Reconstructing the Strength of Photosynthetic Endosymbiosis in Caribbean Corals before the Closure of the Isthmus of Panama

Coral reef ecosystems are highly sensitive to environmental change, and their long-term persistence depends in part on flexible feeding strategies and symbiotic associations. A well-documented example of a major environmental perturbation is the progressive closure of the Isthmus of Panama during the Pliocene epoch (ca. 4.6–4.1 Ma), which initiated the transformation of the Caribbean Sea from a relatively nutrient-rich to a more oligotrophic marine environment. This reorganization imposed strong selective pressures on reef organisms, particularly corals, to adapt to declining nutrient availability.

Fossil records indicate that many modern Caribbean coral taxa originated before the Pliocene–Pleistocene transition. It remains unclear whether these species had already developed strong host-endosymbiont nutrient coupling prior to the closure of the Isthmus or whether these traits evolved in response to it. Here, we investigate this question by analyzing stable isotope records from fossil corals spanning the Late Miocene to the present in the Caribbean Sea. Coral-bound nitrogen isotope ratios (CB-δ¹⁵N) are used to infer changes in internal nitrogen recycling and host-endosymbiont coupling, while coral-bound oxygen isotope ratios (CB-δ¹⁸O) provide constraints on past seawater temperatures.

We hypothesize that many coral lineages had already developed tighter host-endosymbiont nutrient coupling before the Isthmus closure, and that species with intermediate levels of symbiosis facilitated adaption to more oligotrophic condition. This pre-adaptation may explain both the successful establishment of the modern Caribbean coral fauna after the closure and its present-day vulnerability to rapid anthropogenic stressors such as warming and nutrient pollution. By placing modern reef ecology in an evolutionary and paleoenvironmental context, this study aims to improve our understanding of coral resilience and inform future conservation strategies.