Algae share insights into quantum biology

Tiny single-celled algae called cryptophytes are providing insights into the quantum biology that allows them to efficiently gather light to fuel their growth.

Cryptophytes thrive in the bottom of pools of water, or under thick ice, where light is scarce. How they survive in these very low levels of light led to the discovery by a UNSW-led team of researchers that the algae switch on and off a quantum phenomenon, known as coherence, during photosynthesis.

This phenomenon is usually only observed under tightly controlled laboratory conditions where 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.

The function of this quantum effect remains a mystery, but it is thought it could increase the efficiency of photosynthesis and allow the algae to exist on almost no light.

“Most cryptophytes have a light-harvesting system where quantum coherence is present. 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,” said Professor Paul Curmi from the UNSW School of Physics.

The researchers used X-ray crystallography to work out the crystal structure of the light-harvesting phycobiliprotein (PBP) complexes from three different species of cryptophytes. They found a genetic mutation in two species that has led to the insertion of an extra amino acid that changes the structure of the protein complex, disrupting coherence.

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

“It was assumed the energy gets to the reaction centre in a random fashion, like a drunk staggering home. But quantum coherence would allow the energy to test every possible pathway simultaneously before travelling via the quickest route.”

The next step will be to compare the biology of different cryptophytes, such as whether they inhabit different environmental niches, to work out whether the quantum coherence effect is assisting their survival.

“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.”

This work is part of an emerging field called quantum biology, in which evidence is growing that quantum phenomena are operating in nature, not just the lab. Working out its role in a living organism could lead to technological advances, such as organic solar cells and quantum-based electronic devices.

The research was recently published in Proceedings of the National Academy of Sciences.