Key Terms & Concepts — UPSC Mains
Zooxanthellae
"Microscopic symbiotic dinoflagellate algae living within coral tissue that provide up to 90% of the coral's energy needs through photosynthesis"
Zooxanthellae (singular: zooxanthella) are microscopic photosynthetic dinoflagellate algae (genus Symbiodinium and related genera) that live symbiotically within the cells of reef-building corals and other marine invertebrates (giant clams, sea anemones, jellyfish). The relationship is mutually beneficial: zooxanthellae receive shelter and nutrients (carbon dioxide, nitrogen) from the coral host, while providing the coral with up to 70–90% of its organic energy through photosynthesis — primarily glucose, glycerol, and amino acids. The photosynthetic activity of zooxanthellae is also responsible for the intense colours of healthy corals. When ocean temperatures rise more than 1°C above the seasonal maximum for an extended period, corals expel their zooxanthellae in a stress response called coral bleaching. Without zooxanthellae, corals appear stark white, lose their primary energy source, and face starvation — they can recover if temperatures normalise quickly, but prolonged bleaching leads to coral death.
Zooxanthellae are foundational to understanding coral reef ecology — a high-priority GS3 (environment) topic. The zooxanthellae–coral symbiosis explains why coral reefs occupy only 0.1% of the ocean floor yet support ~25% of all marine species. For UPSC, this connects to coral bleaching, climate change impacts on marine ecosystems, Great Barrier Reef, Lakshadweep reef crises, and India's marine biodiversity conservation efforts. The term is also used in environment essay context — reef destruction as a climate indicator.
- 1 Microscopic dinoflagellate algae (genus Symbiodinium) living within coral tissue
- 2 Provide 70–90% of coral's energy needs via photosynthesis
- 3 Give corals their characteristic colours — brown, gold, green hues
- 4 Relationship is mutualistic — algae get shelter and CO₂; coral gets photosynthates (glucose, glycerol)
- 5 When expelled under thermal stress → coral bleaching (white appearance)
- 6 [object Object]
- 7 Corals can survive bleaching if temperatures recover within ~4–6 weeks; prolonged bleaching = mortality
- 8 Also found in giant clams, sea anemones, and some jellyfish species
During the 2016 mass coral bleaching event in Lakshadweep — triggered by sustained warming in the Arabian Sea — over 60% of reefs in the archipelago were bleached as corals expelled their zooxanthellae under thermal stress. Recovery was partial, but the event marked the most severe bleaching in the region's recorded history, illustrating how just 1°C of warming can cascade into an ecosystem collapse.
