Algae Show a Knack for Quantum Mechanics

3 February 2010 (All day)

Tihana Mirkovic/University of Toronto (main image); Gregory Scholes/University of Toronto (inset)

Nanotechs. These humble algae use their protein molecules (inset) to exploit quantum effects.

Algae might have mastered the science of particle physics billions of years ago. Researchers have found that the microscopic organisms have evolved a molecular structure that boosts the efficiency of photosynthesis by taking advantage of an important property of quantum mechanics. Experts say the discovery will open up a new field of research, and it could lead to a new generation of superefficient light-sensitive devices.

Plants and algae use sunlight to break the chemical bonds of water and carbon dioxide and make oxygen and sugar. Although scientists have long suspected that the efficiency of this process depends on quantum mechanics--the weird behavior of particles at the subatomic level--they have been unable to figure out how. One mystery concerns the function of certain proteins, known as antennas, which intercept photons and channel their energy to reaction centers, where a cell converts water and carbon dioxide to oxygen and sugar. But the reaction centers can do the job themselves, so why the extra hardware?

To try to solve the mystery, researchers performed a series of experiments on cryptophytes, ubiquitous algae that inhabit marine and freshwater environments. The team fired ultrafast, low-power laser pulses at the molecular antennas within the algal cells and then measured changes in the light energy.

The results astounded the team. The way the antennas reacted with the photons revealed unmistakable signs of quantum mechanics. When photons strike a molecule, they transfer energy by vibrating the molecule's electrons. But that vibration slows down rapidly if the electron is transferred to another molecule. For that reason the researchers expected that the energy from the laser could not easily be transferred from the antenna molecules to the reaction-center molecules. Yet the experiments showed that the electron vibrations resulting from the photons striking the antennas persisted at full strength four times longer than expected. The reason, the researchers report this week in Nature, is that quantum mechanics controls the energy.

"It was an utter surprise," says physical chemist and co-author Gregory Scholes of the University of Toronto in Canada. For the results to have occurred, he explains, a property called quantum coherence must have been operating. In other words, the molecular structure of the antennas essentially converts incoming light into a wave that travels from an antenna to a reaction center without losing energy.

"It shows that quantum effects can influence biological function," Scholes says. Next, he says researchers need to ask deeper questions, such as whether quantum mechanical effects are active in other biological processes.

Biophysicist Gregory Engel of the University of Chicago in Illinois calls the paper "a fantastic piece of work." Engel, who also has studied the role of quantum mechanics in photosynthesis, agrees that the research "will open an entirely new area of biophysics." And that effort should have "huge implications," he says, "not only for how we think about biophysics, but also light harvesting and light-sensitive devices."