Protists
Leeuwenhoek and Early Microscopy
Leeuwenhoek
Developed single lens brass microscopes capable of up to 288x magnification.
Corresponded extensively, writing 200+ letters to England's Royal Society.
Over 100 letters published despite initial skepticism about his findings.
Known as the “Father of microbiology.”
Historical Classification of Protists
Protista
Former kingdom comprising mostly unicellular eukaryotes.
Although the term protists is still in use, the formal classification has been abandoned.
Protists exhibit more structural and functional diversity than any other eukaryotic group.
Evolutionary relationships among protist groups remain unclear.
Eukaryotes
Most eukaryotes are single-celled organisms.
Protists, being eukaryotes, have organelles and are more complex than prokaryotes.
Single-celled protists are complex; all biological functions are executed by organelles within individual cells.
In multicellular organisms, certain biological activities are limited to specific tissue types, whereas unicellular organisms engage in all biological processes at the sub-cellular level.
Nutritional Diversity of Protists
Nutritional Modes of Protists
Protists are the most nutritionally diverse group of eukaryotes, including:
Photoautotrophs
Contain chloroplasts; resemble plants.
Heterotrophs
Absorb organic molecules or ingest larger food particles.
Can be:
Animal-like (ingestive).
Fungi-like (absorptive).
Mixtrophs
Combine photosynthesis and heterotrophic nutrition.
Endosymbiosis in Eukaryotic Evolution
Evidence supports that much protist diversity is rooted in endosymbiosis, defined as:
A process where a unicellular organism engulfs another cell, resulting in the engulfed cell becoming an endosymbiont that evolves into organelles in the host cell.
Mitochondria are believed to have evolved from an aerobic prokaryote (specifically, an alpha proteobacterium).
Plastids developed through the endosymbiosis of photosynthetic cyanobacteria.
Characteristics of plastids:
Plastids have a double membrane structure.
The DNA in plastids found in algae is akin to that of cyanobacteria.
Transport proteins of plastid membranes closely resemble those found in cyanobacteria.
Secondary endosymbiosis occurred when algae were ingested into food vacuoles.
Chlorarachniophytes:
The engulfed cell evolves into a plastid with a vestigial nucleus known as a nucleomorph, which shares a sequence similarity with that of green algae.
Protist Supergroups
Eukaryotic evolutionary relationships are rapidly evolving, and current hypotheses classify all eukaryotes (including protists) into 4 supergroups.
1. Excavates
Include protists featuring modified mitochondria and unique flagella.
Diplomonads and Parabasalids:
Lack plastids and have modified mitochondria; primarily thrive in anaerobic environments.
Diplomonads:
Possess mitosomes, which are modified chromosomes.
Derive energy from anaerobic biochemical pathways; feature two equal-sized nuclei and multiple flagella.
Commonly parasitic (example: Giardia, causing “Beaver Fever” with severe diarrhea, treatable with metronidazole).
Parabasalids:
Contain reduced mitochondria termed hydrogenosomes that generate energy anaerobically.
Example: Trichomonas vaginalis, a pathogen responsible for vaginitis in human females.
2. Euglenozoa
A diverse clade that encompasses predatory heterotrophs, photosynthetic autotrophs, and pathogenic parasites.
Distinguishing feature: a spiral or crystalline rod of unknown function located within their flagella.
A. Kinetoplastids
Characterized by a single mitochondrion containing an organized mass of DNA known as a kinetoplast.
Examples include Trypanosoma, which causes sleeping sickness in humans, transmitted by the tsetse fly.
Trypanosomes exhibit a “bait-and-switch” defense mechanism, termed “antigenic variation,” in which 1/3 of their genome encodes surface proteins, enabling them to switch between thousands of variants, thus evading the adaptive immune response.
B. Euglenids
Possess 1 or 2 flagella that emerge from a pocket at one end of the cell, with the capability to crawl through a shape-changing mechanism called metaboly.
Many are photosynthetic but can also perform heterotrophy, actively using light detectors to find optimal light intensity.
Some utilize both nutritional strategies, termed mixotrophic behavior, being photosynthetic during the day and heterotrophic at night.
3. SAR Group
A highly diversified collection of protists defined by DNA similarities.
A. Stramenopiles
Include significant phototrophs and numerous clades of heterotrophs characterized by having “hairy” flagellum paired with a “smooth” flagellum.
Important members include:
Diatoms:
Unicellular algae with a unique 2-part silica wall.
Primarily reproduce asexually, occasionally sexually; major components of phytoplankton.
Diatomaceous earth composed mainly of fossilized diatom walls.
When diatom populations bloom, the resultant dead individuals accumulate on the ocean floor, undecomposed, contributing to the biological carbon pump.
Biological Carbon Pump:
CO2 removal from the atmosphere over thousands of years, where dead phytoplankton act as carbon vessels as they sink to the ocean floor.
B. Iron Hypothesis of Ocean Gardening
John Martin proposed that fertilizing oceans with iron could boost carbon sequestration, potentially mitigating global warming effects.
Challenges:
Small amounts reach the ocean floor, indicating a slow process.
Potentially leads to anoxia in deep-sea environments (dead zones).
C. Diatoms: Taxonomy and Impact
Two major groups of diatoms: pennates (pen-shaped) and centric (cylinder-shaped).
Pseudo-Nitzchia: Produces domoic acid, a neurotoxin causing amnesic shellfish poisoning, linked to short-term memory loss, seizures, and death.
D. Brown Algae
The largest and most complex of algae, all being multicellular and primarily marine.
Includes various seaweeds; possesses the most complex anatomy of any algae, with a plant-like body structure called a thallus.
The reproductive cycle involves alternation of generations, characterized by multicellular haploid and diploid stages.
Diploid sporophyte resides below the low tide mark, with cells on blades developing into sporangia, releasing haploid flagellated spores (zoospores) that differentiate into male and female gametophytes.
4. Alveolates
Distinguished by membrane-bound sacs (alveoli) situated just beneath the plasma membrane.
A. Dinoflagellates
Characterized by armored cellulose plates and a flagellum that lies in a groove, leading to a unique spiraling motion.
They can cause red tides, which are toxic blooms of dinoflagellates.
Harmful Algal Blooms (HABs) are on the rise due to factors like nutrient runoff, pollution, and global warming, causing significant ecological impacts, including losses in tourism and food sources.
They form symbiotic relationships with corals, hosting photosynthetic dinoflagellates known as zooxanthellae; coral bleaching happens when zooxanthellae exit the coral.
B. Apicomplexans
A group of animal parasites, responsible for significant diseases in humans, such as malaria and toxoplasmosis.
Characterized by an apex containing organelles specialized for host penetration; lack photosynthetic plastids, exhibiting an apicoplast likely of red algal origin.
Malaria details:
Caused by Plasmodium falciparum.
The World Health Organization (WHO) reported 216 million cases and 655,000 deaths from malaria in 2010, predominantly affecting children.
Symptoms arise from asexual erythrocytic parasites, as they develop within red blood cells, leading to the release of toxic substances upon cell lysis and subsequent symptoms.
Malaria vector cycle:
Anopheles mosquito bites and injects sporozoites.
Sporozoites enter hepatocytes and develop into merozoites.
Merozoites invade Red Blood Cells (RBCs), undergoing reproduction every 48-72 hours before lysing the cells.
Gametocytes are formed and transferred to another mosquito upon biting, repeating the life cycle.
C. Ciliates
Uniquely utilize cilia for locomotion and feeding.
Conjugation: involves genetic variation through the exchange of haploid micronuclei between ciliates.
Results in the production of a new macronucleus through mitosis.
Binary fission results in four daughter cells.
Rhizarians
Defined by DNA similarities, encompassing various protist groups.
A. Radiolarians
Marine organisms with tests (shells) consisting of silica.
Engulf microorganisms through phagocytosis using pseudopodia that radiate from a central body.
B. Foraminiferans (Forams)
Characterized by porous, multi-chambered shells called tests, each test is typically a single piece of organic material hardened with calcium carbonate.
Pseudopodia extend through the pores in the test functioning for swimming and feeding, which can reach up to 1 cm in diameter, forming extensive fossil records in marine sediments.
Archaeplastidia
Includes red algae and green algae, which are the closest relatives to land plants.
Originated from a heterotrophic protist that acquired a cyanobacterial endosymbiont, evolving into red and green algae, with plants ultimately deriving from green algae.
A. Red Algae
Exhibit a color due to the accessory pigment phycoerythrin, varying from greenish-red in shallow water to nearly black in deeper waters, capable of absorbing blue and green light due to their pigmentation.
Typically multicellular and notably largest are the seaweeds abundant in tropical coastal waters.
Corralline algae have hardened calcium carbonate cell walls.
Example: Porphyra, a foliose red algae known as “Nori” used in sushi wrappers.
B. Green Algae
Named for their green chloroplasts; subdivided into two main groups: Chlorophytes and Charophytes.
Plants share a lineage with green algae.
Chlorophytes:
Display forms ranging from unicellular and colonial to multicellular.
Example: Chlamydomonas, which demonstrates the archaeplastidal context where chloroplasts possess two membranes, similar to those found in plants.
Habitat Variability:
Majority of Chlorophytes are found in freshwater; some reside in marine or terrestrial environments.
Example: Chlamydomonas nivalis, which develops a bright red carotenoid pigment, indicating melting ice and accelerated thawing.
C. Evolutionary Trends in Green Algae
Evolution towards larger and more complex forms exhibited through:
Formation of colonies or filamentous masses;
Example: Colonial structures forming hollow balls composed of numerous biflagellated cells embedded in a gelatinous matrix.
True multicellularity with cellular differentiation;
Edible chlorophytes develop multicellular thalli differentiated into leaf-like blades and root-like holdfasts.
Repeated nuclear division without cell division;
Example: Caulerpa, a green algae forming branched filaments that are multinucleate and exhibit a thallus that resembles a single large “supercell.”
Example: Caulerpa taxifolia, which has become an invasive species.