What mutualistic microscopic algae live inside the gastrodermal tissues of reef-building stony corals, providing corals with photosynthetic energy in exchange for shelter and metabolic nutrients?
Answer and explanation
Correct answer: D. Zooxanthellae
- Step 1: Reef-building hermatypic corals thrive in warm, shallow, nutrient-poor tropical waters through an obligate endosymbiotic relationship.
- Step 2: Zooxanthellae (photosynthetic dinoflagellates of the family Symbiodiniaceae) translocate up to 90% of their photosynthetically synthesized organic carbon to the coral host; thermal stress causes corals to expel them, leading to 'coral bleaching'.
Answer
Zooxanthellae
Why not the others: 'Cyanobacteria' Cyanobacteria are free-living photosynthetic blue-green bacteria capable of nitrogen fixation, not the primary coral endosymbionts; 'Diatoms' Diatoms are unicellular algae with silica cell walls (frustules) that constitute phytoplankton in aquatic environments; 'Phytoplankton' Phytoplankton is an overarching broad ecological category of drifting aquatic autotrophs, not the specific endosymbiont inside coral polyps.
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