Data for the phycosphere pH of unicellular nano- and micro- phytoplankton cells and consequences for iron speciation

Surface ocean pH is declining due to anthropogenic atmospheric CO2 uptake with a global decline of ~0.3 possible by 2100. Extracellular pH influences a range of biological processes, including nutrient uptake, calcification and silicification. However, there are poor constraints on how pH levels in the extracellular microenvironment surrounding phytoplankton cells (the phycosphere) differ from bulk seawater. This adds uncertainty to biological impacts of environmental change. Furthermore, previous modelling work suggests that phycosphere pH of small cells is close to bulk seawater, and this has not been experimentally verified. Here we observe under 140 μmol photons·m-2·s-1 the phycosphere pH of <em>Chlamydomonas concordia</em> (5 µm), <em>Emiliania huxleyi </em>(5 µm), <em>Coscinodiscus radiatus </em>(50 µm) and <em>C. wailesii </em>(100 µm) are 0.11, 0.20, 0.41 and 0.15 higher than bulk seawater (pH 8.00), respectively. Thickness of the pH boundary layer of <em>C. wailesii </em>increases from 18 to 122 µm when bulk seawater pH decreases from 8.00 to 7.78. Phycosphere pH is regulated by photosynthesis and extracellular enzymatic transformation of bicarbonate, as well as being influenced by light intensity and seawater pH and buffering capacity. The pH change alters Fe speciation in the phycosphere, and hence Fe availability to phytoplankton is likely better predicted by the phycosphere, rather than bulk seawater. Overall, the precise quantification of chemical conditions in the phycosphere is crucial for assessing the sensitivity of marine phytoplankton to the ongoing ocean acidification and Fe limitation in surface oceans.

受人类活动驱动的大气CO₂海洋吸收效应影响，表层海水pH值正逐步降低，预计至2100年全球表层海水pH将累计下降约0.3个单位。细胞外pH会调控诸多生物学过程，包括营养摄取、钙化作用与硅化作用。然而，学界目前对浮游植物细胞周围胞外微环境（即藻际环境（phycosphere））的pH水平与主体海水的差异缺乏精准约束，这一认知缺口进一步加剧了海洋环境变化对生物影响评估的不确定性。此外，过往建模研究表明小型浮游植物的藻际pH接近主体海水，但该结论尚未通过实验得到验证。
本研究在140 μmol光子·米⁻²·秒⁻¹的光照条件下，测得<em>康氏衣藻（Chlamydomonas concordia）</em>（直径5 μm）、<em>赫氏颗石藻（Emiliania huxleyi）</em>（直径5 μm）、<em>辐射圆筛藻（Coscinodiscus radiatus）</em>（直径50 μm）以及<em>威氏圆筛藻（Coscinodiscus wailesii）</em>（直径100 μm）的藻际pH分别较主体海水（pH 8.00）高出0.11、0.20、0.41和0.15个单位。当主体海水pH从8.00降至7.78时，<em>威氏圆筛藻（Coscinodiscus wailesii）</em>的pH边界层厚度从18 μm增加至122 μm。
藻际pH主要受光合作用与碳酸氢盐的胞外酶促转化调控，同时也受光照强度、海水pH及缓冲能力的影响。藻际pH的变化会改变铁的化学形态，因此浮游植物可利用的铁生物有效性，或许更适合通过藻际环境而非主体海水的pH水平来预测。总而言之，精准量化藻际环境的化学条件，对于评估海洋浮游植物对当前表层海洋酸化及铁限制的响应敏感性至关重要。