Reef-building corals and their aragonite (CaCO3) skeletons support entire reef ecosystems, yet their formation mechanism is poorly understood. Here we used synchrotron spectromicroscopy to observe the nanoscale mineralogy of fresh, forming skeletons from six species spanning all reef-forming coral morphologies: Branching, encrusting, massive, and table. In all species, hydrated and anhydrous amorphous calcium carbonate nanoparticles were precursors for skeletal growth, as previously observed in a single species. The amorphous precursors here were observed in tissue, between tissue and skeleton, and at growth fronts of the skeleton, within a low-density nano- or microporous layer varying in thickness from 7 to 20 μm. Brunauer-Emmett-Teller measurements, however, indicated that the mature skeletons at the microscale were space-filling, comparable to single crystals of geologic aragonite. Nanoparticles alone can never fill space completely, thus ion-by-ion filling must be invoked to fill interstitial pores. Such ion-by-ion diffusion and attachment may occur from the supersaturated calcifying fluid known to exist in corals, or from a dense liquid precursor, observed in synthetic systems but never in biogenic ones. Concomitant particle attachment and ion-by-ion filling was previously observed in synthetic calcite rhombohedra, but never in aragonite pseudohexagonal prisms, synthetic or biogenic, as observed here. Models for biomineral growth, isotope incorporation, and coral skeletons’ resilience to ocean warming and acidification must take into account the dual formation mechanism, including particle attachment and ion-by-ion space filling.
|Number of pages||12|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - 1 Dec 2020|
Bibliographical noteFunding Information:
ACKNOWLEDGMENTS. We thank Guy Oei and Solomon Wong at the Albany Aquarium for maintaining aquaria and all living corals used here; Wenming Dong and Carl Steefel for the Brunauer-Emmett-Teller experiments at Lawrence Berkeley National Laboratory; Richard Celestre, Jun Zhang, and Drue Hood-McFadden for sample preparations; and Alexander Venn for discussions. P.U.P.A.G. acknowledges 80% support from the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under Award DE-FG02-07ER15899, and 20% support from National Science Foundation Grant DMR-1603192. T.M. acknowledges that this project has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Grant 755876). The PhotoEmission Electron Microscopy experiments were done at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, US Department of Energy under Contract DE-AC02-05CH11231.
© 2020 National Academy of Sciences. All rights reserved.
- Coral skeleton formation | PEEM | spectromicroscopy | biomineral | aragonite
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