Light-induced phycobilisome dynamics in Halomicronema hongdechloris

Abstract

The phototrophic cyanobacterium Halomicronema hongdechloris efficiently adapts to varying light conditions. The Chl f content of photosystem II (PS II) reversibly changes from a very low level under white-light culture conditions to a Chl f :Chl a ratio of up to 1:8 under illumination with far-red light (FRL, 720−730 nm). The time- and wavelength-resolved ps fluorescence data show the emergence of light-induced red energy traps containing Chl f in the light-harvesting antenna of some PS II complexes. From there, an efficient uphill excitation energy transfer (EET) from Chl f to Chl a supported by an entropy gain leads to charge separation under FRL conditions.Questions arise about the role of the phycobilisomes (PBS) during this process as they contain FRL-absorbing species of allophycocyanin, but the PBS seem to decouple from PS II during FRL acclimation. After adaption to FRL, the PBS of H. hongdechloris are localized in separated clusters or remain functionally coupled mainly to Chl a containing PS II. Short illumination with white light, blue light (405 nm) or red light (630 nm) leads to a mobilization of the PBS on the time scale of seconds visible by fluorescence microscopy and time-resolved fluorescence. Decay-associated spectra reveal that the PBS shortly appear functionally decoupled for several seconds and subsequently recouple to Chl f-containing PS II, thereby re-establishing efficient EET from PBS to Chl f. This process occurs homogeneously over the whole cell, but with quite different kinetics between different cells of H. hongdechloris indicating a trigger mechanisms dependent on the individual metabolic state of the cell which might be related to reactive oxygen species (ROS) as the process accelerates when the cells are incubated with DCMU.