Eukaryotic Microorganisms: Fungi, Algae, Protozoans, Slime Molds, and Helminths

Fungi

Characteristics of Fungi

Fungi are a diverse group of eukaryotic organisms that play essential roles in ecosystems and human society. They are distinct from plants and animals, primarily due to their heterotrophic nutrition, cell wall composition, and reproductive strategies.

• Mycology is the study of fungi.

• Most fungi are non-pathogenic, but some can cause disease, especially in immunocompromised individuals.

• Fungi are primary decomposers of dead plant material, especially cellulose.

• They form mycorrhizae with plant roots, aiding in nutrient and water absorption.

• Fungi are used in food production (e.g., mushrooms, truffles, bread, cheese, alcoholic beverages) and pharmaceuticals (e.g., penicillin, cephalosporin).

• Fungal cell walls are composed of chitin.

• They can grow in acidic environments and are more resistant to osmotic pressure than bacteria.

• Fungi do not contain chlorophyll and do not perform photosynthesis.

• Fungi can synthesize toxins such as ergot alkaloids, aflatoxins, and psychotropic agents.

Fungal Morphology and Types

• Fungi may be unicellular (yeasts) or multicellular (molds, mushrooms).

• Hyphae are long filaments of cells; a mass of hyphae is called a mycelium.

• Hyphae may be septate (with cross walls) or coenocytic (without cross walls).

• Three main groups: mushrooms, yeasts, and molds.

Yeasts

Yeasts are unicellular fungi that reproduce primarily by budding. Some can form multicellular structures called pseudohyphae.

• Common infection: Candida albicans (candidiasis).

• Yeasts are classified by capsule presence, cell shape, spore formation, and genetic data.

• They can grow with or without oxygen and do not require light.

Molds

• Molds grow as hyphae, which can be septate or coenocytic.

• They reproduce rapidly and form characteristic colonies.

• Hyphae can grow to large sizes and form visible mycelia.

Dimorphic Fungi

Some fungi can exist as both yeast and mold forms, depending on environmental conditions such as temperature. These are called dimorphic fungi.

• As molds at 25°C (environmental) and as yeasts at 37°C (human tissues).

• Some dimorphic fungi cause systemic mycoses.

Fungal Reproduction

Fungi reproduce both asexually and sexually, contributing to their adaptability and genetic diversity.

• Asexual reproduction occurs via budding (yeasts), fragmentation, or spore formation (sporangia, conidia).

• Sexual reproduction involves fusion of haploid cells from different mating types, formation of a dikaryon (n+n), nuclear fusion to form diploid (2n), and meiosis to restore haploid state.

Types of Asexual Spores

• Sporangiospores: Formed inside a sac (sporangium).

• Conidia (conidiospores): Produced at the tips or sides of hyphae, not enclosed in a sac.

• Other types: Arthrospores, chlamydospores, blastospore, phialospore, microconidia, macroconidia, porospore.

Classification of Fungi

• Zygomycota: Multinucleate molds, mostly saprobes or obligate parasites.

• Ascomycota: Includes food spoilage fungi and beneficial species like Penicillium (penicillin) and Saccharomyces (fermentation).

• Basidiomycota: Mushrooms and fruiting bodies; includes edible and toxic species.

• Deuteromycota: "Imperfect fungi" with unknown sexual reproduction.

Algae

Characteristics of Algae

Algae are a diverse group of photosynthetic, aquatic eukaryotes. They are not a unified taxonomic group but are classified based on pigment composition, storage products, and cell wall structure.

• Major component of plankton, forming the base of aquatic food chains.

• Found in freshwater, marine environments, soil, rocks, and even extreme habitats.

• Some produce toxins that can cause food poisoning (e.g., dinoflagellates).

Medical and Environmental Importance

• Algal toxins can accumulate in shellfish, causing shellfish poisoning in humans.

• Overgrowth of dinoflagellates can cause "red tide," leading to fish kills and human health risks.

• Pfiesteria piscicida is a toxic algal form implicated in fish kills and human illness.

Life Cycle of Algae

• All algae can reproduce asexually; unicellular forms divide by mitosis, while multicellular forms may fragment.

• Sexual reproduction involves gamete fusion and zygote formation, followed by meiosis to restore the haploid state.

Classification of Algae

• Chlorophyta (Green algae): Contain chlorophyll a and b; mostly unicellular or filamentous; common in freshwater.

• Rhodophyta (Red algae): Contain red pigments; cell walls with agar; used in food and microbiology.

• Phaeophyta (Brown algae): Contain brown pigments; important in marine environments.

• Chrysophyta (Golden algae, diatoms): Have silica cell walls; major source of global oxygen production.

Protozoans

Characteristics of Protozoans

Protozoans are unicellular, eukaryotic organisms lacking cell walls. They are highly diverse in form and function.

• Most are motile via cilia, flagella, or pseudopodia.

• Require moist environments; found in water, soil, and decaying matter.

• Most are chemoheterotrophs, feeding on bacteria, organic matter, or host tissues.

• Some are photoautotrophic (e.g., dinoflagellates, euglenoids).

• Exist in active (trophozoite) and dormant (cyst) stages.

Life Cycle of Protozoans

• Life cycles range from simple to complex, often involving both trophozoite and cyst stages.

• Main reproduction is asexual (binary fission, schizogony).

• Some reproduce sexually by conjugation (e.g., Paramecium), involving exchange of micronuclei.

Classification of Protozoans

• Classification is based on molecular genetics and locomotion mechanisms.

• Archaezoa: Lack mitochondria; include Trichomonas vaginalis and Giardia lamblia.

• Microspora: Obligate intracellular parasites; lack mitochondria.

• Ciliophora: Move by cilia; include Balantidium coli.

• Euglenozoa: Flagellated; include Trypanosoma brucei (sleeping sickness).

• Amoebozoa: Move by pseudopodia; include Entamoeba histolytica (amoebic dysentery).

• Apicomplexa: Complex life cycles; include Plasmodium (malaria), Toxoplasma gondii, Cryptosporidium.

Slime Molds

Cellular and Plasmodial Slime Molds

Slime molds share characteristics with both fungi and amoebae. They are more closely related to amoebae.

• Cellular slime molds: Live as single amoeboid cells; aggregate under stress to form a multicellular structure (slug).

• Plasmodial slime molds: Form a multinucleate mass (plasmodium); distribute nutrients by cytoplasmic streaming; form sporangia under unfavorable conditions.

Helminths

Characteristics of Helminths

Helminths are multicellular eukaryotic worms, important in microbiology due to their microscopic eggs and larvae.

• Two main groups: Platyhelminths (flatworms) and Nematodes (roundworms).

• Adults are macroscopic, but eggs and larvae are microscopic.

• Life cycles often involve multiple hosts and complex development stages.

Classification of Helminths

• Platyhelminths: Includes trematodes (flukes) and cestodes (tapeworms).

• Nematodes: Includes pinworms (Enterobius vermicularis), roundworms (Ascaris lumbricoides), and hookworms (Necator americanus, Ancylostoma duodenale).