This study investigated how Rhodospirillum rubrum forms intracytoplasmic membranes during phototrophic growth using freeze-etch electron microscopy. Chemotrophically grown cells were transferred to anaerobic light conditions, and samples were collected at intervals to correlate bacteriochlorophyll content with membrane structure. Freeze-etching revealed indentations on convex fracture faces and protuberances on concave faces of the cytoplasmic membrane in photosynthetically active cells, which the authors interpreted as invagination sites for intracytoplasmic membranes. These structures were absent in chemotrophic cells. The number of invaginations per square micrometer of membrane increased as bacteriochlorophyll content rose, though no strict proportionality existed between these parameters. Initially, bacteriochlorophyll synthesis occurred without detectable invaginations, but as pigment levels increased, invaginations appeared and their number continued to rise. This demonstrates that the cytoplasmic membrane participates in continuous formation of new intracytoplasmic membranes throughout the adaptation process, supporting the hypothesis that these organelles form by invagination rather than by elongation of a single initial invagination.

Key findings

• Freeze-etch microscopy revealed indentations and protuberances on cytoplasmic membrane fracture faces that represent sites of membrane invaginations in phototrophically grown R. rubrum
• The number of invaginations increased significantly as cellular bacteriochlorophyll content increased, indicating ongoing formation of new intracytoplasmic membranes
• Early adaptation to phototrophic conditions showed bacteriochlorophyll synthesis without detectable invaginations, suggesting initial pigment incorporation into the cytoplasmic membrane
• No fixed proportionality between bacteriochlorophyll accumulation and invagination formation was established, suggesting complex regulation of membrane development
• Data support a model where both elongation of existing invaginations and formation of new invaginations drive intracytoplasmic membrane proliferation