Cyanobacterial thylakoid membranes represent the active sites for both photosynthetic and respiratory electron transport. We used high-resolution atomic force microscopy to visualise the native organisation and interactions of photosynthetic complexes within the thylakoid membranes from the model cyanobacterium Synechococcus elongatus PCC 7942. The thylakoid membranes are heterogeneous and assemble photosynthetic complexes into functional domains to enhance their coordination and regulation. Under high light, the chlorophyll-binding proteins IsiA are strongly expressed and associates with Photosystem I (PSI) forming highly variable IsiA−PSI supercomplexes to increase the absorption cross-section of PSI. There are also tight interactions of PSI with Photosystem II (PSII), cytochrome b 6 f, ATP synthase, and NAD(P)H dehydrogenase complexes. The organisational variability of these photosynthetic supercomplexes permits efficient linear and cyclic electron transport and bioenergetic regulation. Understanding the organisational landscape and environmental adaptation of cyanobacterial thylakoid membranes may help inform strategies for engineering efficient photosynthetic systems and photo-biofactories.

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Nitrogen is one of the most important nutrients needed for plants and algae to survive, and the photosynthetic ability of algae is related to nitrogen abundance. Red algae are unique photosynthetic eukaryotic organisms in the evolution of algae, as they contain phycobilisomes (PBSs) on their thylakoid membranes. In this report, the in vivo chlorophyll (Chl) a fluorescence kinetics of nitrogen-starved Porphyridium cruentum were analyzed to determine the effects of nitrogen deficiency on photosynthetic performance using a multi-color pulse amplitude modulation (PAM) chlorophyll fluorometer. Due to nitrogen starvation, the photochemical efficiency of PSII and the activity of PSII reaction centers (RCs) decreased, and photoinhibition of PSII occurred. The water-splitting system on the donor side of PSII was seriously impacted by nitrogen deficiency, leading to the inactivation of the oxygen-evolving complex (OEC) and decreased light energy conversion efficiency. In nitrogen-starved cells, a higher proportion of energy was used for photochemical reactions, and thermal dissipation was reduced, as shown by qP and qN. The ability of nitrogen-starved cells to tolerate and resist high photon flux densities was weakened. Our results showed that the photosynthetic performance of P. cruentum was severely impacted by nitrogen deficiency.

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