A new study reveals that photosynthesis starts with a single photon

The research team was led by Graham Fleming, a senior faculty scientist at Lawrence Berkeley National Laboratory and professor of chemistry at UC Berkeley, and Birgitta Whaley, a senior faculty scientist in the Energy Sciences Area at Berkeley Lab.

The findings provide new insights into one of the most fundamental processes and the key to life on Earth.

A brief history of photosynthesis

Photosynthesis produces glucose (sugar), which plants use as a source of energy, and oxygen, used by other living things, such as animals, to breathe. This process happens with the help of chlorophyll, the green pigment, which helps to capture the sun's energy or photons.

During photosynthesis, these photons give energy to electrons, which then move around and interact with other molecules to create the necessary components for making glucose.

Scientists have long suspected a single photon is enough to start the process of photosynthesis. This is because the number of photons reaching a chlorophyll molecule is about one thousand, which is not much. 

Despite the low number of photons reaching chlorophyll molecules, photosynthesis still occurs. However, scientists have not found evidence to support their assumptions, that is, until now.

Previous studies on this phenomenon involve using lasers, which are very different from sunlight. 

"There’s a huge difference in intensity between a laser and sunlight – a typically focused laser beam is a million times brighter than sunlight. Even if you manage to produce a weak beam with an intensity matching that of sunlight, they are still very different due to quantum properties of light called photon statistics," explained first author Quanwei Li in a press release.

A team effort to observe individual photons

Understanding photosynthesis on atomic and subatomic scales meant taking an interdisciplinary approach. This meant bringing together a team of experimentalists and theorists from different fields, such as biology and quantum physics, to design a cutting-edge experiment. 

Scientists developed a photon source capable of producing a pair of photons through spontaneous parametric down-conversion. They used sensitive detectors to monitor the first photon, known as the herald, to confirm the arrival of the second photon at the light-absorbing molecule. Additionally, they detected the emitted photon.

They used a photosynthetic purple bacteria called the LH2 (light-harvesting complex 2) as the light-absorbing molecule. The LH2 structure absorbs 800 nanometers (nm) photons with a ring of 9 bacteriochlorophyll molecules. The absorbed energy transfers to another ring of 18 bacteriochlorophyll molecules, emitting 850 nm fluorescent photons, and the process continues until photosynthesis is initiated. 

However, in the experiments, the team used isolated the LH2 molecule. This meant that detection of the 850 nm photon was enough to confirm the activation or start of the photosynthesis process. And that is precisely what the team observed.

The team carefully analyzed over 17.7 billion herald photon detection events and 1.6 million heralded fluorescent photon detection events to confirm that the observations were solely due to single-photon absorption and eliminate any potential external influences.

Photosynthetic organisms, sharing a common evolutionary ancestor, assure the team that photosynthesis operates similarly in plants and algae. 

"Nature invented a very clever trick," said Fleming in a press release.

The findings of this study are published in the journal Nature.