The Diversity of Protists (Chapter 21): Structure, Function, and Ecological Roles

Case Study: The Green Monster – Caulerpa

Introduction to Caulerpa and Its Ecological Impact

• Caulerpa is a green seaweed bred for use in saltwater aquariums, originally distributed from the Wilhelma Zoo in Germany.

• It became invasive after being dumped into the Mediterranean Sea, where it rapidly spread and excluded native organisms.

• Despite its ecological threat, Caulerpa is a protist, illustrating the diversity and adaptability of this group.

Chapter 21 At a Glance

• 21.1 What Are Protists?

• 21.2 What Are the Major Groups of Protists?

What Are Protists?

Definition and General Characteristics

• Protists are eukaryotic organisms that do not fit into the plant, animal, or fungal kingdoms.

• They are a diverse group, not forming a true clade (not all descendants of a single common ancestor).

• Most protists are single-celled and microscopic, though some are multicellular.

• The term "protist" is used for convenience, not as a formal taxonomic group.

Reproduction in Protists

• Most protists reproduce asexually by mitosis, producing genetically identical offspring.

• Some can reproduce sexually, forming haploid gametes via meiosis; gametes fuse to form a diploid zygote.

• Sexual reproduction is less frequent and produces genetically diverse offspring.

• Non-reproductive genetic exchange (e.g., conjugation) also occurs in some protists.

Nutrition in Protists

• Protists exhibit diverse nutritional strategies:

  • Heterotrophs: Obtain energy by ingesting or absorbing organic molecules (predators, parasites, saprotrophs).

  • Phototrophs: Capture solar energy via photosynthesis in chloroplasts.

  • Some are mixotrophic, switching between autotrophy and heterotrophy.

• Predatory protists use pseudopods or cilia to capture food; digestion occurs in food vacuoles.

• Absorptive protists may be free-living decomposers or parasites inside host organisms.

Photosynthesis and Endosymbiosis

• Photosynthetic protists (e.g., algae) are abundant in aquatic environments and are key primary producers.

• Chloroplasts in protists originated from endosymbiosis—the engulfment of photosynthetic bacteria.

• Secondary endosymbiosis: A non-photosynthetic protist engulfs a photosynthetic protist, resulting in chloroplasts with four membranes.

Ecological and Human Importance

• Positive impacts: Photosynthetic protists produce oxygen and form the base of aquatic food webs.

• Negative impacts: Parasitic protists cause diseases in humans, animals, and plants.

• Modern classification uses DNA sequencing to understand evolutionary relationships (phylogeny).

Major Groups of Protists

Overview of Protist Diversity

• Protists are grouped by shared traits, not strict evolutionary relationships.

• Algae: Photosynthetic protists.

• Protozoa: Single-celled, non-photosynthetic protists.

Excavates

• Named for a feeding groove; most are anaerobic and lack typical mitochondria.

• Include diplomonads, parabasalids, euglenids, and kinetoplastids.

Diplomonads

• Have two nuclei and multiple flagella.

• Giardia causes chronic diarrhea ("beaver fever").

• Form cysts that survive outside the host and spread via contaminated water.

Parabasalids

• Anaerobic, flagellated; possess a parabasal body for support and movement.

• Live inside animals as mutualists (e.g., digesting wood in termites) or as parasites (e.g., Trichomonas vaginalis causes trichomoniasis).

Euglenids

• Single-celled, mostly freshwater; lack a rigid cell wall.

• Move using flagella; have a contractile vacuole for osmoregulation.

• Many are photosynthetic (autotrophic), some are heterotrophic or mixotrophic.

• Possess an eyespot (stigma) for detecting light direction.

Kinetoplastids

• Have unique kinetoplasts (complex mitochondrial DNA).

• Include free-living predators and parasites.

• Trypanosoma causes African sleeping sickness, transmitted by tsetse flies.

Stramenopiles

• Characterized by hair-like projections on flagella.

• Include water molds, diatoms, and brown algae.

Water Molds (Oomycetes)

• Form filamentous structures; many are decomposers.

• Cause plant diseases such as downy mildew and potato blight.

Diatoms

• Photosynthetic; encased in silica cell walls ("glass houses").

• Major component of phytoplankton; contribute to about 50% of Earth's photosynthetic activity.

• Fossilized diatoms form diatomaceous earth, used in products like toothpaste and metal polish.

Brown Algae

• Multicellular seaweeds (e.g., kelp); have brownish-yellow pigments for photosynthesis.

• Important in marine ecosystems as habitat and food source.

Alveolates

• Single-celled; have small cavities (alveoli) beneath the cell surface.

• Include dinoflagellates, apicomplexans, and ciliates.

Dinoflagellates

• Mostly photosynthetic; have two flagella.

• Some cause "red tides," which can be toxic to marine life and humans.

Apicomplexans

• All are parasitic; form infectious spores.

• Plasmodium causes malaria, transmitted by Anopheles mosquitoes.

Ciliates

• Most complex unicellular protists; covered in cilia for movement and feeding.

• Examples: Paramecium (responds to environment), Didinium (predator).

Rhizarians

• Have thin, thread-like pseudopods; include foraminiferans and radiolarians.

Foraminiferans

• Produce calcium carbonate shells; important in marine sediments (e.g., White Cliffs of Dover).

Radiolarians

• Have glassy silica shells; often host photosynthetic symbionts.

Amoebozoans

• Move and feed using thick pseudopods; lack shells.

• Include amoebas (predators, some parasites) and slime molds (decomposers).

Slime Molds

• Acellular slime molds: Form multinucleate plasmodium; produce spores in fruiting bodies.

• Cellular slime molds: Live as individual cells but aggregate into a pseudoplasmodium when food is scarce.

Red Algae

• Multicellular, photosynthetic; contain red pigments that allow them to live in deep water.

• Source of commercial products like carrageenan (thickener).

Chlorophytes (Green Algae)

• Closest relatives to land plants; can be unicellular, colonial, or multicellular.

• Important in freshwater and marine environments; some are cultivated for biofuels.

Summary Table: Major Groups of Protists

Group	Main Features	Examples	Ecological Role
Excavates	No typical mitochondria, often anaerobic	Giardia, Euglena, Trypanosoma	Parasites, mutualists, free-living
Stramenopiles	Hairy flagella, photosynthetic or not	Diatoms, Brown algae, Water molds	Primary producers, decomposers
Alveolates	Alveoli under membrane, diverse nutrition	Dinoflagellates, Plasmodium, Paramecium	Phytoplankton, parasites, predators
Rhizarians	Thin pseudopods, shells	Foraminiferans, Radiolarians	Marine sediments, symbionts
Amoebozoans	Thick pseudopods, no shells	Amoebas, Slime molds	Predators, decomposers
Red Algae	Red pigments, multicellular	Chondrus, Porphyra	Primary producers, commercial uses
Chlorophytes	Green pigments, closest to plants	Ulva, Volvox	Primary producers, biofuels