Photosynthesis discovered to be initiated by single photons

The latest findings published in the journal Nature have shed light on the intricate process of photosynthesis, where organisms harness the energy from light to sustain life. This groundbreaking study, released on June 14, reveals that photosynthesis is remarkably sensitive, capable of responding to even the tiniest unit of light—the photon.

This discovery solidifies our existing understanding of photosynthesis and promises to address fundamental inquiries about the mechanics of life on the microscopic level, where the realms of quantum physics and biology intersect.

Co-lead author Graham Fleming, a distinguished faculty scientist at Lawrence Berkeley National Laboratory and professor of chemistry at UC Berkeley, emphasized the significant amount of global research undertaken to comprehend the events that follow photon absorption. Surprisingly, the initial step had not received the same level of attention until now. Fleming explained that the question regarding this crucial first step remained unanswered in detail.

In their comprehensive study, Fleming, together with co-lead author Birgitta Whaley, a senior faculty scientist at Berkeley Lab, and their respective research groups, demonstrated that the first phase of photosynthesis can indeed be initiated by a solitary photon in photosynthetic purple bacteria. Given the shared evolutionary ancestry and similar processes employed by all photosynthetic organisms, the team confidently asserts that the same mechanism applies to photosynthesis in plants and algae. Fleming remarked, “Nature devised an incredibly ingenious mechanism.”

How living systems use light

Scientists have long assumed that a single photon is all it takes to initiate the highly efficient process of photosynthesis, where photons transfer energy to electrons, leading to the production of energy-rich molecules. Despite this assumption, no concrete evidence had been presented to support this notion. Additionally, much of the research on photosynthesis focused on later stages and utilized intense laser pulses to investigate the process.

First author Quanwei Li, a joint postdoctoral researcher working with the Fleming and Whaley groups, emphasized the lack of demonstration backing the assumption about the role of a single photon. The challenge is further complicated by the vast disparity in intensity between laser beams and sunlight, with lasers being significantly brighter. Even when attempting to match the intensity of sunlight, the quantum properties of light known as photon statistics come into play, making it unclear how different types of photons affect the absorption process. Li emphasized the need to understand the quantum properties of living systems to gain true insights into their functioning and to develop efficient artificial systems for renewable fuel generation, drawing an analogy to the necessity of understanding each particle to build a quantum computer.

Co-senior author Graham Fleming, left, and first author Quanwei Li near some of the equipment used in their cutting-edge experiment. Credit: Henry Lam/Fleming Lab

Initially, the comprehension of photosynthesis, along with other chemical reactions, was based on a macroscopic understanding. Researchers possessed knowledge of the overall inputs and outputs, enabling them to make inferences about the interactions occurring between individual molecules. However, with the advent of technological advancements in the 1970s and 80s, scientists gained the ability to study chemicals at the individual level during reactions. This marked a significant leap forward.

Now, scientists are venturing into a new realm—the realm of individual atoms and subatomic particles. This frontier is being explored through the utilization of even more sophisticated and cutting-edge technologies. By delving into the microscopic scale, researchers aim to unravel the intricacies of chemical processes with unprecedented precision and insight. These advancements hold great potential for pushing the boundaries of our understanding further and unlocking novel scientific discoveries.

From assumption to fact

To facilitate the observation of individual photons, a unique collaboration between theorists and experimentalists was formed, combining state-of-the-art tools from quantum optics and biology. This interdisciplinary approach brought together experts who were not accustomed to employing these techniques in their respective fields.

The scientists established a photon source capable of generating a single pair of photons via spontaneous parametric down-conversion. Using a highly sensitive detector, they observed the first photon known as “the herald,” which confirmed the imminent arrival of the second photon to the assembled sample of light-absorbing molecular structures derived from photosynthetic bacteria. Another photon detector placed near the sample was employed to measure the lower-energy photon emitted by the photosynthetic structure after absorbing the second “heralded” photon from the original pair.

The light-absorbing structure employed in the experiment, referred to as LH2, had been extensively studied. It was understood that photons with an 800 nanometer (nm) wavelength were absorbed by a ring of nine bacteriochlorophyll molecules within LH2. This absorption process transferred energy to a second ring of 18 bacteriochlorophyll molecules capable of emitting fluorescent photons at 850 nm. In native bacteria, the energy from these photons would continue to transfer to subsequent molecules until it initiated the chemistry of photosynthesis. However, in the experimental setup where the LH2 structures were separated from other cellular components, the detection of the 850 nm photon served as a definitive indication that the activation of the process had occurred.

Over 17.7 billion herald photon detection events and 1.6 million heralded fluorescent photon detection events were meticulously analyzed by the scientists to ensure that the observations were solely attributable to single-photon absorption. They rigorously eliminated any potential influences from other factors that could affect the results.

Co-author Birgitta Whaley emphasized the significance of the experiment, highlighting that it demonstrated the feasibility of working with individual photons—a groundbreaking achievement. Looking ahead, the researchers aim to explore further possibilities. Their objective is to investigate the energy transfer from individual photons through the photosynthetic complex at the smallest conceivable temporal and spatial scales, pushing the boundaries of scientific understanding in the field.

Source: Lawrence Berkeley National Laboratory

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