Strong excitonic interactions are a key design strategy in photosynthetic light harvesting, expanding the spectral cross-section for light absorption and creating considerably faster and more robust excitation energy transfer. These molecular excitons are a direct result of exceptionally densely packed pigments in photosynthetic proteins. The main light-harvesting complexes of diatoms, known as fucoxanthin-chlorophyll proteins (FCPs), are an exception, displaying surprisingly weak excitonic coupling between their chlorophyll (Chl) a's, despite a high pigment density. Here, we show, using single-molecule spectroscopy, that the FCP complexes of Cyclotella meneghiniana switch frequently into stable, strongly emissive states shifted 4-10 nm toward the red. A few percent of isolated FCPa complexes and ∼20% of isolated FCPb complexes, on average, were observed to populate these previously unobserved states, percentages that agree with the steady-state fluorescence spectra of FCP ensembles. Thus, the complexes use their enhanced sensitivity to static disorder to increase their light-harvesting capability in a number of ways. A disordered exciton model based on the structure of the main plant light-harvesting complex explains the red-shifted emission by strong localization of the excitation energy on a single Chl a pigment in the terminal emitter domain due to very specific pigment orientations. We suggest that the specific construction of FCP gives the complex a unique strategy to ensure that its light-harvesting function remains robust in the fluctuating protein environment despite limited excitonic interactions.

Department of Physics and Astronomy Vrije Universiteit Amsterdam 1081 HV Amsterdam The Netherlands

Department of Physics and Astronomy Vrije Universiteit Amsterdam 1081 HV Amsterdam The Netherlands Faculty of Mathematics and Physics Charles University 121 16 Prague 2 Czech Republic

Department of Physics University of Pretoria Hatfield 0028 South Africa

Department of Physics University of Pretoria Hatfield 0028 South Africa Department of Physics and Astronomy Vrije Universiteit Amsterdam 1081 HV Amsterdam The Netherlands

Faculty of Mathematics and Physics Charles University 121 16 Prague 2 Czech Republic

Institute of Molecular Biosciences Department of Biosciences Goethe University Frankfurt 60438 Frankfurt Germany

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