Lhcf2 in the peripheral antenna is essential for non-photochemical quenching and Lhcx1 accumulation in the diatom Chaetoceros gracilis

Photosynthetic organisms must continuously balance efficient light harvesting with protection against excess excitation energy, a challenge met by nonphotochemical quenching (NPQ). Although the molecular components involved in NPQ have been extensively studied, how the essential energy-quenching site is assembled remains poorly understood, particularly in marine diatoms. Here, we show that in the centric diatom Chaetoceros gracilis, which belongs to one of the most abundant and diverse genera in marine phytoplankton, the light-harvesting complex protein Lhcf2 is required for energy-dependent NPQ (qE). Targeted knockout of Lhcf2 abolished qE by preventing the stable accumulation of Lhcx1, the canonical NPQ effector in this species. Lhcf2 localizes to the peripheral antenna system and associates with Lhcx1 in a higher-order complex suggested by biochemical and functional analyses. In contrast, other established NPQ-related factors, including the trans-thylakoid proton gradient and the accumulation of diatoxanthin, were not affected by the loss of Lhcf2. These results identify a non-Lhcx-type light-harvesting complex protein as an essential structural component for qE-NPQ and establish a general design principle for the cooperative assembly of photoprotective energy-quenching sites in eukaryotic photosynthesis, with implications for marine carbon fixation. Significance StatementPhotosynthetic organisms must balance efficient light harvesting with protection against excess excitation energy. Nonphotochemical quenching (NPQ) is a conserved photoprotective mechanism, yet how the essential energy-quenching site is assembled remains unclear. Here, we show that in the marine diatom Chaetoceros gracilis, the peripheral light-harvesting complex protein Lhcf2 is strictly required for energy-dependent NPQ by enabling the stable accumulation and functional organization of the canonical NPQ effector Lhcx1. Our results reveal that photoprotective energy dissipation depends on cooperative interactions between distinct classes of light-harvesting complex proteins rather than on a single specialized factor. This study establishes a general design principle for NPQ machinery in eukaryotic photosynthesis, with implications for marine carbon fixation under fluctuating light environments.