OCEAN CHANGE: IMPACT ON PHOTOSYMBIOTIC REEF ECOSYSTEMS OF THE PAST

Many tropical calcified reef organisms engage in photosymbiosis, a process strongly selected in both the animal host and symbiont. A variety of photosymbionts inhabit different hosts but most reef organisms today partner with the one-celled dinoflagellate Symbiodinium, whose phylogeny goes back to the early Cenozoic. Dinoflagellates of close affinities to Symbiodinium are known from the Triassic and Jurassic and could have been symbiotic. However much less is known about photosymbionts in Paleozoic reef corals, stromatoporoids, mollusks and foraminifers. Detecting ancient symbionts is problematic because they are not preserved so their former presence is a working hypothesis based on analysis of functional morphology, stable isotopes, size and massive skeletons, unusual or complex morphology, and tropical paleobiogeography. Our analysis shows widespread photosymbiosis in the Late Ordovician to Devonian reef interval and also during parts of the Mesozoic, while Early Cambrian and late Paleozoic intervals yield weaker evidence. Today reef corals and many other reef taxa require warm, insolated oligotrophic conditions and most cannot survive without their symbionts. Brought on by the rise in CO₂ and rapid global warming, the breakdown of photosymbiosis is revealed in today’s reef assault through bleaching and coral diseases, while ocean acidification will continue to have increasingly deleterious effects. We propose that failures of most ancient reef ecosystems during and after mass extinctions were precipitated by the breakdown of photosymbiosis and rise of ocean acidification. Reef and carbonate gaps in the geologic record following mass extinctions, indicate that breakdown. The collapse of oligotrophic reefs also coincides with the disruption of the carbon cycle and anoxia. Just as ocean warming and acidification are causing death to both symbionts and hosts on reefs, a similar process likely accounted for the global and simultaneous reef extinctions observed in the geological record of ancient reefs. Photosymbiosis is recognized as a driving force in reefs and carbonate rocks, and the geologic record is providing important connections with the present in understanding causes of ancient reef collapse.