Natural high pCO2 increases autotrophy in Anemonia viridis (Anthozoa) as revealed from stable isotope (C, N) analysis

Contemporary cnidarian-algae symbioses are challenged by increasing CO2 concentrations (ocean warming and acidification) affecting organisms' biological performance. We examined the natural variability of carbon and nitrogen isotopes in the symbiotic sea anemone Anemonia viridis to investigate dietary shifts (autotrophy/heterotrophy) along a natural pCO2 gradient at the island of Vulcano, Italy. delta 13C values for both algal symbionts (Symbiodinium) and host tissue of A. viridis became significantly lighter with increasing seawater pCO2. Together with a decrease in the difference between delta 13C values of both fractions at the higher pCO2 sites, these results indicate there is a greater net autotrophic input to the A. viridis carbon budget under high pCO2 conditions. delta 15N values and C/N ratios did not change in Symbiodinium and host tissue along the pCO2 gradient. Additional physiological parameters revealed anemone protein and Symbiodinium chlorophyll a remained unaltered among sites. Symbiodinium density was similar among sites yet their mitotic index increased in anemones under elevated pCO2. Overall, our findings show that A. viridis is characterized by a higher autotrophic/heterotrophic ratio as pCO2 increases. The unique trophic flexibility of this species may give it a competitive advantage and enable its potential acclimation and ecological success in the future under increased ocean acidification.

当前刺胞动物-藻类共生体系正面临二氧化碳浓度持续升高（即海洋变暖和酸化）带来的环境胁迫，该变化会影响生物的生理功能与生存表现。本研究以意大利沃尔卡诺岛的自然二氧化碳分压梯度为研究场景，针对共生海葵华丽海葵（Anemonia viridis）体内碳、氮同位素的自然变异特征展开分析，以此探究其营养策略（自养/异养）的变化规律。随着海水二氧化碳分压升高，华丽海葵的藻类共生体虫黄藻（Symbiodinium）以及宿主组织的δ¹³C值均显著降低（即同位素δ值变轻）。同时，在高二氧化碳分压位点处，共生体与宿主组织的δ¹³C值差值出现缩小，结合前述结果表明：在高二氧化碳分压条件下，华丽海葵的碳收支中净自养碳输入占比更高。沿二氧化碳分压梯度，虫黄藻与宿主组织的δ¹⁵N值以及碳氮比（C/N）均未发生显著变化。额外的生理参数检测结果显示，不同采样位点的海葵总蛋白含量与虫黄藻叶绿素a（chlorophyll a）水平均无显著差异。不同采样位点的虫黄藻密度无显著差异，但高二氧化碳分压条件下的海葵体内虫黄藻有丝分裂指数显著升高。综合来看，本研究结果表明：随着海水二氧化碳分压升高，华丽海葵的自养/异养营养比显著提升。该物种所具备的独特营养可塑性，可能为其带来竞争优势，并使其有望在未来海洋酸化加剧的环境中实现适应性驯化与生态存续。