Protists are mostly single-celled eukaryotes. They are not plants, animals, or fungi.
This group is not a kingdom because some protists are more closely related to plants, fungi, or animals than other protists.
Protists and other eukaryotes possess cells with a nucleus and other membrane-enclosed organelles.
Protists account for a significant portion of eukaryotic diversity.
Most organisms in eukaryotic lineages are protists.
The majority of protists are unicellular.
Nutritional Diversity
Protists exhibit the most nutritional diversity among eukaryotes.
Photoautotrophs contain chloroplasts.
Heterotrophs absorb organic molecules or ingest larger food particles.
Mixotrophs combine photosynthesis and heterotrophic nutrition.
Modes of Reproduction
Some protists reproduce asexually only.
Others have both asexual and sexual phases in their life cycle.
All three basic types of sexual life cycles (animal, plant, and fungal) are represented among protists.
Alternation of Generations
Multicellular algae exhibit a variety of life cycles.
Some have alternation of generations, where both haploid and diploid stages are multicellular.
The diploid generation is called a sporophyte because it produces spores.
Haploid spores develop into multicellular haploid gametophytes that produce haploid gametes.
Heteromorphic species (e.g., Laminaria) have structurally different gametophytes and sporophytes.
Isomorphic species have gametophytes and sporophytes that look similar.
Fertilization of gametes results in a diploid zygote, which develops into a new sporophyte.
Laminaria Life Cycle
Haploid (n) and Diploid (2n) stages alternate.
Sporophyte (2n) produces sporangia via meiosis.
Sporangia release zoospores.
Zoospores develop into male and female gametophytes (n).
Gametophytes produce sperm and eggs.
Fertilization results in a zygote (2n).
Zygote develops into a new sporophyte.
Endosymbiosis
Endosymbiosis is a symbiotic (mutualistic) relationship in which one organism lives within another.
The first eukaryotes were likely heterotrophs.
All eukaryotes have mitochondria, but not all have plastids (plant organelles).
Serial Endosymbiosis: A eukaryotic heterotroph absorbs a proteobacteria, which becomes mitochondria, then absorbs a photosynthetic cyanobacteria, resulting in a eukaryote that can now photosynthesize.
Examples: euglena, brown algae, diatoms, red algae, green algae, plants.
Evidence for Endosymbiosis
Mitochondria and plastids have their own DNA.
They can transcribe and translate their own DNA into proteins, using a slightly different genetic code than the eukaryotic host.
Inner membranes of mitochondria and plastids have enzymes and transport systems homologous to those in bacteria.
Replication of mitochondria and plastids occurs via binary fission.
The size, RNA sequences, and antibiotic sensitivity of mitochondrial/chloroplast ribosomes are more similar to bacteria than eukaryotic cytoplasmic ribosomes.
Secondary Endosymbiosis
Red and green algae were ingested by heterotrophic eukaryotes, a process called secondary endosymbiosis.
"Algae" is not a valid taxonomic name.
The following can all be considered "algae": Protist, Algae, Plankton
Supergroups Including Algae
SAR (Stramenopiles, Alveolates, Rhizarians)
Excavata: Euglenozoans
Rhizara: Cercozoa
Colonies vs. Multicellularity
Colonies: Collections of cells connected to one another, but all cells are roughly the same; this is NOT true multicellularity.
Colonial species are often found in fossils.
Living colonial species usually have rigid cell walls shared to form the colonies.
Examples: Red, green, and brown algae, plants, fungi, animals.
Complex multicellularity has evolved multiple times in evolutionary history.
Animals have a single-celled common ancestor.
Fungi have a different single-celled common ancestor.
Thus, animals and fungi evolved multicellularity independently.
Plants, Animals, and Fungi are truly multicellular.
Some algae (e.g., brown algae, green algae) independently derived multicellularity, different from that in plants.
Eukaryotic Diversity
'Protists' is not a taxonomically valid group.
They have not been a monophyletic clade since the 1960s.
"Kingdom Protista does not exist."
'Protists' are probably nonmonophyletic because of multiple independent events of endosymbiosis.
Different protist lineages are actually the ancestral forms of multicellular eukaryote kingdoms: plants, animals, and fungi.
Major Supergroups of Eukaryotes
Phylogeny is supported by genetic systematics.
There is significant morphological diversity within supergroups, making it challenging to align morphological traits with molecular phylogenies.
It is difficult to tell an evolutionary story based solely on morphology.
These protist supergroups do NOT share a common ancestor.
Supergroup Excavata
General Feature: "Excavated" groove on one side of the cell body.
Clades:
Parabasilids and Diplomonads have highly reduced mitochondria (adapted for anaerobic conditions).
Euglenozoans have unique flagella.
Examples: Trichonympha, Trypanosoma, Euglena.
Excavata: Diplomonads and Parabasilids
General Features:
Reduced mitochondria that do NOT have electron transport chains.
Cannot use oxygen; must use anaerobic respiration.
Many are parasites.
Examples:
Diplomonad: Giardia (intestinal parasite that causes Giardiasis).
Parabasilid: Termite gut mutualist Trychonympha.
Excavata: Euglenozoans
General Features:
Unifying morphological feature: Flagella composed of an internal supporting rod.
Extremely diverse; includes heterotrophs, autotrophs, parasites.
Subclades:
Kinetoplastids have a kinetoplastid (giant mitochondria).
Aquatic heterotrophs and parasites.
Example: Trypanosoma (causes African Sleeping Sickness, via Tsetse fly as vector).
Euglenids have a 'pocket' at one end where 1 or 2 flagella emerge.
Euglena can switch between photosynthesis and heterotrophy.
Supergroup SAR
SAR is a monophyletic supergroup named for the first letters of its three major clades: Stramenopiles, Alveolates, and Rhizarians.
SAR: Stramenopiles (“Straw Hair”)
General Features: Straw-like hair projections found in the long flagellum.
Ecology and Evolution:
Stramenopiles share a common ancestor with Alveolates.
Some photosynthesizers in this group possibly gained the ability to carry out photosynthesis via secondary endosymbiosis of a heterotrophic protist engulfing a red algae.
Stramenopiles includes: Diatoms and Brown Algae.
SAR: Stramenopiles: Diatoms
Anatomy:
Unique perforated silicon “armor” cell walls, which have two parts that overlap like the two halves of a Petri dish.
Ecology:
Diatoms are diverse and abundant unicellular phytoplankton; they are photosynthetic protists.
With over 100,000 species – possibly one of the most important photosynthesizers on Earth.
Human Application:
"Diatomaceous earth" is fossilized remains of diatoms.
Applications include filtration aid, mild abrasive (e.g., toothpaste), absorbent for liquids, reinforcing filler in plastics and rubber, anti-block in plastic films, cat litter, activator in blood clotting studies, a stabilizing component of dynamite, a thermal insulator, soil for potted plants and trees (e.g., bonsai).
SAR: Stramenopiles: Brown Algae
Anatomy, Physiology, and Reproduction:
Largest and most complex algae; almost all species are multicellular.
All are marine, typically cold-water.
Can carry out alternation of generations.
Carotenoids in plastid are dominated by brown pigments named fucoxanthin.
Brown algae carry out photosynthesis on their blades.
Photosynthesis occurs via chlorophyll a and c (there is no chlorophyll b).
Excess glucose is stored as complex polysaccharides, known as laminaran (unlike plants which store glucose as starch).
SAR: Alveolata
General Features: Have membrane-enclosed sacs (alveoli) just under the plasma membrane.
Systematics: Includes 3 clades: Dinoflagellates, Ciliates, Apicomplexans.
SAR: Alveolata: Dinoflagellates
General Features:
Unicellular.
Reinforced cellulose “armor” plates with 2 flagella in the groove between plates.
Some dinoflagellates release toxins.
Locomotion: Peculiar rolling motion caused by the 2 flagella.
Ecology:
Includes autotrophs, heterotrophs, and mixotrophs.
Toxins released by dinoflagellates are usually harmless at small population densities but dangerous at high population densities, which can kill fish and molluscs (and close beaches).
Huge accumulations can lead to red tides (appear red because of carotenoids).
SAR: Alveolata - Ciliates
General Feature: Use cilia to move and feed.
Two types of nuclei: macronucleus and micronucleus.
Anatomy and Ecology:
Unicellular.
Most are heterotrophic predators (e.g., Paramecium).
Reproduction:
Reproduce asexually via binary fission.
Genetic variation accomplished by conjugation.
SAR: Alveolata: Apicomplexans
General Feature: Almost all are animal parasites that possess a complex of organelles at one end (“apex”) of the cell that facilitates entry into a host cell.
Physiology: Parasitic; NOT photosynthetic.
Reproduction: Often require multiple hosts.
Malaria life cycle: Plasmodium vivax causes malaria. It travels via mosquitoes (vector) to the final host, humans.
SAR: Rhizarians
General Feature: Possess very thin pseudopodia.
Anatomy and Physiology:
Some are amoeboid that move and feed using these thin pseudopodia.
Others are flagellated non-amoeboids that use pseudopodia mainly for feeding.
Systematics: Major clades: Foraminifera (Forams) and Cercozoans.
SAR: Rhizarians: Foraminiferans (Forams)
General Feature: Have porous shells called tests (“armor” made of calcium carbonate).
Forams can extend their thin pseudopodia through holes.
Ecology:
Mostly marine, but also found in brackish, freshwater, and even terrestrial habitats.
Includes red algae, green algae (2 lineages, one of which evolved into plants).
Most recent common ancestor of this supergroup was probably a heterotrophic protist that absorbed a cyanobacterium (which became the chloroplast).
Archaeplastida: Red Algae
General Features:
Red algae have red photosynthetic pigment phycoerythrin.
Store excess glucose in carbohydrate form outside of the chloroplast as starch.
Anatomy and Ecology:
Photosynthetic autotrophs.
Generally multicellular.
Chloroplasts contain chlorophyll a.
Store excess glucose in carbohydrate form outside of the chloroplast as starch.
Isomorphic alternation of generations.
Human Interest:
In addition to cellulose, their cell walls contain carrageenan, used as a thickener in foods, and agar, used as a microbiological growth medium.
Dried red algae is also wrapped around rice in sushi.
Archaeplastida: “Green Algae”: (Chlorophytes and Charophytes)
General Features:
Photopigments are chlorophyll a, chlorophyll b, and carotenoids.
Excess glucose is stored as amylose starch contained within the chloroplast.
Cell walls composed of cellulose.
Charophytes (NOT Chlorophytes) are the sister group to plants.
"Plants" are defined as being multicellular and can live on land (In contrast, algae always need to be in a watery/moist environment).
Green Algae:
"Green algae" is a paraphyletic grouping.
Unicellular, colonial, multicellular.
Have complex life cycles with sexual and asexual stages.
Chlorophytes have alternation of generations:
Ulva (a chlorophyte) has isomorphic alternation of generations (sporophyte and gametophyte generations look exactly alike).
Derbesia (a chlorophyte) has heteromorphic alternation of generations.
Strangely, charophytes do not have alternation of generations.
Supergroup Unikonta
Includes Animals, Fungi, and other eukaryotes.
Two major clades: Amoebozoans (tubulinids and slime molds) and Opisthokonts (animals, fungi, and closely related ‘protists’).
Unikonts: Amoebozoans
General Features:
Amoeba-like protists.
Compared to rhizarians, amoebozoans have thicker pseudopodia that can be lobe-shaped or tube-shaped.
Cytoplasmic streaming.
Ecology: Most are predaceous heterotrophs.
Systematics:
“True” amoeba belong to this group.
Includes slime molds: slime molds were once thought to be part of fungi, but are NOT fungi; slime molds have different unicellular ancestors than fungi.
Unikonts: Opisthokonts
Includes animals and their protist relative (choanoflagellates) and fungi and their protist relative (nucleariids).
Thus, the common ancestor of plants, fungi, and animals was probably a single-celled eukaryote, suggesting the independent evolution of multicellularity in all three multicellular lineages.
Conclusion: Multicellularity in plants, fungi, and animals resulted from convergent evolution.
Protists as the Base of Aquatic Food Chains
Many different photosynthetic ‘protists’ comprise the base of aquatic food chains.