Chlorosomes are light-harvesting antennae that enable exceptionally efficient light energy capture and excitation transfer. They are found in certain photosynthetic bacteria, some of which live in extremely low-light environments. In this work, chlorosomes from the green sulfur bacterium Chlorobaculum tepidum were studied by coherent electronic two-dimensional (2D) spectroscopy. Previously uncharacterized ultrafast energy transfer dynamics were followed, appearing as evolution of the 2D spectral line-shape during the first 200 fs after excitation. Observed initial energy flow through the chlorosome is well explained by effective exciton diffusion on a sub-100 fs time scale, which assures efficiency and robustness of the process. The ultrafast incoherent diffusion-like behavior of the excitons points to a disordered energy landscape in the chlorosome, which leads to a rapid loss of excitonic coherences between its structural subunits. This disorder prevents observation of excitonic coherences in the experimental data and implies that the chlorosome as a whole does not function as a coherent light-harvester.

Absorption of sunlight is the first step in photosynthesis, which provides energy for the vast majority of organisms on Earth. The primary processes of photosynthesis have been extensively studied in isolated light-harvesting complexes and reaction centres. However, to fully understand biological light capturing it is crucial to reveal also the functional relations between the individual complexes. This information was scarce thereby preventing a full understanding of the light-capture functionality. Here we report direct tracking of the excitation energy flow through the entire photosynthetic system of green sulfur bacteria by means of two-dimensional electronic spectroscopy. We unravel functional organization of individual complexes in the photosynthetic unit and show that whereas energy is transferred within subunits on a sub-and few picoseconds timescale, energy flows at a timescale of tens of picoseconds between them. Thus, we demonstrate that the bottleneck of the energy transfer is between the constituents.

Chlorosomes of green photosynthetic bacteria constitute the most efficient light harvesting complexes found in nature. In addition, the chlorosome is the only known photosynthetic system where the majority of pigments (BChl) is not organized in pigment-protein complexes but instead is assembled into aggregates. Because of the unusual organization, the chlorosome structure has not been resolved and only models, in which BChl pigments were organized into large rods, were proposed on the basis of freeze-fracture electron microscopy and spectroscopic constraints. We have obtained the first high-resolution images of chlorosomes from the green sulfur bacterium Chlorobium tepidum by cryoelectron microscopy. Cryoelectron microscopy images revealed dense striations approximately 20 A apart. X-ray scattering from chlorosomes exhibited a feature with the same approximately 20 A spacing. No evidence for the rod models was obtained. The observed spacing and tilt-series cryoelectron microscopy projections are compatible with a lamellar model, in which BChl molecules aggregate into semicrystalline lateral arrays. The diffraction data further indicate that arrays are built from BChl dimers. The arrays form undulating lamellae, which, in turn, are held together by interdigitated esterifying alcohol tails, carotenoids, and lipids. The lamellar model is consistent with earlier spectroscopic data and provides insight into chlorosome self-assembly.

Coherent two-dimensional (2D) spectroscopy at 80 K was used to study chlorosomes isolated from green sulfur bacterium Chlorobaculum tepidum. Two distinct processes in the evolution of the 2D spectrum are observed. The first being exciton diffusion, seen in the change of the spectral shape occurring on a 100-fs timescale, and the second being vibrational coherences, realized through coherent beatings with frequencies of 91 and 145 cm(-1) that are dephased during the first 1.2 ps. The distribution of the oscillation amplitude in the 2D spectra is independent of the evolution of the 2D spectral shape. This implies that the diffusion energy transfer process does not transfer coherences within the chlorosome. Remarkably, the oscillatory pattern observed in the negative regions of the 2D spectrum (dominated by the excited state absorption) is a mirror image of the oscillations found in the positive part (originating from the stimulated emission and ground state bleach). This observation is surprising since it is expected that coherences in the electronic ground and excited states are generated with the same probability and the latter dephase faster in the presence of fast diffusion. Moreover, the relative amplitude of coherent beatings is rather high compared to non-oscillatory signal despite the reported low values of the Huang-Rhys factors. The origin of these effects is discussed in terms of the vibronic and Herzberg-Teller couplings.

The excited-state structure and energy-transfer dynamics, including their dependence on temperature and redox conditions, were studied in chlorosomes of the green sulfur bacterium Chlorobium tepidum at low temperatures by two independent methods:  spectral hole burning in absorption and fluorescence spectra and isotropic one-color pump−probe spectroscopy with ∼100 fs resolution. Hole-burning experiments show that the lowest excited state (LES) of BChl c aggregates is distributed within approximately 760−800 nm, while higher excitonic states of BChl c (with absorption maximum at 750 nm) possess the main oscillator strength. The excited-state lifetime determined from hole-burning experiments at anaerobic conditions was 5.75 ps and most likely reflects energy transfer between BChl c clusters. Isotropic one-color absorption difference signals were measured from 720 to 790 nm at temperatures ranging from 5 to 65 K, revealing BChl c photobleaching and stimulated emission kinetics with four major components, with lifetimes of 200−300 fs, 1.7−1.8 ps, 5.4−5.9 ps, and 30−40 ps at anaerobic conditions. The lifetimes are attributed to different relaxation processes of BChl c, taking into account their different spectral distributions as well as limitations arising from results of hole burning. Evidence for at least two spectral forms of BChl c in chlorosome is reported. There is a striking similarity between the spectrum and kinetics of the 5.4−5.9 ps component with those of the LES determined from hole burning. A pronounced change of isotropic decays was observed at around 50 K. The temperature dependence of the isotropic decays is correlated with temperature-dependent changes of BChl c fluorescence emission. Further, the temperature decrease leads to an increase in the relative amplitude of the 200−300 fs component. At aerobic conditions, both hole burning and pump−probe spectroscopy show that the lifetime of the LES shortens to ∼2.6 ps, as a result of excitation quenching by a mechanism presumably protecting the cells against superoxide-induced damage. This mechanism operates on at least two levels, the second one being characterized by a 14−16 ps lifetime.