General Characteristics of Eukaryotic Organisms

Overview of Eukaryotic Microorganisms

Eukaryotic microorganisms include protozoa, fungi, algae, slime molds, and water molds. These organisms are diverse, encompassing both human pathogens and species vital for human life and ecological processes.

• Protozoa: Unicellular, lack cell walls, motile by cilia, flagella, or pseudopods.

• Fungi: Chemoheterotrophic, cell walls of chitin, related to animals.

• Algae: Photoautotrophic, possess chlorophyll, important for oxygen production.

• Slime molds & Water molds: Share features with fungi and protozoa, important decomposers.

Reproduction in Eukaryotes

Eukaryotic reproduction is more complex than in prokaryotes, involving both asexual and sexual methods. Most eukaryotic DNA is packaged as chromosomes within the nucleus.

• Asexual reproduction: Common in many eukaryotes; methods include mitosis, budding, and fragmentation.

• Sexual reproduction: Involves formation of gametes and zygotes; meiosis produces genetic diversity.

• Ploidy: Haploid (one genome copy) or diploid (two genome copies).

Nuclear Division: Mitosis and Meiosis

Nuclear division in eukaryotes occurs via mitosis or meiosis. Mitosis maintains ploidy, while meiosis reduces ploidy and increases genetic diversity.

• Mitosis: Four phases—prophase, metaphase, anaphase, telophase. Results in two genetically identical nuclei.

• Meiosis: Two stages (meiosis I and II), each with four phases. Results in four haploid nuclei from a diploid nucleus.

Cytokinesis (Cytoplasmic Division)

Cytokinesis typically occurs with telophase of mitosis. In some algae and fungi, it may be postponed or absent, resulting in multinucleated cells (coenocytes). Mechanisms vary by organism:

• Cell plate formation: In plants and algae, new cell wall forms between daughter nuclei.

• Cleavage furrow: In protozoa, fungi, and animals, microfilaments contract to divide the cell.

• Budding: In yeast, a bud receives one daughter nucleus and separates.

• Schizogony: In Plasmodium species, multiple mitoses without cytokinesis form a schizont, which then divides into many merozoites.

Classification of Eukaryotic Organisms

Early taxonomy grouped organisms by structural similarities. Modern classification relies on nucleotide sequence similarities, reflecting evolutionary relationships.

Protozoa

General Characteristics

Protozoa are a diverse group of unicellular, eukaryotic organisms lacking cell walls. Most are motile via cilia, flagella, or pseudopods, except apicomplexans.

• Require moist environments; found in aquatic habitats and soil.

• Critical members of plankton; few are pathogenic.

Morphology

Protozoa exhibit great morphological diversity. Some have two nuclei (macronucleus and micronucleus), variable mitochondria, contractile vacuoles, and distinct life cycle stages (trophozoite and cyst).

Nutrition

Most protozoa are chemoheterotrophic, obtaining nutrients by phagocytosis. Some are photoautotrophic (dinoflagellates, euglenoids).

Reproduction

Protozoa primarily reproduce asexually (binary fission, schizogony). Some reproduce sexually via gametocytes or conjugation.

Classification

Protozoa are classified into six groups based on nucleotide sequences:

• Parabasala: Lack mitochondria, single nucleus, parabasal body. Example: Trichomonas vaginalis.

• Diplomonadida: Lack mitochondria, have mitosomes, two nuclei, multiple flagella. Example: Giardia intestinalis.

• Euglenozoa: Plant and animal characteristics, flagella with crystalline rod, disk-shaped mitochondrial cristae. Includes euglenids and kinetoplastids (Trypanosoma, Leishmania).

• Alveolates: Membrane-bound alveoli. Subgroups: ciliates, apicomplexans, dinoflagellates.

• Rhizaria: Amoebae with threadlike pseudopods. Includes foraminifera and radiolaria.

• Amoebozoa: Amoebae with lobe-shaped pseudopods, no shells. Includes slime molds.

Fungi

General Characteristics

Fungi are chemoheterotrophic eukaryotes with cell walls composed of chitin. They do not perform photosynthesis and are closely related to animals.

Significance

• Decompose dead organisms and recycle nutrients.

• Assist plants in water and mineral absorption.

• Used in food production, antibiotics, and research.

• 30% cause diseases in plants, animals, and humans.

Morphology

Fungi have two basic body shapes: molds (hyphae) and yeasts (single cells). Some are dimorphic, changing shape based on environment.

Nutrition

Fungi absorb nutrients, mostly as saprobes. Some trap and kill nematodes, while others use haustoria to extract nutrients from living hosts.

Reproduction

Fungi reproduce asexually (mitosis, budding, spore formation) and sexually (mating types, spore formation).

Classification

• Division Zygomycota: Saprobes, obligate parasites, reproduce via sporangiospores. Includes microsporidia.

• Division Ascomycota: Form ascospores in asci, reproduce by conidiospores. Includes Penicillium, Saccharomyces.

• Division Basidiomycota: Mushrooms, basidiocarps, decomposers, some produce toxins.

• Deuteromycetes: Fungi with unknown sexual stages, mostly ascomycetes.

Lichens

Lichens are symbiotic partnerships between fungi and photosynthetic microbes. They are abundant, grow in diverse habitats, and occur in foliose, crustose, and fruticose shapes.

Algae

General Characteristics

Algae are simple, eukaryotic photoautotrophs with sexual reproductive structures. They differ widely in distribution, morphology, and biochemical traits.

Distribution and Morphology

Most algae are aquatic, living in the photic zone. Morphologies include unicellular, colonial, and multicellular forms. Marine algae may have holdfasts for anchoring.

Reproduction

Unicellular algae reproduce asexually by mitosis and cytokinesis; sexually by gamete fusion and meiosis. Multicellular algae reproduce asexually by fragmentation and sexually with alternation of generations.

Classification

• Division Chlorophyta (green algae): Share characteristics with plants, chlorophylls a and b, cell walls of cellulose.

• Kingdom Rhodophyta (red algae): Red pigment phycoerythrin, cell walls of agar or carrageenan, most are marine.

• Phaeophyta (brown algae): Chlorophylls a and c, carotene, xanthophylls, cell walls of cellulose and alginic acid.

• Chrysophyta (golden algae, diatoms): Use chrysolaminarin, major source of oxygen, component of phytoplankton.

Water Molds

General Characteristics

Water molds differ from fungi in several ways: tubular mitochondrial cristae, cell walls of cellulose, spores with two flagella, and true diploid bodies. They decompose dead animals and recycle nutrients; some are crop pathogens.

Other Eukaryotes of Microbiological Interest: Parasitic Helminths and Vectors

Parasitic Helminths and Arthropod Vectors

Parasitic worms have microscopic infective and diagnostic stages. Arthropod vectors carry pathogens and are classified as mechanical or biological vectors. Disease vectors belong to Arachnida (ticks, mites) and Insecta (fleas, lice, flies, mosquitoes, kissing bugs).

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