Eukaryotic Microorganisms

Fungi

Fungi are a diverse group of eukaryotic organisms that play essential roles in decomposition, symbiosis, and sometimes disease. The study of fungi is known as mycology. While most fungi are not pathogenic, some can cause disease in humans, especially when the immune system is compromised.

• Symbiotic Relationships: Fungi often form symbiotic associations, such as mycorrhizae with plant roots, aiding nutrient absorption.

• Decomposition: Fungi are major decomposers, recycling organic material in ecosystems.

Fungal Characteristics

• Vegetative Colonies: Fungi grow as multicellular filaments (hyphae) or as unicellular organisms.

• Mycelium: A mass of hyphae forms the mycelium, the main body of the fungus.

• Cell Wall: Composed of chitin, a strong, flexible polysaccharide.

• Heterotrophic: Fungi obtain nutrients from organic matter.

• Forms: Yeasts (unicellular), molds (filamentous), and fleshy fungi (e.g., mushrooms).

• Differences from Plants: Fungi lack chlorophyll and do not perform photosynthesis.

• Ribosomal RNA: Fungi have 80S ribosomes.

• Toxins: Some fungi produce toxins such as ergot alkaloids, aflatoxins, and psychotropic agents.

Yeasts

• Unicellular: Yeasts reproduce by budding; some may form multicellular structures.

• Classification: Based on capsule presence, cell size/shape, hyphae formation, sexual spores, and genetic data.

• Oxygen Requirements: Yeasts may be aerobic or anaerobic.

Molds

• Rapid Growth: Molds reproduce asexually and develop hyphae.

• Hyphae Types: Septate hyphae have cross walls; coenocytic hyphae lack septa.

• Mycelium Formation: Hyphae grow to form a mycelium in suitable environments.

Dimorphic Fungi

• Definition: Fungi that can exist as both yeast and mold forms, depending on temperature.

• Growth Conditions: Mold form at 25ºC; yeast form at 37ºC (in tissues).

• Medical Importance: Some dimorphic fungi cause systemic mycoses.

Fungal Reproduction

• Asexual Reproduction: All fungi reproduce asexually; methods include budding (yeasts), fragmentation (filamentous fungi), and spore formation.

• Sporangia: Structures that release spores into the environment.

• Sexual Reproduction: Occurs under unfavorable conditions, introduces genetic variation; mating types designated as “+” and “–”.

Asexual Spores

• Arthrospore: Formed when septate hyphae break at cross wall.

• Chlamydospore: Spherical, thickened hyphal cell.

• Blastospore: Buds from parent cell.

• Phialospore: Buds from vase-shaped cell.

• Microconidium/Macroconidium: Single or multiple cells, formed under different conditions.

• Porospore: Grows through small pores in spore-bearing cell.

Classification of Fungi

Algae

Algae are photosynthetic, aquatic eukaryotes found in diverse environments. They are a major component of plankton and play a crucial role in aquatic food webs.

• Distribution: Found in fresh/marine waters, soil, rocks, plants, hot springs, and snow banks.

• Medical Concerns: Some marine algae produce toxins (e.g., dinoflagellates) causing food poisoning and "red tide" events.

• Toxic Algal Forms: Pfiesteria piscicida implicated in fish kills and human disease; agricultural runoff can promote blooms.

Characteristics of Algae

• Photosynthetic: All algae perform photosynthesis.

• Aquatic: All algae are aquatic.

• Diversity: Algae differ in morphology, reproduction, and biochemistry; classification is complex and evolving.

Life Cycle of Algae

• Asexual Reproduction: Unicellular algae divide by mitosis and cytokinesis; multicellular algae reproduce by fragmentation.

• Sexual Reproduction: Unicellular algae use gametes; fusion forms zygote, which undergoes meiosis. In multicellular algae, all cells in reproductive structures become gametes; sexual and asexual cycles may alternate.

Classification of Algae

Protozoans

Protozoans are unicellular, eukaryotic organisms lacking a cell wall. They are highly diverse, with many free-living and some parasitic species.

• Motility: Most protozoans are motile via cilia, flagella, or pseudopodia.

• Habitat: Found in aquatic environments, moist soil, and decaying matter.

• Nutritional Modes: Most are chemoheterotrophs; some are photoautotrophic (e.g., dinoflagellates, euglenids).

• Life Stages: Active feeding stage (trophozoite); dormant stage (cyst) for survival and transmission.

Life Cycle of Protozoans

• Asexual Reproduction: Main method; includes schizogony (multiple fission).

• Sexual Reproduction: Some protozoans (e.g., Paramecium) reproduce by conjugation, involving macronucleus (growth) and micronucleus (conjugation).

Classification of Protozoans

Slime Molds

Slime molds are eukaryotic organisms with characteristics of both fungi and amoebae. They are more closely related to amoebae.

• Cellular Slime Molds: Live as single amoeboid cells; aggregate under unfavorable conditions.

• Plasmodial Slime Molds: Form masses of protoplasm with many nuclei (plasmodium).

Helminths

Helminths are multicellular parasitic worms, not microbes, but are studied in microbiology due to their microscopic eggs and larvae. They often infect humans and produce diagnostic stages detectable in clinical samples.

• Life Cycle: Includes fertilized egg, larvae, and adult; may involve intermediate and definite hosts.

• Host Specificity: Larval development in intermediate host; adulthood and mating in definite host.

Classification of Helminths

• Eggs and Larvae: Microscopic; often found in blood, feces, urine.

• Adult Helminths: Large enough to be seen with the naked eye.

Example: Plasmodium (Apicomplexa) causes malaria, transmitted by mosquitoes; Enterobius vermicularis (pinworm) spreads via fomites.

Additional info: Classification of protozoans and algae is evolving due to advances in molecular genetics; some details inferred for completeness.