Photo Credit: Roy Zeigerman
Scientific Frontline: Extended "At a Glance" Summary: Coral Photosymbiosis and Evolution
The Core Concept: The evolutionary advantage of photosymbiosis in corals is not a fixed biological trait but is contingent upon environmental context, as demonstrated by a 500-million-year analysis of coral survival.
Key Distinction/Mechanism: Corals are divided into symbiotic (Z) corals, which rely on photosynthetic algae for energy in shallow waters, and non-symbiotic (AZ) corals, which thrive in deeper, darker environments without algae. The evolutionary success of Z corals has been driven historically by the origination of new species, whereas AZ coral success relies on avoiding extinction during environmental upheavals.
Origin/History: During the Paleozoic era, AZ corals outpaced Z corals, with Z corals failing to recover after the Late Devonian extinction. The evolutionary advantage shifted decisively during the Triassic period with the rise of scleractinian corals, establishing photosymbiosis as a primary driver of diversification.
Major Frameworks/Components:
- Bayesian Modeling and Artificial Intelligence: Researchers utilized advanced modeling and AI to analyze extensive fossil datasets spanning geological time.
- Environmental Contingency: The study tests how different coral groups responded to environmental stressors like warming and anoxia, demonstrating that the benefits of symbiosis fluctuate with global climate conditions.
- Bleaching Vulnerability: Shallow-water Z corals are highly sensitive to short-term temperature changes, forcing them to expel algae and bleach, while deeper-water AZ corals are more resilient to such fluctuations.
Branch of Science: Paleobiology, Marine Biology, and Evolutionary Biology.
Future Application: These findings can be integrated into modern conservation strategies, enabling scientists to refine predictions regarding marine biodiversity loss and design targeted mitigation efforts for warming oceans.
Why It Matters: The research challenges the long-held assumption that coral-algae symbiosis is inherently superior, providing crucial deep-time geological context for understanding modern coral bleaching and forecasting ecosystem resilience under anthropogenic climate change.
Coral reef ecosystems, widely seen as a climate change bellwether, are more complex than previously understood. A new international study by the universities of Bristol, Wuhan in China, and Erlangen-Nuremberg, Germany, reveals that the evolutionary advantage of coral-algae symbiosis is not fixed; it depends entirely on environmental context.
The findings, published in the Proceedings of the National Academy of Sciences, offer a 500-million-year perspective on how photosymbiosis, the relationship between corals and photosynthetic algae, has shaped coral success. The new research challenges the assumption that this biological innovation has always been beneficial, showing instead that its advantage has shifted dramatically over deep time.
Rising temperatures cause bleaching, and ocean acidification attacks skeletons. But not all corals respond the same way, and the rules governing survival have changed fundamentally over Earth's history.
Of the roughly 6,000 living coral species, only half require sunlight. These colorful corals host photosynthetic algae for energy. The other half, often overlooked, thrive without algae in deeper, darker waters. Scientists classify these as Z (symbiotic) and AZ (non-symbiotic) corals.
Led by Zhengsheng Wei (China University of Geosciences, Wuhan), who began this work as a visiting student at the University of Bristol under Professor Mike Benton, with Professor Zhong-Qiang Chen (CUG-Wuhan) and Professor Wolfgang Kiessling (FAUE-Nürnberg), the team used Bayesian modeling and AI to analyze vast fossil datasets to test how each group responded to environmental changes across geological time.
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), and 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.
Published in journal: Proceedings of the National Academy of Sciences
Title: The contingent advantage of photosymbiosis in coral evolution
Authors: Zhensheng Wei, Wolfgang Kiessling, Zhen Guo, Michael J. Benton, Lewei Su, Yuangeng Huang, and Zhong-Qiang Chen
Source/Credit: University of Bristol
Edited by: Scientific Frontline
Reference Number: pal062326_01
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