Photosynthesis starts with a single photon, the study

A cutting-edge experiment has revealed the quantum dynamics of one of nature’s most crucial processes.

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Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar. For the first time, researchers have observed the beginnings of photosynthesis, starting with a single photon.

Thanks to a new study, scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) showed that a single photon can indeed initiate the first step of photosynthesis in photosynthetic purple bacteria. The team is convinced that photosynthesis occurs similarly in plants and algae since all photosynthetic species use related mechanisms and have a common evolutionary past.

The finding strengthens our understanding of photosynthesis and will contribute to understanding how life functions at the atomic scale, where quantum physics and biology converge.

Co-lead author Graham Fleming, a senior faculty scientist in the Biosciences Area at Lawrence Berkeley National Laboratory (Berkeley Lab) and professor of chemistry at UC Berkeley, said, “A huge amount of work, theoretically and experimentally, has been done around the world trying to understand what happens after a photon is absorbed. But we realized that nobody was talking about the first step. That was still a question that needed to be answered in detail.”

It took a special group of theorists and experimentalists who combined cutting-edge technologies from quantum optics and biology to design an experiment that would enable viewing individual photons. It was novel for researchers studying photosynthesis because they don’t typically utilize these tools, and it was novel for quantum optics researchers since we don’t necessarily consider using these methods with sophisticated biological systems.

The researchers set up a photon source that, using a technique known as spontaneous parametric down-conversion, produces a single pair of photons. An extremely sensitive detector was used to observe the first photon, or “the herald,” of each pulse, confirming that the second photon was on its way to collecting light-absorbing molecules from photosynthetic bacteria. A different photon detector was placed up close to the sample to measure the lower-energy photon that the photosynthetic structure emits after absorbing the second “heralded” photon of the first pair.

The LH2 is a light-absorbing structure employed in the experiment and has undergone significant research. It is well known that in LH2, a ring of 9 bacteriochlorophyll molecules absorbs photons at the 800 nanometers (nm) wavelength, passing the energy to a second ring of 18 bacteriochlorophyll molecules, which can release fluorescent photons at the 850 nm wavelength.

The energy from the photons would continue to be transferred to subsequent molecules in the native bacterium until it was used to start the process of photosynthesis. In contrast, the experiment’sexperiment’s separation of the LH2s from other cellular machinery and subsequent detection of the 850 nm photon served as unmistakable evidence that the process had been initiated.

Co-lead author Graham Fleming, a senior faculty scientist in the Biosciences Area at Lawrence Berkeley National Laboratory (Berkeley Lab), said, “If you’ve only got one photon, it’s easy to lose it. So that was the fundamental difficulty in this experiment, and that’s why we used the herald photon. The scientists analyzed more than 17.7 billion herald photon detection events and 1.6 million heralded fluorescent photon detection events to ensure that the observations could only be attributed to single-photon absorption and that no other factors were influencing the results.”

Co-lead author Birgitta Whaley, a senior faculty scientist in the Energy Sciences Area at Berkeley Lab, said, “I think the first thing is that this experiment has shown that you can do things with individual photons. So that’s a very, very important point. The next thing is, what else can we do? Our goal is to study the energy transfer from individual photons through the photosynthetic complex at the shortest possible temporal and spatial scales.”

Journal Reference:

  1. Li, Q., Orcutt, K., Cook, R.L. et al. Single-photon absorption and emission from a natural photosynthetic complex. Nature (2023). DOI: 10.1038/s41586-023-06121-5
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