Laser and Light: Photosynthetic Algae's Secrets Revealed by Ultrafast Lasers

Researchers from Princeton University have discovered a way to improve the light harvesting rates of the cryptophyte algae Chroomonas mesostigmatica: quantum coherence.

In a study published in the journal Chem, the scientists wanted to further enhance photosynthetic algae's technique for capturing light that results in one of nature's most potent light-harvesting systems and turns it into energy. The primary goal was to understand the process and recreate it as a source of renewable energy.

The researchers' findings, which explain the design of artificial light-harvesting systems such as molecular sensors and solar energy collectors, were obtained by observing how cryptophyte algae that live underneath other organisms could still absorb most of the sun's rays. The algae had, in fact, evolved to thrive on wavelengths of light that aren't captured by their neighbors above. They gather yellow-green light energy and pass it through a network of molecules then convert it to the red light that chlorophyll molecules require to perform important photosynthetic chemistry.

The speed of the process, however, was a source of confusion for the scientists at first. "The timescales that the energy is moved through the protein. We could never understand why the process so fast," said corresponding author Gregory Scholes, the William S. Tod Professor of Chemistry at Princeton University upon noting that the Scholes lab's predictions were three times slower than the observed rates.

It was in 2010 that Scholes and his team determined that the reason behind the remarkable speed was quantum coherence. In this phenomenon, molecules could share electronic excitation and transfer energy according to quantum mechanical probability laws instead of classical physics.

It was only recently that the research team was able to discover how coherence worked. With the help of ultrafast lasers, the molecules' light absorption was easily measured and essentially energy flow through the system could finally be tracked.

The researchers observed the system as energy was transferred from molecule to molecule with excess energy lost as vibrational energy. These experiments revealed a particular spectral pattern that was a 'smoking gun' for vibrational resonance, or vibrational matching, between the donor and acceptor molecules. This vibrational matching allowed energy to be transferred much faster and provided a mechanism for the previously reported quantum coherence.

"Finally the prediction is in the right ballpark," Scholes said, revealing that he hopes to create light-harvesting systems by taking inspiration from the light-harvesting proteins.  "Turns out that it required this quite different, surprising mechanism. This mechanism is one more powerful statement of the optimality of these proteins."