Abstract
Studies of marine microalgal photosynthesis are heavily molded on legacy research from organisms like Arabidopsis and Chlamydomonas, despite the differences between primary and secondary endosymbionts. Nonphotochemical quenching (NPQ) protects photosystem II from excessive light and, in pennate diatoms, requires the xanthophyll pigment diatoxanthin and Lhcx proteins. Although NPQ’s relationship with diatoxanthin is straightforward, the role of Lhcx proteins has been unclear, leading to unnecessarily complex models due to comparisons with green organisms. We used 14 Phaeodactylum tricornutum strains, including 13 transgenic lines with variable Lhcx1 expression levels, grew them under two non-stressful light conditions, and modulated diatoxanthin levels through short light stress. The resulting Lhcx1-diatoxanthin matrices were used to demonstrate that NPQ is proportional to the product of the Lhcx1 concentration and the proportion of diatoxanthin in the xanthophyll pool. This indicates that the interaction between diatoxanthin and Lhcx1, likely through the binding of diatoxanthin on Lhcx1, creates a homogeneous Stern-Volmer quencher responsible for NPQ. Additionally, we demonstrated that the photosynthetic unit in pennate diatoms follows a “lake” model, with discrepancies in the NPQ-photochemistry relationship arising from unconsidered assumptions, one possibility being cellular heterogeneity. This underscores pennate diatoms as natural reductionist system for studying marine photosynthesis.