2.4.3: Photosynthetic "Protists"

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Page ID: 31926

Melissa Ha, Maria Morrow, & Kammy Algiers
Yuba College, College of the Redwoods, & Ventura College via ASCCC Open Educational Resources Initiative

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Photosynthetic lineages can be found across the phylogenetic tree of protists. Most of the groups are unrelated to each other, as each major lineage acquired its chloroplast (and therefore the ability to photosynthesize) from different endosymbiotic events. For example, red and green algae belong to the same lineage (Archaeplastida). A red algal ancestor engulfed a cyanobacterial ancestor, leading to the first chloroplasts. These were inherited vertically--through reproduction and over evolutionary time--by the lineages of green algae. However, brown algae and diatoms inherited their chloroplasts from a secondary endosymbiotic event where their heterotrophic ancestor (something like an oomycete) engulfed a red alga. This is a horizontal transmission of chloroplasts between lineages, where genes (and here an organelle!) move between organisms through the environment, not through reproduction.

Endosymbiosis

A small green cell is engulfed by a larger blue cell with a nucleus, eventually becoming a chloroplast. — Figure \(\PageIndex{1}\): In primary endosymbiosis, a photosynthetic prokaryote (the free cyanobacterium) is engulfed by a heterotrophic eukaryote. After it is engulfed, the prokaryote is surrounded by a phagosomal membrane. The cyanobacterium is shown to have a peptidoglycan cell wall encompassed by two cell membranes. Eventually, genes are transferred between the cyanobacterium and eukaryote genomes. The phagosomal membrane disappears and we are left with a double-membraned chloroplast. Image by Kelvinsong, CC BY-SA 3.0, via Wikimedia Commons

A cell with a chloroplast and nucleus is engulfed by another eukaryotic cell, becoming a chloroplast with 3 membranes. — Figure \(\PageIndex{2}\): In secondary endosymbiosis, it is a photosynthetic eukaryote that is engulfed. After it is engulfed, it is surrounded by four membranes: the outer is the phagosomal membrane (epiplastid membrane), inside that is the cell membrane of the photosynthetic eukaryote that was engulfed (periplastid membrane), and inside that are the two membranes from the original chloroplast. In some secondary endosymbioses, the periplastid membrane is lost, resulting in a triple-membraned chloroplast. Image by Kelvinsong, CC BY-SA 3.0, via Wikimedia Commons

The horizontal transmission of chloroplasts between lineages via endosymbiosis resulted in several unrelated groups of photosynthetic protists. The lineages covered in this section are:

Dinoflagellates (secondary endosymbiosis)
Brown algae and diatoms (secondary endosymbiosis)
Red and green algae (primary endosymbiosis)

Attribution

Content by Maria Morrow, CC-BY-NC