Lowering O Interacts with Photoperiod to Alter Physiological Performance of the Coastal Diatom .

Exacerbating deoxygenation is extensively affecting marine organisms, with no exception for phytoplankton. To probe these effects, we comparably explored the growth, cell compositions, photosynthesis, and transcriptome of a diatom under a matrix of O levels and Light:Dark cycles at an optimal growth light. The growth rate (μ) of under a 8:16 L:D cycle was enhanced by 34% by low O but reduced by 22% by hypoxia. Under a 16:8 L:D cycle, however, the μ decreased with decreasing O level. The cellular Chl content decreased with decreasing O under a 8:16 L:D cycle, whereas the protein content decreased under a 16:8 L:D cycle. The prolonged photoperiod reduced the Chl but enhanced the protein contents. The lowered O reduced the maximal PSII photochemical quantum yield (F/F), photosynthetic oxygen evolution rate (Pn), and respiration rate (Rd) under the 8:16 or 16:8 L:D cycles. Cellular malondialdehyde (MDA) content and superoxide dismutase (SOD) activity were higher under low O than ambient O or hypoxia. Moreover, the prolonged photoperiod reduced the F/F and Pn among all three O levels but enhanced the Rd, MDA, and SOD activity. Transcriptome data showed that most of 26 differentially expressed genes (DEGs) that mainly relate to photosynthesis, respiration, and metabolism were down-regulated by hypoxia, with varying expression degrees between the 8:16 and 16:8 L:D cycles. In addition, our results demonstrated that the positive or negative effect of lowering O upon the growth of diatoms depends on the O level and is mediated by the photoperiod.