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How does endosymbiosis explain the origin of algal chloroplasts?
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Algae
The macroscopic genus of algae known as Acetabularia is commonly called “mermaid's wine glass” because of the distinctive umbrella-like shape of the tips of its stalks.
The endosymbiotic theory explains the origin of algal chloroplasts as the result of a process where a non-photosynthetic eukaryotic host cell engulfed a photosynthetic prokaryote, specifically a cyanobacterium. Over evolutionary time, this internalized organism underwent genome reduction and became integrated into the host cell as an organelle, providing the host with the ability to perform photosynthesis.123
This process is categorized into two types:
- Primary Endosymbiosis: This initial event involved the engulfment of a free-living cyanobacterium by a eukaryotic cell, leading to the formation of chloroplasts with a double membrane. This is considered the origin of the photosynthetic apparatus in red algae and other lineages.13
- Secondary Endosymbiosis: This involves a non-photosynthetic eukaryotic host engulfing a primary alga that already possessed chloroplasts. This secondary event resulted in plastids surrounded by three or four membranes, as seen in groups such as diatoms, dinoflagellates, and euglenoid algae.41
Evidence for this theory includes the fact that chloroplasts contain their own circular DNA, replicate independently of the cell nucleus, and possess membranes that resemble those of prokaryotes.1