Diversity of Life: Protists: UNIT 6

Topic 6: Diversity of Life – Protists

Introduction

Protists are a diverse group of mostly unicellular eukaryotic organisms that do not fit into the kingdoms of plants, animals, or fungi. They play essential ecological roles and exhibit a wide range of nutritional strategies, life cycles, and cellular structures.

Learning Objectives

• Define the characteristics of protists

• Describe the four major protist supergroups and their features

• Give examples of organisms in each supergroup

• Differentiate between major clades within the SAR supergroup

• Distinguish between plasmodial and cellular slime molds

• Explain the use of algae as a renewable biodiesel source

• Define gametophyte, sporophyte, diploid, and haploid

• Describe the evolution of multicellularity in protists

Protists: General Characteristics

• Eukaryotes that are not classified as plants, animals, or fungi

• Mostly unicellular, but some are multicellular or colonial

• Exhibit diverse nutritional modes:

  • Algae: autotrophs, photosynthetic

  • Protozoans: heterotrophs, ingest food

  • Heterotrophic protists: absorb nutrients

  • Parasites: harm host organisms

  • Mixotrophs: combine photosynthesis and heterotrophy

• Live in moist environments or as endosymbionts

• Possess membrane-bound organelles and a nucleus

• Flagella and cilia with a 9+2 microtubule arrangement

Protist Supergroups

Protists are classified into four monophyletic supergroups, each containing multiple clades. Some lineages are more closely related to plants, fungi, or animals than to other protists.

• SAR (Stramenopila, Alveolata, Rhizaria)

• Excavata

• Unikonta

• Archaeplastida

SAR Supergroup

• Composed of three major clades: Stramenopiles, Alveolates, and Rhizaria

Stramenopiles

• Diatoms: Unicellular algae with glassy silica cell walls; important photosynthesizers in aquatic environments; store food as lipids or carbohydrates; used to make diatomaceous earth.

• Brown algae: Multicellular marine algae (seaweeds); lack true roots, stems, and leaves; kelp forests are ecologically significant.

• Water molds: Heterotrophic, unicellular; decompose dead matter; resemble fungi; some are plant parasites (e.g., potato blight).

Alveolates

• Dinoflagellates: Unicellular; can be autotrophic, heterotrophic, or mixotrophic; cause red tides; some produce neurotoxins; symbiotic with corals.

• Ciliates: Use cilia for movement and feeding; heterotrophic or mixotrophic; example: Paramecium.

• Parasites: Plasmodium causes malaria.

Rhizaria

• Amoebas: Move and feed using thread-like pseudopodia.

• Foraminiferans (forams): Marine and freshwater; calcium carbonate shells; fossilized shells used as geological markers.

• Radiolarians: Silica skeletons; marine; contribute to ocean sediments (radiolarian ooze).

Algae as a Renewable Energy Source

• Fossil fuels are derived from ancient organic matter (oil from diatoms, coal from plants).

• Algae can be cultivated for oil production and processed into biodiesel.

• Challenges include identifying productive species, genetic engineering, and cost-effective harvesting.

Excavata Supergroup

• Some have a feeding groove and modified mitochondria (lacking functional electron transport chains; use anaerobic metabolism).

• Heterotrophs: Example – termite endosymbionts.

• Autotrophs and mixotrophs: Example – Euglena.

• Parasites:

  • Trichomonas vaginalis: Sexually transmitted; feeds on host cells; treated with metronidazole.

  • Giardia intestinalis: Waterborne; causes diarrhea.

  • Trypanosoma: Transmitted by insects; causes sleeping sickness.

Unikonta Supergroup

• Includes protists closely related to fungi and animals.

• Two clades: Amoebozoans, and animals & fungi.

• Amoebozoans:

  • Free-living amoebas: Move with lobe-shaped pseudopodia.

  • Parasitic amoebas: Cause diseases like amoebic dysentery.

  • Slime molds: Exhibit unique life cycles and feeding strategies.

Plasmodial Slime Molds

• Grow on decaying organic matter; brightly pigmented.

• Form a multinucleate mass (plasmodium) without cell membranes.

• Engulf food by phagocytosis; cytoplasmic streaming distributes nutrients.

• Reproduce by forming spores when conditions are unfavorable.

Cellular Slime Molds

• Common on decaying logs; free-living unicells.

• Aggregate into a slug-like form when food is scarce; some cells form a stalk, others become spores.

Archaeplastida Supergroup

• Almost all are autotrophic.

• Includes red algae, green algae, and land plants.

Red Algae

• Accessory pigments mask chlorophyll; mostly multicellular.

• Cells may be encrusted with calcium carbonate (important for coral reefs).

• Produce carrageenan (used as a stabilizer in foods), nori (sushi wrap), and agar.

Green Algae

• Grass-green chloroplasts; can be unicellular, colonial, or multicellular.

• Examples:

  • Chlamydomonas: Unicellular, biflagellate, freshwater.

  • Volvox: Colonial, spherical colonies of biflagellate cells.

  • Ulva: Multicellular, exhibits alternation of generations.

• Alternation of generations:

  • Sporophyte: Multicellular diploid form, produces spores by meiosis.

  • Gametophyte: Multicellular haploid form, produces gametes by mitosis.

Key Definitions

• Gametophyte: Multicellular haploid organism that produces gametes by mitosis.

• Sporophyte: Multicellular diploid organism that produces spores by meiosis.

• Diploid (2n): Having two sets of chromosomes.

• Haploid (n): Having one set of chromosomes.

Evolution of Multicellularity

• Multicellularity evolved independently in several protist lineages (brown algae, fungi, animals, red and green algae, land plants).

• Involves specialized, interdependent cells performing different functions.

Hypothesis on Protist Phylogeny

• Two unikont lineages led to animals and fungi:

  • Choanoflagellates: Closest relatives of animals; resemble sponge collar cells.

  • Nucleariids: Closest relatives of fungi.

• Green algae led to land plants:

  • Charophytes: Closest living relatives of land plants.

Summary Table: Major Protist Supergroups and Examples

Supergroup	Main Clades	Examples	Key Features
SAR	Stramenopiles, Alveolates, Rhizaria	Diatoms, Brown algae, Dinoflagellates, Forams	Photosynthetic, heterotrophic, complex cell walls, pseudopodia
Excavata	Diplomonads, Parabasalids, Euglenozoans	Giardia, Trichomonas, Euglena, Trypanosoma	Feeding groove, modified mitochondria, parasitism
Unikonta	Amoebozoans, Opisthokonts	Amoebas, Slime molds, Fungi, Animals	Lobe-shaped pseudopodia, multicellularity
Archaeplastida	Red algae, Green algae, Land plants	Chlamydomonas, Volvox, Ulva, Charophytes	Autotrophic, chloroplasts, alternation of generations