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

Henry C Wu,Delphine Dissard,François Thil,Aline Tribollet,Eric Douville,Florence Le Cornec,Aurelie Moya

doi:10.3389/fmars.2017.00129

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 29oC) and pCO2 (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 δ11B composition measured by laser ablation MC-ICP-MS was significantly affected by pCO2 (p = 0.0024) and water temperature (p = 1.46 x 10-5). Reconstructed pH of the primary polyp skeleton using the δ11B 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 pCO2. Primary polyp juvenile Sr/Ca ratio indicates a bias in reconstructed sea surface temperature (SST) under higher pCO2 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.

Highlights

Scleractinian corals, the building blocks of coral reefs in the tropical oceans, are known for the formation of extremely productive and diverse ecosystems
As our cultured primary polyp juveniles metamorphosed within the water column, they are different than those found in the field that metamorphosed on settlement substrates
With sample picking procedure being equal across all conditions, the brittle and disintegrating nature of the juvenile skeletons under Ocean acidification (OA) and OW is not an artifact of sample handling

Summary

Introduction

Scleractinian corals, the building blocks of coral reefs in the tropical oceans, are known for the formation of extremely productive and diverse ecosystems. These marine calcifying organisms are threatened and in most areas in decline. The results of increasing anthropogenic CO2 emissions are elevated sea surface temperatures (SST) and decreases in ocean pH (e.g., Pandolfi et al, 2011). The effects of ocean warming or thermal stress have been widely documented in coral reefs around the world by mass coral bleachings and mortalities that are becoming more frequent and severe (Hoegh-Guldberg et al, 2007; Eakin et al, 2009; Pandolfi et al, 2011). Is warming impacting corals worldwide, but the effect of OA can affect coral calcification (Gattuso et al, 1999; Kleypas et al, 1999; Anthony et al, 2008)

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