Distribution of excitation energy among photosystem I and photosystem II in red algae. I. Action spectra of light reactions I and II.

Action spectra of light reaction I and light reaction II from red algae (marine members of Florideae and Bangiales) were measured with 550 nm (light 2) or 699 nm light 1) background light, using a Teflon-covered platinum electrode for O2 measurement. Care was taken to ensure that maximum enhancement was reached by the background light. The action spectra of light reaction I, we found under these conditions, are very similar to the thallus absorption, whilst the action spectra of light reaction II show, besides strong bands of the phycobilins, only minor bands of chlorophyll a, which account for only 10-20% of the total chlorophyll. The spectra are discussed on the basis of two main types of models of energy distribution over both photosynthetic systems. If this distrubution is considered to be invariable (models 1a and b), one has to assume that almost exactly half of the total chlorphyll is not involved in the supply of the non-cyclic electron transport with excitation energy. This part, however, has to be thought of as incorporated in the thylakoid membrane in a similar manner to the chlorophyll in photosystem I. However, if one supposes an almost complete equilibration in the energy distribution over both systems as long as the primary absorption in photosystem II prevails (models 2a and b), there is no need for the assumption of such photosynthetically 'inactive' or less active chlorophyll. Some evidence is shown that strongly supports model 2.