FIGURE 1.

Schematic representation of oxygenic photosynthesis and the generation of superoxide under light conditions inside a cyanobacterial cell. The thylakoid membrane contains photosystem II (PSII) connected to phycobilisomes, photosystem I (PSI), cytochrome b6f (cyt b6f) and electron transporters plastoquinone (PQ) and plastocyanin (PC). Phycobilisomes capture light energy and transfer it to PSII, resulting in the hydrolysis of water and transfer of electrons to PQ, with the concomitant generation of molecular oxygen (O₂) and protons (H⁺). PQ then transfers the electrons to PSI via cyt b6f and PC, thereby translocating two protons (2H⁺) across the thylakoid membrane into the lumen. The resulting proton gradient powers the synthesis of adenosine triphosphate (ATP) via ATP synthase. PSI transfers electrons to ferredoxin (Fd), which in turn transfers electrons to ferredoxin NADP+ oxidoreductase (FNR) to generate reduced nicotinamide adenine dinucleotide phosphate (NADPH). Both NADPH and ATP are used to power cellular processes, such as CO₂ fixation (Hagemann et al. 2016). Increased light exposure may result in excess electrons being fed into the electron transport chain, resulting in the generation of superoxide (O₂ ^•−) (Latifi, Ruiz, and Zhang 2009). Dismutation of O₂ ^•− to hydrogen peroxide (H₂O₂) potentially occurs in different species via superoxide dimutases (SODs), in the cytoplasm, thylakoid lumen or periplasmic space (Herbert et al. 1992; Li et al. 2002; Napoli et al. 2021; Raghavan, Rajaram, and Apte 2013, 2015). The image was generated in BioRender©.