Seawater carbon chemistry and calcification rate of Palau corals

In Palau, calcification rates of two reef-building coral genera (Porites and Favia) are maintained across a strong natural gradient in aragonite saturation state (Omega ar) ranging from 3.7 to 2.3. This observation contrasts the strong sensitivity to decreasing ?ar that these genera demonstrate in both laboratory CO2 manipulation experiments and in field studies. Moreover, in contrast to other naturally more acidic coral reefs, benthic communities in Palau's low-Omega ar (Omega ar = 2.3) Rock Island reefs display ecological indices consistent with healthy communities. A laboratory CO2 manipulation experiment and a field-based reciprocal transplant were used to investigate whether the apparent lack of sensitivity to ocean acidification of Palau's Porites corals can be attributed to local adaptation to chronic acidification or to environmental factors that allow corals to thrive despite extreme pH conditions. In a two-month laboratory incubation, calcification rates of Palau Porites from both environments were insensitive to changes in Omega ar over the range 1.5 to 3.0, suggestive of an adaptive, rather than environmental, mechanism for acidification tolerance. However, in the reciprocal transplant, corals transplanted between reefs at different ambient ?ar levels showed significant declines in calcification rates and high mortality, while corals returned back to their reef of origin were alive after 17 months in the field. Interpreted within the framework of the experimental result, the failure of pH/Omega ar-tolerant corals to successfully transplant between different reef sites hints at local adaptation to other (non-pH) environmental factors such as light, temperature, and/or flow that co-vary with Omega ar across Palau's natural acidification gradient.

在帕劳海域，两种造礁珊瑚属——微孔珊瑚属（Porites）与蜂巢珊瑚属（Favia）的钙化速率，在文石饱和度状态（aragonite saturation state，Ω_ar）从3.7到2.3的强烈自然梯度下保持稳定。这一观测结果与这两个类群在实验室CO₂调控实验及野外研究中所表现出的对Ω_ar下降的强敏感性形成鲜明反差。此外，与其他天然酸性更强的珊瑚礁不同，帕劳低Ω_ar（Ω_ar=2.3）的岩礁群岛礁区底栖生物群落，其生态指标与健康群落的特征相符。本研究通过实验室CO₂调控实验与野外互易移植实验，探究帕劳微孔珊瑚对海洋酸化表观上的不敏感性，究竟源于对长期酸化的本地适应，还是得益于珊瑚能在极端pH条件下存活的其他环境因素。为期两个月的实验室培养实验显示，来自两种生境的帕劳微孔珊瑚，其钙化速率在Ω_ar为1.5至3.0的范围内均不受其变化的影响，这暗示其酸化耐受机制属于适应性而非环境性的。然而，在互易移植实验中，在不同环境Ω_ar的礁区间移植的珊瑚，出现了钙化速率显著下降且死亡率较高的情况；而被移回原栖息礁的个体，则在野外17个月后仍存活。结合实验结果进行解读：耐pH/Ω_ar的珊瑚无法在不同礁区间成功移植的现象，暗示这些珊瑚对与帕劳自然酸化梯度下Ω_ar协同变化的非pH环境因素（如光照、温度和/或水流）产生了本地适应。