Path of light in photosynthesis

Nature has made use of the basic reaction centre architecture that emerged over 3 billion years ago

June 21, 2021
The Scitech

Heliobacteria, a type of bacteria thought to be similar to those of the common ancestors for all photosynthetic organisms. A University of Michigan team has determined the first steps in converting light into energy for this bacterium. “Our study highlights the different ways in which nature has made use of the basic reaction center architecture that emerged over 3 billion years ago,” said lead author and U-M physicist Jennifer Ogilvie. Photosynthetic organisms contain “antenna” proteins that are packed with pigment molecules to harvest photons. The collected energy is then directed to “reaction centers” that power the initial steps that convert light energy into food for the organism. These initial steps happen on incredibly fast timescales—femtoseconds, or one millionth of one billionth of a second.  When light hits a photosynthetic organism, pigments within the antenna gather photons and direct the energy toward the reaction center. In the reaction center, the energy bumps an electron to a higher energy level, from which it moves to a new location, leaving behind a positive charge. This is called a charge separation.  In the reaction centers of plants and most photosynthetic organisms, the pigments that orchestrate charge separation absorb similar colors of light, making it difficult to visualize charge separation. Using the heliobacteria, the researchers identified which pigments initially donate the electron after they’re excited by a photon, and which pigments accept the electron.

Heliobacteria is a good model to examine, Ogilvie said, because their reaction centers have a mixture of chlorophyll and bacteriochlorophyll, which means that these different pigments absorb different colors of lights.  To probe reaction centers in heliobacteria, Ogilvie’s team uses a type of ultrafast spectroscopy called multidimensional electronic spectroscopy, implemented in Ogilvie’s lab by lead author and postdoctoral fellow Yin Song. Each time the laser pulse hits the sample, the light excites the reaction centers within. The researchers vary the time delay between the pulses, and then record how each of those pulses interacts with the sample. When pulses hit the sample, its electrons are excited to a higher energy level. The pigments in the sample absorb specific wavelengths of light from the laser - specific colors - and the colors that are absorbed give the researchers information about the energy level structure of the system and how energy flows through it. Getting a clearer picture of this energy transport and charge separation allows the researchers to develop more accurate theories about how the process works in other reaction centers.

Source: University of Michigan news release