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Link to original content: http://pubmed.ncbi.nlm.nih.gov/28842582/
Detecting Photosymbiosis in Fossil Scleractinian Corals - PubMed Skip to main page content
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. 2017 Aug 25;7(1):9465.
doi: 10.1038/s41598-017-09008-4.

Detecting Photosymbiosis in Fossil Scleractinian Corals

Chiara Tornabene  1 , Rowan C Martindale  2 , Xingchen T Wang  3 , Morgan F Schaller  4
Affiliations

Detecting Photosymbiosis in Fossil Scleractinian Corals

Chiara Tornabene et al. Sci Rep. .

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.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1
Scanning electron microscope (SEM SE) images of samples; note aragonite bundles. A-B are modern, C is Holocene, D is Miocene, E is Oligocene, and F is Triassic in age. (a) Zooxanthellate Diploria labynthoformis; (b) Azooxanthellate Desmophyllium dianthus; (c) Zooxanthellate Diploria strigosa; (d) Azooxanthellate-like Caryophyllia sp.; (e) Zooxanthellate-like Antiguastrea lucasiana; note complete recrystallization to blocky calcite; (f) Distichomeandra sp. (2).
Figure 2
Figure 2
Mean δ15N of the organic matrix of coral samples. White and grey boxes differentiate samples, whereas dashed lines separate trials. Black squares mark specimen analyzed using the dialysis/combustion method; triangles mark samples analyzed using the persulfate/denitrifier method. Error bars are one standard deviation from the mean. Z = zooxanthellate coral (modern), AZ = azooxanthellate coral (modern), Z-like = zooxanthellate-like coral (fossil), AZ-like = azooxanthellate-like coral (fossil). Holo. = Holocene, Mio. = Miocene, Olig. = Oligocene.
Figure 3
Figure 3
δ15N of modern and fossil corals plotted with δ15N values of the regional N source. Fossil data are compared to the δ15N values of modern corals from different locations, and the δ15N of their regional nitrogen source, , displaying the typical ~7‰ offset. Early Miocene (18–20Ma) corals (orange and yellow squares) from adjacent sites in Indonesia (note the offset of the Z-like and AZ-like corals) and Triassic corals from Turkey (green rhombi) are plotted outside of the box as the δ15N of their regional N source is unknown. The δ15N values of Miocene, zooxanthellate coral Acropora papillare (yellow squares) and azooxanthellate coral Caryophyllia sp. (orange squares) indicate that the δ15N of their regional N source was approximately 2.5‰ (arrow), indicative of oligotrophic waters .
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