Primary Life Stage Boron Isotope and Trace Elements Incorporation in Aposymbiotic Acropora millepora Coral under Ocean Acidification and Warming

Abstract

Early-life stages of reef-building corals are vital to coral existence and reef maintenance. It is therefore crucial to study juvenile coral response to future climate change pressures. Moreover, corals are known to be reliable recorders of environmental conditions in their skeletal materials. Aposymbiotic Acropora millepora larvae were cultured in different seawater temperature (27 and 29 • C) and pCO 2 (390 and 750 µatm) conditions to understand the impacts of "end of century" ocean acidification (OA) and ocean warming (OW) conditions on skeletal morphology and geochemistry. The experimental conditions impacted primary polyp juvenile coral skeletal morphology and growth resulting in asymmetric translucent appearances with brittle skeleton features. The impact of OA resulted in microstructure differences with decreased precipitation or lengthening of fasciculi and disorganized aragonite crystals that led to more concentrations of centers of calcifications. The coral skeletal δ 11 B composition measured by laser ablation MC-ICP-MS was significantly affected by pCO 2 (p = 0.0024) and water temperature (p = 1.46 × 10 −5 ). Reconstructed pH of the primary polyp skeleton using the δ 11 B proxy suggests a difference in coral calcification site and seawater pH consistent with previously observed coral pH up-regulation. Similarly, trace element results measured by laser ablation ICP-MS indicate the impact of pCO 2 . Primary polyp juvenile Sr/Ca ratio indicates a bias in reconstructed sea surface temperature (SST) under higher pCO 2 conditions. Coral microstructure content changes (center of calcification and fasciculi) due to OA possibly contributed to the variability in B/Ca ratios. Our results imply that increasing OA and OW may lead to coral acclimation issues and species-specific inaccuracies of the commonly used Sr/Ca-SST proxy.

Mineralogy of Deep-Sea Coral Aragonites as a Function of Aragonite Saturation State

“…In parallel with these crystallographic structural distortions, we also find some evidence of increased surface etching and roughness (more corrosion) for corals growing within waters with decreasing sw (Supplementary Figure S1). While previous studies have observed marked distortions in coral skeletons and crystallite morphologies in shallow corals growing in undersaturated conditions Tambutté et al, 2015;Foster et al, 2016;Wu et al, 2017), overall skeleton morphology shifts in deep-sea corals are not as obvious. Instead, L. pertusa colonies express only minor outward morphological differences in undersaturated sw conditions with hidden internal crystal morphology toward less ordered, smaller crystallites observable via electron backscatter diffraction (EBSD) (Hennige et al, 2015).…”

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

“…However, while elevated temperature increased δ 11 B (i.e. increased pH), it did not affect B/Ca or U/Ca (Wu et al, 2017), potentially reflecting the lower degree of thermal stress (culture temperatures were within the seasonal range at the collection site) compared to Foster et al (2015) and/or that [Ca 2+ ] cf played an important role in our Raman data. Nevertheless, our results further highlight that Raman spectroscopy can detect changes in coral calcifying fluid Ar , and that this information is complementary to other analyses, including boron systematics (Sect.…”

“…Finally, Foster et al (2016) found pitted skeletal surfaces indicative of dissolution, which may have compounded the measured CO 2 effects on calcification and led to their dominance over any apparent effect of temperature in the "+CO 2 + T " treatment. A recent study employing a similar experimental design found that juvenile Acropora B/Ca and U/Ca increased with elevated CO 2 (Wu et al, 2017) based on abiogenic partitioning Holcomb et al, 2016). However, while elevated temperature increased δ 11 B (i.e.…”

Response to Referee Comment #1

“…We collected 440 spectra from various grains spread onto a glass slide. Although JCp-1 exists in ground form, we note that analysis of intact coral skeletons is also possible (Wall and Nehrke, 2012;Hennige et al, 2014) and that peak widths apparently do not depend on whether the sample is powdered or intact (Zakaria et al, 2008), as long as the grain size exceeds the laser spot size.…”

“…These observations imply that Mg content was not the primary source of lattice defects broadening Raman peaks, an interpretation supported by the absence of a correlation between Mg/Ca and ν 1 FWHM (after accounting for the effect of Ar ; Table 1). The lack of correlation between ν 1 FWHM and position also indicates that ν 1 FWHM was unaffected by crystallite size (Urmos et al, 1991;Zakaria et al, 2008).…”