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Contrasting True Fungi (Eumycota) and Oomycetes

Several terrestrial plant diseases and aquatic animal (fish, frog) diseases attributed to fungal attack are actually caused by Oomycetes.
In plants they are often better known by their symptoms and referred to as root rotting fungi, seedling dampening mold, blister rusts, white rusts, downy mildew or late blights.

There are many identification or naming situations - where Oomycetes get lumped into "fungi" for gross morphological reasons, much the same way dolphins and whales were lumped together with fish. Yet the taxonomic difference, or divergence from the last common ancestor, is several magnitudes more extreme than that between mammals and fish. The divergence is the order of billions rather than millions of years.

Oomycetes are like fungi. They have the same filamentous, branching, indeterminate bodies and absorb food by excreting digestive enzymes and absorbing the resultant soup. This can be referred to as the fungal way of life or absorptive nutrition. But ecologically and by using ultra structural characters, the differences can be seen to be far more fundamental:

  • Cell walls
    All fungi have cell walls composed of chitin, similar to insects, whereas Oomycete cell walls are made of something more akin to that of plants, cellulose, or more accurately "cellulosic compounds mixed with glycan"1.
  • Compartmentalization
    The filaments of Oomycetes are coenocytic meaning they lack internal cell walls or septae, the nuclei divide like most familiar eukaryotic cells except in Oomycetes there is no pinching off or even a constriction in the membrane, the cytoplasm is a continuous living fluid. The nuclear control centers free-wheel around spitting out orders, apparently without a delineated juristriction . This situation is not true of the majority of fungi where septae act like toll gates allowing passage of subnuclear objects. The exceptions being except fungi such as bread molds -Zygomycota and also in Chytrids.
  • Ploidy
    Like most life we know on first name basis, Oomycetes are diploid, each somatic cell inheriting a genetic copy from two parents. In contrast, True fungi hyphae are haploid from spore2 and become dikaryotic - not diploid - on mating. Diploidy in true fungi only occurs for a single cell division during the creation of reproductive propagules. At this time, inside the hyphal tips, the fraternal nuclei composing the heterokaryon, fuse. This brief liasion of oneness, analogous to the formation of a zygote, is the only moment of diploidy, and terminated by the meitotic creation of 4 haploid daughter cells ready to become spores.
  • Sex
    Oomycetes have haploid reproductive cells -gametes or spores similar to the sperm and eggs we have ourselves. Like our own, male gametes are motile but have TWIN propulsion units3. When meeting a Significant Other, true fungi don't mess around with the wet messy stuff, its strictly a meet and merge affair4, two selves becoming a continuing branching double nucleated self. Fungal spores are more like embryos inactive until chancing on a substrate fit to absorb nutrients from.
  • Habitat
    Oomycetes, because of their motile sperm, demand wet sex in a free aqueous phase. Differing from true fungi, they require liquid water to reproduce. This is why the infamous Irish potato blight of the 1840's was partly due to bad weather and wet water-logged soils. Many Oomycetes are aquatic, Anyone referring loosely to an underwater fungus, be it one wasting away a tropical fish or turning a decaying piece of debris into a white pincushion, is most probably referring to an Oomycete rather than a member of the Fungal Kingdom.3
  • Biochemistry
    Members of Kingdom Fungi have an almost unique method for synthesizing lysine, they use AAA as the upstream metabolite, while Oomycetes use DAP in an almost universal route found in all plants and algae. Significantly there is no evidence of any relationship between the DAP and AAA enzymes, that is, the pathways evolve completely separately from each other.
  • Polyhydric alcohols (polyols)
    Oomycetes lack polyols entirely while Kingdom Fungi members, polyols such as Glycerol are often stored in high concentrations in cells to counter the effects of water loss5. Because of this, true fungi can survive very dry conditions or places with extremely low osmotic potential while Oomycetes are still tied closely to an abundance of liquid water for reproduction and growth.
  • Mitochondrial ultrastructure
    The cristae, the inner membrane and alchemical anvil inside Oomyete and Kingdom fungi mitochondria have a different morphology. In fungi the, cristae are flat or laminar, while within members of the Oomycota cristae are tubular. What significance the topology has has on function is unclear but it is probably a marker of ancient divergence.

Looking at the boundaries of the Oomycetes taxonomically

After a recent shake up in taxonomy, current thinking (circa 2002) is that Oomycetes are distinctly different from other fungi and also that the paraphyleic kingdom chromista (that they were once subjects of) was inadequate to describe them - or group them sufficiently to bound like with like. They now reside in the Kingdom Stramenopila along with Hyphochytriomycetes and Labyrinthulomycetes, two other fungoid members in addition to some oddball algae. The fledgling Kingdom is bound together by molecular data and ultra structure and once found in high school text books will signify one more growth ring between those who did biology and those still actively involved.

Taxonomically, Oomycetes can be lumped into to two large groups, the Saproleginalean and Peronosporalean galaxies.

The Water Molds

  • Saprolegniales
    The most primitive order commonly known as water molds. Most work on turning dead into dirt a saprobic lifestyle. In the aquaria they create the floating stars of mycelium, fluffy pom-poms of decay. Some such as S.parasiticea are parasites on fish and amphibians causing lesion in overcrowded aquaria. My own experience shows axolotls due to their external gills are particularly susceptible to infections in captivity. Aquatic Invertebrate are also prey to this order, including rotifers, nematodes, arthropods and diatoms.
  • Important genera include:Achlya, Aphanomyces, Aplanes, Aplanopsis, Brevilegnia, Calyptralegnia, Dictyuchus, Isoachlya, Leptolegnia, Plectospira, Pythiopsis, Saprolegnia, Scoliolegnia and Thraustotheca.

  • Leptomitales
    (a group seperated from the above due to blocked celled wall constrictions and hence having the appearance of septae
    e.g. Leptomitus, Apodachlya
  • Peronsoporales
      Family Albuginaceae
      -(white rusts and blights)
      e.g.Albugo candida (white rust on crucifers)
      Family Peronosporaceae - the downy mildews
      • Bremia
        e.g.Bremia lactucae (lettuce downy mildew)
      • Plasmopara
        e.g.Plasmopara viticola
        Originally a North American Downy Mildew of grapes vines that almost wiped out the French wine industry
      Family Pythiaceae -
      • Pythium -fast growing seed and seedling rots.
      • Phytophthora (60 Species mostly invaders of healthy tissue in natural communities and cultivated plants of all ages, including adult trees.
      • Halophytophtora -marine varieties of the above genus.
    Other minor Orders:
    • Lagenidiales
      e.g.Lagenidium
    • Rhiphidiales
      e.g.Sapromyces
    • Sclerosporales
      e.g. Pachymetra


    1There are exceptions, oomycetes with chitin in their cell walls, but they are an extreme minority.

    2 Some fungal spores are not haploid, they are dikaryotic but these 'spores' can be considered clonal fragments of the mother genet.

    3 The 'male' free-swimming spores produced bear two dissimilar flagella, one with mastigonemes, tinsel, a feature common to Stramenopiles.

    4 There are true fungi with free swimming spores and even cellulose in their walls: the Chytrids. Chytrids are considered fungi while oomycetes are not. Aquatic Oomycetes are difficult to confuse with Chytrids, as Chytrids are not filamentous, and are always microscopic.
  • 5Glycerol appears to protect fungal enzymes from accumulation of sodium and water loss, both of which may denature the enzymes. Polyols may also protect membranes. Osmophilic fungi utilize compatible solutes to maintain water potential in the cell, though their rates of metabolism and thus growth are extremely slow.

    6Phytophtora infestans is the all time number one misidentified Oomycete. Due to its fame in causing the Eighteenth century Irish disporia. Here are nodes you will see it / would have seen it mentioned as a 'fungus'. My opinion is that the distinction is important, especially when addressing a plant pathogen, because treatment requires an accurate diagnosis. A parallel would be that antibiotics aren't able to treat colds because antibiotics are semisynthetic derivatives of fungal metabolites whose function is to inhibit the growth of competing bacterial microfauna, and not to prevent viral replication.

    bordeaux mixture, Irish potato famine,Phytophthora infestans, fungi
    If you update the node let me know, this is just to show how common the misnomer is.

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