Acclimatization of the crustose coralline alga Porolithon onkodes to variable pCO2

Ocean acidification (OA) has important implications for the persistence of coral reef ecosystems, due to potentially negative effects on biomineralization. Many coral reefs are dynamic with respect to carbonate chemistry, and experience fluctuations in pCO2 that exceed OA projections for the near future. To understand the influence of dynamic pCO2 on an important reef calcifier, we tested the response of the crustose coralline alga Porolithon onkodes to oscillating pCO2. Individuals were exposed to ambient (400 µatm), high (660 µatm), or variable pCO2 (oscillating between 400/660 µatm) treatments for 14 days. To explore the potential for coralline acclimatization, we collected individuals from low and high pCO2 variability sites (upstream and downstream respectively) on a back reef characterized by unidirectional water flow in Moorea, French Polynesia. We quantified the effects of treatment on algal calcification by measuring the change in buoyant weight, and on algal metabolism by conducting sealed incubations to measure rates of photosynthesis and respiration. Net photosynthesis was higher in the ambient treatment than the variable treatment, regardless of habitat origin, and there was no effect on respiration or gross photosynthesis. Exposure to high pCO2 decreased P. onkodes calcification by >70%, regardless of the original habitat. In the variable treatment, corallines from the high variability habitat calcified 42% more than corallines from the low variability habitat. The significance of the original habitat for the coralline calcification response to variable, high pCO2 indicates that individuals existing in dynamic pCO2 habitats may be acclimatized to OA within the scope of in situ variability. These results highlight the importance of accounting for natural pCO2 variability in OA manipulations, and provide insight into the potential for plasticity in habitat and species-specific responses to changing ocean chemistry.海洋酸化（Ocean Acidification, OA）会对珊瑚礁生态系统的存续产生关键影响，因其可对生物矿化过程造成潜在负面效应。多数珊瑚礁的碳酸盐化学环境处于动态变化状态，其二氧化碳分压（pCO₂）的波动幅度已超出近期海洋酸化的预测水平。为探明动态二氧化碳分压对重要造礁钙化生物的影响，本研究针对结壳珊瑚藻类物种多孔石壳藻（Porolithon onkodes）对周期性波动二氧化碳分压的响应开展了受控实验。实验共设置3组处理：环境对照组（400微大气压）、高二氧化碳分压组（660微大气压）以及波动二氧化碳分压组（在400与660微大气压间周期性波动），实验周期为14天。为探究珊瑚藻类的适应性驯化潜力，我们于法属波利尼西亚茉莉雅岛一处具备单向水流特征的礁后区域，分别从二氧化碳分压波动程度较低和较高的采样位点（依次为上游与下游区域）采集了结壳珊瑚藻样本。本研究通过浮力称重法量化了各处理对藻类钙化作用的影响，并通过密封培养实验测定光合与呼吸速率，以此评估藻类代谢状态。无论样本源自何种原始生境，环境对照组的净光合速率均高于波动处理组，且各实验处理对呼吸速率与总光合速率均无显著影响。无论样本的原始生境如何，暴露于高二氧化碳分压环境下的多孔石壳藻，其钙化作用均下降了70%以上。在波动处理组中，源自高波动生境的结壳珊瑚藻的钙化速率比低波动生境样本高出42%。原始生境对结壳珊瑚藻在波动、高二氧化碳分压环境下的钙化响应具有显著调控作用，这表明生活在动态二氧化碳分压生境中的个体，可在原位波动范围内适应海洋酸化。本研究结果凸显了在海洋酸化操控实验中纳入自然二氧化碳分压波动因素的重要性，并为理解不同生境与物种对海洋化学变化的响应可塑性提供了重要科学参考。