Abstract
Many cyanobacteria fix atmospheric N2 only in specialised cells called heterocysts1,2. Besides containing nitrogenase2, heterocysts are distinguished biochemically from vegetative cells by their inability to evolve O2 or to perform photosystem II activities1,3,4and 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 photosynthesis1,6. 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.
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