Energy Transfer in Light-harvesting Complexes of Green Sulphur Bacteria

Donatas Zigmantasa, Jakub Dostála, Erling Thyrhauga, Karel Žídeka, David Bínab, Jakub Pšencíkc
aDepartment of Chemical Physics, Lund University, P.O. Box 124, 221 00 Lund, Sweden; bBiology Centre CAS, Branisovska 31, and Faculty of Science, University of South Bohemia, Branisovska 1760, 370 05 Ceske Budejovice, Czech Republic; cFaculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 3, 121 16 Prague, Czech Republic

Green sulfur bacteria (GSB) are photosynthetic organisms that can be found 100 meters below the sea level, where the number of photons available for light harvesting and utilization is extremely limited [1]. Thus, not surprisingly, these bacteria developed sophisticated light-harvesting structures. Here we studied energy transfer in GSB Chlorobium tepidum, which has gigantic light-harvesting antenna chlorosome, intermediate Fenna-Matthews-Olson (FMO) complex and FeS-type reaction centers. Interestingly, very high energy transfer efficiency to the reaction centers has never been reported for this organism [2].

Figure 1: Absorptive 2D spectra of the FMO complex at 40 fs and 5 ps with the dashed lines indicating identified eight excitonic levels.

We used two-dimensional electronic spectroscopy to disentangle and characterize energy transfer efficiencies in different parts of the GSB photosynthetic unit. We discovered the excitonic structure in the baseplate, which is an integral part of the chlorosome [3] and determined accurate excitonic energy levels (including elusive eight bacteriochlorophyll) and energy transfer rates in the FMO complex [4]. Comparing the energy levels of the baseplate, FMO complex and core antenna of the RC raises a question regarding the light-harvesting strategies employed by photosynthetic bacteria.

References

[1] J. Overmann, H. Cypionka, & N. Pfennig, Limnol. Oceanogr. 1992, 37, 50–155.
[2] J. Dostál, J. Pšenčík & D. Zigmantas, Nature Chem. 2016, 8, 705-710.
[3] J. Dostál, F. Vácha, J. Pšenčík & D. Zigmantas, J. Phys. Chem. Lett. 2014, 5, 1743-1747.
[4] E. Thyrhaug, K. Žídek, J. Dostál, D. Bína & D. Zigmantas, J. Phys. Chem. Lett. 2016, 7, 1653-1660.