Algae can switch quantum coherence on and off

Could allow the energy to test every possible pathway simultaneously before traveling via the quickest route
June 18, 2014

Scanning electron microscope image of cryptophytes (credit: CSIRO)

Algae that survive in very low levels of light and are able to switch quantum coherence on and off have been discovered by a UNSW-led team of researchers.

The function for this effect, which occurs during photosynthesis, remains a mystery. But working out its role in a living organism could lead to technological advances, such as better organic solar cells and quantum-based electronic devices.

The research is published in the journal Proceedings of the National Academy of Sciences.

Quantum biology

The research is part of an emerging field called quantum biology, in which evidence is growing that quantum phenomena are operating in nature, not just the laboratory, and may even account for how birds can navigate using the earth’s magnetic field.

“Most cryptophytes have a light-harvesting system where quantum coherence is present,” says senior author, Professor Paul Curmi, of the UNSW School of Physics. “But we have found a class of cryptophytes where it is switched off because of a genetic mutation that alters the shape of a light-harvesting protein.

“This is a very exciting find. It means we will be able to uncover the role of quantum coherence in photosynthesis by comparing organisms with the two different types of proteins.”

A system that is coherent — with all quantum waves in step with each other — can exist in many different states simultaneously, an effect known as superposition. This phenomenon is usually only observed under tightly controlled laboratory conditions.

The same effect has been found in green sulphur bacteria that also survive in very low light levels. “The assumption is that this could increase the efficiency of photosynthesis, allowing the algae and bacteria to exist on almost no light,” says Curmi.

A powerful genetic switch to control coherence

“Once a light-harvesting protein has captured sunlight, it needs to get that trapped energy to the reaction center in the cell as quickly as possible, where the energy is converted into chemical energy for the organism.

“Quantum coherence would allow the energy to test every possible pathway simultaneously before traveling via the quickest route.”

This is similar to how quantum computers (QC) are able to solve the classic “traveling salesman” problem to find the the shortest possible route among cities visited. Curiously, bees are also able to solve this kind of problem. Could bees also be using quantum coherence?

“No,” says Curmi. “There are many ways to solve the traveling salesman problem.  QC is only one proposal.  Bees, slime molds and other systems do it by having vast numbers of individuals searching for a solution simultaneously and communicating their finding to each other.  In this way, they collectively solve the problem via consensus.  They have no need for any quantum mechanical effects.”

In the new study, the team used x-ray crystallography to work out the crystal structure of the light-harvesting complexes from three different species of cryptophytes.

They found that in two species a genetic mutation has led to the insertion of an extra amino acid that changes the structure of the protein complex, disrupting coherence.

“This shows cryptophytes have evolved an elegant but powerful genetic switch to control coherence and change the mechanisms used for light harvesting,” says Curmi.

Practical uses

“This is the first time anyone has seen a protein system where through evolution a structural change has occurred that simultaneously changes the structure of the protein and switches the QC properties of the system,” Curmi told KurzweilAI in an email.

“Our discovery will open the way to determining whether photosynthetic algae use quantum coherent (QC) effects in light harvesting. Our goal now is to determine the biological consequences of a photosynthetic organism possessing a light harvesting system which has QC versus one that does not.  Second, we want to learn whether a single organism can switch QC on and off and under what conditions it does so.

“If it turns out that QC is biologically important to light harvesting, then it may be utilized in synthetic light harvesting systems such as in photocells, photovoltaics, etc.”

But don’t expect this to show up as a Best Buy gadget anytime soon. “This is basic research.  We are a long way from understanding the system and utilizing the knowledge for applications.”

Scientists from the University of Toronto, the University of Padua, the University of British Columbia, the University of Cologne and Macquarie University were also involved in the study.

UPDATE 6/19/2014 Comment by Curmi relating to bees added.

Abstract of Proceedings of the National Academy of Sciences paper

Observation of coherent oscillations in the 2D electronic spectra (2D ES) of photosynthetic proteins has led researchers to ask whether nontrivial quantum phenomena are biologically significant. Coherent oscillations have been reported for the soluble light-harvesting phycobiliprotein (PBP) antenna isolated from cryptophyte algae. To probe the link between spectral properties and protein structure, we determined crystal structures of three PBP light-harvesting complexes isolated from different species. Each PBP is a dimer of αβ subunits in which the structure of the αβ monomer is conserved. However, we discovered two dramatically distinct quaternary conformations, one of which is specific to the genus Hemiselmis. Because of steric effects emerging from the insertion of a single amino acid, the two αβ monomers are rotated by ∼73° to an “open” configuration in contrast to the “closed” configuration of other cryptophyte PBPs. This structural change is significant for the light-harvesting function because it disrupts the strong excitonic coupling between two central chromophores in the closed form. The 2D ES show marked cross-peak oscillations assigned to electronic and vibrational coherences in the closed-form PC645. However, such features appear to be reduced, or perhaps absent, in the open structures. Thus cryptophytes have evolved a structural switch controlled by an amino acid insertion to modulate excitonic interactions and therefore the mechanisms used for light harvesting.