ULTRASTRUCTURE AND IMMUNOLOCALIZATION OF PHYCOBILIPROTEINS AND RIBULOSE 1,5‐BISPHOSPHATE CARBOXYLASE/OXYGENASE IN THE MARINE CYANOBACTERIUM TRICHODESMIUM THIEBAUTII1

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Abstract
Trichodesmium thiebautii Gomont, a marine planktonic diazotrophic cyanobacterium, has an unusual subcellular arrangement. To identify subcellular structures related to photosynthesis, antibodies against phycoerythrin, phycocyanin, and ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) were used together with an immuno‐gold labeling technique and electron microscopy. Thylakoid membranes, identified by transmission electron microscopy and phycobiliprotein labeling, were arranged as a loose network throughout all cells. Rubisco showed a particularly intense localization in medium electron‐dense polyhedral bodies, therefore identified as carboxysomes. The average density of the carboxysomal Rubisco label was about five times higher than that in the cytoplasm. The carboxysomes (4–11 per cell section) were scattered throughout the cytoplasm. These data, together with those obtained from double immunolabeling experiments using nitrogenase (Fe‐protein) and Rubisco antibodies, revealed that Trichodesmium contains both N2‐ and CO2‐fixing proteins within the same cell. This is in contrast to the previous concept of a spatial segregation of the two processes in Trichodesmium and demonstrate that nitrogenase‐containing cells are not comparable to heterocysts in this context.