Changes in aggregation states of light-harvesting complexes as a mechanism for modulating energy transfer in desert crust cyanobacteria

In this paper we propose an energy dissipation mechanism that iscompletely reliant on changes in the aggregation state of thephycobilisome light-harvesting antenna components. All photosyntheticorganisms regulate the efficiency of excitation energy transfer(EET) to fit light energy supply to biochemical demands. Not many dothis to the extent required of desert crust cyanobacteria. Followingpredawn dew deposition, they harvest light energy with maximumefficiency until desiccating in the early morning hours. In thedesiccated state, absorbed energy is completely quenched. Timeand spectrally resolved fluorescence emission measurements of thedesiccated desert crust Leptolyngbya ohadii strain identified (i) reducedEET between phycobilisome components, (ii) shorter fluorescencelifetimes, and (iii) red shift in the emission spectra, comparedwith the hydrated state. These changes coincide with a loss of theordered phycobilisome structure, evident from small-angle neutronand X-ray scattering and cryo-transmission electron microscopy data.Based on these observations we propose a model where in the hydratedstate the organized rod structure of the phycobilisome supportsdirectional EET to reaction centers with minimal losses due tothermal dissipation. In the desiccated state this structure is lost, givingway to more random aggregates. The resulting EET path will exhibitincreased coupling to the environment and enhanced quenching