Abstract
Cyanobacteria, the progenitors of plant and algal chloroplasts, enabled aerobic life on earth by introducing oxygenic photosynthesis. In most cyanobacteria, the photosynthetic membranes are arranged in multiple, seemingly disconnected, concentric shells. In such an arrangement, it is unclear how intracellular trafficking proceeds and how different layers of the photosynthetic membranes communicate with each other to maintain photosynthetic homeostasis. Using electron microscope tomography, we show that the photosynthetic membranes of two distantly related cyanobacterial species contain multiple perforations. These perforations, which are filled with particles of different sizes including ribosomes, glycogen granules and lipid bodies, allow for traffic throughout the cell. In addition, different layers of the photosynthetic membranes are joined together by internal bridges formed by branching and fusion of the membranes. The result is a highly connected network, similar to that of higher-plant chloroplasts, allowing water-soluble and lipid-soluble molecules to diffuse through the entire membrane network. Notably, we observed intracellular membrane-bounded vesicles, which were frequently fused to the photosynthetic membranes and may play a role in transport to these membranes.