Effects of ocean acidification and global warming on reef bioerosion-lessons from a clionaid sponge

Coral reefs are under threat, exerted by a number of interacting effects inherent to the present climate change, including ocean acidification and global warming. Bioerosion drives reef degradation by recycling carbonate skeletal material and is an important but understudied factor in this context. Twelve different combinations of pCO2 and temperature were applied to elucidate the consequences of ocean acidification and global warming on the physiological response and bioerosion rates of the zooxanthellate sponge Cliona orientalis-one of the most abundant and effective bioeroders on the Great Barrier Reef, Australia. Our results confirm a significant amplification of the sponges' bioerosion capacity with increasing pCO2, which is expressed by more carbonate being chemically dissolved by etching. The health of the sponges and their photosymbionts was not affected by changes in pCO2, in contrast to temperature, which had significant negative impacts at higher levels. However, we could not conclusively explain the relationship between temperature and bioerosion rates, which were slightly reduced at both colder as well as warmer temperatures than ambient. The present findings on the effects of ocean acidification on chemical bioerosion, however, will have significant implications for predicting future reef carbonate budgets, as sponges often contribute the lion's share of internal bioerosion on coral reefs.珊瑚礁正面临当前气候变化固有的多种交互作用所带来的威胁，具体包括海洋酸化与全球变暖。生物侵蚀（bioerosion）通过循环利用碳酸盐骨骼材料驱动珊瑚礁退化，是该背景下一项重要却未被充分研究的影响因子。本研究设置了12组不同的二氧化碳分压（pCO2）与温度组合，以阐明海洋酸化与全球变暖对虫黄藻共生海绵（zooxanthellate sponge）Cliona orientalis——澳大利亚大堡礁（Great Barrier Reef）上丰度最高、侵蚀能力最强的生物侵蚀类群之一——的生理响应及生物侵蚀速率的影响。研究结果证实，随着pCO2升高，该海绵的生物侵蚀能力显著增强，具体表现为通过蚀刻作用化学溶蚀的碳酸盐量增加。海绵及其光合共生体的健康状况并未受pCO2变化的影响；与之形成对比的是，温度在升高至较高水平时会对其产生显著负面影响。不过，我们无法明确阐释温度与生物侵蚀速率之间的关联：相较于环境温度，无论温度偏低还是偏高，生物侵蚀速率均略有下降。鉴于海绵通常在珊瑚礁内部生物侵蚀中占据主导份额，本研究关于海洋酸化对化学性生物侵蚀影响的发现，将对未来珊瑚礁碳酸盐收支的预测产生重要意义。