Download 5b Eukaryotic Microbial S and F Fungi

Eukaryotes: The Other Guys
Part II: Fungi
There’s a Fungus Among Us!
• The study of fungi is called mycology
• Fungi have traditionally been included in the plant kingdom
– However, there are significant differences between fungi and plants
• Fungi are heterotrophs
• Fungi have filamentous bodies
• Fungi have nonmotile sperm
• Fungi have cell walls made up of chitin
• Fungi have nuclear mitosis
•
•
•
The number of serious fungal infections is increasing.
Fungi are aerobic or facultatively anaerobic chemoheterotrophs.
Most of them are decomposers and a few have parasitic relationships with plants and
animals.
73,000 species of fungi have been named so far.
There are divided into four phyla.
– Zygomycota
– Ascomycota
– Basidiomycota
– Chitrydiomycota
• These are distinguished primarily by their mode of reproduction.
•
•
•
Fungal Characteristics
Fungi can grow in acidic, low moisture environments.
They are able to metabolize complex carbohydrates.
A fungal (mold or fleshy fungus) body (thallus) consists of filaments of cells called
hyphae.
These can grow to large proportions.
•
•
•
•
A mass of these hyphae form a mycelium.
Most molds have hyphae that contain septa.
These are called septate hyphae.
Those that have no septa are called coenocytic hyphae.
•
A scanning electron micrograph of the hyphae cells.
•
•
•
Yeasts
• Yeasts are nonfilamentous, unicellular fungi.
• They are generally spherical or oval in shape.
• They are capable of facultative anaerobic growth.
• If provided oxygen, the yeast will perform aerobic respiration and metabolize
carbohydrates to carbon dioxide and water.
• If denied oxygen, they ferment these carbohydrates and produce ethanol and carbon
dioxide.
• In their reproduction, fission yeasts divide symmetrically, whereas, budding yeasts do
not.
• Fission yeasts divide evenly to form to daughter cells.
• Budding yeasts will form protuberances on its outer surfaces.
• As the bud elongates, the mother cell’s nucleus divides and one migrates to the
growing bud.
•
•
•
Eventually, the bud breaks away.
Mother yeast cells can make up to 24 daughter cells by the budding process.
Buds that do not separate from the mother cell form structures called pseudohyphae.
•
A scanning electron micrograph of yeast cells, and the budding reproductive process .
•
•
•
•
Dimorphic Fungi
These are fungi that exhibit two forms of growth, either as a mold or as a yeast.
In pathogenic fungi, dimorphism is temperature-dependent.
They are yeast-like at 37°C and mold-like at 25°C.
However, some dimorphism will occur with changes in carbon dioxide concentration.
•
•
•
Life as a Fungus
Fungi are classified according to the type of sexual spore that they form.
Sexual spores are usually produced in response to changes in the environment.
These result from sexual reproduction and consist of three phases.
– Plasmogamy- Haploid nucleus of a donor cell penetrates the cytoplasm of a
recipient cell.
– Karyogamy- The two nuclei fuse to form a diploid zygote nucleus.
– Meiosis- Diploid nucleus gives rise to haploid nuclei.
• Spores can also be produced asexually, however.
• These spores include sporangiospores, conidiospores, and chlamydospores.
•
•
•
•
Fungal Nutritional Adaptations
Fungi usually grow better in acidic (pH=5) environments.
Most of them are resistant to osmotic pressure, thereby making it possible for them to
grow in high salt or high sugar concentrations.
They can grow in environments that have a low water activity (low moisture content).
Fungi can metabolize complex carbohydrates such as lignin, which most bacteria
cannot.
Zygomycetes
Zygomycetes are unique fungi.
– Their hyphae lack septa
– Fusion of their hyphae produces a zygote (one 2n nucleus)
• The other fungi produce a heterokaryon (two n nuclei)
• Zygomycetes make up < 1% of named fungi.
– They include…
• Bread molds
• Some pathogens
– Candida
• Zygomycetes typically undergo asexual reproduction.
•
•
Sexual spore formation involves the fusion of two parental nuclei followed by meiosis.
Ascomycetes
• Phylum Ascomycota, the ascomycetes, is the largest of the four phyla.
– Yeasts, morels and truffles
– Many plant fungal pathogens
• Dutch elm disease and chestnut blight
• Reproduction is usually asexual.
– Spores, or conidia, are cut off by septa at ends of hyphae.
• Ascomycetes are named after a characteristic sexual reproductive structure, the ascus
(plural asci).
– The ascus differentiates within the ascocarp.
Basidiomycota (club fungi)
• Phylum Basidiomycota contains the most familiar of the fungi.
– Many plant fungal pathogens
• Rusts and smuts
• Asexual reproduction is rare
• Basidiomycetes are named after a club-shaped sexual reproductive structure, the
basidium.
– There, haploid nuclei fuse, forming a diploid zygote
– Zygote undergoes meiosis releasing haploid basidiospores
•
Mushrooms undergo sexual spore formation.
Subkingdoms of Fungi
•
•
Amastigomycota
– Perfect
– Imperfect
Mastigomycota
•
Black bread mold is classified as a Amastigomycota (Perfect).
•
Penicillium is another example Amastigomycota (Perfect).
•
Mycelium is classified as an Amastigomycota (Imperfect).
•
Fungi are capable of causing superficial and systemic infections.
Fungal Diseases
• A mycosis (pl. mycoses) is a fungal disease.
• Systemic mycoses are fungal infections deep within the body that affect multiple
tissues and organs.
• Subcutaneous mycoses are those that are beneath the skin.
• Cutaneous mycoses affect tissues that contain keratin, such as hair, nails, and skin.
•
•
•
•
•
Superficial mycoses are localized on hair shafts and superficial skin cells.
Opportunistic mycoses are caused by “normal” fungi.
They cause disease in people who are immunocompromised.
These mycoses can affect any tissues and are usually systemic.
Some examples include candidiasis and aspergillosis.
Ecological Roles of Fungi
• Fungi, together with bacteria, are the principal decomposers in the biosphere.
– Fungi are virtually the only organisms that can break down lignin.
• Fungi cause animal diseases.
• Fungi are the most harmful pests of living plants.
• Many commercial products are dependent on the biochemical activities of fungi
– Bread, beer, cheese, soy sauce, and penicillin.
Fungal Associations
• Mutualism is a form of symbiosis in which each partner benefits
• Two kinds of mutualistic associations between fungi and autotrophic organisms are
ecologically important
– 1. Mycorrhizae
– 2. Lichens
Mycorrhizae
• Symbiotic association between a fungus and the roots of plants
– Fungus helps plant absorb minerals and nutrients
– Plant provides fungus with food (organic molecules).
•
•
•
•
Lichens
Symbiotic association between a fungus and a green algae or cyanobacterium.
Lichens are found in the harshest of habitats.
Indeed, they are often the first colonists in such harsh areas.
Lichens are pollution indicators.