Simply Science, Part II: Coral Bleaching

By Stephanie Wolek

Healthy coral reef. Source: Pixabay under a CC license.

We’ve all heard about the issues with our planet’s coral reefs—they’re being damaged by pollution, climate change, habitat loss, and a seemingly endless list of other human-driven factors. It’s easy to become discouraged when hearing about the latest coral losses and some articles have gone so far as to (mistakenly!) declare reefs dead. There’s good news and bad news. The bad news is our coral reefs are in sharp decline but the good news is that they’re not dead and can still be saved!

When articles mention coral declines, they’re usually talking about “coral bleaching” and “massive bleaching events” accompanied by scary images of ghostly reefs that lack even a semblance of life. The coral are white or gray and appear dead. Luckily, this isn’t necessarily true. We’ll take a look at what coral bleaching really means and how it affects coral.

Bleached coral. Source: unknown author, found on Wikipedia under a CC license.

Coral are actually part of a team and rely on another living being to survive—zooxanthellae. Don’t worry, the name isn’t important! Zooxanthellae are algae that are friendly to coral; they’ve actually built a partnership. The algae live in the coral, benefitting from the protection and giving the coral those bright colors we’ve seen and admired. As the algae use photosynthesis to convert light into food, sugars and other by-products from the process are passed on to the coral. Meanwhile, the coral consumes those products and releases its own by-products—which the algae then use in photosynthesis. If the environment is healthy, this cycle can continue for many years.

Problems occur when the environment isn’t healthy. Zooxanthellae are sensitive to water conditions, especially temperature changes, and the coral will ditch the damaged algae when things get too stressful. The coral may begin to starve and will also lose its color because the algae were responsible for the coloration in the first place. This gives the appearance of a “dead” coral. If conditions improve within a reasonable amount of time, however, the zooxanthellae can actually return—allowing the coral to bounce back and eventually regain full health. So, while bleaching events are a big deal, they’re not always a death sentence for the coral or reef in question.

This coral is only partially bleached and could easily recover if environmental conditions improve. Credit: Samuel Chow.

Our reefs are definitely in trouble but beware of articles that claim a reef is dead simply because it has experienced a bleaching event. Bleached reefs can and do recover as long as the environment allows it. While the situation looks grim, coral bleaching on its own does not kill a reef.