Monitoring energy conversion in photosystem I of cyanobacterial heterocysts by photoacoustic spectroscopy

Photoacoustic spectroscopy was used to monitor photosynthetic energy storage in heterocysts isolated from the cyanobacterium Anabaena 7120. Three main energy-conversion pathways have been identified in these specialized cells: cyclic electron transport in photosystem I, nitrogen fixation, and respiration. The aim of this study was to define the conditions for estimating their interdependence. The gas phase (air, Ar, N2, H2) under which the sample was prepared or incubated, as well as the presence of electron donors (NADPH, NADH, H2) and KCN (an inhibitor of respiration), determined the relative flux through the pathways. Heterocysts in which respiration and nitrogen fixation are inhibited have afforded a useful system for specific study of cyclic electron transport around photosystem I. Under optimal conditions for photosystem I turnover, photochemical losses of above 20% were found.