Quantum Algae: Photosynthesis in Low Light
Scientists have discovered that some algae can survive in low-light environments by taking advantage of a ‘weird quantum phenomenon.’ The study comes from a relatively new field of science dubbed ‘quantum biology,’ where scientists explore the idea that quantum processes occur in nature and might actually be pivotal to life.
People might be most familiar with the idea of a quantum process through quantum physics – or at least familiar with the term. But now biologists are turning toward quantum theory to explore the nature of life on Earth.
“Quantum biology covers processes where uniquely quantum mechanical phenomena are utilized in a biological system,” explained corresponding author Paul Curmi of the University of New South Wales in Australia. “Usually, the term is restricted to ‘non-trivial’ quantum phenomena.”
Some examples of what scientists call ‘trival quantum phenomena’ are chemical bonding and enzymatic reactions – chemical interactions that occur constantly in our cells.
“For quantum biology, we usually mean things like long-lived superpositions of states, coherence, etc.,” said Curmi.
Superpositions refer to the idea that a physical system with many different states (think of excitation energy which classically wouldbe located on a single chromophore molecule inside a complex protein system) exists in all the theoretically possible states it can at any one time. If you could look at a light harvesting protein in a cell, then the excitation energy would be in all possible locations at thesame time (hence the term “superposition”). Once that protein interacts with the target reaction centre, the energy will “localize” into one site and transfer to the reaction centre. Thus, the superposition “decohers” or collapses.
“The term “quantum coherence” is vague – probably on purpose!” Curmi told Astrobiology Magazine. “It means a quantum superposition of states that remain “in phase”, usually implying for a long time. A coherence can be electronic or vibrational or other forms. Usually, quantum coherence means electronic coherence, as this would be something that is not expected to be long-lived under biological conditions.”
The terminology is complicated, but what the team basically uncovered in their research is that coherence might exist in the transfer of energy between photosynthetic molecules in two different types of algae. This is important, because quantum coherence in a system is typically only observed under strictly controlled conditions in a laboratory. It’s not until recently that scientists thought to look for it in living organisms.
The team studied cryptophytes, which are single-celled algae that can survive with very little light. Most Cryptophytes are photosynthetic and they can be found in almost any aqueous environment on Earth. However, they are especially adapted for low light, and can survive in places where other photosynthetic organisms might find it difficult – such as below ice or even in snow.
The study focuses on a specific type of photosynthetic protein used by cryptophytes, called phycobiliproteins or PBPs for short. Proteins are composed of long chains of smaller molecules called amino acids. The ways in which amino acids interact with one another (and the surrounding environment) cause the chain to twist and bend into a 3-D structure. This shape is known as the protein’s quaternary structure, and it affects how the protein is used inside a cell.
It turns out that the PBPs in the study can be twisted into two different shapes – dubbed ‘open’ and ‘closed’ – and the difference between these two shapes comes down to just a single amino acid being inserted in the chain. The shape of PBPs controls how these proteins harvest light for photosynthesis.
The research team decided to compare algae that use the two different structures of PBPs in order to determine the role of quantum coherence in photosynthesis. What they found was pretty astonishing.
When light is captured by a PBP, the energy needs to be moved into the reaction center of the cell. Here it is converted to chemical energy for the organism to survive. Previously, this transfer of energy was though to be random. However, by taking advantage of quantum coherence, the energy tests every possible route to the reaction center at the same time – and then takes the fastest one. This improves the efficiency of energy production, and helps and organism survive in dim conditions where cells need to quickly capture and use what little light is available.
“Current work is focused on gaining watertight experimental evidence that some of the oscillations observed in the electronic spectra of these proteins are really reporting on electronic coherence,” said Curmi. “If they represent electronic coherence, it means that the excited state of electrons storing the light energy is delocalized – meaning it is essentially at two (or more) places at the same time. This is exciting as it means that the energy can leave the light-harvesting molecule from several different locations depending on circumstances. This may assist the energy find the best path to its ultimate target location.”
The Astrobiology Connection
Astrobiology is the study of life’s potential in the Universe, and a major topic of research is to understand how life works on Earth, which is our only example thus far of an inhabited planet.
One of the key questions for astrobiologists today is how life on Earth survives at the extreme edges of habitability. They study microbes that survive in the coldest locations to the hottest environments on Earth – from surfaces blasted by solar radiation to areas deep below the ground where sunlight cannot reach.
The Sun provides most of the energy for life as we know it on our planet, and it’s possible that the same would hold true for extrasolar planets. After all, stars are a good source of free energy. Studying the limits at which photosynthesis can occur in living cells could yield clues about how life might thrive on planets where conditions are different than the Earth – for instance, on planets that are bathed in less energy from their host stars than the Earth receives from the Sun.
“Cryptophytes and other organisms that can survive on very low light levels may provide clues as to how life evolved,” said Curmi. “Presumably this may have implications for other scenarios.”
The study was published in the Proceedings of the National Academy of Sciences.
Professor Paul Curmi explains some of his research in the field of biophysics. Credit: University of New South Wales (YouTube)