Acclimation to white light in a far-red light specialist: insights from Acaryochloris marina MBIC11017.

The Chl d-containing cyanobacterium, Acaryochloris marina MBIC11017, is constitutively adapted to far-red light (FRL). However, it occasionally encounters white light (WL) in its natural habitat. Using biochemical and spectroscopic techniques, we investigated how this organism acclimates to WL and analysed the excitation energy trapping dynamics of its photosystems and complex antenna system, comprised of both membrane-embedded and soluble antenna. When grown in WL, A. marina MBIC11017 doubles its Photosystem I/Photosystem II (PSI/PSII) ratio and increases its phycobilisome content compared with FRL, without altering their composition, while the number of membrane-embedded antennae decreases. Under both light conditions, phycobilisomes primarily transfer excitation energy to PSII, but a smaller fraction transfers to PSI. The PSI trapping time is fast (35 ps), confirming the absence of red-shifted forms. By contrast, PSII trapping is slower, with two components of c. 115 and c. 480 ps. Simulations based on the PSII structure suggest that this slow trapping arises mainly from the PSII antenna arrangement rather than from the use of Chld as a primary donor. These results reveal how A. marina MBIC11017 dynamically adjusts photosystem ratios and antenna composition to changes in light quality, offering insights into the ecological and functional implications of Chld-driven photosynthesis and chromatic acclimation.