Protists: Diversity, Structure, and Importance

Protists: Diversity, Structure, and Importance

Definition and Classification of Protists

Protists are a diverse group of mostly unicellular eukaryotic organisms that do not fit into the traditional kingdoms of plants, animals, or fungi. They are considered a paraphyletic group because they include some, but not all, descendants of their most recent common ancestor.

• Paraphyletic group: A group that includes an ancestor and some, but not all, of its descendants.

• Protists are found in the domain Eukarya and are defined more by exclusion than by shared derived traits.

Fundamental Features of Eukaryotes

All protists are eukaryotes and share several key cellular features:

• Membrane-bound nucleus

• Membrane-bound organelles (e.g., mitochondria, chloroplasts in some)

• Cytoskeleton

• Linear chromosomes

• Complex cell division (mitosis and sometimes meiosis)

• Endomembrane system (e.g., endoplasmic reticulum, Golgi apparatus)

Ecological and Medical Importance of Protists

Impacts on Human Health and Welfare

• Malaria: Caused by the apicomplexan protist Plasmodium, transmitted by Anopheles mosquitoes.

• Life cycle: Involves both mosquito and human hosts, with complex asexual and sexual stages.

• Health impacts: Symptoms include fever, chills, and anemia; can be fatal if untreated.

• Harmful algal blooms: Caused by rapid growth of certain photosynthetic protists (e.g., dinoflagellates), producing toxins that can affect human health and marine life.

Ecological Importance

• Protists are key primary producers in aquatic food chains (e.g., diatoms, dinoflagellates).

• Food chain: Linear sequence of organisms through which nutrients and energy pass.

• Food web: Complex network of interconnected food chains.

• Some protists are autotrophic (photosynthetic), others are heterotrophic (ingest or absorb food).

• Protists are consumed by a variety of aquatic organisms, forming the base of many food webs.

• Some protists (e.g., diatoms) play a role in carbon cycling and may help limit global climate change by sequestering carbon.

Evolutionary Innovations in Protists

Endosymbiosis and Organelle Origins

Endosymbiosis is a key evolutionary process in the origin of mitochondria and chloroplasts:

• Mitochondria: Evolved from engulfed aerobic bacteria (likely alpha-proteobacteria).

• Chloroplasts: Evolved from engulfed photosynthetic cyanobacteria.

$\text{Endosymbiotic Theory:}$

$\text{Eukaryotic cell} + \text{engulfed prokaryote} \rightarrow \text{organelle (mitochondrion or chloroplast)}$

• Evidence includes double membranes, their own DNA, and similarities to prokaryotes.

Morphological Innovations

• All eukaryotes have mitochondria (or mitochondrial genes), a nucleus, and a cytoskeleton.

• The nuclear envelope and endomembrane system evolved through infolding of the plasma membrane.

• Protists have evolved diverse structures for support and protection (e.g., silica shells in diatoms, cellulose plates in dinoflagellates).

Innovative Structures for Support and Protection

Protist Group	Structure/Behavior
Diatoms	Silica cell walls (frustules)
Dinoflagellates	Cellulose plates (theca)
Foraminiferans	Calcium carbonate shells (tests)
Alveolates	Membrane-bound sacs (alveoli) under plasma membrane

Multicellularity

• Multicellular organisms contain specialized, interdependent cells.

• Most multicellular species are plants, animals, or fungi, but multicellularity evolved independently in several protist lineages (e.g., brown algae, red algae, green algae).

Protist Diversity: Feeding, Locomotion, and Reproduction

Feeding Strategies

• Ingestive feeding: Engulfing food particles (e.g., amoebas use pseudopodia).

• Absorptive feeding: Absorbing nutrients directly from the environment (e.g., slime molds).

• Photosynthesis: Using chloroplasts to convert light energy to chemical energy (e.g., euglenids, algae).

• Some protists are mixotrophic (can switch between autotrophy and heterotrophy).

Locomotion

• Amoeboid motion: Movement via pseudopodia ("false feet").

• Cilia: Short, hair-like structures for movement (e.g., paramecium).

• Flagella: Long, whip-like structures for movement (e.g., euglena).

• Closely related protists may use different forms of locomotion.

Reproduction

• Asexual reproduction: Mitosis and cell division or binary fission.

• Sexual reproduction: Meiosis and fusion of gametes; increases genetic diversity.

• Sexual reproduction may be an adaptation to fight disease and parasites due to increased genetic variability.

• Life cycles: May be haploid-dominated, diploid-dominated, or involve alternation of generations.

Comparison of Sexual and Asexual Reproduction

Type	Advantages	Disadvantages
Sexual	Genetic diversity, adaptation to changing environments	Requires more energy, slower
Asexual	Rapid population growth, less energy required	Less genetic diversity

Key Lineages of Protists

There are seven major lineages of eukaryotes (six described here), each with unique traits and ecological roles. Multiple independent origins of parasitism, photosynthesis, and multicellularity have occurred among protists.

Amoebozoa

• Definition: Protists with lobe-shaped pseudopodia.

• Traits: Move and feed using pseudopodia; lack cell walls.

• Impacts: Some cause disease (e.g., Entamoeba histolytica causes dysentery).

• Examples: Amoebas, slime molds.

Excavata

• Definition: Protists with an "excavated" feeding groove.

• Traits: Many have reduced or modified mitochondria.

• Impacts: Some are human parasites (e.g., Giardia, Trypanosoma).

• Examples: Euglena, trypanosomes.

Plantae

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

• Traits: All have chloroplasts derived from primary endosymbiosis.

• Impacts: Important primary producers; some are edible (e.g., nori from red algae).

• Examples: Chlamydomonas, Spirogyra, red algae.

Rhizaria

• Definition: Protists with threadlike pseudopodia.

• Traits: Often have elaborate shells (tests).

• Impacts: Important in marine sediments; some are indicators of past climate.

• Examples: Foraminiferans, radiolarians.

Alveolata

• Definition: Protists with membrane-bound sacs (alveoli) under the plasma membrane.

• Traits: Includes ciliates, dinoflagellates, and apicomplexans.

• Impacts: Some cause disease (e.g., Plasmodium causes malaria, Toxoplasma causes toxoplasmosis).

• Examples: Paramecium, dinoflagellates, Plasmodium.

Stramenopila (Heterokonta)

• Definition: Protists with "hairy" and "smooth" flagella.

• Traits: Includes both unicellular and multicellular forms.

• Impacts: Includes important photosynthetic organisms (diatoms, brown algae) and pathogens (oomycetes, e.g., cause of potato famine).

• Examples: Diatoms, brown algae, oomycetes.

Review Questions and Key Facts

• Locomotion structures: Cilia and flagella.

• Control center of unicellular organisms: Nucleus.

• Organism moving by pseudopodia: Amoeba.

• Protist responsible for malaria: Plasmodium; carrier is Anopheles mosquito.

• Organisms with chloroplasts: Euglena, Spirogyra.

• Organelle for removing excess water: Contractile vacuole.

• Diagnosis of malaria: Blood test.

• Waste exit in paramecium: Anal pore.

• Amoeba: Unicellular, heterotrophic.

• Euglena: Unicellular, mixotrophic (can be autotrophic or heterotrophic).

• Paramecium: Unicellular, heterotrophic.

• Unicellular algae example: Chlamydomonas.

• Mixotrophic organism: Euglena.

• Potato famine cause: Oomycete Phytophthora infestans.

• Disease from tsetse fly: African sleeping sickness (Trypanosoma).

• Disease from Anopheles mosquito: Malaria.

• Organelle for sexual reproduction in paramecium: Micronucleus.

• Main symptom of malaria: Cyclic fever; treated with drugs like chloroquine.

• Conjugation: Sexual process in ciliates involving exchange of genetic material.

• "False foot": Pseudopodium.

• Animal-like protists: Protozoa.

• Organelle for photosynthesis: Chloroplast.

• Organism causing dysentery: Entamoeba histolytica.

• Can malaria be fatal? Yes.

• Organism with mouth and anal pore: Paramecium.

• Organism moving by cilia: Paramecium.

• Binary fission: Asexual reproduction by cell division.

• Spirogyra: Multicellular, autotrophic (green algae).

• Habitat of amoeba and paramecium: Freshwater ponds and streams.

• Diseases caused by protists: Malaria, dysentery.

• "Protozoa" meaning: "First animals" (animal-like protists).

• Do protists have a nucleus? Yes; they are eukaryotic.

• Amoeba in unfavorable conditions: Forms a cyst.

• Three types of multicellular algae: Brown, red, and green algae.

• How amoeba gets food: Engulfs food by phagocytosis.

• Fungus-like protists: Slime molds, water molds.

Additional info: For diagrams of protist reproductive cycles, refer to standard biology textbooks for alternation of generations and haploid/diploid life cycles.