Divergent responses of the diatom <i>Thalassiosira weissflogii</i> to ocean acidification during light and dark periods

Abstract Given the limited understanding of discrepancies in responses of diatoms to ocean acidification (OA), we comparatively investigated the physiological and transcriptional performances of a diatom Thalassiosira weissflogii acclimated to OA (pH t drop of 0.35–0.41) between day and night periods. We found that OA enhanced its specific growth rate (up to 10%) in the light period by upregulating light reaction, Calvin cycle and H + pumps to cope with the decreased pH. On the other hand, OA reduced its apparent specific growth rate (14%) in the dark period due to additive pH drop caused by OA‐enhanced respiratory CO 2 release. In the dark period, the cells could not effectively cope with the decreased pH since H + pumps were downregulated. Consequently, OA did not affect cell growth during a 24 h diel cycle. These findings suggest that daytime positive and night negative effects of OA on diatoms could be responsible for differential results observed under different conditions, with implications for possible seasonal and latitudinal effects of OA.