MnSOD Diversifies, while FeSOD Dominates Superoxide Defense in Cyanobacteria

Cyanobacteria are unique among prokaryotes in possessing photosynthetic thylakoid membranes enclosing an internal lumen, where superoxide production is expected to be substantial. Thylakoid-associated superoxide dismutases (SODs) would seem the obvious first line of defense, yet many cyanobacteria rely instead on a single soluble cytoplasmic enzyme, raising the question of what role thylakoid-associated SOD isoforms actually play. Using Cyanothece sp. PCC 7425, which encodes both a cytoplasmic FeSOD and a thylakoid-associated MnSOD, we combined sequence analysis, structural, genetics, and biochemical approaches to characterise FeSOD and MnSOD cellular localisation and assess each enzyme's contribution to superoxide defense. Cyanobacterial MnSODs display diverse N-terminal architectures, consistent with soluble, transmembrane, or lipid-anchored forms, yet contribute only modestly to superoxide defense in Cyanothece PCC 7425 and, very likely, across cyanobacteria broadly. Soluble FeSOD instead provides the major line of defense, showing that thylakoid association does not guarantee physiological importance. This predominance of a cytoplasmic enzyme highlights the central role of cytoplasmic superoxide detoxification, consistent with superoxide being generated mainly at the PSI acceptor side, and extends to the chloroplast stroma of plants. Our findings support a model in which SOD distribution reflects metal availability, enzyme properties, and subcellular organisation : FeSOD protects the compartment harbouring the main targets of PSI-associated superoxide, whereas MnSOD is retained where thylakoid association offers additional local protection, but becomes dispensable when the local ROS burden is limited. Spatial matching between superoxide production, antioxidant localisation, and target vulnerability may thus shape the diversification and maintenance of antioxidant systems across cyanobacteria and chloroplasts.