Did you know that some species of algae use quantum mechanical behavior to maximize the light they collect? Well, a paper in the Proceedings of the National Academy of Sciences reveals that certain species have the ability to switch this capacity off, prompting questions of both why and how that could ultimately lead to improved energy harvesting technologies.
Quantum coherence has been defined by the great mathematician Sir Roger Penrose as, “Circumstances when large numbers of particles can collectively cooperate in a single quantum state”. Moreover, these particles can exist in multiple quantum states simultaneously, known as superposition, most famous from the thought experiment known as Schrodinger's Cat. Perhaps the most familiar coherence application is the laser, but is is also responsible for superconductivity and superfluidity.
Certain algae, and green sulfur bacteria, were found in 2010 to be able to transfer energy internally in a coherent manner. “The assumption is that this could increase the efficiency of photosynthesis, allowing the algae and bacteria to exist on almost no light,” says Professor Paul Curmi of the University of New South Wales. “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. 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 single-celled algal species found capable of this are cryptophytes, and live in places where little light penetrates, such as under ice or at the bottom of deep ponds, making every scrap of energy vital.
However, Curmi has now found two species of Hemiselmis cryptophytes with an extra amino acid in their energy sharing proteins compared to the coherence inducing ones. X-ray crystallography of the light harvesting structure showed the alteration disrupted the coherence. “This shows cryptophytes have evolved an elegant but powerful genetic switch to control coherence and change the mechanisms used for light harvesting,” Curmi says.
It's not clear what the advantage of doing without the additional efficiency coherence supplies might be. Curmi told IFLS, "First, we do not yet know whether quantum coherence is actually relevant to biology. It is possible that this is an evolutionary spandrel - a property that is a by product of the evolution of some other trait. Assuming it is not an evolutionary spandrel, we do not know what the function of quantum coherence may be. People have guessed "increasing efficiency" but there is no hard evidence for this." This makes it even harder to know whether there is an evolutionary advantage to its disruption. Moreover, Curmi says the team have not yet studied the Hemiselmis cryptophytes enough to know whether they have lost the capacity of coherence entirely. "It is possible that rather than losing quantum coherence, Hemiselmis has gained the ability to switch it on and off," he says. "We're currently investigating this."
Curmi also plans to study differences in the habitats and ecological niches between the species with and without coherence. Coherence is much easier to achieve at lower temperatures, so it would not be surprising if species that operate in warmer conditions couldn't make it work, but that does not explain an adaptation to disrupt it, if that is what he has found.
The discovery of biological coherence sparked interest in the possibility of incorporating the phenomenon into organic solar cells. Knowing how, and maybe even why, some species block it could help in this application.
1