AM

Protist Diversity and Life Cycles — SAR Supergroup (Plant-like, Fungus-like, Animal-like Protists)

SAR supergroup overview

  • Organisms covered are diverse and can be confusing; many are grouped into SAR supergroup and further categorized as plant-like (algae), animal-like (protozoa), and fungus-like (slime molds).
  • Key idea: while they look very different, many share similar metabolic strategies and life cycle patterns with other groups (e.g., bacteria as a reference point for metabolism and reproduction).
  • Emphasis of lecture: recognize major organism types, their characteristic structures, and the phyla they belong to; understand how life cycles and metabolism compare to other groups studied (bacteria, plants, animals).
  • Note on common misnomers: cyanobacteria are bacteria (blue-green algae) and not true algae; chlorophyta are true green algae (distinct from cyanobacteria).

Plant-like protists (algae)

  • Broad idea: photosynthetic, mostly aquatic, and eukaryotic; sometimes multicellular (brown/red algae) and sometimes unicellular (green algae, diatoms).
  • Life cycles and diversity vary widely by group; many have both sexual and asexual reproduction.

Chlorophyta (green algae)

  • True green algae; most are unicellular; some multicellular examples include sea lettuce (Ulva).
  • Habitat: many require wet environment to avoid desiccation; related to land plants (close relatives).
  • Life cycles: both sexual and asexual life cycles present; mitosis used to produce gametes; fertilization leads to a diploid phase, followed by meiosis to return to haploid spores.
  • Haploid-dominant life cycle discussion:
    • Most of life is haploid: n
    • Gametes produced by mitosis: n
      ightarrow n (gametes)
    • Fertilization: two gametes fuse to form diploid zygote: n + n
      ightarrow 2n
    • Zygote undergoes meiosis to produce haploid spores: 2n
      ightarrow n
    • Spores germinate to grow into haploid individuals: n
      ightarrow ext{haploid organism}
  • Mating types are often described as plus and minus (two mating types within a species); not the same as male/female in animals, but serve as compatible partners for sexual reproduction.
  • Life cycle timing variations are important for understanding differences with the human haploid/diploid pattern (humans: meiosis to produce haploid gametes, then fertilization to restore diploidy).
  • Designation: chlorophytes are the true green algae and are distinct from the blue-green (cyanobacteria) confusion.

Brown algae (Phaeophyta)

  • Typically multicellular; forms including large seaweeds and giant kelp (kelp forests).
  • Attachment: commonly attach to rocks via a holdfast; buoyant air bladders keep bodies upright, aiding light capture.
  • Size range: can be very large; exemplified by giant kelp beds.
  • Uses: brown algae and their extracts are useful as food thickeners and in other applications; often discussed in ecological and biotechnological contexts.

Red algae (Rhodophyta)

  • Multicellular protists; typically inhabit marine environments and deeper water compared to some green/light-loving algae.
  • Structure: diverse shapes; many have glass-like silica shells? (Note: actual red algae lack silica shells; the silica shells are a feature of diatoms—see below.)
  • Important features: contribute to marine ecosystems and human uses (agar and carrageenan derived from some red algal species).

Diatoms

  • Unicellular algae with silica-based cell walls (frustules) made of silicon dioxide (SiO₂).
  • Frustule composition gives diatoms their glassy, intricate appearance and makes them important for various industrial uses.
  • Geological/industrial relevance: contribute to diatomaceous earth; used in toothpaste abrasives, filtration, reflective paints, and as a silica source in soils and gardening media.

Dinoflagellates

  • Planktonic, often photosynthetic; some species are bioluminescent when disturbed.
  • Structure: two flagella and plates around cells made of cellulose; can have a characteristic shape with a sulcus and grooves.
  • Ecological roles: many are photosynthetic; some form mutualistic relationships with corals (zooxanthellae live inside corals).
  • Zooxanthellae: specific dinoflagellates that live symbiotically inside corals; supply carbohydrates via photosynthesis; corals provide a protected environment and access to light.
  • Notable phenomena:
    • Red tides: blooms of dinoflagellates that can produce neurotoxins (brevet toxins) harmful to marine life and humans by inhibiting nerve function.
    • Bioluminescence: many dinoflagellates produce light when agitated, a possible predator-deterrence mechanism.
  • Some dinoflagellates are parasitic in marine environments, including effects on fish and other organisms.

Zooxanthellae and coral symbiosis

  • Zooxanthellae are dinoflagellates living within corals; they photosynthesize and provide organic carbon to corals.
  • This mutualistic relationship is essential for coral reef health and growth; disruption can contribute to coral bleaching.

Fungi-like protists (slime molds)

  • Not true fungi, but fungus-like protists with similar heterotrophic lifestyles; commonly found in moist environments.
  • Nutritional mode: heterotrophs; decompose dead organic matter and recycle nutrients.
  • Cell walls present; moist habitats; feed on bacteria and other microorganisms.
  • Major interest due to unusual life cycles and remarkable morphologies.

Cellular slime molds (Dictyostelium-type)

  • Two-stage lifestyle: single-celled amoeboid cells (individual organisms) that can aggregate into a multicellular structure when resources are scarce.
  • Aggregation forms a slug-like pseudoplasmodium that migrates and then forms a stalked fruiting body with spores;
    • Spores germinate under favorable conditions to produce new haploid amoeboid cells (via mitosis).
  • Conceptual note: colonies can resemble multicellular organisms, but the individual organisms can still be genetically identical or nearly so.

Plasmodial slime molds (Physarum-type)

  • A single, multinucleate mass called a plasmodium (one cell with many nuclei) that moves and engulfs food by phagocytosis.
  • Plasmodium eventually differentiates to form sporangia on stalks; spores are produced and released.
  • Spores germinate into haploid amoeboid or flagellated cells, which can later fuse to form a diploid phase and restart the cycle.

Animal-like protists (protozoa)

  • Not animals, but single-celled eukaryotes with nucleus and organelles; typically heterotrophic and not photosynthetic.
  • Movement: many use flagella or cilia; others rely on pseudopods.
  • Reproduction: mostly asexual reproduction via mitosis or binary fission in many groups; sexual cycles exist in some groups.

Rhizopods (Amoebae)

  • Move and feed using pseudopods (extensions of the cell membrane).
  • Feeding method: phagocytosis, engulfing prey (bacteria, other protists) to form food vacuoles for digestion.
  • Water balance: possess contractile vacuoles to regulate intracellular water.
  • Reproduction: primarily by mitosis and binary fission; some complex life cycles involve sexual processes.

Foraminifera (forams)

  • Single-celled protists with calcium carbonate shells called tests; tests often elaborate and contribute to fossil records.
  • Usually have reticulopodia or filopodia for feeding and movement; tests can be composed of calcite and other materials.
  • Ecological significance: abundant in marine sediments; important for paleoceanography.

Euglena-like (Euglenozoa) protists

  • Often unicellular with flagellum; contain a photosynthetic plastid and an eyespot (stigma) used to detect light (though not a true eye).
  • Nutritional strategy: mixotrophic—can photosynthesize in light and ingest prey in the dark; not strictly autotrophic.
  • Reproduction: primarily asexual via mitosis; sexual reproduction is not typical in Euglena but can occur in some related groups.

Apicomplexa and malaria life cycle (parasites in animal-like protists)

  • Group including Plasmodium (the malaria parasite) within animal-like protists; complex life cycles.
  • Life cycle highlights:
    • Sporozoites are transmitted by mosquito vectors to vertebrate hosts.
    • Sporozoites invade liver cells and develop into hepatic stages (asexual replication).
    • Release of merozoites infects red blood cells (erythrocytic cycle); cycles of asexual replication and possible sexual stages occur in mosquitoes.
  • Note: sexual and asexual phases can occur in the parasite's life cycle; the stages may involve fusion events and stage differentiation within hosts and vectors.

Reproductive foundations and terminology

  • Zygospore: a spore formed by the fusion of two gametes (fertilization) to produce a diploid zygote that can later undergo meiosis.
  • Syngamy: fusion of two gametes to form a zygote.
  • Isogamy: gametes appear similar in size and form; anisogamy: gametes differ in size; both concepts relate to how fertilization occurs in various algae and protists.
  • Zygospore formation is a strategy to endure harsh conditions (desiccation, temperature, etc.).

Key life cycle patterns to compare across groups

  • Green algae (Chlorophyta): haplontic or haploid-dominant life cycles with haploid gametes produced by mitosis, fertilization to form diploid zygote, meiosis to return to haploid spores.
    • Emphasis on relative timing of fertilization and meiosis; helps explain diversity of life cycles among eukaryotes.
  • Diatoms and other algae with diverse life cycles may switch between forms and reproduction modes depending on environment.
  • Slime molds illustrate a spectrum from unicellular to multicellular-like structures; plasmodial slime molds form a multinucleate plasmodium that differentiates into sporangia to produce spores.
  • Protozoa exhibit a range of reproductive strategies from simple asexual division to complex sexual cycles (as seen in Apicomplexa).

Connections to real-world relevance and applications

  • Brown algae: ecological importance as kelp forests; economic uses in food and industry.
  • Diatoms: silica-based shells with industrial uses; ecological indicators in aquatic systems.
  • Dinoflagellates: red tides and marine toxins affecting fisheries and human health; zooxanthellae are essential for coral reef ecosystems.
  • Slime molds: model systems for studying cellular communication, aggregation, and differentiation.
  • Foraminifera: important for paleoclimatology and marine sediment studies.
  • Euglena and mixotrophs: examples of flexible nutritional strategies in plankton.
  • Apicomplexa (Plasmodium): major public health relevance due to malaria; understanding life cycles guides interventions and treatments.

Terminology recap for quick study

  • Phyla/groups: Chlorophyta (green algae), Phaeophyta (brown algae), Rhodophyta (red algae), Diatoms (silica shells), Dinoflagellates, Euglenids, Foraminifera, Rhizopods (Amoebae), Apicomplexa (e.g., Plasmodium), Slime molds (Cellular and Plasmodial).
  • Key structures: holdfast, air bladders; frustule (diatoms); cell plates in dinoflagellates; tests (calcium carbonate shells in forams); pseudopods in rhizopods; eyespots in Euglena.
  • Reproduction/ life cycle terms: mitosis, meiosis, fertilization, zygote, spores, sporangia, zygospore, syngamy, isogamy, anisogamy.
  • Ecological concepts: plankton, symbiosis (zooxanthellae), bioluminescence, red tides, desiccation tolerance, multicellularity in brown/red algae, photosynthetic vs heterotrophic lifestyles.

Quick notes to remember from today

  • Cyanobacteria are bacteria, not true algae; chlorophyta are true green algae and closest relatives to land plants.
  • Algae exhibit a spectrum from unicellular to multicellular; life cycles vary from haploid-dominant to diploid-dominant patterns.
  • Brown algae form large kelp forests with holdfasts and air bladders; diatoms have silica shells used in many applications.
  • Dinoflagellates can be photosynthetic, bioluminescent, and form red tides that produce neurotoxins (brevet toxins).
  • Zooxanthellae-fungi symbiosis drives coral reef productivity; reef health depends on this relationship.
  • Slime molds demonstrate interesting life cycles that blur lines between unicellular and multicellular organization.
  • Protozoa include diverse forms like amoebae (Rhizopoda), foraminifera (with calcium carbonate tests), and the malaria parasite Plasmodium with complex host-vector life cycles.

Synthesis prompt for exam prep

  • Be able to describe: what defines algae as plant-like protists, the differences among green, brown, and red algae, and where diatoms fit in.
  • Explain the main life cycle patterns (haplontic, diplontic, alternation of generations) with examples from Chlorophyta or other groups.
  • Identify key structural features: silica frustules (diatoms), cellulose plates (dinoflagellates), holdfasts/air bladders (brown algae), tests (foraminifera).
  • Distinguish bacterial cyanobacteria from true algae; understand the ecological roles of dinoflagellates and their toxins.
  • Describe slime mold life cycles and how they differ from true fungi.
  • Understand protozoan diversity: pseudopods, flagella, cilia, phagocytosis, and the relevance of Plasmodium’s life cycle to human disease.