When ocean temperatures rise even slightly above a coral’s normal summer maximum, the microscopic algae living inside coral tissue begin to malfunction. The coral, unable to tolerate the toxic byproducts, expels these algae and turns ghostly white. This process, coral bleaching, has affected roughly 84% of the world’s reef area since January 2023, making it the most widespread bleaching event ever recorded.
The Partnership That Keeps Coral Alive
Coral animals are not self-sufficient. Each coral polyp hosts millions of single-celled algae (called zooxanthellae) inside its own tissue. These algae photosynthesize, converting sunlight into sugars and other nutrients that they pass directly to the coral. This energy transfer covers most of the coral’s nutritional needs, which is why reef-building corals thrive in the nutrient-poor tropical waters where they’re found. In return, the algae get a protected home and access to compounds they need to grow.
The algae are also what give coral its color. A healthy coral’s browns, greens, and purples all come from pigments in the algae, not from the coral animal itself. When those algae disappear, the transparent coral tissue reveals the white limestone skeleton underneath.
What Heat Does to the Algae’s Machinery
The algae inside coral are tiny solar-powered factories, and heat damages their equipment. Specifically, elevated temperatures interfere with photosystem II, a key component of the photosynthetic process. Under normal conditions, this system constantly sustains minor damage from light exposure and constantly repairs itself. The two rates stay in balance. But when water temperature climbs past about 31 to 32°C, the repair process stalls while damage continues at its usual pace. The result is a net breakdown of the photosynthetic machinery.
Research published in PNAS pinpointed why: heat damages the internal membranes of the algae’s chloroplasts, making them leaky. This disrupts the chemical gradients the cell needs to build new repair proteins. It also suppresses the production of those proteins directly. So the algae can’t fix their solar panels even as sunlight keeps degrading them, creating a vicious cycle of worsening dysfunction.
From Damaged Algae to Bleaching
As the algae’s photosynthetic machinery breaks down, it starts producing harmful molecules called reactive oxygen species. These are essentially molecular shrapnel, unstable oxygen compounds that damage proteins, membranes, and DNA. Under heat stress, the algae’s production of these molecules jumps by roughly 69% compared to normal conditions.
The prevailing theory for decades was that these toxic molecules leak out of the algae and poison the coral host, forcing it to eject the algae in self-defense. More recent single-cell research has complicated this picture. A study in The ISME Journal found that in the early stages of heat stress, before severe photosynthetic damage occurs, the toxic molecules don’t necessarily leak into the host tissue in damaging quantities. This suggests the coral may begin expelling algae through other stress-sensing pathways before outright poisoning occurs.
Regardless of whether the initial trigger is toxic leakage or another cellular alarm, the outcome is the same. The coral ejects its algae, losing both its color and its primary energy source. By expelling the algae, the coral removes the source of potential chemical damage, but it also cuts off its own food supply.
How Little Warming It Takes
Coral bleaching doesn’t require dramatic temperature spikes. NOAA’s Coral Reef Watch tracks thermal stress using a metric called Degree Heating Weeks, which accumulates every day that sea surface temperature exceeds 1°C above a location’s normal summer maximum over a rolling 12-week window. When that accumulated stress reaches 4°C-weeks, substantial bleaching typically begins. At 8°C-weeks, severe bleaching with significant coral death becomes likely.
To put that in perspective, a reef that sits just 1.5°C above its normal peak for about three weeks can cross the threshold into bleaching territory. These are not extreme temperatures in absolute terms. They’re extreme only relative to what corals at that location have adapted to over centuries. A reef in the Caribbean might bleach at 29.5°C while a reef in the Persian Gulf tolerates 34°C, because each population is tuned to its own local baseline.
Global conditions have made these thresholds easier to cross. In 2024, global surface temperature was 1.29°C above the 20th-century average, the highest on record, beating 2023 by 0.10°C. That background warming means every seasonal temperature peak starts from a higher baseline, making bleaching-level anomalies more frequent and more severe.
What Happens to a Bleached Coral
Once bleached, a coral is alive but starving. The algae that supplied most of its energy are gone. The coral can still capture tiny food particles from the water using its tentacles, but this heterotrophic feeding provides only a fraction of what it needs. Bleached corals become more vulnerable to disease, stop growing, and cease reproducing.
If water temperatures drop back to normal relatively quickly, corals can reabsorb algae from the surrounding water and recover. A two-year study tracking three Caribbean coral species found that two of them fully recovered their algal populations, energy reserves, and growth rates within about a year after a bleaching event. The third species, however, could not restore its protein and carbohydrate stores in that timeframe, suffering cumulative damage that weakened its ability to survive future stress.
This uneven recovery is critical. When bleaching events happen year after year, even resilient species may not have enough time between episodes to fully rebuild their energy reserves. Species that recover slowly face compounding damage, and eventually die. Over time, this selective pressure reshapes entire reef communities, favoring fast-recovering species over slower ones and reducing the diversity that makes reefs ecologically productive.
Why Some Corals Handle Heat Better
Not all coral-algae partnerships are equally vulnerable. Some algal species are substantially more heat-tolerant than others, and the coral colonies that host them gain a real survival advantage. In the eastern tropical Pacific, researchers documented this playing out in real time during the 2015-2016 marine heatwave. One lineage of Pocillopora coral increasingly partnered with a heat-tolerant algal species called Durusdinium glynnii during the stress event. That lineage experienced lower bleaching and mortality compared to a closely related lineage that didn’t acquire the tougher algae.
The heat-tolerant algae raise the bleaching threshold by about 1.5°C, a significant buffer. Modeling suggests that reefs dominated by corals hosting these symbionts could maintain high cover through warming trends continuing into the 2060s or 2070s. After that point, even the tougher partnerships would face unsustainable annual bleaching. This natural adaptation buys time, but it doesn’t eliminate the problem if ocean temperatures keep climbing.
What Reefs Stand to Lose
Coral reefs support roughly 25% of all marine species despite covering less than 1% of the ocean floor. The cascading effects of widespread bleaching ripple through entire food webs, from the small fish that shelter in reef structures to the larger predators that depend on them.
The economic stakes are enormous. In the United States alone, coral reef services are valued at over $3.4 billion annually, including $1.8 billion in flood protection from wave energy absorption and $200 million in commercial and recreational fisheries. Globally, hundreds of millions of people in coastal communities depend on reef fisheries for protein and income.
Since January 2023, bleaching-level heat stress has been documented across reefs in at least 83 countries and territories, touching every major reef region on Earth. The current global bleaching event, the fourth on record, has affected approximately 84.4% of the world’s reef area. Each successive event narrows the window corals have to recover before the next one hits, making the relationship between ocean warming and bleaching not just a biological curiosity but one of the most consequential ecological crises of this century.