Expansion microscopy visualizes the photosystem distribution in Synechocystis cells

Area of Science:

• Cellular Biology
• Microscopy
• Photosynthesis

Background:

• Photosynthesis in cyanobacteria involves photosystem I (PSI), photosystem II (PSII), and phycobilisomes (PBS).
• The spatial organization of these complexes within thylakoid membranes is not fully understood, with potential microdomain formation suggested.
• Environmental conditions may influence the distribution of photosynthetic complexes.

Purpose of the Study:

• To visualize and map the nanoscale distribution of PSI, PSII, and PBS in cyanobacteria.
• To assess the utility of cryo-expansion microscopy (cryo-ExM) for studying cyanobacterial thylakoid organization.

Main Methods:

• Application of cryo-expansion microscopy (cryo-ExM) to dark-adapted Synechocystis sp. PCC 6803 cells.
• Cryo-fixation, rehydration, and physical expansion of cells by 5.5-fold.
• Immunolabeling for simultaneous localization of PSI, PSII, and PBS within the expanded thylakoid network.

Main Results:

• Cryo-ExM achieved nanoscale resolution of thylakoid compartments in intact cyanobacterial cells.
• Overall spatial distributions of PSI, PSII, and PBS signals were similar.
• Phycobilisomes (PBS) were notably excluded from the neck region between dividing cells, whereas PSI and PSII were present in this area.

Conclusions:

• Cryo-ExM is a powerful technique for visualizing cyanobacterial thylakoid membranes at high resolution.
• The study provides insights into the spatial organization of photosynthetic complexes, particularly the exclusion of PBS from cell division necks.
• This method complements existing approaches for dissecting the spatial organization of photosynthesis in cyanobacteria.