Impacts of food availability and pCO2 on planulation, juvenile survival, and calcification of the azooxanthellate scleractinian coral Balanophyllia elegans

Ocean acidification, the assimilation of atmospheric CO2 by the oceans that decreases the pH and CaCO3 saturation state (Omega) of seawater, is projected to have severe adverse consequences for calcifying organisms. While strong evidence suggests calcification by tropical reef-building corals containing algal symbionts (zooxanthellae) will decline over the next century, likely responses of azooxanthellate corals to ocean acidification are less well understood. Because azooxanthellate corals do not obtain photosynthetic energy from symbionts, they provide a system for studying the direct effects of acidification on energy available for calcification. The solitary azooxanthellate orange cup coral Balanophyllia elegans often lives in low-pH, upwelled waters along the California coast. In an 8-month factorial experiment, we measured the effects of three pCO2 treatments (410, 770, and 1220 µatm) and two feeding frequencies (3-day and 21-day intervals) on "planulation" (larval release) by adult B. elegans, and on the survival, skeletal growth, and calcification of newly settled juveniles. Planulation rates were affected by food level but not pCO2. Juvenile mortality was highest under high pCO2 (1220 µatm) and low food (21-day intervals). Feeding rate had a greater impact on calcification of B. elegans than pCO2. While net calcification was positive even at 1220 µatm (~3 times current atmospheric pCO2), overall calcification declined by ~25-45%, and skeletal density declined by ~35-45% as pCO2 increased from 410 to 1220 µatm. Aragonite crystal morphology changed at high pCO2, becoming significantly shorter but not wider at 1220 µatm. We conclude that food abundance is critical for azooxanthellate coral calcification, and that B. elegans may be partially protected from adverse consequences of ocean acidification in habitats with abundant heterotrophic food.

海洋酸化（Ocean acidification）指海洋吸收大气二氧化碳，进而降低海水的pH值与碳酸钙（CaCO₃）饱和状态（Omega, Ω），据预测将对钙化生物造成严重不利影响。尽管大量证据表明，带有藻共生体（虫黄藻zooxanthellae）的热带造礁珊瑚的钙化作用将在未来百年内下降，但无虫黄藻珊瑚（azooxanthellate corals）对海洋酸化的潜在响应却尚未得到充分研究。由于无虫黄藻珊瑚无法从共生体获取光合能量，因此它们为研究酸化对钙化可用能量的直接影响提供了理想实验体系。独居无虫黄藻橙色杯珊瑚（Balanophyllia elegans）通常栖息于加利福尼亚沿岸的低pH上升流海域。在一项为期8个月的析因实验中，我们设置了3组二氧化碳分压（pCO₂）处理组（410、770与1220 μatm）以及2种投喂频率（3天间隔、21天间隔），测定了上述因素对成年B. elegans的"排幼"（planulation，即幼体释放）行为，以及新附着幼体的存活率、骨骼生长与钙化作用的影响。排幼速率仅受食物丰度调控，与pCO₂水平无关。高pCO₂（1220 μatm）配合低食物供给（21天间隔投喂）的条件下，幼体死亡率达到峰值。相较于pCO₂，投喂频率对B. elegans钙化作用的影响更为显著。即便在1220 μatm（约为当前大气pCO₂的3倍）环境中，净钙化作用仍维持正值，但随着pCO₂从410 μatm升高至1220 μatm，总钙化量下降约25%~45%，骨骼密度降低约35%~45%。高pCO₂条件下文石晶体形态发生改变：在1220 μatm组中，晶体长度显著缩短，但宽度无明显变化。本研究得出结论：食物丰度对无虫黄藻珊瑚的钙化作用至关重要；在异养食物充足的栖息环境中，B. elegans可部分抵御海洋酸化带来的不利影响。