Scatterings image

Microscopy image of cryophyte algae. [Image: Desmond Toa/Princeton University]

A group of U.S. scientists, led by Princeton University’s Gregory Scholes, used 2-D electronic spectroscopy (2DES) to better understand the quantum-scale mechanisms that subsurface marine algae use for ultrafast energy-transfer during sunlight harvesting (Chem, doi: 10.1016/j.chempr.2016.11.002). Information about how these tiny photosynthetic organisms make the most of the small amount of light they have access to could help scientists design new and highly efficient bio-inspired materials for solar-energy collection.

Light-harvesting antennae

The molecular structure of photosynthetic reaction centers (i.e., the biological equivalent of solar cells) is fairly standard for all organisms that get their fuel from the sun. However, the molecular structure of light-harvesting antennae varies among species.

Antennae consist of light-absorbing molecules attached to a protein scaffold. Their arrangement, scientists are learning, is not random, but instead finely tuned to funnel as much light as possible from the organism’s environment into its reaction centers.

Scholes and his team wanted to find out how, on a quantum-mechanical level, light-harvesting antennae from Chroomonas mesostigmatica—a cryophyte algae that hovers below the ocean’s surface—are able to gather enough photons to fuel the organism’s survival.

Tracking energy transfer

For their experiments, the researchers cultured C. mesostigmati and extracted its light-absorbing molecules, or chromophores, from the algae’s light-harvesting complex. Using 2DES in conditions similar to the algae’s natural environment, Scholes’ team noted vibrionic coherence between two remote sets of chromophores when excited by short laser pulses.

Spectra from the 2DES sessions showed patterns that, according to the researchers, can be explained by vibrational resonance linking remote chromophores and “partially delocalizing the excitation.” Delocalization leads to a spectral overlap that ensures energy conservation during the energy-transfer process “while inherently incorporating environmental fluctuations borne out in the spectral line shapes.”

The process results in a measurable enhancement in the rate of energy transfer between the chromophores. More simply, as light enters the antenna, photon energy hops from chromophore to chromophore, causing them to vibrate. This vibration, the researchers say, can more than triple the speed at which the algae can capture light and convert it into fuel for consumption.

The authors say that this work provides hints about how the molecular structure of this particular light-harvesting antenna enhances energy transfer efficiency in low-light conditions. “If researchers could learn how to move energy with such precision and efficiency over comparable distance,” says Scholes, “then enormous leaps in the development of cheap organic solar cell technology would ensue.”