Light Harvesting in the Deep Ocean – Tuning Energy Transfer Efficiency in Marine Cyanobacteria in Response to Light Intensity

Energy transfer processes in photosynthetic light-harvesting antennae demonstrate remarkable efficiency. Traditionally this mechanism was modeled through dipole-dipole interaction between adjacent chromophores using Forster resonance energy transfer (FRET) amidst an incoherent path. This classic approach has difficulties justifying the relatively high efficiency of the process, while recent experimental results using 2D photon-echo spectroscopy suggest that underlying these processes is long-lived quantum coherence. It was shown theoretically that a combination of quantum and classical processes could enhance the total light collection efficiency. Moreover, in this limit between classical and quantum regime, small structural changes could control the energy transfer efficiency. If indeed photosynthetic organisms have a way to reduce decoherence and increase efficiency, we expect this control mechanism to become evident where light is a limited resource. In the oceans, marine cyanobacteria Synechococcus WH8102 live through-out the water column, under different light regimes, and survive vertical mixing of the water layers, by photo-acclimating to the environmental conditions. The light harvesting antennae rods of Synechococcus WH8102 are long, way beyond the optimal length expected by classical approaches. In this study, we compare the same bacteria culture grown under different light intensities, studying the different organization and mechanisms enabling efficient harvesting at different ocean depths. A clear change is seen between growth at different light conditions, and we have shown how lower light intensities lead to major morphological changes, higher quantum yield of the photochemistry, and faster energy transfer. The results may point to quantum-classical control of energy transfer achieved by changing the coupling strength between the pigments through conformational changes.