Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 28.4 Red Algae and Green Algae Are the Closest Relatives of Plants
The red algae and green algae—two major protist lineages—share a common ancestor with plants and together form the monophyletic supergroup Archaeplastida. These lineages are crucial to understanding the origin of photosynthetic eukaryotes and the evolution of land plants.
Red Algae (Rhodophytes)
- Over 6,000 species, mostly multicellular and abundant in warm, tropical oceans.
- Color comes from phycoerythrin, a pigment allowing photosynthesis at greater depths:
- Shallow-water species: greenish-red
- Deep-water species: nearly black
- Some are heterotrophic parasites that lack pigmentation.
- Examples:
- Porphyra ("nori"): edible seaweed used in sushi
- Palmaria palmata (dulse): leafy red alga
- Lack flagellated gametes; rely on water currents for fertilization.
- Exhibit alternation of generations, with diverse reproductive cycles.
Green Algae
- Have chloroplasts similar in structure and pigment to those in land plants.
- With land plants, they form the group Viridiplantae.
- Divided into:
- Charophytes – Closest relatives of land plants
- Chlorophytes – Over 7,000 species; mostly freshwater, also marine and terrestrial
- Structural diversity:
- Unicellular (e.g., Chlamydomonas)
- Colonial (e.g., Pediastrum)
- Multicellular (e.g., Ulva, sea lettuce)
- Mechanisms for complexity:
- Formation of colonies (e.g., pond scum)
- Development of true multicellular bodies (e.g., Ulva)
- Multiple nuclear divisions without cytokinesis forming "supercells" (e.g., Caulerpa)
- Most reproduce asexually and sexually with biflagellated gametes.
- Some show alternation of generations, like land plants.
Archaeplastida: A Monophyletic Group
- Red algae, green algae, and plants descend from a common ancestor that engulfed a cyanobacterium.
- This primary endosymbiosis event produced plastids and gave rise to photosynthetic eukaryotes.
- Red and green algae represent early evolutionary branches leading to plants.
In a Nutshell
Red and green algae are the closest algal relatives of plants. Red algae are multicellular, pigment-rich marine organisms, while green algae show extensive diversity and structural complexity. Both lineages descend from a shared ancestor with land plants, forming the supergroup Archaeplastida—united by a common origin of photosynthesis through primary endosymbiosis.