Scott K. Johnson

Underwater photograph of gray and white coral.
Enlarge / These pale corals were bleached by high temperatures.

The outlook for coral reefs in our warming world is truly dismal. The latest IPCC report stated that more than 99 percent of coral reefs will likely be lost if the world warms by 2°C—a level we are still on track to exceed. The Great Barrier Reef has given us a preview of this future in recent years, with bleaching due to marine heatwaves in 2016, 2017, and 2020.

Claims about small-scale conservation steps like artificial reef habitat creation or coral-safe sunscreen lotion frequently appear with descriptions ranging from hopeful to breathless. But the reality is that these things are dwarfed in importance by the real problem—climate change.

Still, a team led by CSIRO’s Patrick Buerger wondered if there might be a way to help, at least to some extent, by raising heat-tolerant strains of the corals’ symbionts. And the good news is that there may be.


Corals are colonies of individual invertebrate animals that host symbiotic algae which produce food for the corals. The corals “bleach” in hot temperatures because they are forced to spit out the algae, which take their color with them. The reason the corals risk starvation is that the algae’s biology starts to go haywire at those temperatures. So the heat tolerance of the coral ultimately depends on the heat tolerance of the algae.

Algae have pretty short generation times, so it’s tempting to think that directed evolution in a laboratory environment could result in sturdier strains of algae that are ahead of the global warming curve. But this had been tried once before, and corals with the experimental algae sadly fared no better.

This time, the researchers took more shots, evolving 10 strains at the same time rather than just one. Starting with a wild strain, they grew two populations in comfortable 27°C water and put 10 populations in pushing-the-limits 31°C water. They kept the specimens there for fully four years, enough time for 120 (asexual) generations.

For the test, all the algae were first brought down to 27°C for six weeks (to get all the strains on equal footing) before bumping them all up to 31°C for three weeks. The 10 warm-water strains showed about a 66 percent increase in cell count during the higher-temperature period, with steady numbers for biochemical metrics. The other two strains saw cell counts drop by almost 80 percent in the warmer water, with clear biochemical signs of distress. So obviously the experimental strains picked up some heat-tolerance adaptations in the previous four years.

Back together

But the next stage of the test was the most important—getting them inside coral hosts for a week and setting the thermostat to 31°C. Many of the 10 experimental strains weren’t any help, but three were. Those three strains continued growing and multiplying while all the others declined to varying degrees, triggering some level of bleaching.

Some measurements of gene activity in the experiment provide clues as to why only a few of the experimental strains worked out. The three winners seemed to have boosted their conversion of CO2 into organic carbon—a process typically hit hard by heat stress. So it may be that a more robust conversion process keeps them chugging along when the water gets hot and, in a way, that particularly helps their coral hosts.

This is an encouraging result that suggests it might be possible to release heat-tolerant algae into reefs, helping corals survive to slightly higher temperatures. But there are huge caveats to consider. The researchers note that they don’t know if those strains would retain their heat-tolerant traits after release into the wild. Similarly, it’s not clear how effectively these algae would spread.

Even the repeated, severe bleaching events we’ve seen in recent years only involved high temperatures for a short period of the year. The rest of the year, milder temperatures would prevail. Would the adaptations of these lab-evolved strains fade in relaxed conditions? And would they grow to a big-enough share of the population to matter when the heatwaves did arrive? Those are open questions.

But even so, it’s at least possible the strains could have some benefit. “Coral stock with enhanced climate resilience,” the researchers write, “provides an opportunity to assist corals in coping with climate change until global warming is halted.” Provided that warming is halted very soon.

Science Advances, 2020. DOI: 10.1126/sciadv.aba2498 (About DOIs).

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