Diffusion boundary layers ameliorate the negative effects of ocean acidification on the temperate coralline macroalga Arthrocardia corymbosa

Anthropogenically-modulated reductions in pH, termed ocean acidification, could pose a major threat to the physiological performance, stocks, and biodiversity of calcifiers and may devalue their ecosystem services. Recent debate has focussed on the need to develop approaches to arrest the potential negative impacts of ocean acidification on ecosystems dominated by calcareous organisms. In this study, we demonstrate the role of a discrete (i.e. diffusion) boundary layer (DBL), formed at the surface of some calcifying species under slow flows, in buffering them from the corrosive effects of low pH seawater. The coralline macroalga Arthrocardia corymbosa was grown in a multifactorial experiment with two mean pH levels (8.05 'ambient' and 7.65 a worst case 'ocean acidification' scenario projected for 2100), each with two levels of seawater flow (fast and slow, i.e. DBL thin or thick). Coralline algae grown under slow flows with thick DBLs (i.e., unstirred with regular replenishment of seawater to their surface) maintained net growth and calcification at pH 7.65 whereas those in higher flows with thin DBLs had net dissolution. Growth under ambient seawater pH (8.05) was not significantly different in thin and thick DBL treatments. No other measured diagnostic (recruit sizes and numbers, photosynthetic metrics, %C, %N, %MgCO3) responded to the effects of reduced seawater pH. Thus, flow conditions that promote the formation of thick DBLs, may enhance the subsistence of calcifiers by creating localised hydrodynamic conditions where metabolic activity ameliorates the negative impacts of ocean acidification.由人类活动驱动的pH降低现象，被称为海洋酸化（ocean acidification），可能对钙化生物的生理机能、种群规模与生物多样性构成重大威胁，并削弱其提供的生态系统服务功能。近期学界的讨论焦点集中于，亟需开发应对手段以遏制海洋酸化对钙化生物主导生态系统的潜在负面影响。本研究证实了在缓流环境下部分钙化物种体表形成的离散（即扩散）边界层（diffusion boundary layer, DBL），能够为其抵御低pH海水的腐蚀作用提供缓冲。本实验以珊瑚藻（Arthrocardia corymbosa）为材料，设置两水平pH梯度（8.05为环境基准，7.65为预测至2100年的极端海洋酸化情景），并针对每个pH梯度设置两种海水流速（快流与缓流，对应形成薄与厚的扩散边界层），开展多因素培养实验。结果显示，在pH 7.65的环境中，缓流环境下形成厚扩散边界层的珊瑚藻仍可维持净生长与钙化过程；而高流速下形成薄扩散边界层的个体则出现净溶解现象。在环境pH（8.05）条件下，薄与厚扩散边界层处理组的珊瑚藻生长速率无显著差异。其余所有检测指标（幼体大小与数量、光合参数、碳含量百分比、氮含量百分比、碳酸镁百分比）均未对海水pH降低产生显著响应。因此，能够促进厚扩散边界层形成的流态条件，可通过构建局部水动力环境，借助生物代谢活动缓解海洋酸化的负面影响，从而提升钙化生物的生存能力。