In situ mapping of the energy flow through the entire photosynthetic apparatus.

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 studied extensively in isolated light-harvesting complexes and reaction centres, however, to understand fully the way in which organisms capture light it is crucial to also reveal the functional relationships between the individual complexes. Here we report the use of two-dimensional electronic spectroscopy to track directly the excitation-energy flow through the entire photosynthetic system of green sulfur bacteria. We unravel the functional organization of individual complexes in the photosynthetic unit and show that, whereas energy is transferred within subunits on a timescale of subpicoseconds to a few picoseconds, across the complexes the energy flows at a timescale of tens of picoseconds. Thus, we demonstrate that the bottleneck of energy transfer is between the constituents.