Altered Excitation-Energy Transfer in the Dark-Adapted Leptolyngbya boryana Variant dg211

Dark-heterotrophic growth allows photosynthetic pigment-protein complexes to be examined when photosynthetic energy conversion is not required for growth. A dark-adapted variant, dg211, of Leptolyngbya boryana has been reported to exhibit altered carotenoid composition, but its effect on excitation-energy transfer remains unclear. Here, we analyzed light-grown wild-type (WT-light) and dg211 (dg211-light), and dark-grown dg211 (dg211-dark) cells using immunoblotting, pigment analysis, absorption spectroscopy, 77 K steady-state and time-resolved fluorescence spectroscopy, and fluorescence decay-associated spectra. β-Carotene was detected in the WT-light and dg211-light cells but not in the dg211-dark cells, although chlorophyll a and phycobilisome-related spectral features were retained. In the dg211-dark cells, D1 accumulation was markedly reduced, whereas PsaA/B remained detectable. PSII-delayed fluorescence disappeared, while PSI-associated 729-730 nm fluorescence persisted, indicating a severe depletion of functional PSII reaction-center units with retention of PSI-associated features. Pronounced 687 nm nanosecond fluorescence was attributed mainly to prolonged excitation-energy retention in phycobilisome-related emitting states. These findings reveal substantial reorganization of photosynthetic excitation-energy transfer in dark-grown dg211.