Seawater carbonate chemistry and boron isotope and trace elements incorporation in aposymbiotic Acropora millepora coral

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 delta 11B 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 ?11B 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.

造礁珊瑚的早期生命阶段对于珊瑚种群存续与珊瑚礁维护至关重要，因此研究幼龄珊瑚对未来气候变化压力的响应具有重要科学意义。此外，众所周知珊瑚的骨骼组织可作为环境条件的可靠记录载体。本研究将无共生体多孔鹿角珊瑚（*Acropora millepora*）幼体置于不同海水温度（27℃与29℃）和二氧化碳分压（pCO₂，390与750 µatm）条件下培养，以探究“世纪末”海洋酸化（OA）与海洋变暖（OW）环境对珊瑚骨骼形态与地球化学特征的影响。
实验条件对初级水螅体幼龄珊瑚的骨骼形态与生长产生了显著影响，使其呈现不对称半透明外观，并伴随骨骼脆弱的特征。海洋酸化可导致珊瑚骨骼微观结构出现差异：钙化沉积速率降低、骨束延长，同时文石晶体排列紊乱，进而导致钙化中心数量增多。通过激光剥蚀多接收等离子体质谱（MC-ICP-MS）测得的珊瑚骨骼δ¹¹B（delta 11B）组成，显著受到pCO₂（p=0.0024）与海水温度（p=1.46×10⁻⁵）的影响。利用δ¹¹B替代指标重建的初级水螅体骨骼pH值，表明珊瑚钙化位点pH与海水pH存在差异，这与此前观测到的珊瑚pH上调现象一致。
类似地，通过激光剥蚀电感耦合等离子体质谱（ICP-MS）测得的微量元素结果，同样体现了pCO₂的调控作用。初级水螅体幼体的锶钙（Sr/Ca）比值表明，在高pCO₂条件下，海表温度（SST）的重建结果存在偏差。海洋酸化引发的珊瑚微观结构组分变化（钙化中心与骨束），可能是硼钙（B/Ca）比值出现波动的重要诱因。本研究结果显示，不断加剧的OA与OW可能会引发珊瑚适应性问题，同时也会导致常用的Sr/Ca-SST替代指标出现物种特异性的重建偏差。