Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification

Ocean acidification (OA) and its associated decline in calcium carbonate saturation states is one of the major threats that tropical coral reefs face this century. Previous studies of the effect of OA on coral reef calcifiers have described a wide variety of outcomes for studies using comparable partial pressure of CO2 (pCO2) ranges, suggesting that key questions remain unresolved. One unresolved hypothesis posits that heterogeneity in the response of reef calcifiers to high pCO2 is a result of regional-scale variation in the responses to OA. To test this hypothesis, we incubated two coral taxa (Pocillopora damicornis and massive Porites) and two calcified algae (Porolithon onkodes and Halimeda macroloba) under 400, 700 and 1000 µatm pCO2 levels in experiments in Moorea (French Polynesia), Hawaii (USA) and Okinawa (Japan), where environmental conditions differ. Both corals and H. macroloba were insensitive to OA at all three locations, while the effects of OA on P. onkodes were location-specific. In Moorea and Hawaii, calcification of P. onkodes was depressed by high pCO2, but for specimens in Okinawa, there was no effect of OA. Using a study of large geographical scale, we show that resistance to OA of some reef species is a constitutive character expressed across the Pacific.

海洋酸化（Ocean Acidification, OA）及其伴随的碳酸钙饱和态下降，是本世纪热带珊瑚礁面临的主要威胁之一。过往针对OA对珊瑚礁钙化生物影响的研究，在使用相近二氧化碳分压（partial pressure of CO2, pCO2）梯度的实验中得到了多样化的结果，这表明诸多核心科学问题仍未得到解答。其中一项尚未验证的假说提出，珊瑚礁钙化生物对高pCO2的响应异质性，乃是不同区域对OA的响应存在区域尺度差异所导致的。为验证该假说，我们在环境条件各异的法属波利尼西亚茉莉雅岛、美国夏威夷以及日本冲绳开展实验，将两种珊瑚类群（鹿角杯形珊瑚*Pocillopora damicornis*与团块状滨珊瑚*Porites*）以及两种钙化藻类（*Porolithon onkodes*与*Halimeda macroloba*）置于400、700和1000 µatm的pCO2环境中进行培养。在三个实验区域中，两种珊瑚以及*H. macroloba*均对OA表现出不敏感性；而OA对*P. onkodes*的影响则呈现区域特异性：在茉莉雅岛与夏威夷，高pCO2会抑制*P. onkodes*的钙化作用，但冲绳的实验样本则未受OA的显著影响。通过这项大地理尺度的研究，我们证实，部分珊瑚礁物种对OA的耐受性乃是一种固有性状，在整个太平洋区域均有体现。