Photosystem II core quenching in desiccated Leptolyngbya ohadii.

Cyanobacteria living in the harsh environment of the desert have to protect themselves against high light intensity and prevent photodamage. These cyanobacteria are in a desiccated state during the largest part of the day when both temperature and light intensity are high. In the desiccated state, their photosynthetic activity is stopped, whereas upon rehydration the ability to perform photosynthesis is regained. Earlier reports indicate that light-induced excitations in Leptolyngbya ohadii are heavily quenched in the desiccated state, because of a loss of structural order of the light-harvesting phycobilisome structures (Bar Eyal et al. in Proc Natl Acad Sci 114:9481, 2017) and via the stably oxidized primary electron donor in photosystem I, namely P700+ (Bar Eyal et al. in Biochim Biophys Acta Bioenergy 1847:1267-1273, 2015). In this study, we use picosecond fluorescence experiments to demonstrate that a third protection mechanism exists, in which the core of photosystem II is quenched independently.

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 is completely reliant on changes in the aggregation state of the phycobilisome light-harvesting antenna components. All photosynthetic organisms regulate the efficiency of excitation energy transfer (EET) to fit light energy supply to biochemical demands. Not many do this to the extent required of desert crust cyanobacteria. Following predawn dew deposition, they harvest light energy with maximum efficiency until desiccating in the early morning hours. In the desiccated state, absorbed energy is completely quenched. Time and spectrally resolved fluorescence emission measurements of the desiccated desert crust Leptolyngbya ohadii strain identified (i) reduced EET between phycobilisome components, (ii) shorter fluorescence lifetimes, and (iii) red shift in the emission spectra, compared with the hydrated state. These changes coincide with a loss of the ordered phycobilisome structure, evident from small-angle neutron and X-ray scattering and cryo-transmission electron microscopy data. Based on these observations we propose a model where in the hydrated state the organized rod structure of the phycobilisome supports directional EET to reaction centers with minimal losses due to thermal dissipation. In the desiccated state this structure is lost, giving way to more random aggregates. The resulting EET path will exhibit increased coupling to the environment and enhanced quenching.

An easily reversible structural change underlies mechanisms enabling desert crust cyanobacteria to survive desiccation.

Biological desert sand crusts are the foundation of desert ecosystems, stabilizing the sands and allowing colonization by higher order organisms. The first colonizers of the desert sands are cyanobacteria. Facing the harsh conditions of the desert, these organisms must withstand frequent desiccation-hydration cycles, combined with high light intensities. Here, we characterize structural and functional modifications to the photosynthetic apparatus that enable a cyanobacterium, Leptolyngbya sp., to thrive under these conditions. Using multiple in vivo spectroscopic and imaging techniques, we identified two complementary mechanisms for dissipating absorbed energy in the desiccated state. The first mechanism involves the reorganization of the phycobilisome antenna system, increasing excitonic coupling between antenna components. This provides better energy dissipation in the antenna rather than directed exciton transfer to the reaction center. The second mechanism is driven by constriction of the thylakoid lumen which limits diffusion of plastocyanin to P700. The accumulation of P700(+) not only prevents light-induced charge separation but also efficiently quenches excitation energy. These protection mechanisms employ existing components of the photosynthetic apparatus, forming two distinct functional modes. Small changes in the structure of the thylakoid membranes are sufficient for quenching of all absorbed energy in the desiccated state, protecting the photosynthetic apparatus from photoinhibitory damage. These changes can be easily reversed upon rehydration, returning the system to its high photosynthetic quantum efficiency.

Balancing photosynthetic electron flow is critical for cyanobacterial acclimation to nitrogen limitation.

Nitrogen limitation forces photosynthetic organisms to reallocate available nitrogen to essential functions. At the same time, it increases the probability of photo-damage by limiting the rate of energy-demanding metabolic processes, downstream of the photosynthetic apparatus. Non-diazotrophic cyanobacteria cope with this situation by decreasing the size of their phycobilisome antenna and by modifying their photosynthetic apparatus. These changes can serve two purposes: to provide extra amino-acids and to decrease excitation pressure. We examined the effects of nitrogen limitation on the form and function of the photosynthetic apparatus. Our aim was to study which of the two demands serve as the driving force for the remodeling of the photosynthetic apparatus, under different growth conditions. We found that a drastic reduction in light intensity allowed cells to maintain a more functional photosynthetic apparatus: the phycobilisome antenna was bigger, the activity of both photosystems was higher and the levels of photosystem (PS) proteins were higher. Pre-acclimating cells to Mn limitation, under which the activity of both PSI and PSII is diminished, results in a very similar response. The rate of PSII photoinhibition, in nitrogen limited cells, was found to be directly related to the activity of the photosynthetic apparatus. These data indicate that, under our experimental conditions, photo-damage avoidance was the more prominent determinant during the acclimation process. The combinations of limiting factors tested here is by no means artificial. Similar scenarios can take place under environmental conditions and should be taken into account when estimating nutrient limitations in nature.