Functional organisation of the photo-synthetic apparatus in heterocysts of nitrogen-fixing cyanobacteria

Many cyanobacteria fix atmospheric N₂ only in specialised cells called heterocysts^1,2. Besides containing nitrogenase², heterocysts are distinguished biochemically from vegetative cells by their inability to evolve O₂ or to perform photosystem II activities ^1,3,4and by a deficiency in the CO₂ fixing enzyme ribulose-1,5-bisphosphate carboxylase⁵. These cells also lack phycobiliproteins and phycobilisomes, that are part of the major light-collecting system for the light reactions of photosynthesis^1,6. The phycobilipigments, light-collecting chlorophyll a and the photochemical reaction centres constitute a photosynthetic unit (PSU) - the minimum number of pigments necessary to drive photochemistry, oxygen evolution and photosynthetic electron transport. Therefore, the PSU represents an energy generating unit. In cyanobacteria, photosynthesis, and hence the PSU, contributes both ATP and probably a strong reductant to drive nitrogen fixation⁸. Because of the distinct photosynthetic properties of these cells, especially in pigmentation, we have compared the functional organisation of the pigments and photochemical activity of the thylakoid membranes from vegetative and heterocyst cells of Nostoc muscorum and Anabaena cylindrica. We demonstrate that the PSU is modified during the differentiation of heterocysts to support the specialised energetic requirements of these nitrogen fixing cells.