Echinometra sea urchins acclimatised to elevated pCO2 at volcanic vents outperform those under present-day pCO2 conditions

Rising atmospheric CO2 concentrations will significantly reduce ocean pH during the 21st century (ocean acidification, OA). This may hamper calcification in marine organisms such as corals and echinoderms, as shown in many laboratory-based experiments. Sea urchins are considered highly vulnerable to OA. We studied an Echinometra species on natural volcanic CO2 vents in Papua New Guinea, where they are CO2-acclimatized and also subjected to secondary ecological changes from elevated CO2. Near the vent site, the urchins experienced large daily variations in pH (> 1 unit) and pCO2 (> 2000 ppm) and average pH values (pHT 7.73) much below those expected under the most pessimistic future emission scenarios. Growth was measured over a 17-month period using tetracycline tagging of the calcareous feeding lanterns. Average-sized urchins grew more than twice as fast at the vent compared with those at an adjacent control site, and assumed larger sizes at the vent compared to the control site and two other sites at another reef near-by. A small reduction in gonad weight was detected at the vents, but no differences in mortality, respiration, or degree of test calcification were detected between urchins from vent and control populations. Thus, urchins did not only persist but actually 'thrived' under extreme CO2 conditions. We suggest an ecological basis for this response: increased algal productivity under increased pCO2 provided more food at the vent, resulting in higher growth rates. The wider implication of our observation is that laboratory studies on non-acclimatized specimens, which typically do not consider ecological changes, can lead to erroneous conclusions on responses to global change.

21世纪大气CO₂浓度持续攀升将显著降低海洋pH值，即海洋酸化（ocean acidification, OA）。现有大量室内实验证实，海洋酸化可抑制珊瑚、棘皮动物等海洋生物的钙化过程。海胆被视作对海洋酸化高度敏感的类群。本研究以巴布亚新几内亚天然火山CO₂喷口区域的长海胆属（Echinometra）物种为研究对象，该区域的海胆不仅已长期适应高CO₂环境，同时还经受了由CO₂升高引发的次生生态变化。喷口附近海域的pH与pCO₂每日波动幅度极大：pH波动超过1个单位，pCO₂超过2000 ppm，平均海水pH（pHT 7.73）远低于最悲观未来排放情景下的预测值。研究通过四环素标记钙质摄食咀嚼器（亚里士多德提灯），对海胆开展了为期17个月的生长监测。结果显示，喷口区域的中等体型海胆生长速率是邻近对照区域的两倍以上，且其体型显著大于对照区域及另一处邻近珊瑚礁的两个采样点的海胆。喷口区域海胆的性腺重量存在小幅降低，但喷口与对照种群的死亡率、呼吸速率及海胆骨壳钙化程度均无显著差异。由此可见，海胆不仅能在极端高CO₂环境中存续，甚至实现了“旺盛生长”。本研究提出该响应的生态学机制：pCO₂升高可提升藻类生产力，为喷口区域的海胆提供更为充足的食物，进而提高其生长速率。本观测结果的更广泛启示在于：未考虑生态变化的非驯化个体室内实验，可能会得出关于全球变化生物响应的错误结论。