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Fungi: The Decomposers

  Nature works through balanced ecological cycles. Plants and photosynthetic microbes, for example, remove carbon from the air and trap it in carbohydrates. If these were the earth ‘s only organisms, all carbon would soon be locked up in their cells and in their remains. This doesn’t happen, however, because other organisms eventually decompose organic wastes and the remains of dead organisms, releasing into the environment compounds containing carbon, nitrogen, phosphorus, and other elements required for life. The earth’s major decomposers are the 100,000 or more species of fungi (including the familiar mushrooms molds, and puffballs) along with the various decomposing bacteria and protists discussed in next passage.

                  

NUTRITION IN FUNGI

  Fungi are heterotrophs and must obtain performed organic molecules from the environment and modify them. Fungi secrete powerful enzymes that enable the cells to digest organic matter in their environment, breaking down large molecules into smaller ones. The cells then absorb the nutrient molecules through their cell membranes. Because the actual digestive process takes place outside the organism’s body, it is called extracellular digestion. A similar type of food breakdown takes place in animal’s hollow gut.

  Most fungi are saprobes, organisms that decompose nonliving organic matter. Fungi consume everything from leather and cloth to paper, wood, paint, and other materials, slowly reducing old buildings, books, and shoes to crumbled ruin. During the long evolution of the fungi, however, some species changed from decomposers of nonliving tissue to parasites, organisms that live off other living things, with about 5000 different fungal species attacking crops in fields, gardens, and orchards. The fungus that Andrea Stierle and her colleagues isolated from the yew tree and found to produce taxol is probably a parasite on the tree. As you will see, other types of parasitic fungi attack people and domestic animals, causing athlete’s foot, ringworm, and vaginal yeast infections, to name a few.

 

STRUCTURE OF FUNGI

  To survey the fungi, we will begin with a familiar mushroom like the common type on salads or pizzas. The body of a typical button mushroom looks solid, but is actually made up of cells joined end to end like the cars of a passenger train and forming filaments called hyphae (Highfee; singular hypha). Hyphae pack tightly together into a mat called a mycelium, rather like steel wool (figure below). The aboveground portion of a mushroom is made up of densely packed hyphal filaments. Belowground, an extensive, loose mycelium spreads below the mushroom, penetrating the soil for many square meters. Whether aboveground or below, each filament has tubular side walls made mostly of chitin (ki-tin), a nitrogencontaining polysaccaride. The chitinous side walls surround the cell’s plasma membrane (see figure below). These tough walls make fungi the most resilient organisms of all the eukaryotes. The cell walls that separate adjacent cells in a filament are called septa (singular, septum), and they are perforated. These perforations in the septa permit the cytoplasm of one cell to flow into the next.

In the early 1990s, some fungal biologists suggested that this one individual fungus may be the largest living organisms in the world. As we see later, plant biologists were quick to challenge this suggestion.

ASCOMYCOTA: YEASTS, MORELS, AND TRUFFLES

This largest division of fungi includes mostly saprobes but some important plant parasites as will. Many grow in dense, fuzzy mats; they have hyphae with perforated cross walls; and they have hypae with perforated cross walls; and they can reproduce both asexually and sexually. During sexual reproduction, ascomycotes produce spores in a little sac called an ascus. In many species, rows of these sacs are borne in a cup-shaped sturcture.

While the fungi we have discussed so far are generally multicellular prganisms, there are numerous singlecelled yeasts in the group Ascomycota. Although yeasts usually reproduce asexually by pinching off new smaller cells , they can also reproduce sexually by forming ascospores, the products of a meiotic divesion. Yeasts, which are used in the brewing and baking industries, are probably the most economically useful fungi. Another yeast, Candida albicans, causes vaginal yeast infections.

Some ascmycotes are used directly as food, including the morels and truffles so favored by gourmet cooks. Other ascomycotes, however, attack our food crops, and powdery mildews parasitize apple and cherry trees as well as grain crops. Bread baked with rye infected by one particular ascomycote causes ergot poisoning in humans, characterized by hallucinations, convulsions, premature labor, and gangrene of the arms and legs. Some historians now believe that around 1690 , the “possessed” witches of Salem, Massachusetts, were actually suffering from ergot poisoning. Still other ascomycotes caused Dutch elm disease and chestnut blight, diseases that have robbed us of millions of our most beautiful hardwood trees.

Fungal reproduction: with or without sex

   As we discussed in the above, many multicellular organisms can reproduce by either asexual or sexual means, depending on external conditions. In the fungi, asexual reproductions is the most common mode. Pieces can break off the hyphal meshwork and grow into new individuals; hyphal cells can divide or bud; or the fungus can produce asexual spores, cells that are dispersed and divide mitotucally to form new fungi. The hyphae and the asexual spores are uaually haploid phrase dominates the fungal life cycle. Spores are adaptations of dryness, heat or cold, then produce a new fungus when conditions improve.

   Fungi can also reproduce sexually. While fungi are neither male nor female each haploid individual is one of two mating types, plus or a minus. The haploid cells of either mating type can grow into hyphae and reproduce asexually. Eventually, a haploid cell of the minus mating type, forming a single cell containing two haploid nuclei. (Note that a dikaryon, a cell with two haploid nuclei, is different from a diploid cell containing two haploid chromosome sets in a single nucleus.) These doublenucleated cells can divide and form hyphae, and these threadlike structures can join and give rise to an organ of reproduction, the fruiting body. The typical mushroom of forest and field is an aboveground fruiting body. Inside the fruiting body, the haploid nuclei can become gametes, and the nuclei of two gametes can fuse, forming a diploid zygote. The diploid zygote usually undergoes meiosis, producing haploid cells of the two mating types, and the cycle is complete. There are many variations of the fungal life cycle, but the essential feature is that the haploid stage dominates, and the diploid phase is often relegated to a single cell, the zygote.

MAJOR GROUPS OF GUNGI

Fungi cannot move on their own and lack flagella at all stages of their life cycles. They are usually classified by the shapes of their spore-producing structures. Spores are the only means a fungus has of dispersing to new areas, and thus they are of prime importance.

Zygomycota: bread molds and others

    Rhizopus, the common fuzzy, whitish or grayish mold that grows on bread, is a zygomycote. The typical members of this group are filaments and grow in cottony masses, but the filaments lack cross walls between adjacent cells. Sexual fusion produces diploid zygotes that form dark, spore-forming structures; these structures may remain dormant for several months. When growth conditions are favorable, the zygotes undergoes meiosis and the resulting an asexual spore-producing sphere that can split open and spread haploid spores. Spores can lie dormant and withstand extremes of drought and cold , germinating later and perhaps producing a new hyphal mat in another slice of bread.