Photosynthetic organisms have developed vital strategies which allow them to switch from a light-harvesting to an energy dissipative state at the level of the antenna system in order to survive the detrimental effects of excess light illumination. These mechanisms are particularly relevant in diatoms, which grow in highly fluctuating light environments and thus require fast and strong response to changing light conditions. We performed transient absorption spectroscopy on FCPa, the main light-harvesting antenna from the diatom Cyclotella meneghiniana, in the unquenched and quenched state. Our results show that in quenched FCPa two quenching channels are active and are characterized by differing rate constants and distinct spectroscopic signatures. One channel is associated with a faster quenching rate (16ns⁻¹) and virtually no difference in spectral shape compared to the bulk unquenched chlorophylls, while a second channel is associated with a slower quenching rate (2.7ns⁻¹) and exhibits an increased population of red-emitting states. We discuss the origin of the two processes in the context of the models proposed for the regulation of photosynthetic light-harvesting. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.
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