May 112010
 

Purple bacteria were among the first life forms on Earth. They are single celled microscopic organisms that play a vital role in sustaining the tree of life. This tiny organism lives in aquatic environments like the bottom of lakes and the colorful corals under the sea, using sunlight as their source of energy. Its natural design seems the best structural solution for harvesting solar energy. Neil Johnson, a physicist and head of the inter-disciplinary research group in complexity in the College of Arts and Sciences at the University of Miami, thinks its cellular arrangement could be adapted for use in solar panels and other energy conversion devices to offer a more efficient way to garner energy from the sun.

“These bacteria have been around for billions of years, you would think they are really simple organisms and that everything is understood about them. However, purple bacteria were recently found to adopt different cell designs depending on light intensity,” says Johnson. “Our study develops a mathematical model to describe the designs it adopts and why, which could help direct design of future photoelectric devices.”

Johnson and his collaborators from the Universidad de los Andes in Colombia share their findings in a study entitled “Light-harvesting in bacteria exploits a critical interplay between transport and trapping dynamics,” published in the current edition of Physical Review Letters.

Solar energy arrives at the cell in “drops” of light called photons, which are captured by the light-gathering mechanism of bacteria present within a special structure called the photosynthetic membrane. Inside this membrane, light energy is converted into chemical energy to power all the functions of the cell. The photosynthetic apparatus has two light harvesting complexes. The first captures the photons and funnels them to the second, called the reaction center (RC), where the solar energy is converted to chemical energy. When the light reaches the RCs, they close for the time it takes the energy to be converted.

According to the study, purple bacteria adapt to different light intensities by changing the arrangement of the light harvesting mechanism, but not in the way one would think by intuition.

“One might assume that the more light the cell receives, the more open reaction centers it has,” says Johnson. “However, that is not always the case, because with each new generation, purple bacteria create a design that balances the need to maximize the number of photons trapped and converted to chemical energy, and the need to protect the cell from an oversupply of energy that could damage it.”

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