Protists Notes

Protists

Chapter 28: Protists

Diversity and Evolution

  • Endosymbiosis: Much of protistan diversity can be traced to endosymbiosis.

    • Engulfed aerobic bacterium $\rightarrow$ Mitochondrion.
    • Engulfed photosynthetic bacterium $\rightarrow$ Plastid.

  • Supergroups of Eukaryotes:

    • Excavata
    • SAR (Stramenopila, Alveolata, Rhizaria)
    • Archaeplastida
    • Unikonta

Concept 28.1: Eukaryotic Diversity

  • Protists: Informal term for eukaryotes that are not plants, animals, or fungi.
  • Protists are not a kingdom because some are more closely related to plants, fungi, or animals than to other protists.
  • Eukaryotic cells (including protists) have a nucleus and membrane-enclosed organelles.
  • Organelles isolate functions, making eukaryotic cells more complex than prokaryotic cells.
  • Well-developed cytoskeleton allows for asymmetric shape and shape change over time.
  • Protists make up much of the diversity of eukaryotes.
  • Most eukaryotic lineages consist of protists.
  • Most protists are unicellular.

Structural and Functional Diversity

  • Protists exhibit more structural and functional diversity than any other group of eukaryotes.
  • Most are unicellular, but some are colonial or multicellular.
  • Unicellular protists are the most complex cells because each cell must carry out all functions of life.
  • Some unicellular protists have organelles not found in most other eukaryotic cells (e.g., ocelloid in dinoflagellates).
  • Protists are the most nutritionally diverse of all eukaryotes.
    • Photoautotrophs: Contain chloroplasts.
    • Heterotrophs: Absorb organic molecules or ingest larger food particles.
    • Mixotrophs: Combine photosynthesis and heterotrophic nutrition.
  • Some protists reproduce only asexually; others have both asexual and sexual phases.
  • All three basic types of sexual life cycles (animal, plant, and fungal) are represented among protists.

Endosymbiosis in Eukaryotic Evolution

  • Abundant evidence suggests that much of protistan diversity originated from endosymbiosis.
  • Endosymbiosis: A relationship where one organism lives inside the cell or cells of another organism (the host).
  • Mitochondria and plastids are derived from bacteria engulfed by ancestors of early eukaryotes.
  • Mitochondria evolved before plastids and arose from an alpha proteobacterium.
  • Molecular analysis indicates that mitochondria and plastids each evolved only once.
  • The ancestral host was a relatively complex cell with eukaryotic features, such as a cytoskeleton.
  • The host cell lineage is uncertain, but lokiarchaeotes, the archaean sister group to the eukaryotes, is a candidate taxon.

Plastid Evolution

  • Evolution of mitochondria gave rise to eukaryotes.
  • Plastids arose later when a heterotrophic eukaryote engulfed a photosynthetic cyanobacterium.
  • Red and green algae evolved from the plastid-bearing ancestor.
  • Plastids of red algae and green algae have two membranes, like cyanobacteria.
  • Transport proteins in these membranes are homologous to those in cyanobacteria membranes.
  • Red and green algae were ingested by heterotrophic eukaryotes through secondary endosymbiosis.
  • Chlorarachniophytes likely evolved when a heterotrophic eukaryote engulfed a green alga.
  • The engulfed cell contains a vestigial nucleus called a nucleomorph.

Four Supergroups of Eukaryotes

  • Understanding of evolutionary relationships among protist groups is rapidly changing.
  • One hypothesis divides all eukaryotes (including protists) into four supergroups:
    • Excavata
    • SAR
    • Archaeplastida
    • Unikonta
  • Root of the eukaryotic tree is not known.
  • Some major new groups of protists have been recently discovered, but their relationship to the supergroups is unresolved (e.g., haptophytes, cryptophytes, and hemimastigophores).

Concept 28.2: Excavates

  • Excavata is characterized by its cytoskeleton.
  • Some members have an "excavated" feeding groove.
  • The excavates include three monophyletic groups: Diplomonads, Parabasalids, and Euglenozoans.

Diplomonads and Parabasalids

  • Diplomonads and parabasalids lack plastids and have reduced mitochondria.
  • Most live in anaerobic environments.
  • Diplomonads:
    • Reduced mitochondria called mitosomes that lack electron transport chains.
    • Energy derived from anaerobic pathways.
    • Two equal-sized nuclei and multiple flagella.
    • Many are parasites (e.g., Giardia intestinalis).
  • Parabasalids:
    • Reduced mitochondria called hydrogenosomes that generate energy anaerobically.
    • Hydrogen gas released as a by-product.
  • Trichomonas vaginalis is a sexually transmitted parasite (infects about 140 million people per year).

Euglenozoans

  • Euglenozoa is a diverse clade including predatory heterotrophs, photosynthetic autotrophs, mixotrophs, and parasites.
  • Main feature: a spiral or crystalline rod inside each flagella.
  • Includes kinetoplastids and euglenids.

Kinetoplastids

  • Have a single mitochondrion containing an organized mass of DNA called a kinetoplast.
  • Free-living species are consumers of prokaryotes.
  • Some species parasitize animals, plants, and other protists.
  • Trypanosoma infects humans, causing sleeping sickness (about 10,000 people per year) and Chagas’ disease.
  • Trypanosomes have a single cell-surface protein that changes from one generation to the next (bait-and-switch defense).

Euglenids

  • Have one or two flagella that emerge from a pocket at one end of the cell.
  • Some species are mixotrophs (switch between autotrophic and heterotrophic modes).

Concept 28.3: SAR Supergroup

  • SAR is a monophyletic supergroup named for the first letters of its three major clades: Stramenopiles, Alveolates, and Rhizarians.

Stramenopiles

  • Include some of the most important photosynthetic organisms on Earth.
  • Most have a "hairy" flagellum paired with a "smooth" flagellum.
  • Diatoms, oomycetes, and brown algae are three important groups of stramenopiles.

Diatoms

  • Unicellular algae with a unique two-part, glass-like wall of silicon dioxide (SiO2SiO_2).
  • The wall withstands pressure up to 1.4 million kg/m21.4 \text{ million } kg/m^2.
  • About 100,000 species, compose much of the phytoplankton in the ocean and lakes.
  • Photosynthetic activity affects global CO2CO_2 levels.
  • After a diatom bloom, many dead individuals fall to the ocean floor, where decomposition is slow.
  • Promoting diatom blooms by fertilizing the ocean with essential nutrients is a proposed method to reduce atmospheric CO2CO_2 levels.

Brown Algae

  • Largest and most complex multicellular algae.
  • Carotenoids in the plastids produce the brown color.
  • Most are marine, including many species commonly called “seaweeds”.
  • Brown algal seaweeds have plantlike structures: holdfast, stipe, and blades.
  • Lack the true tissues and organs found in plants.
  • Important commodities for humans (e.g., Laminaria, algin).

Alternation of Generations

  • A variety of life cycles have evolved among the multicellular algae.
  • Some have alternation of generations, in which both haploid and diploid stages are multicellular.
  • Diploid generation is called a sporophyte because it produces spores.
  • Haploid spores develop into multicellular haploid gametophytes that produce haploid gametes.
  • Fertilization of gametes results in a diploid zygote, which develops into a new sporophyte.
  • Heteromorphic species (e.g., Laminaria) have structurally different gametophytes and sporophytes.
  • Isomorphic species have gametophytes and sporophytes that look similar.

Oomycetes

  • Include water molds, white rusts, and downy mildews.
  • Misidentified as fungi due to their multinucleate filaments that resemble fungal hyphae.
  • Oomycetes cell walls are composed of cellulose, rather than chitin.
  • Oomycetes and fungi are not closely related.
  • Related to plastid-bearing groups, but do not have plastids or perform photosynthesis.
  • Acquire nutrients through parasitism or decomposition.
  • Phytophthora infestans is a parasite that causes potato late blight.

Alveolates

  • Have membrane-enclosed sacs (alveoli) just under the plasma membrane.
  • Three clades included in the alveolates: Dinoflagellates, Apicomplexans, and Ciliates.

Dinoflagellates

  • Abundant components of marine and freshwater phytoplankton.
  • Have two flagella housed in the grooves of armor-like cellulose plates that surround the cell.
  • Beating of the spiral flagella causes dinoflagellates to spin as they move through the water.
  • Dinoflagellate blooms cause “red tides”.
  • Red tides are toxic and can cause massive kills of invertebrates and fishes.
  • Ocean warming caused by climate change is facilitating more frequent red tides.

Apicomplexans

  • Nearly all apicomplexans are parasites of animals.
  • Spread through the host as infectious cells called sporozoites.
  • Apex (cell end) contains a complex of organelles specialized for penetrating host cells and tissues.
  • Most life cycles include both sexual and asexual stages, and require two or more different hosts.
  • Plasmodium, the parasite causing malaria, lives in both mosquitoes and humans.
  • Plasmodium evades the host immune system by living inside cells and continually changing its surface proteins.
  • Approximately 220 million people in the tropics are infected, and 450,000 die each year from malaria.
  • The first licensed malarial vaccine was recently approved, and routine use began in Africa in 2019.
  • The vaccine provides only partial protection.
  • Research into other potential vaccine targets is ongoing.

Ciliates

  • Named for their use of cilia to move around and feed on bacteria or other protists.
  • Cilia may completely cover the cell surface or be clustered in a few rows or tufts.
  • Have two types of nuclei: tiny micronuclei and large macronuclei.
  • Macronuclei have multiple copies of the genome.
  • Micronuclei may be diploid or haploid, depending on the life stage.
  • Conjugation produces genetic variation without reproduction through the exchange of micronuclei.
  • Asexual reproduction occurs by binary fission.
  • During binary fission, the macronucleus dissolves, and a new one is formed from micronuclei.

Rhizarians

  • Many species of rhizarians are amoebas.
  • Amoebas are protists that move and feed using pseudopodia.
  • Rhizarian amoebas differ from amoebas in other clades by having threadlike pseudopodia.
  • Three clades included in the rhizarians: Radiolarians, Forams, and Cercozoans.

Radiolarians

  • Have delicate, symmetrical internal skeletons typically made of silica.
  • Pseudopodia reinforced by microtubules radiate from the central body.
  • Prey are engulfed by cytoplasm in the pseudopodia and carried into the cell by cytoplasmic streaming.
  • Most radiolarians are marine organisms.

Forams

  • Named for their porous calcium carbonate shells, called tests.
  • Pseudopodia that extend through pores in the test are used in swimming, test formation, and feeding.
  • Some host mutualistic photosynthetic algae within their tests.
  • Live in the ocean and fresh water, and their fossils make up part of the marine sediments.
  • Fossilized tests are used for correlating the age of sedimentary rocks.
  • The magnesium content of fossilized tests is used to estimate the change in ocean temperature over time.

Cercozoans

  • Amoeboid and flagellated protists that feed using threadlike pseudopodia.
  • Common in marine, fresh water, and soil ecosystems.
  • Most cercozoans are heterotrophic parasites or predators.
  • Chlorarachniophytes are a small group of mixotrophs.
  • Paulinella chromatophora has a unique photosynthetic structure called a chromatophore.
  • Chromatophores are derived from endosymbiosis with a cyanobacteria different from the one that gave rise to plastids.

Concept 28.4: Red and Green Algae

  • Plastids arose when a heterotrophic protist acquired a cyanobacterial endosymbiont.
  • The photosynthetic descendants evolved into red algae and green algae.
  • Plants are descended from the green algae.
  • Archaeplastida is the supergroup that includes red algae, green algae, and plants.

Red Algae

  • An accessory pigment called phycoerythrin masks the green of chlorophyll, giving red algae its color.
  • Color varies from greenish-red in shallow water to dark red or almost black in deep water.
  • Most are multicellular; the largest are seaweeds.
  • Reproduction is sexual, and life cycles often include alternation of generations.
  • Red algae are common in coastal waters of tropical oceans.
  • Some species are consumed by humans (e.g., Porphyra).

Green Algae

  • Named for their green chloroplasts, which are structurally and chemically similar to those found in plants.
  • Green algae form a paraphyletic group that includes the charophytes and the chlorophytes.
  • Charophytes include the algae most closely related to plants.
  • Most chlorophytes live in fresh water, although there are many marine and some terrestrial species.
  • Various unicellular species are free-living while others live symbiotically with other eukaryotes.
  • Some live in environments exposed to intense visible and ultraviolet radiation.
  • Larger size and greater complexity evolved in green algae by three different mechanisms:
    • Formation of colonies from individual cells (Pediastrum).
    • Formation of true multicellular bodies by cell division and differentiation (Volvox and Ulva).
    • Repeated division of nuclei with no cytoplasmic division (Caulerpa).
  • Most chlorophytes have complex life cycles with both sexual and asexual reproductive stages.
  • Nearly all species have biflagellated gametes with cup-shaped chloroplasts.
  • Alternation of generations has evolved in some chlorophytes, including Ulva.

Concept 28.5: Unikonta

  • The supergroup Unikonta includes animals, fungi, and some protists.
  • The two major clades of unikonts are the amoebozoans and the opisthokonts.
  • The root of the eukaryotic tree is uncertain.
  • One controversial hypothesis is that unikonts were the first to diverge from other eukaryote groups.

Amoebozoans

  • Amoebozoans are amoebas that have lobe- or tube-shaped, rather than threadlike, pseudopodia.
  • They include tubulinids, slime molds, and entamoebas.

Tubulinids

  • A diverse group of amoebozoans with lobe- or tube-shaped pseudopodia.
  • Common unicellular protists in soil as well as freshwater and marine environments.
  • Most tubulinids are active predators of bacteria and other protists; others feed on detritus.

Slime Molds

  • Slime molds, or mycetozoans, were once thought to be fungi due to their spore-producing fruiting bodies.
  • This resemblance is a result of convergent evolution.
  • Slime molds have diverged into two lineages: plasmodial slime molds and cellular slime molds.
Plasmodial Slime Molds
  • Many plasmodial slime molds are brightly colored, often yellow or orange.
  • They form a large feeding mass called a plasmodium.
  • The plasmodium is a single “supercell” that contains many diploid nuclei undivided by plasma membranes.
  • The plasmodium forms a fruiting body for sexual reproduction in unfavorable environmental conditions.
Cellular Slime Molds
  • Cellular slime molds form multicellular aggregates in which cells are separated by plasma membranes.
  • The feeding stage consists of solitary cells.
  • Solitary cells unite to form a slug-like aggregate for migration when habitat conditions are poor.
  • Ultimately, the aggregated cells form a fruiting body.
  • Dictyostelium discoideum is a model organism for the studying the evolution of multicellularity.
  • Cells in the stalk of the fruiting body die without reproducing; cells at the top survive to reproduce.
  • Some cells have a “cheat” mutation, giving them the reproductive advantage of not forming the stalk.
  • Cheating cells lack a specific surface protein recognized by noncheaters.
  • Non-cheaters avoid exploitation by preferentially aggregating with other non-cheaters.
  • Such a recognition system may have been important in the evolution of other multicellular eukaryotes.

Entamoebas

  • Members of the genus Entamoeba are parasites of all classes of vertebrates and some invertebrates.
  • Humans are host to at least six species, but only E. histolytica is pathogenic.
  • E. histolytica causes amoebic dysentery, the third-leading cause of death due to eukaryotic parasites.

Opisthokonts

  • Opisthokonts are a diverse group including animals, fungi, and several groups of protists.