Cancelling noise and being green

Successful photosynthesis is in part about cancelling out noise, according to a unique new model created by a team of scientists with expertise running from physics to biology.

They say they borrowed ideas from the science of complex networks – which explores efficient operation in mobile phone networks and the power grid – to construct a model that reproduces a general feature of photosynthetic light harvesting.

It seeks to explain the underlying design principle by which photosynthetic organisms, from plants to bacteria, are both able to protect themselves from sudden surges of solar energy when sunlight hits them and make best use of this energy.

“Our model shows that by absorbing only very specific colours of light, photosynthetic organisms may automatically protect themselves against sudden changes – or noise – in solar energy, resulting in remarkably efficient power conversion,” says physicist Nathaniel M Gabor, from the University of California, US.

“Green plants appear green and purple bacteria appear purple because only specific regions of the spectrum from which they absorb are suited for protection against rapidly changing solar energy.”

Gabor and colleague Trevor Arp led the research, which involved scientists from the US, Canada, Scotland and the Netherlands and is described in a paper in the journal Science.

During photosynthesis, photons from the Sun are absorbed by a network of antennae and transferred as electronic excitations to reaction centres, where they are converted into the chemical energy plants and other organisms use to drive their metabolism.

These antennae are remarkably efficient, the researchers say, converting each photon into a chemically usable electron despite ever-changing light and complex physiology.

Light harvesting varies in form and function across the range of photosynthetic life, but it is not clear whether a common set of “design” principles underlies such effective systems.

To test this, Gabor and colleagues applied a network theory model to reveal the most basic requirements needed for optimal light-harvesting under three conditions: full sun, under a canopy of leaves and underwater.

They found, they say, that by using two pigments that absorb slightly different wavelengths of light in a narrow range of the spectrum, photosynthetic organisms in different environments mitigate sudden changes in solar energy and minimise the potential for energy fluctuations or “noise” in the output of light-harvesting antennae.

The results show how light-harvesting antennae can be evolutionarily tuned for maximum power conversion, they say, and provide a basis to explain the variation in wavelength dependence observed among some photosynthetic organisms.

Gabor says they were thus able to make clear, quantitative, and generic statements about highly diverse photosynthetic organisms.

“Our model is the first hypothesis-driven explanation for why plants are green,” he says, “and we give a roadmap to test the model through more detailed experiments.”

If the model “holds up to continued experiments,” he adds, “we may find even more agreement between theory and observations, giving rich insight into the inner workings of nature.”

In a related commentary in Science, Christopher Duffy from Queen Mary University of London says the finding is important because it “suggests that the evolutionary driving force behind the development of photosynthetic antennae is not maximisation of efficiency but the cancellation of noise”.

“Moreover, the finding indicates that to build such a system, one must start from the simple requirements of two similar absorbing species that are tuned to the steepest region (not the strongest) of the spectrum of available light,” he writes.

“Fine structural details are important, but they come as refinements to this simple underlying principle.”