Comprehensive Notes: Eukaryotic Microbes, Endosymbiosis, and Early Evolution (Transcript Summary)

Endosymbiosis

A biological process where one cell engulfs another cell, and the engulfed cell persists within the host, often retaining its metabolic capabilities, leading to a symbiotic relationship. This process is a primary evolutionary driver in the origin and diversification of eukaryotes, explaining the presence of organelles like mitochondria and chloroplasts.

Primary Endosymbiosis

The foundational event where a eukaryotic host cell engulfed a free-living cyanobacterium, which subsequently evolved into a plastid (chloroplast), gaining the ability to perform photosynthesis. This crucial event gave rise to the ancestral red and green algal lineages.

Secondary Endosymbiosis

A subsequent evolutionary event where a eukaryotic host cell engulfed another eukaryote that already contained a plastid (which originated from primary endosymbiosis, such as a green or red alga). This led to a wider distribution of plastids in non-plant lineages and is often characterized by plastids with multiple membranes (typically three or four).

Aliciflorotica (green sea slug)

An exemplary organism demonstrating secondary endosymbiosis. This animal exhibits a unique capability to temporarily become photosynthetic. It consumes algae, digests most algal cells, but strategically retains their chloroplasts. These retained chloroplasts are sequestered in the interstitial spaces between the slug's cells and remain functional for a limited duration, providing photosynthetic products (via 'leakiness') to the slug, reducing its need for foraging.

Nitroplast

A recently discovered, specialized nitrogen-fixing organelle found within certain eukaryotic algal hosts (like Brutus virella). It originated from a cyanobacterial endosymbiont and enables the conversion of inorganic nitrogen into organic nitrogen for the host cell, complementing existing photosynthesis.

Brutus virella

An organism that possesses a nitroplast, a specialized organelle of cyanobacterial endosymbiotic origin that enables nitrogen fixation. The host alga simultaneously performs photosynthesis using its own chloroplasts.

Alpha-1,4 linked glucans

A type of polysaccharide used for energy storage in eukaryotes. Examples include starch, which is the primary storage polysaccharide in higher plants, predominantly composed of \text{α-1,4} linkages, with occasional \text{α-1,6} branches, and glycogen, which is the primary energy storage in animals.

Beta-1,3 linked glucans

A specific type of polysaccharide used for energy storage in certain eukaryotic lineages. An example is paramylon, which is the storage polysaccharide found in Euglena, distinctly differentiating it from starch-based storage in plants.

Green plastids

Plastids characterized by the presence of chlorophylls a and b. Their presence is a crucial trait for categorizing photosynthetic eukaryotes and indicates distinct evolutionary lineages.

Red plastids

Plastids that contain chlorophyll a along with red accessory pigments known as phycobilins. These pigments aid in capturing light energy during photosynthesis in red algae and certain secondary endosymbionts.

Brown plastids

Plastids thought to have descended from red plastids. They contain chlorophyll a and chlorophyll c, in addition to various brown carotenoids, indicative of specific evolutionary lineages within photosynthetic eukaryotes.

Excavata (Excavates)

A major supergroup of eukaryotic microbes, particularly highlighted in the lecture. This group includes important subgroups like Euglenozoa, which encompasses Euglenids (e.g., Euglena) and closely related organisms. They are distinguished by unique morphological and genetic characteristics.

Euglena

A representative organism from the Euglenozoa group, commonly found in freshwater. Key features include: storing energy as paramylon ( \text{\beta-1,3} linked glucan); possessing green plastids acquired through secondary endosymbiosis with a green algal cell, containing chlorophyll a and b; having a flexible outer pellicle composed of interlocking protein strips instead of a rigid cell wall, allowing for shape changes; typically having two flagella with the canonical eukaryotic 9+2 microtubule arrangement, one of which features a crystalline structural rod; an ISpot (eyespot/stigma) for phototaxis acting as a shading mechanism; and a contractile vacuole to actively expel excess water (osmoregulation) in freshwater environments.

Pellicle

A flexible outer covering composed of interlocking protein strips that replaces a rigid cell wall in certain eukaryotes like Euglena. This unique structure allows for cell shape changes and metabolic movement, providing structural support without limiting flexibility.

Contractile Vacuole

An organelle found in protists, particularly those in freshwater environments like Euglena. Its critical function is to actively expel excess water that diffuses into the cell due to osmotic pressure, thereby preventing cell lysis and maintaining osmotic balance ('sump pump' function).

ISpot (Eyespot/Stigma)

A photosensitive organelle in certain unicellular organisms, such as Euglena, critical for phototaxis (movement towards or away from light). It functions primarily as a shading mechanism around photoreceptor proteins embedded in a membrane, enabling effective light detection and orientation, rather than being the photoreceptor itself.

Kinetoplastids

A group within the Euglenozoa defined by the presence of a kinetoplast. This group includes important parasitic genera like Trypanosoma, known for causing significant diseases in humans and animals.

Kinetoplast

A defining characteristic of kinetoplastids; it is a dense, disc-like mass of circular mitochondrial DNA found within the single, large mitochondrion. This structure is typically electron-dense and plays a crucial role in the biology of parasitic kinetoplastids.

Trypanosoma brucei

The causative agent of sleeping sickness (African trypanosomiasis). This parasitic flagellate is transmitted to humans by the bite of the tsetse fly. The disease affects the blood and eventually the central nervous system, leading to severe lethargy and neurological symptoms.

Trypanosoma cruzi

The causative agent of American trypanosomiasis (Chagas disease). This parasitic flagellate is primarily transmitted to humans by the triatomine bug, commonly known as the 'kissing bug' or reduviid bug, which typically feeds at night.

Sleeping Sickness (African Trypanosomiasis)

A serious parasitic disease caused by Trypanosoma brucei and transmitted by the tsetse fly. It leads to severe lethargy, neurological symptoms, and can be fatal if untreated, affecting the host's blood and brain.

Chagas Disease (American Trypanosomiasis)

A potentially life-threatening illness caused by the parasite Trypanosoma cruzi. It is primarily transmitted by the triatomine bug (kissing bug) and can lead to chronic cardiac and gastrointestinal complications if left untreated. Charles Darwin is speculated to have suffered from this disease.

Apicoplasts

Non-photosynthetic plastids that have been retained by Apicomplexans, even though these organisms have lost their photosynthetic function. These vestigial plastids still perform vital non-photosynthetic metabolic