Purple bacteria, one of the first life forms on Earth, give solar power researchers unprecedented reasons to wonder at nature’s ways of doing things efficiently. These bacteria live on lake bottoms or in the corals under the sea, and use sunlight as their source of energy. Scientists thought they had discovered all there is to know about purple bacteria, but recently, Neil Johnson, a physicist from the University of Miami, also found out other properties of these primordial creatures.
The purple bacteria are very flexible when it comes to the intensity of light, arranging themselves in different patterns. “Our study develops a mathematical model to describe the designs it adopts and why, which could help direct design of future photoelectric devices,” says Johnson, who collaborated on his study with colleagues from the Universidad de los Andes in Colombia.
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.”
To make it easier to understand what purple bacteria do, Johnson put up an example: “Imagine a really busy day at the supermarket, if the reaction center is busy it’s like the cashier is busy, somebody is doing the bagging,” Johnson says. “The shopper wonders around to find an open checkout and some of the shoppers may get fed up and leave – The bacteria are like a very responsible supermarket,” he says. “They would rather lose some shoppers than have congestion on the way out, but it is still getting enough profit for it to survive.”
His study developed the first analytical model explaining the “critical light intensity,” below which the cell enhances the creation or reaction centers, as being the spot of highest efficiency for that cell. That happens because the cell contains the greatest number of the best location of open RCs, and the least amount of energy loss – hence the efficiency.
With this discovery, solar panels could render themselves much more efficient if coated with specially adapted photosynthetic bacteria, whose energetic output would become part of the conventional electrical circuit. These solar panels could adapt themselves to different light intensities. Finding better solutions that those provided by nature in purple bacteria is also one or the researchers’ goal. They even use supercomputers to that, but it’s a hard task, as nature developed these systems in billions of years.
