Mechanisms of light harvesting complex proteins in photoprotection of the brown tide alga.

The propensity of Aureococcus anophagefferens to form harmful brown tide blooms has been linked to rapid light responses, but the underlying molecular mechanisms remain elusive. Here, we find that two glutamic residues in plastid luminal and C terminal domains in light harvesting complex (LHC) proteins are crucial to the alga's unique photoadaptation capacity. Specifically, we demonstrate that glutamate residues contribute to the induction of non-photochemical quenching (NPQ). Protein structure analysis further indicates that these acidic residues can form stable hydrogen bonds under protonation, causing changes in the secondary structure of LHC. Our data suggest that this is the initial action of amino acids under light-induced lumen acidification, which then drives the function of NPQ through a complex process. This photoprotection mechanism, along with low light adaptation, enables this alga to thrive throughout water columns with spatially contrasting and temporally fluctuating irradiance, with implications of bloom formation.