By Lulu Allen-Waller
This is a guest post from contributing author Lulu Allen-Waller. If you are interested in contributing an entry, please contact PSPDG.
The headlines roll in like waves, more every year. Half of the great barrier reef is dead. Reefs are battered by climate change and look “ravaged by war.” But how does ocean warming actually affect how corals function? And can some corals take the heat? I’m on a research team at Penn Biology that’s trying to find answers. Our newest results suggest that heat-stressed corals slow their metabolisms and lose the power to regulate their cellular chemistry – even if they appear healthy at first glance.
Tropical coral reefs cover barely three percent of the ocean floor, yet >25% of marine species depend on them for shelter or food. As a result, at least half a billion people worldwide need reefs: to house fish that we eat, but also for income from fishing or tourism, or medicine from bioactive compounds reef organisms make. Reefs also are self-sustaining coastal protection from storm surges and tidal waves, preventing $355 million a year in flood damage to Florida buildings alone. All that economic value adds to reefs’ immeasurable beauty and cultural importance.
Reefs are so productive because the corals that build them have evolved efficient ways to recycle nutrients. Corals are close relatives of anemones and jellyfish, except they build massive 3-D skeletons that undergird every tropical reef. This construction costs a lot of energy. To make ends meet, corals strike a deal with microscopic algae.
The reddish microalgae give corals their color, and so much more. Tucked in the translucent greenhouses of a coral’s cells, the algae spin sunlight into sugar and protein. Here’s where recycling comes in: the algae pass most of the sugar they make back to the coral host. Some corals get as much as 90% of their food from this incredible partnership. They use this energy to build the complex structures that provide critical habitat to all the fish, turtles, sharks, and other species that live on reefs. In turn, the coral gives the microalgae shelter and nutrients that are scarce in seawater, like nitrogen and phosphorus. This intimate relationship between the corals and their algae is called endosymbiosis (literally, “living together inside”).
Unfortunately, this key symbiosis is also sensitive to environmental disturbance, and the ocean is changing faster than corals can keep up. The top threat is climate change. Humans burn carbon, trapping heat in our atmosphere and warming the ocean. Hotter water causes corals to lose their colorful algae in a process called coral bleaching. Without energy from photosynthesis, corals eventually starve; without corals to build them up, reefs crumble and collapse.
Some corals can resist bleaching, though, and we don’t fully understand why. Since reefs face multiple global crises, the scientists who study them often measure health on a large scale: surveys of whole islands or entire seas.
Yet individual differences underlie how species adapt. Coral biologists have begun to investigate variation in heat tolerance, hoping “climate-proof corals'' may be the secret to saving coral reefs. During an extreme heatwave in Hawai’i in 2014, members of our lab carefully marked which corals kept their color. Sure enough, when Pacific water temperatures crept up again five years later, the same corals resisted bleaching. We mobilized to track coral health in real time with scientists from the University of Hawai’i and the University of Rhode Island. This time we tested what makes “heat-tolerant” corals different at the cellular level.
But to our surprise, all the corals were distressed by the end of the summer. Bleaching-resistant ones avoided some of the worst effects, but even their photosynthesis tanked, a sign that their microalgae were less productive. Their respiration was lower, too – the corals were breathing less. The whole symbiosis metabolism was slowing down.
Concerned, we pushed deeper. Besides heat, corals are also threatened by ocean acidification, a distinct but related by-product of increasing atmospheric CO2. Ocean acidification can hinder coral growth and dissolve their skeletons. Because corals in the wild face daily fluctuations in acidity from tides, currents, and even their own metabolism, they have evolved ways to handle these pH changes. If you experimentally acidify any healthy coral cell, pH-sensing enzymes quickly return cell pH to a safe baseline. However, during the heatwave, we found that none of the coral cells could recover at all. It’s possible that months of heat stress had sapped their strength. Even if they managed to hang on to their algae, the corals may have had no energy left to overcome a second stress.
This astonishing disruption of coral cell biology reveals how climate change harms even the hardiest-looking corals. Until we force corporations to quit their fossil fuel habits, the oceans will continue to warm, hurting all reefs. What’s more, our results show how heat stress sensitizes these animals to other disturbances. If corals can't regulate their cell pH when it’s hot, they may not be able to handle the long-term ocean acidification that comes with climate change.
It’s crucial that we understand how the cells of key species work, even when whole ecosystems are threatened. In fact, it’s especially critical in those cases. While a coral cell maintaining the right acidity may look like a tangential thread in biology’s tangled web, it keeps the fabric of the entire reef from unraveling.
It’s not enough to understand, though – we have to act. Marine biologists are quickly learning how corals suffer from climate change. This study joins a growing catalog of results for conservationists and policymakers to use when they propose marine protected areas or invest in reef restoration projects. But these are stopgaps. Even if a coral looks healthy as you snorkel past, our results show it could be fundamentally damaged by heat waves that are becoming frighteningly frequent. Our reefs, coasts, and people need comprehensive climate policy now.
Article: Innis et al. 2021 Global Change Biology. Marine heatwaves depress metabolic activity and impair cellular acid–base homeostasis in reef-building corals regardless of bleaching susceptibility. https://doi.org/10.1111/gcb.15622