Upper Triassic (ca. 220 Ma) scleractinian corals can be amazingly well preserved.  (Credit: Jarosław Stolarski)

Last week came the latest grim news about the Great Barrier Reef: In its remote and treasured northern section, coral death appears to be extensive in the wake of an extreme ocean heat event in March, followed by subsequent severe coral bleaching.

It’s just the latest evidence that a changing climate is driving a wedge between two organisms that have managed to live together for eons in “one of the most successful modes of life that we know,” explains Kim Cobb, a coral reefs expert and climate scientist at Georgia Tech University.

Those two organisms are corals and the photosynthetic algae, or zooxanthellae, that live inside their cells, and provide the corals with energy. The problem is that when ocean waters get too hot, coral “bleaching” occurs, and the corals banish the algae from their system, turning white. In this condition, starved of what it takes to survive, they can only last so long.

Now, a new study in Science Advances gives the full context. In it, Katarzyna Frankowiak and Jarosław Stolarski of the Polish Academy of Sciences and a group of colleagues at institutions in Poland, the U.S., Brazil, and Switzerland studied the fossils of extremely ancient corals in order to try to determine precisely how far back in time this original symbiosis between coral and algae came to exist.

That inquiry took the scientists all the way back to the Tethys Ocean, an ancient body of water that existed during the Triassic Period, between 252 and 201 million years ago, when the continents were in vastly different alignments than where they are today. Fossils from that sea still exist in Turkey, including some well preserved specimens of corals that researchers believed proliferated during this era, despite the poor quality of the waters in which they lived.

“Even if the corals evolved much earlier, then really something strange and dramatic happened in the Triassic that caused the sort of boom of reef evolution,” said Stolarski.

“It’s a situation that, something highly advantageous to these corals happened, they start to proliferate, and we think this is because of the symbiosis,” he continued.

Extremely high-powered analyses of the composition of these coral skeletons, with a focus on the different isotopes of carbon, nitrogen, oxygen of which they’re composed, revealed that roughly 212-million-year-old corals living in this time period had indeed already achieved symbiosis with photosynthetic algae, which lived inside of their own bodies. Apparently, the symbiosis helped the corals thrive despite the murky and nutrient poor waters in which they lived, since after all, all it takes for photosynthesis is light, water, and carbon dioxide.

“If we have the poor nutrient water, then the only way that the animal, the coral animal may survive, is to be symbiotic,” said Stolarski.

That development, in turn, launched corals and their algal partners forward through time, a cooperation for the ages. And although there were many changes over the intervening several hundred million years, and although the symbiosis may have subsequently re-evolved or re-developed multiple times, we still find algae living inside of coral today.

Granted, it’s not as if it was always smooth sailing. There were warm periods before the present one, for instance — but the symbiosis persisted. “The fact that together they have weathered off-scale warming and acidification events like the Paleocene-Eocene Thermal Maximum [55 million  years ago], when much of the world’s reef systems were wiped out, is a testament to the resilience of this model,” said Georgia Tech’s Cobb in a comment on the new study.

However, Stolarski explained that the worrying difference in the present is not that oceans are warmer than they have ever been in the last 200 million years, but rather, that human driven climate change is happening far more rapidly than past climate changes in Earth’s history.

“The problem of course is that if the symbiosis is broken, which happens in modern oceans, because of the temperature and stress, then the corals usually die because they do not have the nutrients that the algae are providing,” said Stolarski. “The speed of the changes, and especially the temperature changes of the oceans, are quite rapid, and really, the issue is whether globally speaking, the corals will be able to survive this.”

Granted, Stolarski thinks that some of them can survive — even though there could be very big losses, as at the Great Barrier Reef. Cobb, too, argues that corals remain quite diverse and there will still be survivors of the latest global bleaching event, and perhaps of all of global warming.

“I am reminded of the few corals that survived a temperature-related mortality event that killed up to 85% of the corals at my study site, Christmas Island,” she wrote by email. “These hardy corals will seed the recovery of this reef in the near-term, and their descendants may seed the global recovery of coral reefs once temperatures and atmospheric carbon dioxide levels stabilize.”

Thomas Lovejoy, a prominent ecologist at George Mason University, told the Post earlier this year — at the peak of the global coral bleaching event — that the “single-species interaction” between coral and algae is a central part of the vulnerability of reefs.

“We can do all the modeling we want, climate modeling, and vegetation modeling, and it’s never going to pick up these sensitivities that come down to in many instances, just a relationship between one or two species, or two or three species,” Lovejoy added Wednesday. “So whatever the models are telling you, they’re not sensitive enough to actually predict the kinds of things we’re seeing with corals. And the consequence of that, you just have to think even more carefully and conservatively.”

“Some may survive, some may rebuild, but entire ecosystems are collapsing in the meantime,” said Lovejoy of coral reefs.

But others are less pessimistic.

“The new study is a powerful testament to the staying power of one of the most successful partnerships on our planet – one that has been honed and refined through repeated planetary-scale crises,” Cobb concluded. “There is no doubt that climate change will profoundly reshape global reefs over the next 100 years, but once the dust settles on our planetary-scale experiment, corals will likely flourish once more.”