Detecting Photosymbiosis in Fossil Scleractinian Corals

Chiara Tornabene,Morgan F Schaller,Rowan C Martindale,Xingchen T Wang

doi:10.1038/s41598-017-09008-4

Chiara Tornabene, Morgan F Schaller + Show 2 more

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https://doi.org/10.1038/s41598-017-09008-4

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Abstract

The evolutionary success of reef-building corals is often attributed to photosymbiosis, a mutualistic relationship scleractinian corals developed with zooxanthellae; however, because zooxanthellae are not fossilized, it is difficult (and contentious) to determine whether ancient corals harbored symbionts. In this study, we analyze the δ15N of skeletal organic matrix in a suite of modern and fossil scleractinian corals (zooxanthellate- and azooxanthellate-like) with varying levels of diagenetic alteration. Significantly, we report the first analyses that distinguish shallow-water zooxanthellate and deep-water azooxanthellate fossil corals. Early Miocene (18–20 Ma) corals exhibit the same nitrogen isotopic ratio offset identified in modern corals. These results suggest that the coral organic matrix δ15N proxy can successfully be used to detect photosymbiosis in the fossil record. This proxy will significantly improve our ability to effectively define the evolutionary relationship between photosymbiosis and reef-building through space and time. For example, Late Triassic corals have symbiotic values, which tie photosymbiosis to major coral reef expansion. Furthermore, the early Miocene corals from Indonesia have low δ15N values relative to modern corals, implying that the west Pacific was a nutrient-depleted environment and that oligotrophy may have facilitated the diversification of the reef builders in the Coral Triangle.

Highlights

Through photosymbiosis, dinoflagellates, called zooxanthellae, live within the tissue of many modern scleractinian corals
Since zooxanthellae live within the soft tissue of corals, they are not directly preserved in the fossil record, making it difficult to determine whether fossil corals had symbionts, when photosymbiosis originated, or how this relationship evolved through time
Triassic samples Gablonzeria sp. and Distichomeandra sp. sample 1 [ noted as Distichomeandra sp. (1)] (Table 1) retained their coralline structures, as visible in thin section, but displayed varying levels of diagenesis; both blocky calcite and aragonite bundles were visible on specimens in scanning electron microscopy (SEM) secondary electron (SE) images and thin sections

Summary

Introduction

Dinoflagellates, called zooxanthellae, live within the tissue of many modern scleractinian corals. Zooxanthellae photosynthesize within the coral tissue providing corals with most of their energy, while the coral hosts in turn live in shallow, clear waters where zooxanthellae have optimal exposure to sunlight for photosynthesis[1,2,3] Both zooxanthellate and azooxanthellate scleractinians exist and can build reefs, a symbiotic relationship with zooxanthellae is an obvious advantage. The regularity of microscopic skeletal growth bands in the fibrous aragonitic fibrous bundles of coral skeletons has been proposed as a signature of photosymbiosis[10, 16, 17] Both growth band proxies have promise, they can be significantly altered by diagenesis and so can only be used in the most pristine samples[10, 15, 17, 18]. In the modern ocean this leads to a ~7‰ δ15N offset between zooxanthellate and azooxanthellate corals[10, 25,26,27]

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