The fluorescence emitted at 710 nm by Phaeodactylum tricornutum (F 710 ) was characterized. Development of F 710 was found to be regulated by the quality of light needed for algal growth: weak red light absorbed mainly by Chl a induced its development, and weak blue-green light absorbed mainly by fucoxanthin and Chl c suppressed it. The difference spectra between cells grown under the two light conditions revealed two Chl a forms, absorption peaks of which were located at 692 nm (Chl a 692 )

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... at 703 nm (Chl a 703 ), respectively, in red-light-grown cells. During cell growth under red light, the appearance and intensification of the emission correlated well with development of Chl a 692 and Chl a 703 suggesting that the two forms of Chl a are involved in the energy flow to F 710 . A clear induction phenomenon characteristic of the PSII fluorescence was observed not only with the emission at 680 nm but also with F 710 , indicating that F 710 is emitted by PSII Chl a. Development of F 710 under red light was sensitive to cycloheximide, indicating that the development of the energy flow to F 710 requires protein synthesis and that the emitter is installed in a protein encoded in the nuclear genome like the lightharvesting complex (LHC). Centrifugal fractionation of pigment-protein complexes revealed F 710 to be located at fractions slightly heavier than the major LHC. Development of F 710 was also found in red-light-grown cells of the diatom Nitzschia closterium.