Capturing final act of photosynthesis offers milestone for clean energy

Essentially, this protein complex is a vital component in the process that allows plants and other photosynthetic organisms to generate their own food and release oxygen into the atmosphere.

However, researchers from the Lawrence Berkeley National Laboratory, the SLAC (Stanford Linear Accelerator Center) National Accelerator Laboratory, and other institutions have finally unlocked a crucial secret of Photosystem II. They have published their findings in the journal Nature, revealing how nature has perfected the process of photosynthesis. 

The discovery is expected to help scientists create artificial systems that imitate photosynthesis, potentially using sunlight to transform carbon dioxide into hydrogen and carbon-based fuels. Considering this context, Interesting Engineering (IE) interviewed Roberto Alonso-Mori, a lead scientist at SLAC’s LCLS (Linac Coherent Light Source) X-ray laser, to learn more. 

How does Photosystem II power life on Earth?

“Photosystem II is the protein responsible for utilizing the Sun’s energy to convert water into oxygen in plants, algae, and cyanobacteria,” explained Alonso-Mori. 

“This process also generates the protons and electrons to drive the consequent photosynthetic CO₂ reaction that happens in other parts of the organism.”

He discussed how the goal is to understand this mechanism at a molecular and atomic level. “In particular, the last step in the reaction, where the molecular oxygen (O₂) gets released, is the most intriguing and challenging aspect of the sequence,” he added. 

“Numerous suggestions have been proposed to explain the mechanism in detail. We were able to study this last step in detail by recording a molecular movie and discovering a new intermediate state. This narrowed down the potential mechanism to two or three possibilities.”

Photosystem II has an oxygen-evolving center comprising four manganese atoms and one calcium atom connected by oxygen atoms. During photosynthesis, this center helps to break apart a water molecule, releasing oxygen. 

When exposed to sunlight, the center goes through four stable states called S₀ to S₃. In a press release, the scientists liken these states to players on a baseball field – S₀ is the player on home base, while S₁ to S₃ are players on first, second, and third bases. 

Every time the complex absorbs a photon of sunlight, the player on the field (the ‘batter’) advances one base. When the fourth photon is absorbed, the player slides into home, releasing a molecule of oxygen.

Essentially, the team studied this center by exciting samples from cyanobacteria with optical light and then examining them with ultrafast X-ray pulses from LCLS and SACLA. This helped them understand the atomic structure of the cluster and the chemical processes around it.

“LCLS at SLAC (US) and SACLA (Japan) are X-ray Free Electron Lasers (XFEL)—only five exist in the world—and LCLS was the first commissioned— around 13 years ago,” Alonso-Mori told IE. 

X-rays capture a 100,000-frame molecular movie of photosynthesis 

The XFELs can produce molecular movies with atomic resolution due to their ultra-bright and ultra-short X-ray pulses, which can capture the atomic motions of molecules in real time. 

“The short pulse duration of XFELs, which lasts only a few femtoseconds, is essential in capturing the atomic motions of molecules by interacting with them before significant motion occurs,” he added. 

He emphasized that in the photosystem II reaction, where changes occur on the order of microseconds, it is essential to use such short pulses to capture the images before any X-ray pulse damage occurs. 

“By repeating this process with a series of XFEL pulses, we could create a molecular movie by capturing multiple snapshots of the molecular motion,” he said.

This technique enabled the first-time capturing of images of the short-lived state called S₄ in Photosystem II – or, as they would describe in their baseball analogy, the home run. This is when two oxygen atoms bond together and form an oxygen molecule that’s released. Significantly, the information revealed new steps in the reaction that were previously unknown.

“Photosystem II is responsible for producing the vast majority of O₂ in our atmosphere, enabling the evolution of complex life relying on respiration,” he added. 

In a previous statement, co-author Uwe Bergmann, a scientist and professor at the University of Wisconsin-Madison, mentioned that other experts argued that these images (what the team captured) were something that could never be captured.

“It’s really going to change the way we think about Photosystem II,” he said. 

“It’s the closest anyone has ever come to capturing this final step”

While the team may not have discovered a revolutionary mechanism, just yet, they can confidently say that some of the existing theories about photosynthesis that have been proposed over the last few decades can be ruled out based on their findings. Think of it like a scientific highlighter that has marked off some possibilities, narrowing down the field and bringing us one step closer to understanding this incredible natural process.

“It’s the closest anyone has ever come to capturing this final step and showing how this process works with actual structural data,” Bergmann argued. 

“This is one more step for understanding the precise mechanism utilized by nature to split water into oxygen and hydrogen in ambient conditions using just the sunlight,” Alonso-Mori told IE. 

In future experiments, the researchers aim to acquire even more precise and detailed images of the reaction steps and include additional “frames” in the “movie” of the reaction. 

“[This will enable us] to discern the exact configuration of the intermediate state that the two oxygen atoms form before releasing O₂ gas and identify the correct mechanism among the remaining proposals,” he said.

“To do this is not an easy task. Many steps in the reaction involve small changes in the position of individual atoms.”

He mentioned to IE that to obtain clear atomic ‘frames’ at different times, it is necessary to measure a large number of images (many 10,000s to 100,000s) for each time-point of interest. This, however, consumes a significant amount of measurement time at the XFEL. 

“Moreover, combining these images into a single picture is computationally demanding. It took approximately six years of work to collect the data necessary for this research,” Alonso-Mori highlighted. 

He revealed that an upcoming upgrade of LCLS (LCLS-II) will allow his team to take these snapshots at a much higher rate (up to 10,000 times faster) and unravel the secrets of the photosynthetic reaction mechanism at a much faster pace.

Replicating the process could harness a cleaner energy future 

IE prompted Alonso-Mori to reveal why his team is motivated to unravel the mysteries of photosynthesis. His answer was simple yet profound: he’s in awe of the remarkable and unparalleled ability of the Photosystem II protein to transform water into oxygen. 

He went on to explain that this process has enormous potential for creating clean energy and promoting sustainability. In essence, the team is ultimately driven by the desire to understand how nature has perfected this incredible feat and how we can harness it to create a brighter future for all.

“By understanding how nature accomplishes this challenging task, we could replicate this process in synthetic systems to produce clean energy from sunlight in the future,” Alonso-Mori concluded.