Scientists uncover a key secret of photosynthesis using X-ray lasers

Researchers from the Lawrence Berkeley National Laboratory, the SLAC National Accelerator Laboratory, Uppsala University, Humboldt University, and other institutions have made a breakthrough in unraveling the mysteries of photosynthesis. Specifically, they have discovered how photosystem II, a protein complex found in plants, algae, and cyanobacteria, uses sunlight to split water and produce oxygen. By utilizing the Linac Coherent Light Source and SPring-8 Angstrom Compact free electron LAser, they were able to observe the final moments leading up to the release of breathable oxygen in atomic detail. This discovery, which was published in the scientific journal Nature, provides insights into how nature has optimized photosynthesis and could aid in the development of artificial photosynthetic systems that can convert carbon dioxide into fuels using natural sunlight.

The machinery responsible for photosynthesis—while commonplace and essential to life on Earth—is still not fully understood. One of its molecular mysteries involves how a protein complex, photosystem II, harvests energy from light and uses it to split water into hydrogen and oxygen. This video explains how, in a previous paper, researchers were able to see two key steps in photosynthetic water splitting under conditions as it occurs in nature, a big step to decoding how the process works in detail. Credit: Chris Smith/SLAC National Accelerator Laboratory

Jan Kern, a scientist at Berkeley Lab and co-author of the study, stated that understanding how nature carries out photosynthesis can assist in developing human-made processes such as artificial photosynthesis that harness natural energy in a sustainable manner. The discovery of the intermediate reaction step in photosystem II, as reported in the study published in Nature, can serve as a blueprint for optimizing clean energy sources and avoiding hazardous byproducts that can harm the system. Another co-author, Junko Yano, also from Berkeley Lab, highlighted that the research results could potentially become a promising avenue for enhancing energy technologies by leveraging fundamental scientific discoveries.

Bases loaded

Photosystem II is responsible for splitting water molecules during photosynthesis and releasing oxygen. The oxygen-evolving center in photosystem II comprises a cluster of four manganese atoms and one calcium atom that are linked by oxygen atoms. This center undergoes a series of complex chemical reactions known as S0 through S3, triggered by exposure to sunlight. S0 represents the start of the process, similar to a player waiting to bat in a baseball game. As the reactions progress, they are akin to players advancing bases, with S1, S2, and S3 representing players on first, second, and third bases, respectively. The last reaction, similar to the fourth hit in a baseball game, releases one molecule of oxygen. Researchers from Berkeley Lab and other institutions examined this center by stimulating samples from cyanobacteria with optical light and then using ultrafast X-ray pulses from LCLS and SACLA to study the chemical process and the cluster’s atomic structure.

A homerun

By using ultrafast X-ray pulses from LCLS and SACLA, the researchers were able to capture the elusive S4 state, the moment when two oxygen atoms bond together and release an oxygen molecule. This discovery challenges previous assumptions that this state could never be captured. Co-author Uwe Bergmann notes that the findings will have a significant impact on our understanding of photosystem II, and while they have not yet identified a unique mechanism, they have ruled out several models proposed over the past few decades. These new insights provide actual structural data on the final step of photosystem II’s process, bringing scientists closer to understanding how nature efficiently produces breathable oxygen.

In photosystem II, the water-splitting center cycles through four stable states, S0–S3. On a baseball field, S0 would be the start of the game when a batter on home base is ready to hit. S1–S3 would be players waiting on first, second, and third. The center gets bumped up to the next state every time it absorbs a photon of sunlight, just like how a player on the field advances one base every time a batter connects with a ball. When the fourth ball is hit, the player slides into home, scoring a run or, in the case of photosystem II, releasing the oxygen we breathe. Credit: Greg Stewart/SLAC National Accelerator Laboratory

Over the past decade, the research team has conducted a series of studies to observe different stages of the photosynthetic cycle at the temperature at which it naturally occurs. The latest study focuses on the final step, where breathable oxygen is produced. Co-author Vittal Yachandra explained that various factors at different stages of photosystem II must come together for the reaction to succeed. Just like how a player’s movements in baseball are influenced by factors such as the ball’s location and the position of the fielders, the protein environment surrounding the catalytic center affects how this reaction plays out.

Brighter X-rays for a brighter future

The researchers aim to further investigate the photosynthetic process by conducting additional experiments to capture more snapshots of the reaction. While this latest study has provided unprecedented insights into the final steps of photosynthesis, there are still gaps in understanding that need to be bridged. The team plans to take advantage of the LCLS-II upgrade, which will significantly increase the repetition rate of X-ray pulses, allowing them to collect large amounts of data quickly. Additionally, soft X-rays will be used to further investigate the chemical changes occurring in the system. These new capabilities are expected to drive the research forward and provide further understanding of photosynthesis.

Source: SLAC National Accelerator Laboratory

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