Light harvesting and photoprotective states in the marine diatom Fragilariopsis sp.: functional implications of chlorophylls c1/c2 in the fucoxanthin–chlorophyll a/c-binding proteins (FCPs)

Abstract

We report pH/pD-dependent fluorescence-excitation spectra of the light-harvesting fucoxanthin–chlorophyll a/c-binding proteins (FCPs) of the marine diatom Fragilariopsis sp. There is a reversible 451 to 455 nm Soret transition accompanied by a 588 to 586 nm Q_x transition of chlorophylls c₁/c₂ in the pH/pD 4.9–8 range with a pK_a = 5.4. The H/D exchangeable site of the 17-acrylate group of chlorophylls c₁/c₂ was characterized and from the pH/pD sensitivity of the Soret and Q_y bands we suggest that the chlorophylls c₁/c₂-acrylate-H₂O moiety can act as a proton acceptor/donor site. Under high intensity light conditions, the acrylate moiety of chlorophylls c₁/c₂ becomes protonated, resembling that observes under acidic conditions. In the photoprotective state, the absorption of chlorophylls c₁/c₂ is red shifted and resembles that observed in the reversible transition from light-harvesting to energy-quenching state at acidic pH. The induced ΔpH and that created from the high intensity light, is responsible for the red-shifted chlorophylls c₁/c₂ due to the protonation of the acrylate group. We present a model that describes an open and a closed form of the protonated/deprotonated chlorophylls c₁/c₂-acrylate-H₂O moiety that controls the proton loading site in the photoprotective and light harvesting state.