Zooxanthellae are microscopic photosynthetic algae that live inside coral tissue in a symbiotic relationship, feeding the coral with sugars from photosynthesis. When ocean warming stresses the coral, it expels the algae and turns white, which is coral bleaching (EK STB-4.G.3).
Zooxanthellae are tiny photosynthetic algae that live inside the tissue of coral polyps. Think of them as solar panels with a lease: the coral gives them shelter and protection, and the algae pay rent in food. Through photosynthesis, zooxanthellae make sugars and pass most of that energy to the coral. That's the deal that keeps a reef alive, and it's a textbook case of symbiosis.
These algae are also why healthy coral looks brown or greenish-brown. The color you see isn't the coral itself, it's the pigment of millions of zooxanthellae living in it. So when the algae leave, the coral's white skeleton shows through. That's the direct link the CED draws in EK STB-4.G.3: ocean warming causes corals to lose their algae, which makes them bleach white. Some corals recover if the algae return; some starve and die without their food source.
Zooxanthellae live in Unit 9: Global Change, specifically topic 9.6 Ocean Warming. They're the mechanism behind learning objective AP Enviro 9.6.A, where you explain the causes and effects of ocean warming. EK STB-4.G.1 says ocean warming comes from rising greenhouse gases. EK STB-4.G.2 says warming hurts marine species through habitat loss and metabolic changes. EK STB-4.G.3 ties it together: warming triggers coral bleaching when corals lose these algae. Knowing zooxanthellae is how you move from a cause (more CO2, warmer water) to an effect (dead reefs) in a single clear chain, which is exactly the kind of cause-and-effect reasoning Unit 9 rewards.
Keep studying AP Environmental Science Unit 9
Coral Bleaching (Unit 9)
Bleaching is what happens when zooxanthellae leave. Heat-stressed coral expels its algae, loses both its color and its food supply, and turns white. Coral bleaching is the effect; losing zooxanthellae is the cause.
Symbiosis (Units 2, 9)
The coral-zooxanthellae relationship is the classic mutualism: both partners benefit. The coral gets sugars and the algae get a protected, sunlit home. Ocean warming is essentially a symbiosis that breaks under stress.
Photosynthesis (Units 1, 6)
Zooxanthellae feed coral by photosynthesizing, turning sunlight and CO2 into sugars. This is the same process that powers food webs everywhere, just happening inside an animal instead of in a leaf.
Ocean Warming and Greenhouse Gases (Unit 9)
EK STB-4.G.1 traces warming back to rising greenhouse gases. That connects bleaching all the way up to fossil fuel combustion and the carbon cycle, so zooxanthellae become the visible end of a much longer chain you can argue in an FRQ.
On multiple-choice questions, expect a scenario like a marine biologist seeing water rise 2°C while coral loses its zooxanthellae and turns white, then asking you to name the phenomenon (coral bleaching). Other stems ask which physiological mechanism explains why warming causes bleaching, and the answer hinges on the coral expelling its symbiotic algae. On free response, you'd use zooxanthellae to explain the cause-and-effect chain for ocean warming under AP Enviro 9.6.A: greenhouse gases warm the water, heat stress makes coral expel its zooxanthellae, the coral loses its food and bleaches. Be ready to note that some corals recover and some die, since the CED makes that distinction explicit.
Zooxanthellae are the organisms (the algae living in coral). Coral bleaching is the event that happens when those algae are lost and the coral turns white. Don't write that zooxanthellae 'are' bleaching; zooxanthellae loss causes bleaching.
Zooxanthellae are photosynthetic algae that live inside coral and feed it sugars through photosynthesis, a mutualistic symbiosis.
Healthy coral looks brown because of its zooxanthellae; when they leave, the white skeleton shows through and the coral has bleached.
Ocean warming, driven by greenhouse gases (EK STB-4.G.1), stresses coral into expelling its zooxanthellae, which causes coral bleaching (EK STB-4.G.3).
After bleaching, some corals recover if the algae return and some die from losing their food source, a distinction the CED specifically makes.
On the exam, use zooxanthellae to build the full cause-and-effect chain for ocean warming under learning objective AP Enviro 9.6.A.
They're microscopic photosynthetic algae that live inside coral tissue in a symbiotic relationship. They photosynthesize and pass most of the resulting sugars to the coral, which is why a healthy reef looks brown and stays alive.
No. Zooxanthellae are the algae; coral bleaching is the event that happens when coral loses those algae and turns white. Losing zooxanthellae causes bleaching, so they're cause and effect, not the same thing.
The brown-green color of coral comes from the pigment in its zooxanthellae, not the coral itself. When heat stress makes the coral expel the algae, only the transparent tissue and white calcium-carbonate skeleton are left, so the coral appears bleached.
Rising greenhouse gases warm the ocean (EK STB-4.G.1), and elevated temperatures stress the coral. As a stress response, the coral expels its zooxanthellae, losing both its color and its main food source, which is coral bleaching (EK STB-4.G.3).
No. The CED is clear that some corals recover if conditions improve and the zooxanthellae return, while others die because they can't survive long without the energy the algae provide.