Diversity of Microbial Eukarya: Structure, Phylogeny, and Major Groups

I. Organelles and Phylogeny of Microbial Eukarya

Overview of Microbial Eukarya

Microbial members of the domain Eukarya are genetically and ecologically diverse, exhibiting complex morphology and ecological roles but limited metabolic diversity. Most are either chemoorganotrophic or phototrophic and obligate aerobes. Microbial eukaryotes are classified as protists, algae, or fungi, unified by their complex cell structure and true organelles.

• Chemoorganotrophs: Organisms that obtain energy by oxidizing organic molecules.

• Phototrophs: Organisms that use light as an energy source.

• Obligate aerobes: Require oxygen for growth.

18.1 Endosymbioses and the Eukaryotic Cell

Endosymbiosis is a key evolutionary process in the origin of eukaryotic organelles. Two major types are recognized: primary and secondary endosymbiosis.

• Primary Endosymbiosis: A bacterial symbiont was acquired directly by the ancestor of Eukarya.

  • Mitochondria evolved from a bacterial cell capable of respiration, leading to mutual dependency and eventual integration as a single cell. Mitochondria contain bacterial DNA.

  • Chloroplasts were acquired when a eukaryotic cell (already containing mitochondria) engulfed a phototrophic cyanobacterium. All phototrophic eukaryotes (plants and algae) require chloroplasts for photosynthesis.

• Secondary Endosymbiosis: Involves engulfing a green or red algal cell, retaining its chloroplast, and becoming phototrophic. Examples include:

  • Euglenids and chlorarachniophytes (from green algae)

  • Alveolates and stramenopiles (from red algae)

• Endosymbioses are common and ongoing in evolution.

Figure: Organellar DNA

Shows the presence of DNA in mitochondria and nucleus, supporting the endosymbiotic origin of mitochondria.

Figure: Endosymbioses

Illustrates the evolutionary steps of primary and secondary endosymbiosis, leading to the diversity of phototrophic eukaryotes.

18.2 Phylogenetic Lineages of Eukarya

The acquisition of mitochondria was foundational for eukaryotic evolution. All extant Eukarya contain mitochondria, homologous structures, or genetic traces thereof. Secondary endosymbioses contributed to the diversity of phototrophic eukaryotes.

• Ribosomal RNA gene sequences recognize five supergroups of Eukarya:

  • Archaeplastida

  • SAR clade

  • Excavates

  • Amoebozoa

  • Opisthokonta

Figure: Phylogenetic Tree of Eukarya

Depicts the evolutionary relationships among major eukaryotic lineages.

II. Protists

Definition and Major Groups

The term protist refers to any microbial eukaryote that is not a plant, animal, or fungus. Major groups include Excavates, Alveolata, Stramenopiles, Rhizaria, Haptophytes, and Amoebozoa.

18.3 Excavates

Key Genera: Giardia, Trichomonas, Trypanosoma, Euglena

• Diplomonads:

  • Two nuclei of equal size

  • Very reduced mitochondria (mitosomes)

  • Giardia intestinalis causes giardiasis, a common waterborne intestinal disease

• Parabasalids:

  • Contain a parabasal body

  • Lack mitochondria but have hydrogenosomes for anaerobic metabolism

  • Most genomes lack introns

• Kinetoplastids:

  • Characterized by a kinetoplast (mass of DNA in a single large mitochondrion)

  • Live in aquatic habitats, feed on bacteria

  • Some are animal parasites:

    • Trypanosoma brucei: causes African sleeping sickness

    • Trypanosoma cruzi: causes Chagas disease

• Euglenids:

  • Nonpathogenic

  • Alternate between chemotrophic and phototrophic lifestyles

  • Contain chloroplasts in light; lose them in dark and become chemoorganotrophs

  • Can feed on bacteria by phagocytosis

18.4 Alveolata

Key Genera: Gonyaulax, Plasmodium, Paramecium

• Characterized by alveoli: sacs under the cytoplasmic membrane, possibly for osmotic balance

• Major groups:

  • Ciliates:

    • Possess cilia for motility and feeding

    • Most widely distributed genus: Paramecium

    • Have two nuclei: macronucleus and micronucleus

    • Conjugation involves exchange of micronuclei

    • Some are animal parasites (e.g., Balantidium coli causes dysentery-like disease)

  • Dinoflagellates:

    • Diverse marine and freshwater phototrophs

    • Two flagella with different insertion points

    • Some are free-living, others symbiotic with corals

    • Dense suspensions cause red tides; some produce neurotoxins (PSP)

    • Pfiesteria piscicida: toxic genus responsible for fish kills

  • Apicomplexans:

    • Obligate animal parasites (e.g., malaria, toxoplasmosis)

    • Produce sporozoites for transmission

    • Contain apicoplasts (degenerate chloroplasts lacking pigments)

18.5 Stramenopiles

Key Genera: Phytophthora, Nitzschia, Ochromonas, Macrocystis

• Includes diatoms, oomycetes, golden algae, and brown algae

• All have many short, hairlike extensions

• Members can be chemoorganotrophic or phototrophic

• Diatoms:

  • Over 100,000 species; found in freshwater and marine habitats

  • Cell walls made of silica (frustules)

  • Exhibit radial and pinnate symmetry

  • Appeared ~200 million years ago

• Oomycetes:

  • Also called water molds; filamentous growth, coenocytic hyphae

  • Cell walls made of cellulose

  • Phytophthora infestans: causes late blight in potatoes

• Golden algae:

  • Chrysophytes; unicellular, motile via two flagella

  • Named for golden-brown color (fucoxanthin pigment)

• Brown algae:

  • Marine, multicellular

  • Color varies with fucoxanthin content

18.6 Rhizaria

• Distinguished by threadlike pseudopodia for movement and feeding

• Includes Chlorarachniophyta, Foraminifera, and Radiolaria

• Chlorarachniophyta:

  • Phototrophic, amoeba-like, use flagella for movement

  • Chloroplasts from secondary endosymbiosis (four membranes)

  • Contain a nucleomorph (remnant of engulfed algae)

• Foraminifera:

  • Exclusively marine

  • Form shell-like structures called tests (organic material + calcium carbonate)

• Radiolarians:

  • Mostly marine, heterotrophic

  • Tests made of silica

  • Radial symmetry

Table: Comparison of Major Protist Groups

Group	Key Features	Representative Genera	Habitat	Pathogenicity
Excavates	Reduced mitochondria, flagella, kinetoplasts	Giardia, Trypanosoma, Euglena	Freshwater, soil, host-associated	Some pathogenic (giardiasis, sleeping sickness)
Alveolata	Alveoli, cilia, flagella, apicoplasts	Paramecium, Plasmodium, Gonyaulax	Freshwater, marine, host-associated	Some pathogenic (malaria, dysentery)
Stramenopiles	Hairlike extensions, silica/cellulose walls	Nitzschia, Phytophthora, Macrocystis	Freshwater, marine, soil	Some pathogenic (potato blight)
Rhizaria	Pseudopodia, tests (shells)	Foraminifera, Radiolaria	Marine	Generally non-pathogenic

Additional info:

• Protists play essential roles in aquatic food webs, nutrient cycling, and as pathogens.

• Endosymbiotic theory is supported by the presence of bacterial-like DNA in mitochondria and chloroplasts.

• Phylogenetic classification is based on molecular data, especially ribosomal RNA sequences.