Spectroscopic Investigation of the In Vivo Light‐Dependent Photodynamics of the Marine Diatom Phaeodactylum tricornutum

Marine diatoms are responsible for fixing CO ₂ into organic carbon and contribute to the regulation of global climate. The conversion of solar energy into chemical energy is catalyzed by photosystem I (PSI) and photosystem II (PSII), located in the highly folded thylakoid membranes. Both PSs are associated with Fucoxanthin‐chlorophyll a / c proteins. Herein, we have investigated the light intensity‐dependent reactions in the cells of Phaeodactylum tricornutum by resonance Raman, fourier transform infrared (FTIR), fluorescence and laser‐induced fluorescence spectroscopy at ambient temperature. The silica‐based frustule bands observed at 472/1072 cm ⁻¹ and the allenic C═C═C band of Fucoxanthin at 1930 cm ⁻¹ are reported. In the low‐light grown cells (LL) the PSII is observed at 680 nm and the PSI at 735 nm. In high intensity light grown cells (HL) the PSI emission was blue‐shifted to 730 nm. The change in the observed relative PSI fluorescence intensity under HL conditions can create changes in the distribution of excitation energy between the two PSs (state transitions). We suggest that the pH decrease in the luminal side in the HL cells induces conformational changes in the red states of PSI including H‐bonding interactions of Chl a/c , shifting the energy dissipation pathways to protect the PSs.