Interactive effects of ocean acidification and warming on coral reef associated epilithic algal communities under past, present-day and future ocean conditions

Epilithic algal communities play critical ecological roles on coral reefs, but their response to individual and interactive effects of ocean warming (OW) and ocean acidification (OA) is still largely unknown. We investigated growth, photosynthesis and calcification of early epilithic algal community assemblages exposed for 6 months to four temperature profiles (-1.1, +/-0.0, +0.9, +1.6 °C) that were crossed with four carbon dioxide partial pressure (pCO2) levels (360, 440, 650, 940 µatm), under flow-through conditions and natural light regimes. Additionally, we compared the cover of heavily calcified crustose coralline algae (CCA) and lightly calcified red algae of the genus Peyssonnelia among treatments. Increase in cover of epilithic communities showed optima under moderately elevated temperatures and present pCO2, while cover strongly decreased under high temperatures and high-pCO2 conditions, particularly due to decreasing cover of CCA. Similarly, community calcification rates were strongly decreased at high pCO2 under both measured temperatures. While final cover of CCA decreased under high temperature and pCO2 (additive negative effects), cover of Peyssonnelia spp. increased at high compared to annual average and moderately elevated temperatures. Thus, cover of Peyssonnelia spp. increased in treatment combinations with less CCA, which was supported by a significant negative correlation between organism groups. The different susceptibility to stressors most likely derived from a different calcification intensity and/or mineral. Notably, growth of the epilithic communities and final cover of CCA were strongly decreased under reduced-pCO2 conditions compared to the present. Thus, CCA may have acclimatized from past to present-day pCO2 conditions, and changes in carbonate chemistry, regardless in which direction, negatively affect them. However, if epilithic organisms cannot further acclimatize to OW and OA, the interacting effects of both factors may change epilithic communities in the future, thereby likely leading to reduced reef stability and recovery.石基藻类群落（Epilithic algal communities）在珊瑚礁生态系统中发挥着关键的生态作用，但其对海洋变暖（OW）与海洋酸化（OA）各自及交互效应的响应仍未得到充分探明。本研究针对暴露于4种温度梯度（-1.1、±0.0、+0.9、+1.6 ℃）、并与4种二氧化碳分压（pCO2）水平（360、440、650、940 µatm）交叉组合的早期石基藻类群落组合，在流水培养条件及自然光照环境下开展了为期6个月的培养实验，测定了其生长、光合作用与钙化作用。此外，本研究还对比了各处理组中高度钙化的结壳珊瑚藻（CCA）与皮丝藻属（Peyssonnelia）浅钙化红藻的盖度。 石基藻类群落的盖度增量在温度适度升高与当前pCO2水平下呈现最优值，而在高温与高pCO2条件下盖度显著下降，这一现象尤以结壳珊瑚藻盖度降低为主要诱因。类似地，在两个测试温度梯度下，高pCO2条件均会显著降低群落钙化速率。尽管在高温与高pCO2条件下（二者产生叠加负效应），结壳珊瑚藻的最终盖度有所下降，但相较于年平均温度与适度升温条件，高温环境下皮丝藻属物种的盖度有所升高。因此，在结壳珊瑚藻盖度较低的处理组合中，皮丝藻属物种的盖度有所升高，这一结果得到了两类生物类群间显著负相关关系的佐证。类群对胁迫因子的不同敏感性，大概率源于钙化强度及/或矿物组成的差异。 值得注意的是，与当前pCO2水平相比，低pCO2条件下石基藻类群落的生长速率与结壳珊瑚藻的最终盖度均显著下降。由此可见，结壳珊瑚藻或已从过往pCO2环境驯化至当前水平，而碳酸盐化学系统的任何方向的变化，均会对其产生负面影响。然而，若石基藻类生物无法进一步适应海洋变暖和海洋酸化，二者的交互效应或将在未来改变石基藻类群落结构，进而可能降低珊瑚礁的稳定性与恢复能力。