Comparative analysis of biochemical and sequence features of PSI-LHCI supercomplexes from Cyanidium caldarium NIES-551 and Cyanidiococcus yangmingshanensis NIES-2137.

Cyanidiophyceae represent basal, extremophilic taxa subdivided into several genetically distinct orders. While photosystem I supercomplexes containing light-harvesting complexes (PSI-LHCI) have been characterized in Cyanidioschyzonales and Galdieriales, their organization in Cyanidiales remains poorly understood. Here, we purified the PSI-LHCI supercomplex from Cyanidium caldarium NIES-551, and analyzed its biochemical and spectroscopic properties. SDS-PAGE and mass spectrometry identified canonical PSI and LHCI subunits. The chloroplast-encoded PSI proteins were identical to those of a previously sequenced Cd. caldarium strain. Comparative sequence analysis revealed substantial divergence in both PSI and LHCI proteins between the NIES-551 strain and Cyanidiococcus yangmingshanensis NIES-2137, a strain recently reclassified from Cd. caldarium. Pigment profiling of PSI-LHCI showed similar species across strains, yet carotenoid-to-chlorophyll a ratios were lower in NIES-551. 77-K fluorescence-emission spectra of PSI-LHCI highlighted a red-shifted emission peak in NIES-551 (734 nm) relative to NIES-2137 (727 nm), suggesting differences in pigment configuration and excitation-energy transfer. These findings indicate that PSI-LHCI in NIES-551 possesses lineage-specific pigment composition and LHCI organization. Together, these characteristics likely represent hallmarks of PSI-LHCI in Cyanidiales. Our results reinforce the evolutionary distinctiveness of Cyanidiales within Cyanidiophyceae and provide a foundation for future structural studies that will clarify the diversification of photosynthetic architecture in early-diverging red algae.