Biochemical characterization of PSI-FCPI and FCP complexes from a haptophyte Pavlova (Pavlovophyceae).

Abstract

Most oxygenic photosynthesis relies on photosystem cores coupled with light-harvesting complexes (LHCs) to capture and funnel solar energy. In haptophytes, LHCs are represented by fucoxanthin chlorophyll (Chl) a/c-binding proteins (FCPs), which assemble into a photosystem I (PSI) supercomplex known as PSI-FCPI. However, the structural and pigment diversity of PSI-FCPI in early-branching haptophytes remains largely unexplored. Here, we present the first biochemical and spectroscopic characterization of PSI-FCPI and FCP complexes isolated from Pavlova sp. OPMS30543X, a member of the basal haptophyte lineage Pavlovophyceae. PSI-FCPI and FCP fractions were separated by glycerol density gradient centrifugation, revealing distinct pigment and protein compositions. Notably, a Chl c2-like pigment, previously detected only in whole-cell extracts of Pavlova, was identified in both complexes. Its red-shifted absorption relative to canonical Chl c2 suggests a specialized pigment-binding environment within the protein scaffolds. Additionally, an unknown carotenoid absorbing at 418, 441, and 469 nm was detected exclusively in the PSI-FCPI supercomplex, though its identity could not be confirmed owing to a mismatch in retention time with known references. Fluorescence-emission spectra revealed excitation-dependent energy transfer heterogeneity among Chl a species, indicating functional differentiation within the antenna system. Altogether, these results suggest that Pavlovophyceae retains ancestral pigment-protein configurations, providing new insights into the early evolution of light-harvesting systems in haptophytes.