Protozoans and Endosymbiosis

Protozoans: Unicellular Eukaryotes

• Protozoans are unicellular eukaryotes.
• They act as the evolutionary origin of animal life.
• The terms "unicellular eukaryotes" and "protozoans" can be used interchangeably.
• Fossil records indicate that protozoans existed 100 million years ago.
• Protozoans are believed to be the first eukaryotic cells.

Endosymbiosis

• Endosymbiosis is the process by which protozoans came about.
• This process led to the development of some organelles, including mitochondria and chloroplasts.
• In endosymbiosis, a larger cell engulfs a smaller cell, such as cyanobacteria.
• Instead of being digested, the smaller cell becomes part of the host cell.
• This results in a symbiotic relationship.
• Primary endosymbiosis occurs when a larger cell engulfs a smaller cell like cyanobacteria.

Secondary Endosymbiosis

• Occurs when a third cell engulfs a cell that has already engulfed another cell.
• This process further complicates the taxonomic classification of protozoans.

Protozoan Classification and Characteristics

• Protozoans are not considered animals because they are unicellular, whereas animals are multicellular.
• They are at the protoplasmic stage of complexity.
• Most protozoans are microscopic and ubiquitous, found in lakes, soil, and the human body.
• They require moisture and favorable environmental conditions to survive.
• The diversity of protozoans makes it challenging for taxonomists to follow their lineage.
• Primary and secondary endosymbiosis and convergent evolution contribute to the difficulty in classification.
• Molecular analysis is increasingly used to differentiate protozoan phyla and species.

Protozoan Diversity

• Most protozoans are microscopic and lack germ layers due to their unicellular nature.
• They possess specialized organelles essential for their survival.
• Protozoans exhibit diverse symmetries, including spherical, radial, and bilateral.
• Lifestyles vary, including sessile and free-floating forms.
• They utilize cilia, flagella, or pseudopodia for locomotion.
• Some have exoskeletons or endoskeletons, while others are naked (without a skeleton).
• They can be commensal, mutualistic, or parasitic.
• Nutritional modes include autotrophic, heterotrophic, and mixotrophic.
• Reproduction can be asexual or sexual.

Nutrient Acquisition

• Protozoans are classified as either animal-like (heterotrophic) or plant-like (autotrophic).
• Heterotrophic protozoans obtain prey through phagocytosis, using a cytostome (designated mouth area).
• Amoebas can perform phagocytosis anywhere on their body.
• In phagocytosis, the amoeba uses receptors to bind bacteria, bringing it into a vesicle called a phagosome.
• The phagosome fuses with a lysosome, which digests the bacteria with enzymes.
• Undigestible material is removed through exocytosis.
• Osmotrophs use passive and active diffusion to absorb already broken-down nutrients.
• Pinocytosis is used to bring in water and liquids from the environment.

Mixotrophs

• Can perform both autotrophic and heterotrophic activities, depending on environmental conditions.
• For example, euglena can switch from autotrophic to heterotrophic in the dark.
• Nutrient acquisition is not a reliable characteristic for classification due to this variability.

Modes of Locomotion

• Protozoans move using cilia, flagella, or pseudopodia.
• Cilia and flagella are facilitated by microtubules.
• Microtubules extend the entire length of the flagella and meet at a kinetosome or basal body inside the cell's cytoplasm.
• Cilia and flagella vary in number and placement depending on the species.
• They are used for feeding, reproduction, excretion of waste, and maintaining osmoregulation.
• Pseudopodia are primarily composed of actin filaments that polymerize on one end and depolymerize on the other.
• Pseudopodia are used for locomotion and feeding, and come in various shapes.

Pseudopodia Mechanism

• The hyaline cap is the leading end of the pseudopodia.
• Endoderm flows into the extension, carrying actin filaments towards the hyaline cap.
• Actin filaments polymerize at the plasma membrane to provide structure.
• At the trailing end, actin is depolymerized, pushing everything towards the hyaline cap.
• Myosin proteins pull the trailing end towards the hyaline cap.

Cellular Respiration and Organelles

• Many protozoans have mitochondria due to endosymbiosis.
• Mitochondria are prokaryotic in origin and have their own DNA separate from the host cell's DNA.
• Mitochondrial DNA sequences have, over time, moved to the nucleus.
• Mitochondria have various structures, including long elongated forms and bean shapes.
• Cristae, the invaginations in mitochondria, also differ in structure.
• The type of mitochondrial cristae is a homologous character.
• Some protozoans lack mitochondria and use a hydrogenasome for anaerobic cellular respiration.
• The kinetoplast, found in some protozoans, is a dense area of mitochondrial DNA located near the flagella and is derived from mitochondria.
• Plastids are organelles containing photosynthetic pigments, such as chloroplasts with chlorophyll.
• Pigment type can be homologous but is not always reliable due to secondary endosymbiosis.
• Many organisms have multiple chloroplasts.

Reproduction

• Asexual reproduction occurs through binary fission (resulting in two organisms) or multiple fission (resulting in more than two organisms).
• Budding is a type of binary fission where one larger cell and one smaller cell are produced.
• Mitosis in protozoans differs from that in animal cells, with variations in nucleus structure and spindle formation.
• Asexual reproduction results in genetically identical offspring (clones).
• Sexual reproduction allows for genetic recombination.
• Gametes from two haploid cells make haploid gametes that undergo fertilization.
• Some protozoans have a haploid adult phase, while others have a diploid phase or alternate between both.
• Meiosis can occur before or after fertilization, resulting in different adult phases.
• Syngamy involves gametes from two different cells, while autogammy involves the fusion of pro nuclei within the same organism.
• Conjugation is when two organisms come together physically to exchange genetic material.

Cyst Formation

• Some protozoans can form cysts, which are highly resistant and dormant cells.
• Cysts have a self-produced wall that protects against environmental dangers.
• Cysts protect against food deficiency, desiccation, changes in osmotic pressure, decreased oxygen concentrations, changes in pH, and temperature.
• Not all protozoans can form cysts.

Diplomonads

• Lack mitochondria but have mitochondrial genes in the nucleus.
• Giardia is a diplomonad that causes severe diarrhea (traveler's diarrhea).
• It lives in the digestive tract as a parasite and is transmitted via the fecal-oral route.

Euglenozoa

• Have a nuclei during mitosis and bicoid mitochondrial cristae.
• Euglenida possess chloroplasts and a red eye spot to sense light.
• They are phototrophic and can switch to heterotrophic means of getting their carbon in the dark.
• Kinetoplasta are parasites with a kinetoplast.
• Examples include trypanosoma brucei (African sleeping sickness), trypanosoma cruisy (chaga's disease), and leash mania (leish maniacism).

Ciliates

• Have cilia at some point in their life cycle.
• Paramecium is a ciliate with a cytostome (mouth area).
• They have two nuclei: a macronucleus (genes transcribed and translated) and a micronucleus (role in sexual reproduction).

Dinoflagellates

• Come in many shapes, sizes, and colors.
• Include autotrophic and pigmented dinoflagellates, as well as heterotrophic and colorless dinoflagellates.
• All dinoflagellates have two flagella and are important in marine environments.
• They are the zooxanthellae in coral reefs, essential for the survival of corals and anemones.
• Some dinoflagellates are bioluminescent.
• High concentrations of dinoflagellates can cause red tide, producing toxins harmful to wildlife and humans.

Apicomplexans

• Most are endoparasites with an apical complex that penetrates host tissues.
• Examples include toxoplasma gondii (toxoplasmosis) and plasmodia (malaria).

Viridiplantae

• "Plant-like" protozoans that can be unicellular, multicellular, or colonial.
• Volvox is an example of a colonial cell organization.
• Green algae is a protozoan found covering rocks in shallow waters.

Amoebozoas

• Include amoebas and plasmodial slime molds.
• Inter-amoeba hypolitica causes dysentery and abscesses.

Opisthokonta

• Includes choanoflagellates, which are sister taxa to animals.
• Similar to coanocytes in sponges.
• Studied to learn more about multicellularity and how it developed in animals.