Seawater carbonate chemistry, gross photosynthesis and metabolically induced rate of pH change during experiments with macroalgae, 2012

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO2. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO2 and HCO3 increase; CO32 decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO3]. We measured the short-term photosynthetic responses of five macroalgal species with various carbon-use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO2, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO3 increased their use of CO2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO2 did so exclusively in all treatments. Although CO2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO2 is less energetically expensive than active HCO3 uptake, and so HCO3-using macroalgae may benefit in future seawater with elevated CO2.

海洋酸化（Ocean acidification, OA）是指海洋因吸收人为产生的二氧化碳而发生pH降低的现象。该变化会改变海水无机碳组分的浓度——大型藻类可利用这些组分完成光合作用与钙化作用：其中二氧化碳（CO₂）与碳酸氢根（HCO₃⁻）浓度升高，碳酸根（CO₃²⁻）浓度降低。
实验中用于降低海水pH的两种常用方法，会对碳酸盐化学体系的其他方面造成差异化影响：通入二氧化碳气体的方式可模拟海洋酸化预计带来的海水化学变化，而添加盐酸（HCl）则会导致海水中碳酸氢根浓度相对更低。
本研究针对五种具有不同碳利用策略的大型藻类，在三组海水pH处理条件下测定了其短期光合响应：分别为通过通入二氧化碳气体将pH降至7.5的处理组、通过添加盐酸将pH降至7.5的处理组，以及环境pH为7.9的对照组。
在所检测的所有藻类物种中，二氧化碳处理组、盐酸处理组与环境pH 7.9组之间的光合速率均无显著差异。不过，大型藻类通过碳吸收提升周围海水pH的能力，在pH 7.5的处理组中表现最强。
对碳同化过程引发的pH变化进行建模分析后发现：与pH 7.9组相比，能够利用碳酸氢根的大型藻类在pH 7.5处理组中增加了对二氧化碳的利用；而仅能利用二氧化碳的物种在所有处理组中均仅以二氧化碳作为碳源。
尽管对于本次检测的大型藻类物种而言，二氧化碳不太可能成为光合作用的限制因子，但二氧化碳的扩散吸收相较于主动吸收碳酸氢根的能量成本更低。因此，在未来二氧化碳浓度升高的海水中，利用碳酸氢根的大型藻类可能会获得竞争优势。