12.1 Fungi

• Fungus are important in the food chain because they decompose dead plant matter, thereby recycling vital elements.

• Nearly all plants depend on symbiotic fungi, known as mycorrhizae, which help their roots absorb minerals and water from the soil.

• The study of fungi is called mycology.

• In most molds, the hyphae contain cross-walls called septa (singular: septum), which divide them into distinct, uninucleate (one-nucleus) cell-like units.

• These hyphae are called septate hyphae.

  • In a few classes of fungi, the hyphae contain no septa and appear as long, continuous cells with many nuclei.

  • These are called coenocytic hyphae.

• When environmental conditions are suitable, the hyphae grow to form a filamentous mass called a mycelium, which is visible to the unaided eye.

• Yeasts are nonfilamentous, unicellular fungi that are typically spherical or oval.

  • Some yeasts produce buds that fail to detach themselves; these buds form a short chain of cells called a pseudohypha.

  • Fission yeasts, such as Schizosaccharomyces, divide evenly to produce two new cells.

• Both sexual and asexual reproduction in fungi occurs by the formation of spores.

  • Asexual spores are formed by the hyphae of one organism.

  • Sexual spores result from the fusion of nuclei from two opposite mating strains of the same species of fungus.

• Two types of asexual spores are produced by fungi.

  • One type is a conidiospore, or conidium (plural: conidia), a unicellular or multicellular spore that is not enclosed in a sac.

    • Conidia are produced in a chain at the end of a conidiophore.

      • Conidia formed by the fragmentation of a septate hypha into single, slightly thick- ened cells are called arthroconidia.

    • Another type of conidium, blastoconidia, are formed from the buds of its parent cell.

• A chlamydoconidium is a thick-walled spore formed by rounding and enlargement within a hyphal segment.

• The other type of asexual spore is a sporangiospore, formed within a sporangium, or sac, at the end of an aerial hypha called a sporangiophore.

• A fungal sexual spore results from sexual reproduction, which consists of three phases:

  • Plasmogamy. A haploid nucleus of a donor cell (+) penetrates the cytoplasm of a recipient cell (-).

  • Karyogamy. The (+) and (-) nuclei fuse to form a diploid zygote nucleus.

  • Meiosis. The diploid nucleus gives rise to haploid nuclei (sexual spores), some of which may be genetic recombinants.

• A zygospore is a large spore enclosed in a thick wall.

• Any fungal infection is called a mycosis.

• Systemic mycoses are fungal infections deep within the body.

• Subcutaneous mycoses are fungal infections beneath the skin caused by saprophytic fungi that live in soil and on vegetation.

• Fungi that infect only the epidermis, hair, and nails are called dermatophytes, and their infections are called dermatomycoses or cutaneous mycoses.

12.2 Lichens

• A lichen is a combination of a green alga (or a cyanobacterium) and a fungus.

  • The lichen’s thallus, or body, forms when fungal hyphae grow around algal cells to become the medulla.

• Fungal hyphae project below the lichen body to form rhizines, or holdfasts.

  • Fungal hyphae also form a cortex, or protective covering, over the algal layer and sometimes under it as well.

12.3 Algae

• The body of a multicellular alga is called a thallus.

  • Thalli of the larger multicellular algae, those commonly called seaweeds, consist of branched holdfasts (which anchor the alga to a rock), stem- like and often hollow stipes, and leaflike blades.

• Algin, a thickener used in many foods (such as ice cream and cake decorations), is extracted from their cell walls.

• Since 1991, hundreds of marine birds and sea lions have died from the same domoic acid toxicosis in California.

• Dinoflagellates are unicellular algae collectively called plankton, or free-floating organisms.

• Dinoflagellates in the genus Alexandrium produce neurotoxins (called saxitoxins) that cause paralytic shellfish poisoning (PSP).

  • Large concentrations of Alexandrium give the ocean a deep red color, from which the name red tide originates.

• A disease called ciguatera occurs when the dinoflagellate Gambierdiscus toxicus passes up the food chain and is concentrated in large fish.

• Seasonal changes in nutrients, light, and temperature cause fluctuations in algal populations; periodic increases in numbers of planktonic algae are called algal blooms.

12.4 Protozoa

• The feeding and growing stage, or trophozoite, feeds upon bacteria and small particulate nutrients.

• Schizogony is multiple fission; the nucleus undergoes multiple divisions before the cell divides.

• The ciliates, such as Paramecium, reproduce sexually by conjugation, which is very different from the bacterial process of the same name.

• Some protozoa produce gametes (gametocytes), which are haploid sex cells.

• Like some other flagellates, T. vaginalis has an undulating membrane, which consists of a membrane bordered by a flagellum.

• Two groups of flagellated cells are included in the Euglenozoa based on common rRNA sequences, disk-shaped mitochondria, and absence of sexual reproduction.

• Euglenoids are photoautotrophs.

  • Euglenoids have a semirigid plasma membrane called a pellicle, and they move by means of a flagellum at the anterior end.

• The hemoflagellates (blood parasites) are transmitted by the bites of blood-feeding insects and are found in the circulatory system of the bitten host.

• The amebae move by extending blunt, lobelike projections of the cytoplasm called pseudopods.

• The trophozoites, called tachyzoites, reproduce sexually and asexually in an infected cat, and oocysts, each containing eight sporozoites, are excreted with feces.

12.5 Slime Molds

• Slime molds are closely related to amebae and are placed in the phylum Amoebozoa.

• There are two taxa of slime molds: cellular and plasmodial.

  • Cellular slime molds are typical eukaryotic cells that resemble amebae. In the life cycle of cellular slime molds.

  • Plasmodial slime molds were first scientifically reported in 1729.

    • A plasmodial slime mold exists as a mass of protoplasm with many nuclei (it is multinucleated). This mass of protoplasm is called a plasmodium.

    • When plasmodial slime molds are grown in laboratories, a phenomenon called cytoplasmic streaming is observed, during which the protoplasm within the plasmodium moves and changes both its speed and direction so that the oxygen and nutrients are evenly distributed.

12.6 Helminths

• Helminths are multicellular eukaryotic animals that generally possess digestive, circulatory, nervous, excretory, and reproductive systems.

• Adult helminths may be dioecious; male reproductive organs are in one individual, and female reproductive organs are in another.

  • Adult helminths may also be monoecious, or hermaphroditic—one animal has both male and female reproductive organs.

• Members of the phylum Platyhelminthes, the flatworms, are dorsoventrally flattened.

• Members of the phylum Nematoda, the roundworms, are cylindrical and tapered at each end.

• Males are smaller than females and have one or two hardened spicules on their posterior ends.

• Spicules are used to guide sperm to the female’s genital pore.

12.7 Arthropods as Vectors

• Arthropods are animals characterized by segmented bodies, hard external skeletons, and jointed legs.

• Arthropods that carry pathogenic microorganisms are called vectors.