Zinc and Sulfur Distributions and Bonding Environments in Scleractinian Corals

Gabriela Farfan, Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, Amy Apprill, Woods Hole Oceanographic Institution, Department of Marine Chemistry & Geochemistry, Woods Hole, MA, United States, Samuel Webb, SLAC National Accelerator Laboratory, Menlo Park, United States and Colleen Hansel, Woods Hole Oceanographic Institution, Department of Marine Chemistry and Geochemistry, Woods Hole, MA, United States
Abstract:
Zinc is a cofactor in many enzymes essential for life and a limiting nutrient in the oligotrophic waters

surrounding most coral reefs. Because corals make up a large percentage of the volume of coral reefs

and may be a significant sink for otherwise bioavailable zinc, we tracked the spatial distributions and

bonding environments of zinc (Zn) and sulfur (S) in the tissues and skeletons of corals using synchrotron-

based micro-X-ray fluorescence (u-XRF) high-resolution mapping and X-ray absorption near edge

structure (XANES) spectroscopy. Micro-X-ray diffraction (XRD) was used to determine how the

distribution and concentration of Zn was related to changes in skeletal crystal structure. Consistent with

previous studies that employed bulk-techniques, our in-situ measurements indicated that metal

concentrations in the tissues greatly exceeded those incorporated into the skeletons. XRF maps showed

that Zn distributions traced tissue morphologies with concentrations especially high around what appear

to be endosimbionts in the tissues. Zinc XANES spectra indicate that Zn is bonded to organics in both the

tissues and skeletons. In speculation, Zn in the skeleton may have originated from carbonic anhydrase,

Zn-containing enzymes known to play a role in coral aragonite precipitation. Indeed, Zn concentrations

are higher in outer, younger skeletal regions in comparison to inner regions of the skeleton. This Zn

gradient also correlated to a small change in the aragonite crystal structure consistent with previous

biogenic aragonite studies that confirm the presence of organics in biogenic aragonite. Sulfur is

observed in both the tissue and skeleton but does not correlate with Zn distributions. Sulfur present as

organo-sulfur complexes differ between the tissues and the skeleton. In sum, this study finds Zn is highly

concentrated within coral tissue, illustrating that corals may be a substantial sink for Zn in oligotrophic

regions of the ocean.

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