Algae master quantum science
Algae that have to survive underwater in low
light conditions have to find a way to make
the most of the light they get.
Image: iStockphoto

Simple single-celled algae use highly sophisticated quantum physics to harvest and convert solar energy for their survival, a new study suggests.

The study, published today in the prestigious science journal Nature, was by an international team of Canadian, Italian and Australian researchers, including two UNSW biophysicists - Professor Paul Curmi and Dr Krystyna Wilk.

It sheds new light on the process of photosynthesis used by green plants and algae to harvest energy from the sun and may open up new avenues to develop organic solar cells and other electronic devices that emit or are initiated by light, such as lasers and visual displays.

The water-dwelling algae are in effect highly miniaturised quantum computers, the study suggests. They have mastered the process of photosynthesis so well that they can convert sunlight into electrical energy with near-perfect efficiency.

They do so by having their light-harvesting proteins "wired" together through a phenomenon known as quantum coherence, enabling them to transfer energy from one protein to another with lightning-fast speed and so reduce energy loss along the energy conversion pathway.

"Photosynthesis makes use of sunlight to convert carbon dioxide into useful biomass and is vital for life on Earth," the paper notes. "Crucial components for the photosynthetic process are antenna proteins, which absorb light and transmit the resultant excitation energy between molecules to a reaction centre.

"The efficiency of these electronic energy transfers has inspired much work on antenna proteins isolated from photosynthetic organisms to uncover the basic mechanisms at play."

Intriguingly, recent work by other researchers has documented that light-absorbing molecules in some photosynthetic proteins capture and transfer energy according to quantum-mechanical probability laws instead of classical laws at temperatures up to 180 degrees Kelvin.

"Where our study breaks new ground is that we observe the same quantum coherence at normal room temperature," says Professor Curmi. "Therefore it is occurring in living algae."

The study is part of a larger, ongoing collaboration between the biophysics lab at the UNSW School of Physics, the Centre for Applied Medical Research, St Vincent's Hospital, Sydney, and Associate Professor Greg Scholes's group at the University of Toronto, says Professor Curmi .

"We are working on understanding how a group of single-celled algae can thrive under low light conditions in marine and freshwater habitats. To do this, they must be incredibly efficient in capturing all solar energy and converting it to chemical energy via photosynthesis.  They cannot afford to let any solar energy escape, so they have evolved elaborate antenna systems that trap light.

"We have been solving the atomic structures of the light-harvesting proteins from these algae, while Greg Scholes's lab has been characterising them using femtosecond laser spectroscopy.

"Our initial structures of these proteins indicated that there was something very unusual in the way their light-trapping molecules were arranged.  The Scholes' lab probed the way light energy moved through these light-trapping molecules using lasers and, to our surprise, we saw that the pathways of energy transport obeyed quantum rules - quantum coherence.

"Once a light-harvesting protein has captured light energy, it needs to get that energy quickly and efficiently to a protein called the reaction centre, which is nature's solar cell.  This is where the light energy is converted to an electrical form that eventually powers all plants, algae and bacteria that use photosynthesis.

"It has been assumed that the energy moves from the light-harvesting protein to the reaction centre by a classical 'random walk' - the way a drunk staggers home from the pub.  What we have discovered is that instead of the classical random walk, the light-harvesting proteins use quantum mechanical methods to 'test every possible path simultaneously' before picking the correct path to transfer the energy.

"This quantum coherence is the way quantum computers work.  Quantum mechanics allows a system to be in many states simultaneously - this is quantum coherence.

"The trouble is that these quantum coherent states usually last for extremely short time periods and extremely short distances, except in special laboratory conditions.  This is why our result is such a surprise.  Evolution has discovered how to establish and use quantum coherence to increase the efficiency of photosynthesis when light energy is scarce."

Editor's Note: Original news release can be found here.