Seawater carbonate chemistry and physiological responses of three temperate coralline algae in a laboratory experiment

Coralline algae are major calcifiers of significant ecological importance in marine habitats but are among the most sensitive calcifying organisms to ocean acidification. The elevated pCO2 effects were examined in three coralline algal species living in contrasting habitats from intertidal to subtidal zones on the north-western coast of Brittany, France: (i) Corallina elongata, a branched alga found in tidal rock pools, (ii) Lithophyllum incrustans, a crustose coralline alga from the low intertidal zone, and (iii) Lithothamnion corallioides (maerl), a free-living form inhabiting the subtidal zone. Metabolic rates were assessed on specimens grown for one month at varying pCO2: 380 (current pCO2), 550, 750 and 1000 µatm (elevated pCO2). There was no pCO2 effect on gross production in C. elongata and L. incrustans but L. incrustans respiration strongly increased with elevated pCO2. L. corallioides gross production slightly increased at 1000 µatm, while respiration remained unaffected. Calcification rates decreased with pCO2 in L. incrustans (both in the light and dark) and L. corallioides (only in the light), while C. elongata calcification was unaffected. This was consistent with the lower skeletal mMg/Ca ratio of C. elongata (0.17) relative to the two other species (0.20). L. incrustans had a higher occurrence of bleaching that increased with increasing pCO2. pCO2 could indirectly impact this coralline species physiology making them more sensitive to other stresses such as diseases or pathogens. These results underlined that the physiological response of coralline algae to near-future ocean acidification is species-specific and that species experiencing naturally strong pH variations were not necessarily more resistant to elevated pCO2 than species from more stable environment.

珊瑚藻（coralline algae）是海洋生境中具有重要生态意义的主要钙化类群，同时也是对海洋酸化（ocean acidification）最为敏感的钙化生物之一。本研究以法国布列塔尼西北海岸潮间带至潮下带不同生境中的3种珊瑚藻为研究对象，探究了升高的二氧化碳分压（pCO₂）对其的影响：（i）长珊瑚藻（Corallina elongata），一种栖息于潮间带岩石池中的分枝状藻类；（ii）嵌合石叶藻（Lithophyllum incrustans），低潮间带的皮壳状珊瑚藻；（iii）珊瑚石藻（Lithothamnion corallioides，maerl），一种栖息于潮下带的自由生活类群。研究人员将各物种的实验样本置于梯度二氧化碳分压条件下培养1个月，设置的梯度分别为380 µatm（当前大气二氧化碳分压基准值）、550、750及1000 µatm（升高的二氧化碳分压水平），随后测定其代谢速率。长珊瑚藻与嵌合石叶藻的总初级生产力未受二氧化碳分压水平的影响，但嵌合石叶藻的呼吸速率随二氧化碳分压升高显著增强。珊瑚石藻的总初级生产力在1000 µatm条件下略有提升，而其呼吸速率未受显著影响。嵌合石叶藻（无论光照与黑暗条件下）与珊瑚石藻（仅光照条件下）的钙化速率均随二氧化碳分压升高而下降，而长珊瑚藻的钙化速率未受显著影响。这一结果与长珊瑚藻较低的骨骼镁钙比（0.17）相符——其余两种供试藻类的骨骼镁钙比均为0.20。嵌合石叶藻的白化现象发生率更高，且该发生率随二氧化碳分压升高而加剧。二氧化碳分压或可通过间接途径改变珊瑚藻的生理状态，使其对疾病、病原体等其他胁迫因素的敏感性显著提升。本研究结果表明，珊瑚藻对近未来海洋酸化的生理响应具有显著的物种特异性；且经历自然强pH波动的物种，未必比栖息于更稳定环境中的物种对升高的二氧化碳分压具有更强的耐受性。