Molecular environments of divinyl chlorophylls in Prochlorococcus and Synechocystis: Differences in fluorescence properties with chlorophyll replacement

A marine cyanobacterium, Prochlorococcus, is a unique oxygenic photosynthetic organism, which accumulates divinyl chlorophylls instead of the monovinyl chlorophylls. To investigate the molecular environment of pigments after pigment replacement but before optimization of the protein moiety in photosynthetic organisms, we compared the fluorescence properties of the divinyl Chl a-containing cyanobacteria, Prochlorococcus marinus (CCMP 1986, CCMP 2773 and CCMP 1375), by a Synechocystis sp. PCC 6803 (Synechocystis) mutant in which monovinyl Chl a was replaced with divinyl Chl a. P. marinus showed a single fluorescence band for photosystem (PS) II at 687 nm at 77 K; this was accompanied with change in pigment, because the Synechocystis mutant showed the identical shift. No fluorescence bands corresponding to the PS II 696-nm component and PS I longer-wavelength component were detected in P. marinus, although the presence of the former was suggested using time-resolved fluorescence spectra. Delayed fluorescence (DF) was detected at approximately 688 nm with a lifetime of approximately 29 ns. In striking contrast, the Synechocystis mutant showed three fluorescence bands at 687, 696, and 727 nm, but suppressed DF. These differences in fluorescence behaviors might not only reflect differences in the molecular structure of pigments but also differences in molecular environments of pigments, including pigment–pigment and/or pigment–protein interactions, in the antenna and electron transfer systems.

Research Highlights
► Three cultured strains of Prochlorococcus marinus showed characteristic fluorescence.
► Synechocystis sp. PCC 6803 with divinyl chlorophyll a changed fluorescence spectrum.
► We discuss the molecular environments of the pigments following replacement of a photosynthetic pigment, monovinyl chlorophyll to divinyl chlorophyll.