Record levels of heat in the ocean are causing a worldwide mass bleaching event on coral reefs. It's the second one this decade, where the delicate skeletons of corals turn a ghostly white.
With mass bleaching only expected to get worse as the climate keeps warming, coral scientists are urgently searching for ways to help reefs endure. Bleaching can kill corals, putting some of the most diverse ecosystems in the world at risk. So scientists are homing in on how bleaching happens.
It boils down to relationship drama between corals and a tiny organism that's too small to see.
Corals are the builders of reefs, their skeletons creating the vast infrastructure that tens of thousands of other species depend on. But corals are powered by the tiny algae that live in their tissue, which provide food for them.
"They're these microscopic, sort of nondescript algae," says Matthew Nitschke, research scientist at the Australian Institute of Marine Science, as he magnifies a few under a microscope, revealing golden-brown circles.
"People are like: why are you so interested in them?" he says. "And it's because they, for me, are really at the foundation of the ecosystem."
The tiny algae and coral make up one of the most productive roommate relationships on the planet. But as the climate gets hotter, that relationship is increasingly going bad. When ocean temperatures rise, corals get stressed and their algae get expelled. Without their roommates, corals can starve and eventually die.
Studies show that if climate change continues at the same pace, 99% of the world's coral reefs are likely to die off by the end of the century. To buy reefs a little extra time, scientists are breeding both algae and corals to withstand more heat, speeding up the natural process of evolution. But with oceans heating up more rapidly than expected, they're racing against the clock.
"I think anyone who wasn't worried, needs to be worried now," says Kate Quigley, coral biologist at James Cook University in Australia and the Minderoo Foundation. "Nature has time to make mistakes and then adjust. We don't have that time."
Natural selection in a bottle
The tanks at the Australian Institute of Marine Science, just outside of Townsville in Queensland, are full of delicate branching corals in a vast array of colors. Another lab there is somewhat less eye-catching – full of scientific flasks with clouds of brown algae in them. They're zooxanthellae, the algae that live in coral, but these have been isolated from their coral homes (the algae can live in the ocean without the coral, but coral can't live without algae).
"If you look at a coral, they look bright, they look colorful," Nitschke says. "They're actually mostly translucent and a lot of the color of the coral that you see comes from the algae."
The algae in Nitschke's lab have been grown over hundreds of generations, subjected to an accelerated version of survival-of-the-fittest. They've been exposed to heat, singling out those best able to handle higher temperatures, which then go to produce future algal generations.
"What we're really doing is natural selection in a bottle," he says. "We're really excited about the possibility for that to help corals persist into the future."
Scientists are still trying to tease out exactly what happens between a coral and its algae when temperatures get hot. They depend on a carefully-balanced living arrangement. The algae get a comfy home and nutrients they need from the coral. In return, they do photosynthesis, using sunlight to produce energy for the coral.
But when the ocean heats up, that balance gets upset. Scientists believe one reason is that the warmer water stresses the coral, upsetting the nutrient exchange between the coral and algae. Another reason could be that the hotter water impairs how cells function, causing them to release too much of certain chemicals. The result is that most algae get the boot, leaving the coral without its main food supply.
"They begin to starve," Nitschke says. "That primary energy source – the loss of that during a heat stress event is potentially catastrophic for an individual coral. They are now in a race against time."
Buying time for coral
If the heat subsides, corals can recover, slowly building back their algae population. But if the heat persists, or if there are too many marine heat waves back-to-back, the corals die.
Bleaching events are becoming more frequent, putting corals on a path for a mass die-off by the end of the century if the planet warms more than 2 degrees Celsius (3.6 degrees Fahrenheit). The effects could be devastating for marine biodiversity and for human communities. Hundreds of millions of people worldwide live near coral reefs, relying on them for food and coastal protection, since reefs can reduce flooding by absorbing wave energy.
It's why Nitschke and his colleagues have focused on breeding algae. They're in the process of testing them, giving them to tiny brain corals the size of walnuts. In trials, they've found corals inoculated with the heat-tolerant algae seem to resist bleaching for longer.
Researchers are also breeding corals themselves to be more heat-tolerant, in the hope that a combination of both a "super coral" and "super algae" can be used to restore reefs someday. Both are "assisted evolution" – a technique to speed up the natural process organisms use to adapt to their environment.
"Assisted evolution is an umbrella term for many things we've been doing in many other systems: agriculture, for pets." Nitschke says. "We're really only just starting to understand what we can do in the coral space."
Not a "get out of jail free" card
Still, in nature, there is no free lunch. Heat-tolerant algae may not share as many nutrients with their coral hosts, which means corals grow more slowly and reproduce later than they would otherwise. That could hamper their ability to restore reefs impacted by climate change. A key step will be testing the corals and algae on the Great Barrier Reef itself to see how they do.
"The last thing we want to do is make things worse," says Line Bay, a research program director at the Australian Institute of Marine Science. "We don't want to produce lab-adapted corals and then put them out in the real world where they don't do well."
Even if the heat-tolerant corals prove to be successful, the number of coral needed to restore impaired reefs could be enormous. The Great Barrier Reef is more than 1,000 miles long. And regulators will need to assess if the corals pose any risk to wild populations or the ecosystem as a whole.
"Coral reefs are magical places," Bay says. "I think we need to be brave and we should use all the tools at our disposal in a humble and sensitive manner."
Coral scientists are clear about one aspect of the work: it's not a long-term solution. At best, it only buys coral reefs extra time until the effects of climate change become too much.
"It's not our 'get out of jail free' card," Quigley says. "Maybe that gets us to 2030, 2050 for a very few number of species that we can work with. If we don't have an ocean to put them back in that's healthy, no amount of incredible technology or money is worth it."
The hope is that giving coral reefs a few extra years, or even decades, will be enough time for humans to slow the pace of climate change. That means cutting heat-trapping emissions from the largest source – burning fossil fuels – and switching to alternative energy sources like solar and wind.
"We could all be despondent and be hopeless if there weren't great solutions on the table to turn climate change problems around," Quigley says. "We just need to get it on, now, really."
