Photosymbiosis Secrets Revealed By 500 Million Year Fossil Record
Coral reef ecosystems, often regarded as primary indicators of climate change, are even more intricate than once thought. A recent international study conducted by the universities of Bristol, Wuhan in China, and Erlangen-Nuremberg in Germany has uncovered that the evolutionary benefits of coral-algae symbiosis are not constant; they are entirely dependent on the surrounding environmental conditions.
The results, published in the Proceedings of the National Academy of Sciences, provide a 500-million-year overview of how photosymbiosis—the partnership between corals and photosynthetic algae—has influenced the success of corals. This new research questions the belief that this biological advancement has always been advantageous, revealing instead that its benefits have varied significantly over extensive periods.
Increasing temperatures lead to bleaching, while ocean acidification undermines coral skeletons. However, not all corals react similarly, and the principles that dictate survival have fundamentally evolved throughout Earth’s history.
Among the approximately 6,000 coral species alive today, only about half depend on sunlight. These vibrant corals rely on photosynthetic algae for their energy needs. The remaining species, often neglected, flourish in deeper, darker waters without algae. Scientists categorise these as Z (symbiotic) and AZ (non-symbiotic) corals.
The research team, led by Zhengsheng Wei from the China University of Geosciences in Wuhan, who initiated this project as a visiting student at the University of Bristol under Professor Mike Benton, along with Professor Zhong-Qiang Chen (CUG-Wuhan) and Professor Wolfgang Kiessling (FAUE-Nürnberg), employed Bayesian modeling and AI to scrutinise extensive fossil datasets, examining how each group adapted to environmental shifts throughout geological history.
During the Paleozoic, AZ corals outpaced Z corals. Z corals even failed to recover after the Late Devonian extinction. Both groups were similarly vulnerable to warming and anoxia.
But everything changed with the rise of scleractinian corals in the Triassic. After that, photosymbiosis became the primary driver of diversification. Z corals gained a clear advantage, not because symbiosis is inherently superior, but because the environmental context had shifted.
The study also found different dynamics: Z success was driven by origination (new species), AZ success by extinction (avoiding death).
Z corals are especially sensitive to short-term temperature changes, likely due to their shallow habitats. High temperatures force them to expel algae, causing bleaching. Deeper-water AZ corals can better weather such storms.
Mike Benton, Professor of Vertebrate Paleontology in the School of Earth Sciences at the University of Bristol, said, “Our work confirms the vulnerability of sunlight-loving Z corals, but shows that deeper-water AZ corals are less at risk. However, like all marine animals, they will eventually move away from waters that become too hot.”
The study underscores that conservation strategies must account for these differences. Understanding how survival rules have changed in the past could help predict, and perhaps mitigate, future biodiversity losses in a warming world.
