Seawater carbonate chemistry and growth of calcifying tubeworm shells (Spirorbis spirorbis)

The calcareous tubeworm Spirorbis spirorbis is a widespread serpulid species in the Baltic Sea, where it commonly grows as an epibiont on brown macroalgae (genus Fucus). It lives within a Mg-calcite shell and could be affected by ocean acidification and temperature rise induced by the predicted future atmospheric CO2 increase. However, Spirorbis tubes grow in a chemically modified boundary layer around the algae, which may mitigate acidification. In order to investigate how increasing temperature and rising pCO2 may influence S. spirorbisshell growth we carried out four seasonal experiments in the Kiel Outdoor Benthocosms at elevated pCO2 and temperature conditions. Compared to laboratory batch culture experiments the benthocosm approach provides a better representation of natural conditions for physical and biological ecosystem parameters, including seasonal variations. We find that growth rates of S. spirorbis are significantly controlled by ontogenetic and seasonal effects. The length of the newly grown tube is inversely related to the initial diameter of the shell. Our study showed no significant difference of the growth rates between ambient atmospheric and elevated (1100 ppm) pCO2 conditions. No influence of daily average CaCO3 saturation state on the growth rates of S. spirorbis was observed. We found, however, net growth of the shells even in temporarily undersaturated bulk solutions, under conditions that concurrently favoured selective shell surface dissolution. The results suggest an overall resistance of S. spirorbis growth to acidification levels predicted for the year 2100 in the Baltic Sea. In contrast, S. spirorbis did not survive at mean seasonal temperatures exceeding 24 °C during the summer experiments. In the autumn experiments at ambient pCO2, the growth rates of juvenile S. spirorbis were higher under elevated temperature conditions. The results reveal that S. spirorbis may prefer moderately warmer conditions during their early life stages but will suffer from an excessive temperature increase and from increasing shell corrosion as a consequence of progressing ocean acidification.

钙质管蠕虫螺旋旋鳃虫（*Spirorbis spirorbis*）是波罗的海广泛分布的龙介虫科物种，通常作为附生生物生长在大型褐藻（墨角藻属*Fucus*）之上。该物种栖息于镁方解石（Mg-calcite）构成的壳体中，可能受到未来大气CO₂升高引发的海洋酸化与温度上升的影响。然而，螺旋旋鳃虫的管体生长在褐藻周围经化学修饰的边界层中，这一结构或可缓解酸化作用。为探究升温与pCO₂分压升高对螺旋旋鳃虫壳体生长的影响，我们在基尔野外底栖中宇宙系统（Kiel Outdoor Benthocosms）中设置了升高pCO₂与温度的条件，开展了四组季节性实验。与实验室分批培养实验相比，底栖中宇宙实验方法能更真实地反映物理与生物生态系统参数的自然状态，包括季节变化。研究发现，螺旋旋鳃虫的生长速率显著受个体发育与季节效应的调控。新生长管体的长度与壳体初始直径呈负相关关系。本研究未观测到背景大气与升高（1100 ppm）pCO₂条件下螺旋旋鳃虫生长速率存在显著差异，也未发现日平均CaCO₃饱和状态对其生长速率存在影响。但我们观测到，即使在暂时不饱和的主体溶液中，当环境同时利于壳体表面选择性溶蚀时，螺旋旋鳃虫的壳体仍能实现净生长。结果表明，螺旋旋鳃虫的生长对波罗的海2100年预测的海洋酸化水平具有整体抗性。与之相反，夏季实验中当季节平均温度超过24 ℃时，螺旋旋鳃虫无法存活。在背景pCO₂的秋季实验中，幼体螺旋旋鳃虫在升温条件下的生长速率更高。本研究结果揭示，螺旋旋鳃虫在早期生活史阶段可能偏好适度升温的环境，但会因过度升温以及持续加剧的海洋酸化引发的壳体腐蚀而受到负面影响。