Decreasing O₂ availability reduces cellular protein contents in a marine diatom

Anthropogenic activities and climate change are exacerbating marine deoxygenation. Apart from aerobic organisms, reduced O₂ also affects photoautotrophic organisms in the ocean. This is because without available O₂, these O₂ producers cannot maintain their mitochondrial respiration, especially under dim-light or dark conditions, which may disrupt the metabolism of macromolecules including proteins. We used growth rate, particle organic nitrogen and protein analyses, proteomics, and transcriptomics to determine cellular nitrogen metabolism of the diatom Thalassiosira pseudonana grown under three O₂ levels in a range of light intensities at nutrient-rich status. The ratio of protein nitrogen to total nitrogen under ambient O₂ level among different light intensities was about 0.54-0.83. At the lowest light intensity, decreased O₂ had a stimulatory effect on protein content. When light intensity increased to moderate and high or inhibitory levels, decreased O₂ reduced the protein content, with maximum values of 56 % at low O₂ and 60 % at hypoxia, respectively. In addition, cells growing under low O₂ or hypoxic status exhibited a decreased rate of nitrogen assimilation associated with decreased protein content, which was associated with downregulated expression of genes related to nitrate transform and protein synthesis and upregulated expression of genes related to protein degradation. Our results suggest that decreased O₂ reduces the protein content of phytoplankton cells, which might degrade grazer nutrition and thus affect marine food chains under the scenario of increasingly deoxygenated/hypoxic waters in future.