Mg2+ limitation leads to a decrease in chlorophyll, resulting in an unbalanced photosynthetic apparatus in the cyanobacterium Synechocytis sp. PCC6803.

Mg²⁺, the most abundant divalent cation in living cells, plays a pivotal role in numerous enzymatic reactions and is of particular importance for organisms performing oxygenic photosynthesis. Its significance extends beyond serving as the central ion of the chlorophyll molecule, as it also acts as a counterion during the light reaction to balance the proton gradient across the thylakoid membranes. In this study, we investigated the effects of Mg²⁺ limitation on the physiology of the well-known model microorganism Synechocystis sp. PCC6803. Our findings reveal that Mg²⁺ deficiency triggers both morphological and functional changes. As seen in other oxygenic photosynthetic organisms, Mg²⁺ deficiency led to a decrease in cellular chlorophyll concentration. Moreover, the PSI-to-PSII ratio decreased, impacting the photosynthetic efficiency of the cell. In line with this, Mg²⁺ deficiency led to a change in the proton gradient built up across the thylakoid membrane upon illumination.