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

SAR Supergroup

A diverse group of organisms including plant-like (algae), animal-like (protozoa), and fungus-like (slime molds) protists, often sharing similar metabolic strategies and life cycle patterns.

Chlorophyta

True green algae, mostly unicellular but some multicellular (e.g., sea lettuce). They are close relatives to land plants and have both sexual and asexual life cycles, often haploid-dominant.

Haploid-dominant life cycle

A life cycle where the majority of an organism's life is spent in the haploid (n) state, with gametes produced by mitosis, fertilization forming a diploid zygote (2n), and meiosis immediately returning to haploid spores.

Brown algae (Phaeophyta)

Typically multicellular protists, including large seaweeds and giant kelp. They attach to rocks via a holdfast and use buoyant air bladders to aid light capture. Extracts are used as food thickeners.

Holdfast

A root-like structure used by brown algae (and some other seaweeds) to attach to rocks, not for nutrient absorption.

Red algae (Rhodophyta)

Multicellular protists that typically inhabit marine environments, often deeper water. They are a source of agar and carrageenan.

Diatoms

Unicellular algae characterized by intricate silica-based cell walls called frustules, composed of silicon dioxide (SiO_2). They contribute to diatomaceous earth and have various industrial uses.

Frustules

The glassy, intricate, silica-based cell walls of diatoms, made of silicon dioxide (SiO_2).

Dinoflagellates

Planktonic protists, often photosynthetic, with two flagella and cellulose plates around their cells. Some are bioluminescent and others form mutualistic relationships with corals (zooxanthellae) or cause red tides.

Zooxanthellae

Specific dinoflagellates that live symbiotically inside corals, providing carbohydrates to the corals through photosynthesis in exchange for a protected environment and access to light. Essential for coral reef health.

Red Tides

Blooms of dinoflagellates that can produce potent neurotoxins (brevet toxins), harmful to marine life and humans by inhibiting nerve function.

Slime Molds

Fungus-like protists that are heterotrophic, decomposing dead organic matter. They are found in moist environments and exhibit unusual life cycles, categorized as cellular or plasmodial.

Cellular Slime Molds (Dictyostelium-type)

Protists that exist as single-celled amoeboid cells but can aggregate into a multicellular slug-like pseudoplasmodium when resources are scarce, eventually forming a fruiting body with spores.

Plasmodial Slime Molds (Physarum-type)

Protists that form a single, multinucleate mass called a plasmodium (one cell with many nuclei) that moves and engulfs food by phagocytosis, eventually differentiating into sporangia to produce spores.

Protozoa (Animal-like protists)

Single-celled, eukaryotic protists with a nucleus and organelles, typically heterotrophic. Many move using flagella, cilia, or pseudopods, and commonly reproduce asexually.

Rhizopods (Amoebae)

Protozoa that move and feed using pseudopods (extensions of the cell membrane). They engulf prey via phagocytosis and regulate water balance with contractile vacuoles.

Foraminifera (Forams)

Single-celled protists with calcium carbonate shells called tests, which are often elaborate and contribute significantly to fossil records. Abundant in marine sediments.

Euglena (Euglenozoa)

Unicellular protists often possessing a flagellum, a photosynthetic plastid, and an eyespot (stigma). They are mixotrophic, capable of photosynthesis in light and ingesting prey in the dark.

Apicomplexa

A group of parasitic protozoa, including Plasmodium (the malaria parasite), characterized by complex life cycles involving both sexual and asexual phases and multiple hosts (e.g., mosquitoes and vertebrates).

Plasmodium

The malaria parasite, an apicomplexan that undergoes a complex life cycle involving transmission by mosquito vectors (sporozoites) to vertebrate hosts, where it infects liver cells and red blood cells (merozoites).

Zygospore

A diploid spore formed by the fusion of two gametes (fertilization), which can later undergo meiosis to produce haploid spores, often as a strategy to endure harsh conditions.

Syngamy

The fusion of two gametes to form a zygote.

Isogamy

A condition in sexual reproduction where gametes appear similar in size and form.

Anisogamy

A condition in sexual reproduction where gametes differ in size, with one typically being larger than the other.

Mixotrophic

An nutritional strategy where an organism can utilize both autotrophic (e.g., photosynthesis) and heterotrophic (e.g., ingestion) modes of nutrition, depending on environmental conditions.