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Lesson Plan: Water Molds (Oomycetes)

Learning objectives:

To learn about a group of fungus-like organisms; the oomycetes, by baiting them from natural sources (water and soil) and observing them.

Exercise description:

Students use a simple procedure to bait oomycetes from water and/ or soil and then examine these organisms with the microscope to see how they look. They will see large hyphae, sporangia and swimming asexual spores (zoospores), the structures involved in sexual reproduction (oogonia and antheridia), and how the shapes and sizes of these structures may vary among different kinds of oomycetes. Students may then design their own biodiversity experiments to see if the use of different natural water sources, different kinds of soils, or different baits affect the variety of oomycetes baited.

Time frame:

Oomycetes will be observable on the baits after 2-4 days (12 hours for pollen baits), and should remain viable for at least several days.

Study questions:

  1. What characteristics make oomycetes different from the true fungi?
  2. Do different baits capture different oomycetes?
  3. Do different water or soil sources contain different kinds of oomycetes?

Answers to Study Questions:

  1. What characteristics make oomycetes different from the true fungi?
    The oomycetes, commonly called water molds, are more closely related to other Protists than to true fungi. True fungi are more closely related to animals, than to plants and protists. There are some important characteristics of oomycetes that are very different from true fungi:
    1. The mycelium cell wall is primarily cellulose, rather than chitin as in true fungi. Also, there are generally no septa or crosswalls dividing the cytoplasm into "sections". (Some do form in injured and old hyphae and sometimes at the base of reproductive structures.) In species studied, the vegetative nuclei are diploid. In true fungi, nuclei are generally haploid or dikaryotic (pairs of genetically different haploid nuclei that divide in tandem).
    2. The sexual spores are oospores (or oospheres) produced from the pairing of antheridia and oogonia. These are an important source of genetic variation from generation to generation. In addition, the thick-walled spores are important survival structures during adverse conditions (drought, winter).
    3. Oomycetes have a motile zoospore stage, which true fungi do not. These fast moving, swimming spores are one reason why they are commonly found in water and wet soils.
  1. Do different baits capture different oomycetes?
  2. Do different water or soil sources contain different kinds of oomycetes?

The oomycetes trapped on various baits will be saprophytes or the saprophytic stage of a parasitic pseudofungus. There are a number of genera and species commonly found in soils and various water sources. They can be differentiated by the structure of the sporangia, the oogonia, and the antheridia which are illustrated in many biology and mycology texts. These structures usually form in less than a week and are easily seen with a compound microscope, so they make a good observational exercise in which students can see what kinds of variation in form exist. Some factors that will affect what species may be trapped include:

  1. bait
  2. water: still pond vs. moving stream, for example
  3. time of year (although found even in water collected in mid-winter)
  4. soil: drainage, depth, soil type, pH