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Autumn 2015

Bringing Bacteria's Secrets to Light

Scientists show how energy moves in bacteria that live in extremely low sunlight

Zachary A. Morseth

(Top) Model of the photosynthetic apparatus used in green sulfur bacteria. The main elements are the chlorosome (composed of bacteriochlorophyll molecules), the baseplate, the Fenna-Matthews-Olson (FMO) complex, and the reaction center. Adapted with permission from Valleau et al. Copyright 2014 American Chemical Society. (Bottom) Schematic illustration of the chlorosome used in the simulations. The chlorosome features four complete inner rolls and eight thin plates stacked on each side. Adapted with permission from Sawaya et al. Copyright 2015 American Chemical Society.

Put simply, photosynthetic organisms (plants, photosynthetic bacteria, and algae) are impressive. They have evolved over billions of years to effectively capture sunlight and nutrients to sustain life. As we continue our pursuit for the ultimate source of energy to satisfy our own needs, energy production in plants serves as an inspiration. Using sophisticated computer simulations, the researchers at the Center for Excitonics (CE) have uncovered some of the elaborate details of the photosynthetic machinery that provide useful guidelines to design human-made systems for light harvesting.

Because sunlight is a vital component to their life cycle, it is bewildering that green sulfur bacteria thrive hundreds of meters below the surface of water where sunlight is nearly absent, yet these organisms are some of the most efficient light harvesters. This incredible feat makes green sulfur bacteria inspirational to scientists that want to efficiently harvest sunlight to produce energy, even on cloudy days. Scientists at CE believe a fundamental understanding of the structural design of their light-harvesting molecular systems, known as chlorosomes, and their influence on the ability to capture light and transport the energy, is warranted.

In chlorosomes, the light-harvesting antennas are composed of a large array of tiny, colorful molecules, known as bacteriochlorophyll (BChl) molecules, attached to a reaction center, as illustrated in the top figure. Researchers have known for years that the BChl molecules are arranged into concentric cylinders at the center of the chlorosome, while the outer layers are composed of thin plates stacked onto the outermost cylinder, resulting in an overall ellipsoidal shape, as depicted in the bottom figure. The absorption of a photon, or "packet" of sunlight, by a BChl molecule generates an exciton, or a confined packet of energy. The exciton migrates within the chlorosome until it reaches the reaction center, at which point the transported energy is used to drive a series of chemical reactions that sustain life in the organism.

But how does the arrangement of the BChl molecules to form this large light-harvesting complex govern the overall exciton motion? With the help of quantum mechanical calculations on a chlorosome containing 74,000 BChl molecules (bottom figure), researchers at CE have uncovered some of the elaborate details regarding the movement of the excitons within the chlorosomes. They found that following excitations localized at the center of the chlorosome, the excitons rapidly spread outwards both axially (along the axis of the roll) and radially (between the layers). The overall migration is governed by the distance, orientations, and number of nearby BChl molecules. Because the concentric layers feature a large spacing of 2.1 nanometers, the center-to-center distances to nearby BChl molecules is greater in the radial direction; hence, transport occurs 2 to 4 times slower in the radial direction than the axial direction.

A key result from the simulations reveals how structural features of the antenna complex govern exciton transport to the outer layers of the chlorosome, where fuel is produced. The initial, rapid delocalization of the exciton leads to efficient, long-range energy transport to the outer layers. The simulations also indicate that while chlorosomes have evolved to take on an ellipsoidal shape, the transport of the excitons would be enhanced if the final structure was a large cylinder composed of concentric rolls with equal length.

With nature as a guide, a detailed understanding of the exciton motion occurring within these nanostructures will aid in the development of light-harvesting antennas for solar energy conversion applications. A salient lesson learned from light-harvesting bacteria is how to improve the solar efficiency to operate under low-light conditions. Through fundamental studies like the one discussed here, we can begin to mimic the light-harvesting properties of natural photosynthetic antennae, even tailoring the design to meet our energy needs.

More Information

Sawaya NPD, J Huh, T Fujita, SK Saikin, and A Aspuru-Guzik. 2015. "Fast Delocalization Leads To Robust Long-Range Excitonic Transfer in a Large Quantum Chlorosome Model." Nano Letters 15:1722-1729. DOI: 10.1021/nl504399d

Valleau S, SK Saikin, D Ansari-Oghol-Beig, M Rostami, H Mossallaei, and A Aspuru-Guzik. 2014. “Electromagnetic Study of the Chlorosome Antenna Complex of Chlorobium tepidium.” ACS Nano 8:3884-3894. DOI: 10.1021/nn500759k

Acknowledgments

This work was supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences.

About the author(s):

  • Zachary A. Morseth is a Ph.D. candidate at the University of North Carolina at Chapel Hill and is a member of the Center for Solar Fuels. He is advised by John M. Papanikolas at the center. There, Zachary employs ultrafast spectroscopic methods and computational tools to study fundamental energy and electron transfer dynamics in multifunctional molecular assemblies for solar energy conversion. Zachary holds a B.S. in chemistry and a B.A. in mathematics from Minnesota State University – Moorhead.

Illuminating the Secrets of Light-Harvesting Bacteria

Scientists show how energy moves in bacteria that live in extremely low sunlight

In green sulfur bacteria, the photosynthetic apparatus can use limited amounts of light to create fuels. The main elements are the chlorosome, the baseplate, the Fenna-Matthews-Olson (FMO) complex, and the reaction center. Adapted with permission from Valleau et al. Copyright 2014 American Chemical Society.

Living underwater, green sulfur bacteria efficiently harvest light and use it to subsist in light illumination comparable to that of moonlight. Scientists have modeled the extremely large antenna structures of these organisms. Inside these structures, the scientists found how the energy of the sunlight moves to reach the reaction center, where the chemistry of life begins. Knowing how the energy moves in these efficient structures could lead to better organic materials for harvesting sunlight. Harvard scientists at the Massachusetts Institute of Technology/Harvard/Brookhaven National Laboratory Center for Excitonics conducted the work.

More Information

Sawaya NPD, J Huh, T Fujita, SK Saikin, and A Aspuru-Guzik. 2015. "Fast Delocalization Leads To Robust Long-Range Excitonic Transfer in a Large Quantum Chlorosome Model." Nano Letters 15:1722-1729. DOI: 10.1021/nl504399d

Valleau S, SK Saikin, D Ansari-Oghol-Beig, M Rostami, H Mossallaei, and A Aspuru-Guzik. 2014. “Electromagnetic Study of the Chlorosome Antenna Complex of Chlorobium tepidium.” ACS Nano 8:3884-3894. DOI: 10.1021/nn500759k

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.