Coral bleaching is when corals lose the symbiotic algae living in their tissues, usually because of warmer ocean water, causing them to turn white. Some corals recover; others die if the stress continues.
Coral bleaching is what happens when corals get stressed and kick out the algae that live inside them. Corals and these algae (called zooxanthellae) have a symbiotic relationship: the algae photosynthesize and feed the coral, and they also give coral its color. When water gets too warm, the coral expels the algae. Without them, the coral loses both its color and its main food source, so it turns bone white. That white look is the "bleaching."
Per EK STB-4.G.3, ocean warming is the main driver, and the outcome isn't always death. Some corals recover if the water cools back down quickly. But if the heat stress drags on, the coral starves and dies. Since algae are the major photosynthetic organisms in aquatic biomes (EK ERT-1.C.3), losing them is a big deal for the whole reef.
Coral bleaching shows up in two different units, which is exactly why it's worth knowing well. In Unit 9 (Global Change), it's a direct effect of ocean warming under learning objective AP Enviro 9.6.A, where you explain the causes and effects. In Unit 1 (The Living World), it connects to topic 1.3 on aquatic biomes, since coral reefs are a marine biome (EK ERT-1.C.2). The big-picture link: greenhouse gases warm the atmosphere, the atmosphere warms the ocean (EK STB-4.G.1), and warmer water bleaches coral. That chain ties human activity to ecosystem collapse, which is the kind of cause-and-effect reasoning the exam loves.
Keep studying AP Environmental Science Unit 1
Symbiotic Relationship (Unit 1)
Bleaching is basically a broken symbiosis. The coral and its algae normally trade food and shelter, so once the coral expels the algae, that partnership ends and both sides lose. Understanding the symbiosis is what makes bleaching make sense.
Ocean Warming (Unit 9)
Warming is the cause; bleaching is the effect. Rising greenhouse gases heat the ocean (EK STB-4.G.1), and that extra heat is the single biggest trigger for mass bleaching events.
Ocean Acidification (Unit 9)
Acidification and bleaching are two separate problems hitting the same reefs. Bleaching comes from heat stress, while acidification comes from the ocean absorbing CO2, which weakens the calcium carbonate corals build with. Together they're a double hit.
Coral Reef Ecosystem (Unit 1)
When coral dies from bleaching, the species that depend on the reef for food and shelter lose their habitat. A bleaching event doesn't just hurt the coral; it ripples out to the whole community living there.
On multiple choice, expect stems that test the mechanism and the consequences. One common angle asks why rising ocean temperatures cause bleaching, where the right answer points to corals expelling their symbiotic algae. Another asks what happens immediately after a mass bleaching event, where you'd connect dead coral to lost habitat and declining reef biodiversity. You may also see bleaching listed as a direct consequence of ocean warming. For FRQs tied to objective 9.6.A, you should be able to explain the full chain from greenhouse gases to warmer water to bleaching, and note that some corals recover while others die. Don't just say "the coral turns white"; explain why by naming the loss of algae.
These get mixed up because both threaten coral reefs and both trace back to greenhouse gases, but the mechanism is different. Bleaching is a heat problem: warm water makes coral expel its algae. Acidification is a chemistry problem: the ocean absorbs CO2, becomes more acidic, and makes it harder for corals to build their skeletons. On the exam, match the cause to the right process.
Coral bleaching is when stressed corals expel their symbiotic algae (zooxanthellae) and turn white, losing both their color and their main food source.
Warmer ocean water is the main trigger, and ocean warming is driven by rising greenhouse gases in the atmosphere (EK STB-4.G.1 and STB-4.G.3).
Bleaching doesn't always mean death; some corals recover if the water cools quickly, but prolonged stress kills them.
A bleaching event harms the whole reef ecosystem because so many species depend on living coral for habitat and food.
Bleaching is a heat-driven effect, while ocean acidification is a separate CO2-driven chemistry problem, even though both threaten reefs.
It's when corals lose the algae living in their tissues, usually because of warmer water, causing them to turn white. Per EK STB-4.G.3, it's a direct effect of ocean warming, and some corals recover while others die.
No. The coral can recover if the water cools back down quickly enough. But if the heat stress lasts too long, the coral starves without its algae and dies, which is why prolonged warming is so dangerous.
Bleaching is caused by heat stress that makes coral expel its algae and turn white. Ocean acidification is caused by the ocean absorbing CO2, which lowers pH and makes it harder for corals to build their skeletons. Different cause, different mechanism, same victim.
Heat stress breaks the symbiotic relationship between the coral and its algae, so the coral expels the algae living in its tissues. Without those algae the coral loses its color (turning white) and its main source of food.
It appears in both Unit 9 (Global Change) under ocean warming, learning objective 9.6.A, and Unit 1 (The Living World) under aquatic biomes in topic 1.3, since coral reefs are a marine biome.