Ocean alkalinity enhancement (OAE) does not cause cellular stress in a phytoplankton community of the subtropical Atlantic Ocean

Abstract. A natural plankton community from oligotrophic subtropical waters of the Atlantic near Gran Canaria, Spain, was subjected to varying degrees of ocean alkalinity enhancement (OAE) to assess the potential physiological effects in the context of the application of ocean carbon dioxide removal (CDR) techniques. We employed nine mesocosms with sediment traps attached to the bottom, each enclosing a volume of 8.3 m3, to create a gradient in total alkalinity (TA). OAE was based on the addition of carbonates (NaHCO3 and Na2CO3). The lowest point on this gradient was 2400 µmol L−1, which corresponded to the natural alkalinity of the environment, and the highest point was 4800 µmol L−1. Over the course of the 33 d experiment, the plankton community exhibited two distinct phases. In phase I (days 5–20), a notable decline in the photosynthetic efficiency (Fv/Fm) was observed. This change was accompanied by substantial reductions in the abundances of picoeukaryotes, small-size nanoeukaryotes (nanoeukaryotes-1), and microplankton. The cell viability of picoeukaryotes, as indicated by fluorescein diacetate hydrolysis by cellular esterases (FDA green fluorescence), slightly increased by the end of phase I, whilst the viability of nanoeukaryotes-1 and Synechococcus spp. did not change. Reactive oxygen species levels (ROS green fluorescence) showed no significant changes for any of the functional groups. In contrast, in phase II (days 21–33), a pronounced community response was observed. Increases in Fv/Fm in the intermediate OAE treatments of Δ900 to Δ1800 µmol L−1 and in chlorophyll a (Chl a), chlorophyll c2 (Chl c2), fucoxanthin, and divinyl Chl a were attributed to the emergence of blooms of large-size nanoeukaryotes (nanoeukaryotes-2) from the genera Chrysochromulina, as well as picoeukaryotes. Synechococcus spp. also flourished towards the end of this phase. In parallel, we observed a significant change of 20 % in the overall metaproteome of the phytoplankton community. This is considered a significant alteration in protein expression, having a substantial impact on cellular functions and the physiology of the organisms. Medium levels of ΔTA showed more upregulated and less-downregulated proteins than higher ΔTA treatments. Under these conditions, cell viability significantly increased in pico- and nanoeukaryotes-1 at intermediate alkalinity levels, while in Synechococcus spp., nanoeukaryotes-2 and microplankton remained stable. ROS levels did not significantly change in any functional group. The pigment ratios DD+DT : FUCO and DD+DT : Chl a increased in medium ΔTA treatments, supporting the idea of nutrient deficiency alleviation and the absence of physiological stress. When all data are taken together, this study shows that OAE did not cause cellular stress in the phytoplankton community studied, and physiological fitness was not impaired. The drawdown in phytoplankton cell numbers that was observed at times seemed to have been most likely caused by nutrient limitation.