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Energy Transfer from the Phycobilisomes to Photosystem II Reaction Centers in Wild Type Cyanidium caldarium¹

^a Institut de Biologie Physico-Chimique, 13, rue Pierre et Marie Curie, 75005 Paris, France

Nonsaturating light at 600 or 436 nanometers was used to excite specifically phycocyanin or chlorophyll a, respectively, both of which participate in light capture in photosystem II of Cyanidium caldarium. The ratio of absorption of light by phycocyanin to chlorophyll in photosystem II in this organism is >20 at 600 nanometers and 0.2 at 436 nanometers.

The distribution of the absorbed light energy at these two wavelengths was followed by detecting the flash yields of O₂ during each illumination. We found that light absorbed by phycocyanin was transferred to only half of the reaction centers of photosystem II. This heterogeneity of energy distribution arises because only half of the centers and their associated antennae of 40 chlorophyll a are attached to phycobilisomes.