• Global warming is aggravating marine deoxygenation, with the deoxygenated extent getting worse with progressive eutrophication. Apart from aerobic organisms, the reduced O₂ influenced marine photoautotrophs; because, without available O₂ these O₂ producers cannot maintain mitochondrial respiration in dim-light or dark conditions and disrupt the metabolism of macromolecules including protein.

• We used growth rates, element and protein analyses, proteomics, and transcriptomics to estimate the cell nitrogen (N) compositions and metabolisms in diatom Thalassiosira pseudonana, growing at three pO₂ levels across a range of growth lights.

• The protein N:total N under ambient pO₂ is c. 0.54-0.83 across growth lights. At the lowest light level, decreased pO₂ exhibited a stimulated effect on cellular protein content (Low pO₂, 24%; Hypoxia, 34%). As the growth light increases to inhibitory levels, decreased pO₂ reduced the protein content, with the maximal reduction of 46% by Low pO₂ and 58% by hypoxia, respectively.

• Cells growing under low pO₂ or hypoxic status exhibit lower growth rates at higher light, coinciding with lowered protein content accompanied with downregulation of genes expression in relation to nitrate transport and reduction and protein synthesis, as well as the upregulation of the protein degradation-related genes. We predict the lowered cell proteins may poorer the nutritional state of phytoplankton, thus influencing marine food chains.