Investigating the photosynthetic antenna during super-excess energy dissipation in the diatom Phaeodactylum tricornutum

Vasco Giovagnetti, Alexander V. Ruban
School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK

Diatoms are unicellular photosynthetic eukaryotes that show a remarkable photoprotective flexibility. When grown under intermittent light regime, the pennate diatom Phaeodactylum tricornutum is able to form non-photochemical chlorophyll fluorescence quenching (NPQ) up to five times larger (‘super-NPQ’) than that observed in the higher plants. Being secondary symbionts, diatoms possess an evolutionarily unique organization and protein/pigment composition of their thylakoid membrane.

However, the molecular and structural changes that govern their mechanism of NPQ are poorly understood. Currently, it has been proposed that NPQ consists of two quenching sites, one located in the antenna attached to photosystem II (PSII) reaction centers (RCII), while the other detaches from RCII and aggregates into oligomeric complexes. To further elucidate how antenna reorganization controls NPQ formation and relaxation, we addressed the transition of the photosynthetic antenna from light harvesting to dissipative state - and vice versa - in P. tricornutum grown under continuous and intermittent light conditions. PSII fast fluorescence induction kinetics and a number of spectroscopic measurements, as well as pigment content analysis, have been performed. Antenna rearrangement and detachment related to NPQ formation/relaxation are discussed in the context of competition between RCII and NPQ trapping capacity in the photosynthetic membrane of diatoms.