Species-specific consequences of ocean acidification for the calcareous tropical green algae Halimeda, 2011

Ocean acidification (OA), resulting from increasing dissolved carbon dioxide (CO2) in surface waters, is likely to affect many marine organisms, particularly those that calcify. Recent OA studies have demonstrated negative and/or differential effects of reduced pH on growth, development, calcification and physiology, but most of these have focused on taxa other than calcareous benthic macroalgae. Here we investigate the potential effects of OA on one of the most common coral reef macroalgal genera,Halimeda. Species of Halimeda produce a large proportion of the sand in the tropics and are a major contributor to framework development on reefs because of their rapid calcium carbonate production and high turnover rates. On Palmyra Atoll in the central Pacific, we conducted a manipulative bubbling experiment to investigate the potential effects of OA on growth, calcification and photophysiology of 2 species of Halimeda. Our results suggest that Halimeda is highly susceptible to reduced pH and aragonite saturation state but the magnitude of these effects is species specific. H. opuntiasuffered net dissolution and 15% reduction in photosynthetic capacity, while H. taenicola did not calcify but did not alter photophysiology in experimental treatments. The disparate responses of these species to elevated CO2 partial -pressure (pCO2) may be due to anatomical and physiological differences and could represent a shift in their relative dominance in the face of OA. The ability for a species to exert biological control over calcification and the species specific role of the carbonate skeleton may have important implications for the potential effects of OA on ecological function in the future.海洋酸化（Ocean acidification, OA）指表层海水溶解二氧化碳（carbon dioxide, CO₂）浓度升高所引发的环境现象，其大概率会对诸多海洋生物造成影响，尤以钙化生物为甚。既往相关研究已证实，海水pH降低会对生物的生长、发育、钙化过程及生理机能产生负面影响或差异化效应，但此类研究大多聚焦于除钙化底栖大型藻类（calcareous benthic macroalgae）之外的类群。本研究针对珊瑚礁最常见的大型藻类属之一——仙掌藻属（Halimeda），探究了海洋酸化对其可能产生的潜在影响。仙掌藻属物种可生成热带海域中占比可观的砂质沉积物，且因其碳酸钙沉积速率快、周转效率高，成为珊瑚礁礁体构建的重要贡献者。我们在太平洋中部的帕尔迈拉环礁（Palmyra Atoll）开展了操控性曝气实验，以探究海洋酸化对两种仙掌藻属物种的生长、钙化过程及光生理特性的潜在效应。研究结果表明，仙掌藻属对海水pH降低及文石饱和度下降具有高度敏感性，但此类影响的强度存在物种特异性。其中，团扇仙掌藻（H. opuntia）出现了净溶解现象，光合能力下降15%；而带形仙掌藻（H. taenicola）虽未发生钙化，但实验处理并未对其光生理机能产生显著改变。这两个物种对二氧化碳分压（carbon dioxide partial-pressure, pCO₂）升高的差异化响应，可能源于其解剖结构与生理机能的差异，亦或预示着未来海洋酸化背景下二者相对优势地位的转变。物种对钙化过程实施生物调控的能力，以及碳酸盐骨骼所发挥的物种特异性作用，可能对未来海洋酸化对生态系统功能的潜在影响具有重要意义。