Extra O₂ evolution reveals an O₂-independent alternative electron sink in photosynthesis of marine diatoms

Following the principle of oxygenic photosynthesis, electron transport in the thylakoid membranes (i.e., light reaction) generates ATP and NADPH from light energy, which is subsequently utilized for CO₂ fixation in the Calvin-Benson-Bassham cycle (i.e., dark reaction). However, light and dark reactions could discord when an alternative electron flow occurs with a rate comparable to the linear electron flow. Here, we quantitatively monitored O₂ and total dissolved inorganic carbon (DIC) during photosynthesis in the pennate diatom Phaeodactylum tricornutum, and found that evolved O₂ was larger than the consumption of DIC, which was consistent with ¹⁴CO₂ measurements in literature. In our measurements, the stoichiometry of O₂ evolution to DIC consumption was always around 1.5 during photosynthesis at different DIC concentrations. The same stoichiometry was observed in the cells grown under different CO₂ concentrations and nitrogen sources except for the nitrogen-starved cells showing O₂ evolution 2.5 times larger than DIC consumption. An inhibitor to nitrogen assimilation did not affect the extra O₂ evolution. Further, the same physiological phenomenon was observed in the centric diatom Thalassiosira pseudonana. Based on the present dataset, we propose that the marine diatoms possess the metabolic pathway(s) functioning as the O₂-independent electron sink under steady state photosynthesis that reaches nearly half of electron flux of the Calvin-Benson-Bassham cycle.