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17.3: Oomycota -- The Water Molds

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  • Oomycetes (a term used to refer to organisms in the phylum Oomycota) are a group of fungus-like organisms that rely on water for completion of their life cycle, hence the common name “water molds”. Members of this group share the following characteristics:

    • Heterotrophic by absorption
    • Morphology: Filamentous
    • Cell wall composition: Cellulose
    • Storage carbohydrate: Glycogen
    • Life cycle: Diplontic
    • Ecology: Many oomycetes are important decomposers in aquatic ecosystems, while others -- namely those in the genus Phytophthora -- are some of the most destructive plant pathogens. You can culture some of the more common decomposers (genus Saprolegnia) by putting pond water into a petri dish, adding a dead insect, covering it, and waiting for a few days.

    Considering what you have learned about root words, how does the name Saprolegnia help you understand this organism’s ecology?

    Saprolegnia Life Cycle

    View prepared slides of Saprolegnia. This organism reproduces asexually by producing zoospores (zoospores are spores that swim, zoo- meaning ‘to live’) inside of an elongated sac called a zoosporangium (-angium meaning vessel, so a zoosporangium is what zoospores are produced inside of). These zoospores grow by mitosis into a diploid thallus, an undifferentiated body.

    Figure \(\PageIndex{1}\): Zoosporangium

    Look for asexual reproduction in the form of elongate zoosporangia releasing zoospores. The life cycle of Saprolegnia is diplontic. Are the zoosporangia and zoospores haploid or diploid?

    The image above shows a mature zoosporangium releasing diploid zoospores. Each of these spores has two flagella, one ornamented and one whiplash.

    Look for Saprolegnia's sexual reproducing structures, the globose oogonium and smaller, pad-like antheridia (singular, antheridium) that attach to the oogonium. Because these structures produce gametes--much like spores are produced in sporangia--the oogonia and antheridia are also referred to as gametangia (gametangium singular). The oogonium produces haploid eggs via meiosis. These eggs are fertilized by the haploid male nuclei produced by meiosis within the antheridium, creating a diploid, thick-walled zygote called an oospore.

    Figure \(\PageIndex{2}\): Unfertilized oogonium
    Figure \(\PageIndex{3}\): Fertilized oogonium

    The oospore will be released and grow by mitosis to create a new multicellular thallus, completing the life cycle.

    Check your Saprolegnia cultures for presence of mycelium (fuzz around the dead insect). Make a wet mount of some of the mycelium and look at it under the compound microscope. It may help to stain the tissue, as you did in the fungal labs.

    Do you see any features that belong to Saprolegnia? Do you see any that belong to a member of the true fungi?

    Draw what you see in the space below and attempt to identify which group(s) of organisms you have in your culture.

    Below is the life cycle diagram of Saprolegnia. Label an oogonium, antheridium, male nuclei, egg, oospore (zygote), zoosporangium, zoospore, and thallus. Indicate where meiosis and fertilization occur. Use a different color to indicate which tissue is haploid or diploid.

    Figure \(\PageIndex{4}\): Saprolegnia life cycle diagram

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