Two Southern Ocean diatoms are more sensitive to ocean acidification and changes in irradiance than the prymnesiophyte Phaeocystis antarctica

To better understand the impact of ocean acidification (OA) and changes in light availability on Southern Ocean phytoplankton physiology, we investigated the effects of pCO2 (380 and 800 µatm) in combination with low and high irradiance (20 or 50 and 200 µmol photons/m2/s) on growth, particulate organic carbon (POC) fixation and photophysiology in the three ecologically relevant species Chaetoceros debilis, Fragilariopsis kerguelensis and Phaeocystis antarctica. Irrespective of the light scenario, neither growth nor POC per cell was stimulated by OA in any of the tested species and the two diatoms even displayed negative responses in growth (e.g. C. debilis) or POC content (e.g. F. kerguelensis) under OA in conjunction with high light. For both diatoms, also maximum quantum yields of PSII (Fv/Fm) were decreased under these conditions, indicating lowered photochemical efficiencies. To counteract the negative effects by OA and high light, the two diatoms showed diverging photoacclimation strategies. While cellular chlorophyll a and fucoxanthin contents were enhanced in C. debilis to potentially maximize light absorption, F. kerguelensis exhibited reduced chlorophyll a per cell, increased disconnection of antennae from photosystem II reaction centers and strongly lowered absolute electron transport rates (ETR). The decline in ETRs in F. kerguelensis might be explained in terms of different species-specific strategies for tuning the available flux of adenosine triphosphate and nicotinamide adenine dinucleotide phosphate. Overall, our results revealed that P. antarctica was more tolerant to OA and changes in irradiance than the two diatoms, which may have important implications for biogeochemical cycling.为深入探究海洋酸化（OA，ocean acidification）与光照可用性变化对南大洋浮游植物生理的影响，本研究针对3种生态相关物种——柔弱角毛藻（Chaetoceros debilis）、凯尔盖朗脆杆藻（Fragilariopsis kerguelensis）及南极海囊藻（Phaeocystis antarctica），设置了不同分压二氧化碳（pCO₂，380与800 µatm）结合低、高辐照度（20或50 µmol光子·m⁻²·s⁻¹与200 µmol光子·m⁻²·s⁻¹）的交互处理实验，分析其对物种生长、颗粒有机碳（POC，particulate organic carbon）固持能力及光生理特性的作用效果。 无论光照条件如何，所有受试物种的生长速率与单位细胞颗粒有机碳含量均未因海洋酸化得到显著促进；两种硅藻在海洋酸化结合高光辐照度的处理组中，甚至分别表现出生长（如柔弱角毛藻）或单位细胞颗粒有机碳含量（如凯尔盖朗脆杆藻）的负向响应。在该处理条件下，两种硅藻的光系统II（PSII，photosystem II）最大量子产量（Fv/Fm）均出现下降，提示其光化学效率有所降低。 为抵消海洋酸化与高光辐照度带来的负面影响，两种硅藻展现出分化显著的光适应策略：柔弱角毛藻通过提升单位细胞内叶绿素a（chlorophyll a）与岩藻黄素（fucoxanthin）的含量，以增强光能吸收能力；而凯尔盖朗脆杆藻则呈现出单位细胞叶绿素a含量降低、光系统II反应中心与捕光天线复合物的解离程度升高，且绝对电子传递速率（ETR，electron transport rates）显著下降的特征。凯尔盖朗脆杆藻的电子传递速率下降现象，或可通过物种特异性的三磷酸腺苷（ATP，adenosine triphosphate）与烟酰胺腺嘌呤二核苷酸磷酸（NADPH，nicotinamide adenine dinucleotide phosphate）可用通量调控策略差异加以解释。 整体而言，本研究结果表明南极海囊藻对海洋酸化及辐照度变化的耐受性优于两种硅藻，该发现可能对海洋生物地球化学循环具有重要的生态学意义。