbonate, so that it ends up in their peridia and capillitia. This "lime present" or "no lime present" character,
then, becomes another useful characteristic for dividing the dark-spored species into two groups.
The Life Cycle
A spore germinates to produce one-to-four haploid, flagellated swarm cells. These feed, grow, and divide. They may change to myxamoebae which, also, feed, grow, and divide and, perhaps, switch back to being swarm cells. Two compatible swarm cells or myxamoebae fuse to form a structure in which the two haploid nuclei soon fuse to form a diploid nucleus. This is the beginning of the plasmodium.
The young plasmodium engulfs bacteria and other particulate material on which it feeds. It grows, its nuclei and other cell organelles divide, and it becomes a multinucleate mass of active, naked protoplasm. This is the mature plasmodium.
If conditions become unfavorable, the plasmodium may respond in two fashions. It may produce a sclerotium, which is a dry, resistant mass of resting protoplasm with no particular form. Sclerotia are capable of resting for many months before becoming active plasmodia again.
Another thing that adverse conditions--or depletion of the food supply--may stimulate in the myxomycetes is the formation of fruiting structures. Their purpose is to produce spores. The spores are highly resistant, with some of them, we know, lasting for over 100 years. They are also the disseminatable units, readily blown about to new locations. If the habitat is favorable and environmental conditions are reasonable, the spore ultimately germinates to release swarm cells and we are back where we started.
Role in Nature and Importance to Man
There is little to be said here. In spite of their profusion and world-wide occurrence, the myxomycetes do not seem to fill any special niches or perform any particular functions. They do play a role in control of bacterial populations, as do the other two groups of slime organisms. They are simply common, regular components of the magnificent natural system which is our Planet Earth.
Nor are they of any great importance directly to man and his enterprises. One of the myxomycetes, Physarum polycephalum, is readily available from biological supply houses and is easily grown in the teaching laboratory so that students can have a good look at what protoplasm is all about. Some people experience allergic reactions when they breathe in myxomycete spores which, of course, are in profusion in the atmosphere. Sometimes, people, who especially value well-tended, beautifully-groomed lawns, get quite excited when some of the myxomycetes produce sporangia on the blades of grass in those lawns. Such can be eye-catching and disconcerting patches of grayish discoloration in the green lawn. But it is totally superficial; there is no damage and the sporangia can be washed off easily with a garden hose.
Except for such generally insignificant intrusions into the human scene, the myxomycetes are of no direct importance to us. They go their way, minding their own business, while making the world a slightly more beautiful and interesting place. Who can ask for more from any group of organisms, slime or otherwise?
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