Abstract
The positive or negative effect of a decrease in dissolved O 2 on the photophysiology of phytoplankton is determined by the duration of light exposure. To uncover the underlying mechanisms, the marine model diatom Thalassiosira pseudonana was cultured under three dissolved O 2 levels (8.0 mg L -1 , ambient O 2 ; 4.0 mg L -1 , low O 2 ; and 1.3 mg L -1 , hypoxia) to compare its growth, cell composition, and physiology between the light and dark periods. The results showed that the growth rate under ambient O 2 was 0.60 ± 0.02 day -1 , which was half of the growth rate during light period and 15-fold of the growth rate during dark period. Decreasing O 2 increased the growth rate during light period but decreased it during dark period and decreased the cell pigment content in both the light and dark periods. In the light, low O 2 increased cell carbon (C) content, while hypoxia decreased it, with the degree of increase and decrease being greater in the dark. Low O 2 had no significant effect on cell nitrogen (N) content, but hypoxia decreased it. Low O 2 had no significant effect on photosynthetic efficiency but decreased the dark respiration rate. In darkness, low O 2 had no significant effect on cell C loss rate but decreased N loss rate, leading to an increase in the POC/PON ratio. In addition, hypoxia exacerbated cell mortality and sinking, suggesting that diatom-derived carbon burial may be accelerated due to marine deoxygenation in the future.