Geokolloqium, Jonathan Jung

- GEOWISSENSCHAFTLICHES KOLLOQUIUM -

Presentation
The emergence of photosymbiosis in corals

From: Jonathan Jung
Max-Planck-Institut for Chemistry

Coral reefs are among Earth’s most enduring and ecologically complex ecosystems, with an evolutionary legacy extending over 450 million years. Long before the rise of modern scleractinian corals in the Triassic, reef frameworks were constructed by tabulate and rugose corals in partnership with stromatoporoid sponges, forming vast Middle Paleozoic reef tracts that rivalled or exceeded the scale and biodiversity of modern reefs. However, the mechanisms underlying the evolutionary success and resilience of these early coral-stromatoporoid assemblages across major extinction boundaries remain unresolved. The Late Ordovician mass extinction (~445 Ma), which eliminated more than half of marine species, provides a key natural experiment to examine how reef ecosystems persisted through episodes of severe climatic oscillation, oceanic anoxia, and nutrient reorganization.
We use coral-bound nitrogen isotope (CB-δ¹⁵N) data from colonial tabulate and solitary rugose corals spanning the Late Ordovician to Early Silurian transition in Laurentia (Anticosti Island) and Baltica (Estonia, Gotland). Our results reveal isotopic signatures consistent with the presence of photosymbiotic nutrient recycling in Paleozoic corals, predating the end-Ordovician mass extinction and emerging during the terminal phase of the Great Ordovician Biodiversification Event (GOBE).
Elevated CB-δ¹⁵N values across the extinction horizon indicate expansion of marine anoxia, likely driven by enhanced phosphorus recycling and redox instability in the late Ordovician ocean. Together, these findings provide the first geochemical evidence for early coral photosymbiosis as an adaptive strategy established well before modern coral evolution. We propose that photosymbiosis acted as both a key innovation and an evolutionary constraint in early reef ecosystems facilitating Paleozoic reef expansion under stable, oligotrophic conditions, yet amplifying vulnerability to environmental
perturbations that foreshadowed later reef crises.