Dynamicantenna and inter‐photosystem spillover enable flexible light harvesting in
Source: PubMed Central Open Access, NCBI / U.S. National Library of Medicine
SUMMARY Photosynthetic light‐harvesting antenna systems of aquatic algae show remarkable evolutionary diversity., a unicellular flagellate that acquired its chloroplast through secondary endosymbiosis with a green alga, exemplifies evolutionary innovation in light harvesting.possesses a distinctive photosynthetic apparatus with loosely stacked thylakoid membranes and atypical light‐harvesting complexes (LHCs). Previous studies proposed shared antenna systems serving both photosystems I and II (PSI and PSII) and inter‐photosystem excitation energy transfer (spillover) as a regulatory mechanism. Here, we investigate the mechanisms and dynamics of light harvesting in isolated photosynthetic protein complexes and whole cells ofby absorption, circular dichroism, fluorescence, and time‐resolved fluorescence spectroscopy. Isolated PSII and PSI–LHCI were found to have spectroscopic characteristics distinct from their green‐lineage counterparts. PSI–LHCI showed exceptionally long excitation lifetimes of 100–200 ps, resulting from energy transfer through a large mosaic LHCI antenna. Time‐resolved fluorescence of intact cells and thylakoid membranes revealed a characteristic 50 ps energy transfer component at room temperature from PSII‐associated antenna to PSI‐associated red chlorophyll forms, providing direct evidence for prevalent spillover. The mobile LHC complex, previously thought to contain both LHCI and LHCII, comprises exclusively multiple LHCI types and likely regulates excita
Abstract
SUMMARY Photosynthetic light‐harvesting antenna systems of aquatic algae show remarkable evolutionary diversity., a unicellular flagellate that acquired its chloroplast through secondary endosymbiosis with a green alga, exemplifies evolutionary innovation in light harvesting.possesses a distinctive photosynthetic apparatus with loosely stacked thylakoid membranes and atypical light‐harvesting complexes (LHCs). Previous studies proposed shared antenna systems serving both photosystems I and II (PSI and PSII) and inter‐photosystem excitation energy transfer (spillover) as a regulatory mechanism. Here, we investigate the mechanisms and dynamics of light harvesting in isolated photosynthetic protein complexes and whole cells ofby absorption, circular dichroism, fluorescence, and time‐resolved fluorescence spectroscopy. Isolated PSII and PSI–LHCI were found to have spectroscopic characteristics distinct from their green‐lineage counterparts. PSI–LHCI showed exceptionally long excitation lifetimes of 100–200 ps, resulting from energy transfer through a large mosaic LHCI antenna. Time‐resolved fluorescence of intact cells and thylakoid membranes revealed a characteristic 50 ps energy transfer component at room temperature from PSII‐associated antenna to PSI‐associated red chlorophyll forms, providing direct evidence for prevalent spillover. The mobile LHC complex, previously thought to contain both LHCI and LHCII, comprises exclusively multiple LHCI types and likely regulates excitation energy redistribution via spillover. Significance Statement is an early diverging photosynthetic organism with a unique mosaic photosynthetic apparatus. Biochemical and spectroscopic analysis reveals a kinetically slower photosystem I sharing excitation energy with photosystem II in the absence of strict photosystem segregation. is an early diverging photosynthetic organism that has acquired, though secondary endosymbiosis, horizontal gene transfer and gene duplication, a mosaic photosynthetic apparatus markedly different from that of other phototrophic clades. Biochemical and spectroscopic analysis reveals a unique functional architecture wherein a kinetically slower photosystem I allows sharing of excitation energy between the two photosystems in the absence of strict photosystem segregation. graphical