Scientists get up to speed on the light-harvesting Secrets of photosynthetic algae

Microscopy Image of cryptophyte algae (c) Desmond Toa

Since millions of years ago, photosynthetic algae have been refining their technique for capturing light.

As a result, their light-harvesting systems (proteins that absorb light to be turned into energy) are so powerful that scientists have sought to understand and mimic them to use in renewable energy applications.

Now, researchers at Princeton University have revealed a mechanism that enhances the light-harvesting rates of the cryptophyte algae Chroomonas mesostigmatica. Their findings, published in the Chem journal recently, provide valuable insight for the design of artificial light-harvesting systems like molecular sensors and solar energy collectors. Cryptophyte algae often live below organisms that absorb most of the sun's rays. As a result, they have evolved to thrive on those wavelengths of light not sought after by the organisms above them – mainly the yellow-green colors. They collect this yellow-green light energy and pass it through a network of molecules that converts it into red light, something that chlorophyll molecules need to perform important photosynthetic chemistry. The scientists have always been fascinated and intrigued by the speed of the energy transfer. Their predictions were always about three times slower than the observed rates. 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. In 2010, his team discovered that these fast rates were due to a phenomenon called quantum coherence, in which molecules shared electronic excitation and transfer energy according to quantum mechanical probability laws instead of classical physics. However, they could not explain exactly how coherence worked to speed up the rates – until now. Using a sophisticated method enabled by ultrafast lasers, the researchers measured the molecules' light absorption and tracked the energy flow through the system. Normally the absorption signals would overlap, making them impossible to assign to specific molecules within the protein complex; however, the team was able to sharpen the signals by cooling the proteins down to very low temperatures, said lead author Jacob Dean, postdoctoral researcher in the Scholes lab. The researchers observed the system as energy was transferred from molecule to molecule, from high-energy green light to lower energy red light, with excess energy being lost as vibrational energy. This showed that a specific spectral pattern that was a "smoking gun" for vibrational resonance (or vibrational matching) between the donor and acceptor molecules, said Dean. Thanks to the vibrational matching, energy was able to transfer much faster than it otherwise would be by distributing the excitation between molecules. The effect provided a mechanism for the previously reported quantum coherence. With this in mind, the researchers recalculated their prediction and arrived at a rate that was about three times faster. The Scholes lab intends to study related proteins to investigate whether this mechanism is found in other photosynthetic organisms. Eventually, the scientists hope to develop light-harvesting systems with perfect energy transfer inspired by the robust light-harvesting proteins. This mechanism is one more powerful statement of the optimality of these proteins, " said Scholes.