Chlorophyll–carotenoid excitation energy transfer and charge transfer in <i>Nannochloropsis oceanica</i> for the regulation of photosynthesis

Abstract

Nonphotochemical quenching (NPQ) is a proxy for photoprotective thermal dissipation processes that regulate photosynthetic light harvesting. The identification of NPQ mechanisms and their molecular or physiological triggering factors under in vivo conditions is a matter of controversy. Here, to investigate chlorophyll (Chl)-zeaxanthin (Zea) excitation energy transfer (EET) and charge transfer (CT) as possible NPQ mechanisms, we performed transient absorption (TA) spectroscopy on live cells of the microalga <i>Nannochloropsis oceanica</i> We obtained evidence for the operation of both EET and CT quenching by observing spectral features associated with the Zea S₁ and Zea^●+ excited-state absorption (ESA) signals, respectively, after Chl excitation. Knockout mutants for genes encoding either violaxanthin de-epoxidase or LHCX1 proteins exhibited strongly inhibited NPQ capabilities and lacked detectable Zea S₁ and Zea^●+ ESA signals in vivo, which strongly suggests that the accumulation of Zea and active LHCX1 is essential for both EET and CT quenching in <i>N. oceanica</i>.