Green algae evolved through a process called endosymbiosis, where a eukaryotic cell engulfed a prokaryotic cell that could perform photosynthesis, ultimately leading to the development of plastids (chloroplasts) within green algae.
Here's a breakdown of the evolutionary process:
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The Primary Endosymbiotic Event: A heterotrophic (non-photosynthetic) eukaryotic cell engulfed a cyanobacterium (a type of photosynthetic bacteria). Instead of digesting the cyanobacterium, the eukaryotic cell established a symbiotic relationship with it.
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Development of Plastids: Over time, the engulfed cyanobacterium evolved into a plastid, specifically a chloroplast, which is the organelle responsible for photosynthesis in green algae and plants. This involved significant genetic transfer from the cyanobacterium to the host cell's nucleus.
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Evolution of Green Algae Lineages: The initial endosymbiotic event gave rise to the Archaeplastida lineage, which includes green algae (Chlorophyta and Charophyta), red algae (Rhodophyta), and land plants (Embryophyta). Different lineages of green algae then diverged, leading to the diversity of green algae we see today.
Key Evidence Supporting Endosymbiosis:
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Double Membranes: Chloroplasts have two membranes, the inner membrane likely originating from the cyanobacterium's plasma membrane, and the outer membrane likely derived from the host cell's membrane during the engulfment process.
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Chloroplast DNA: Chloroplasts contain their own DNA, which is circular and similar to bacterial DNA.
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Ribosomes: Chloroplast ribosomes are more similar to bacterial ribosomes than to eukaryotic ribosomes.
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Genetic Similarity: Genetic analyses reveal a strong relationship between chloroplast genes and cyanobacterial genes (Delwiche, 1999; Delwiche et al., 2004).
In essence, the evolution of green algae involved a symbiotic partnership between a eukaryotic cell and a photosynthetic prokaryote, resulting in the acquisition of chloroplasts and the ability to perform photosynthesis. This event was crucial for the evolution of all green plants and algae.
