Researchers have found that the symbiotic partnership between coral and photosynthetic algae began over 210 million years ago, during the Triassic Period. The findings highlight the importance of this symbiosis in the formation of coral reefs. The details are in a paper that was just published in the journal Science Advances.
Coral are odd animals and actually represent a partnership between two organisms. The majority of coral species contain algae called zooxanthellae in their tissue. Zooxanthellae produce nutrients through photosynthesis, just like plants, and the coral use these nutrients to grow. The algae benefit from having a safe place to live while also consuming byproducts released by the growing coral. This nutrient recycling process allows coral reefs to thrive in nutrient poor areas. As the coral build up their calcium carbonate skeletons, the size of the reef expands and biodiversity increases. Scientists have long believed that coral symbiosis was critical to the development of the first reefs.
An international team of researchers collaborated to investigate the evolution of the coral-algae partnership. The team studied coral fossils to look for signs of symbiosis. Algae would not have been preserved during fossilization but the team found tiny microstructures made by zooxanthellae. They also analyzed the chemical isotopes in the fossils. Isotopes are different versions of the same chemical; they have different molecular weights due to differing numbers of neutrons. Symbiotic coral contain different isotopes and chemical ratios when compared to non-symbiotic species. The team analyzed all of the data and determined that coral-algae symbiosis began at least 210 million years ago. The event was most likely what led to the rapid development of coral reefs during the Triassic Period.
The team’s findings provide new insights into the evolution of coral reefs. Symbiotic relationships between coral and photosynthetic algae allowed the animals to thrive in areas with low nutrient availability, leading to the growth of large coral reefs during the Triassic Period. Modern reefs are currently undergoing bleaching events due to climate change, where the coral expel the photosynthetic zooxanthellae. This eventually kills the coral and can lead to the death of a reef. By understanding the evolution and functions of coral-algae symbiosis, researchers may be able to save the planet’s reefs.
Frankowiak et al. Photosymbiosis and the expansion of shallow-water corals. Science Advances (2016).