Introduction to Protists and Protozoans
Taxonomic Reclassification of Protists
The Concept of Kingdom Protista: Traditionally, "Protista" was used as a catch-all group for eukaryotic organisms that did not fit neatly into the kingdoms of plants, animals, or fungi.
Modern Critique of the Term: Current scientific consensus suggests that calling them a "kingdom" is inaccurate. Kingdoms typically represent a monophyletic group (organisms derived from a single common ancestor).
Polyphyletic Nature: Protists are considered a polyphyletic group. This means the group contains members from different ancestral lineages that do not all tie back to the same immediate ancestor.
Transition to Supergroups: Contemporary laboratory manuals and scientists now divide protists into high-level categories called Supergroups. This hierarchical system replaces the traditional "Kingdom Protista" designation.
Functional Classifications of Protists
While the kingdom designation is outdated, classifying protists by their functional roles remains a useful pedagogical tool:
Plant-like Protists (True Algae):
These organisms perform photosynthesis.
They consist of pigments like chlorophyll used to absorb sunlight.
They possess specialized organelles known as chloroplasts.
Fungus-like Protists (Slime Molds and Water Molds):
Though referred to as "molds," they are distinct from true molds in the Kingdom Fungi.
Similarities to Fungi: They produce spores for reproduction and feed saprophytically (decomposing dead organic matter) or parasitically.
Animal-like Protists (Protozoans):
The term "Protozoan" literally translates to "first animal."
They are heterotrophic, meaning they feed on other organisms or materials made by other organisms rather than performing photosynthesis.
Feeding patterns resemble animals; they are often microscopic predators that consume bacteria or smaller organisms.
They possess cellular structures similar to those found in animal cells.
The Six Protozoan Supergroups
Excavata:
Includes euglenas and flagellates (protists characterized by a tail-like flagella).
Can possess secondary plastids derived from green algae living endosymbiotically within them.
Archaeoplastida:
An algae-focused group including red algae and green algae.
Possess primary plastids—specialized membrane-bound structures like chloroplasts with two envelope membranes.
Use plastids for photosynthesis and storing energy particles such as starch.
Chromalveolata:
A diverse group containing diatoms, ciliates, apicomplexans, and brown algae.
Key characteristic: Presence of alveoli (peripheral membrane sacs).
Includes Plasmodium (the agent of malaria), paramecium (found in pond water), and dinoflagellates (responsible for "red tides").
Plastid origin: Some have secondary plastids from red or green algae; others have tertiary plastids from diatoms.
Rhizaria:
Includes radiolarians and foraminiferans.
Etymology: "Rhiz" means root-like, referring to their thin, root-like cytoplasmic projections called pseudopods.
Often contain endosymbiotic green algae.
Amoebozoa:
Includes slime molds and amoebas.
Key characteristic: Flexible extensions called pseudopods used for movement and engulfing food.
Opisthokonta:
Includes choanoflagellates.
Characterized by swimming cells with a single flagellum.
Physiology of Animal-Like Protists (Protozoans)
Heterotrophic Ecology: Protozoans must consume organic material from other organisms.
Intracellular Digestion: Utilizes food vacuoles to break down nutrients inside the cell.
Osmoregulation: Many freshwater protozoans use contractile vacuoles to expel excess water taken in from the environment, maintaining internal pressure through osmoregulation.
Detailed Study: Amoebozoa
Locomotion and Feeding:
Amoebas use pseudopodia (lobe-shaped extensions of the cell) to move.
They feed via phagocytosis: the pseudopods surround a food particle (like bacteria or algae), fuse together, and trap the food in a food vacuole.
Anatomy:
The shape is constantly changing due to the flow of loose cytoplasm.
Possess a hyaline cap, which is a clear space at the leading edge of the pseudopod.
Historical Context: August Johann Rösel von Rosenhof was the first person to describe the amoeba.
Immune System Parallel: Human macrophages behave similarly to amoebas, moving through the body to engulf bacteria via phagocytosis.
Pathogenic Example (Entamoeba histolytica):
Causes "travelers' diarrhea," intestinal abscesses, and liver damage.
Common in areas with untreated water; locals may be resistant, but travelers often lack immunity.
Detailed Study: Slime Molds (Myxogastria)
Morphology:
Referred to as molds because they produce spores on stalks called sporangia.
The vegetative, non-reproductive form is called a plasmodium.
Plasmodium Characteristics:
A slimy mass that is multinucleate and coenocytic (sharing a continuous cytoplasm without internal cell walls).
Starts as a zygote; the nucleus divides repeatedly while the cytoplasm expands.
Note: Do not confuse this vegetative state with the genus Plasmodium that causes malaria.
Life Cycle:
Found on decomposing logs, bark, and mulch.
In the reproductive phase, the plasmodium grows stalks with sporangia on top.
Nuclei undergo meiosis to produce spores.
Spores germinate into either amoeboid or flagellated cells.
Two cells fuse via syngamy (fusion of similar cells, rather than distinct sperm and egg) to form a new zygote.
Detailed Study: Foraminiferans and Radiolarians (Rhizaria)
Foraminiferans:
Known as "shelled amoebas."
Shells are composed of calcium carbonate ().
Shells have pores through which thin, ray-like pseudopods extend.
Ecological Significance:
The ocean floor is largely composed of foraminiferan and diatom shells.
Used in stratigraphy: Oil companies study the strata of earth for these shells to indicate if an area was once underwater and may contain oil deposits.
Detailed Study: Flagellates (Trypanosomes)
Trypanosomes: Flagellated parasites that cause severe diseases.
African Sleeping Sickness (African Trypanosomiasis):
Vector: The tsetse fly, a small fly roughly the size of a gnat.
At-Risk Populations: Those involved in agriculture, fishing, hunting, or animal husbandry in endemic regions of Africa.
Symptoms: Fever, headache, joint pain, and itching.
The "Sleeping" Stage: In advanced stages, parasites and white blood cells aggregate in the neck and head, restricting blood flow to the brain, leading to extreme lethargy.
Chagas Disease:
Vector: The assassin bug (or kissing bug).
Endemic to South America; it is theorized that Charles Darwin may have contracted this disease during his travels.
Detailed Study: Ciliates (Paramecium)
General Characteristics:
Named for cilia, short hair-like extensions used for locomotion and feeding.
Approximately known species.
Anatomy of a Paramecium:
Cytostome: The "cell mouth."
Oral Groove: A depression that directs food into the cytostome.
Contractile Vacuole: Includes radiating canals that pull in excess water and a central pore to expel it.
Dual Nuclei:
Macronucleus: Contains most genes; handles regular chemical processes and protein production.
Micronucleus: Contains genes specifically for sexual reproduction.
Reproduction:
Asexual: Via transverse fission (a type of mitosis where the cell splits).
Sexual: Via conjugation, where two cells swap genetic material to create new gene combinations.
Vorticella: A sessile (anchored) ciliate. It has a stalk that can snap or coil like a telephone cord when it detects vibrations/predators.
Detailed Study: Apicomplexans
General Characteristics:
Non-motile endoparasites (lack cilia/flagella).
Moved through the host via the host's own systems (e.g., the bloodstream).
Malaria (Plasmodium vivax):
Vector: Female Anopheles mosquitoes (females bite to obtain nutrient-dense blood for egg production).
Symptoms: Malaise (confusion), chills, fever, headache.
Life Cycle:
Sporozoites: Injected into the human by a mosquito.
Liver Stage: Sporozoites travel to the liver and mature into merozoites.
Red Blood Cell Stage: Merozoites enter the bloodstream and hijack red blood cells, causing cycles of fever.
Gametocytes: Some parasites become gametocytes, which are taken up by another mosquito to continue the cycle.
Toxoplasma (Toxoplasmosis):
Often carried by cats (contracted by eating infected prey).
Humans contract it through cat litter or raw meat.
Can cause blindness, congenital defects, or neurological damage in unborn babies.
Estimated of women of childbearing age show serological evidence of exposure.
Pneumocystis jirovecii: A rare form of pneumonia that primarily affects immunocompromised individuals, such as those with HIV/AIDS.
Questions & Discussion
Student: Aren't Trypanosomes also from mosquitoes?
Instructor: No, the vectors vary. African Sleeping Sickness is from the tsetse fly, and Chagas disease is from the assassin bug. Malaria is the primarily discussed disease transmitted by mosquitoes.
Student: Are we answering all the questions in the manual or just the ones you put up?
Instructor: Just the ones on the board. For example, do questions 1e and 1f for procedure 26.1, and the specific questions I've added. I suggest writing out the full questions for your study notes.
Student Discussion on Lab Slides: Students discussed the difficulty of finding the flagella on certain slides and decided to split up the remaining slides among themselves to save time.
Instructor Note on Procedure 26.5: The instructor noted they would not follow the book exactly because the lab has a permanent slide of a paramecium instead of the specific one mentioned in the manual. Students are required to do two sketches for this section.