Core Identity: Algae are thalloid, non-vascular, primarily aquatic, photosynthetic eukaryotes. Classification based on pigment composition, storage product, cell wall chemistry, and habitat.
Chlorophyceae (Green Algae) — Key Points:
- Pigments: Chlorophyll a + b (same second Chl as land plants → evolutionary link)
- Storage: Starch (same as land plants)
- Cell wall: Cellulose only
- Habitat: Freshwater (mainly); some marine
- Sexual reproduction: Isogamy / Anisogamy / Oogamy
- Examples: Chlamydomonas (unicellular, biflagellate), Volvox (colonial), Spirogyra (spiral Chl, conjugation), Ulothrix (unbranched filament, girdle Chl), Chara (calcified, most advanced)
Phaeophyceae (Brown Algae) — Key Points:
- Pigments: Chlorophyll a + c + Fucoxanthin (gives brown colour)
- Storage: Laminarin
- Cell wall: Cellulose + Algin (commercially important)
- Habitat: Marine, cold waters (intertidal and subtidal)
- Commercial product: Algin (from Laminaria, Sargassum) — food thickener, pharmaceutical stabiliser
- Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus
Rhodophyceae (Red Algae) — Key Points:
- Pigments: Chlorophyll a + d + Phycoerythrin (red; absorbs blue-green light → deep ocean adaptation)
- Storage: Floridean starch (branched, like amylopectin, stored outside chloroplast)
- Cell wall: Cellulose
- Habitat: Marine, deep waters
- Commercial product: AGAR (from Gelidium and Gracilaria) — microbiology culture medium, food
- Examples: Polysiphonia, Porphyra (nori food), Gelidium, Gracilaria
NEET Non-Negotiables:
- Agar = RED algae ONLY (NOT brown algae — this is the #1 trap)
- Fucoxanthin is EXCLUSIVE to brown algae
- Phycoerythrin enables red algae to live at depth (blue-green light absorption)
- All three classes share Chlorophyll a — it is the primary photosynthetic pigment
- Algin (brown) vs Agar (red) — always be clear on this distinction