Decreasing available O2 interacts with light to alter the growth and fatty acid content in a marine diatom

Hypoxic zones and oceanic deoxygenation are spreading worldwide due to anthropogenic activities and climate change, greatly affecting marine organisms exposed to lowered O2. Yet, the effects of the lowered O2 on phytoplankton are often neglected when studying O2 effects as they are the O2 producers. Here we showed that low O2 (dissolved O2, 150 ± 10 μmol L−1) enhanced the growth of the marine diatom Thalassiosira pseudonana in limited light but reduced it in moderate to inhibitory light and that hypoxia (40 ± 7.5 μmol L−1) reduced its growth at any growth lights. Low O2 and hypoxia decreased cellular chlorophyll a and carotenoid content under growth light from 30 to 400 µmol photons m−2 s−1, as well as photosynthetic efficiency (FV/FM). Such a decrease effect by low O2 or hypoxia also occurred in the O2 production by photosynthesis and the O2 consumption by dark respiration, especially under moderate to inhibitory growth light. Malondialdehyde, a product of lipid peroxidation, increased by up to 200% under low O2, and by 400% under hypoxia at the highest light. The lowered O2 mechanistically down-regulated the expression of proteins related to chlorophyll synthesis, light energy conversion and transfer, and carbon metabolism. In addition, we found that the lowered O2 generally decreased the fatty acid content by more than 30%, along with the down-regulation of proteins related to the synthesis of these fatty acids and the elongation and desaturation of carbon chains. This suggests the change in the nutrition of marine grazers' food and thus the potential impacts on the food chains in increasingly deoxygenated/hypoxic coastal waters.