The symbiotic relationship between corals and their photosynthetic algal partners (photosymbionts) goes back at least to the Devonian (385 million years ago), according to new research.
The dendroid rugosa coral Dendrostella trigemme and fragments of the tabulate coral Roemerolites brevis rhiphaeus. Image credit: Jung et al., doi: 10.1038/s41586-024-08101-9.
All modern corals belong to a group called the scleractinians, which evolved in the Triassic period.
These corals can be home to symbiotic organisms (for example, photosymbionts) that have a role in nutrient recycling, which can be particularly beneficial in nutrient-poor waters.
However, it is not clear whether earlier, extinct forms of coral had photosymbionts or not.
“The Devonian (419 to 359 million years ago) was a period of higher sea-surface temperatures and atmospheric carbon dioxide than the present,” Max Planck Institute for Chemistry researcher Jonathan Jung and his colleagues wrote in their paper.
“Unlike today, its carbonate chemistry was dominated by calcite precipitation, probably due to lower sea-water magnesium/calcium ratios.”
“The Mid-Devonian hosted the most significant expansion of metazoan reefs in the Phanerozoic, and well-preserved reefs from this period are widespread across present-day Europe, North America, North Africa, Australia, Siberia and China,” they explained.
“In the Devonian, these reefs bordered the Rheic Ocean, which lay at the southern margin of Laurussia and northern border of Gondwana.”
“Along the southern edge of Laurussia, these ancient reef communities reached their greatest extent and highest diversity during the Givetian stage of the Devonian (around 387-382 million years ago).”
“These flourishing metazoan reefs were wiped out diachronically over the course of the Kellwasser Crisis during the Late Frasnian (372.2 million years ago).”
“Afterwards, reefs were mainly built by cyanobacteria/algae but were present only in very reduced numbers until the end of the Famennian (Devonian/Carboniferous boundary).”
“It has been suggested that the ability to host photosymbionts was paramount to the ecological success of ancient reef communities during the Givetian stage and that the subsequent reef collapse during the Late Devonian was associated with a gradual loss of photosymbiotic associations.”
“However, there is still no clear consensus as to whether photosymbiosis was prevalent in the now-extinct coral groups of the Paleozoic.”
In their study, Dr. Jung and co-authors examined fossils of two extinct groups of coral — tabulate and rugose — from mid-Devonian reefs.
They measured coral-bound nitrogen isotopes (15N/14N), which can be used to distinguish whether corals derive their energy from photosynthetic symbionts or not.
Their results suggest that the tabulate corals they studied did have symbionts, but most rugose corals didn’t.
The findings provide conclusive geochemical evidence of the earliest known example of symbiosis in corals.
“Widespread oligotrophy during the Devonian may have promoted coral photosymbiosis, the occurrence of which may explain why Devonian reefs were the most productive reef ecosystems of the Phanerozoic,” they wrote in the paper.
“These early signals of photosymbiosis in corals from the Mid-Devonian indicate that it supported coral productivity under warm climatic conditions.”
“The Late Triassic and Early Miocene — subsequent periods during which coral photosymbiosis has been reconstructed using nitrogen isotopes — were also warmer than today.”
“By contrast, under modern global warming due to anthropogenic greenhouse gas emissions, coral bleaching and associated mass mortality events point to a warming-driven breakdown of their symbiosis as perhaps being the greatest threat to the future of scleractinian coral reefs.”
“The robustness of coral photosymbiosis during past warm climates indicates that the failure of coral symbiosis under ongoing global warming is not due to the elevated surface-ocean temperatures being reached, but rather the rapidity with which surface-ocean temperatures are rising, which may be outstripping the ability of the symbiotic relationship to adapt.”
The team’s paper appears in the journal Nature.
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J. Jung et al. Coral photosymbiosis on Mid-Devonian reefs. Nature, published online October 23, 2024; doi: 10.1038/s41586-024-08101-9