Scientists have finally decoded the process of photosynthesis

After decades of hard work and frustrating setbacks, scientists have finally seen the process by which nature creates the oxygen we breathe. The experts accomplished this herculean task by using SLAC’s X-ray laser. In the intricate process of photosynthesis, Photosystem II, a protein complex present in plants, algae, and cyanobacteria, plays a vital role in harvesting energy from sunlight and splitting water, generating the oxygen that sustains life on Earth.

Despite the fundamental nature of this process, much about Photosystem II’s operation remains enigmatic. Recently, however, a team of researchers from the Department of Energy’s Lawrence Berkeley National Laboratory and SLAC National Accelerator Laboratory, along with collaborators from Uppsala University, Humboldt University, and other institutions, have successfully unlocked a key aspect of Photosystem II’s function.

Giant laser to the rescue

By utilizing the Linac Coherent Light Source (LCLS) at SLAC and the SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan, the researchers were able to capture, in unprecedented atomic detail, the final moments preceding the release of breathable oxygen in the photosynthesis process. The data unveiled a previously unknown intermediate reaction step.

This groundbreaking discovery, published in the journal Nature, provides valuable insight into the optimization of photosynthesis in nature. Moreover, it aids scientists in the development of artificial photosynthetic systems that emulate this natural process to harness sunlight, converting carbon dioxide into hydrogen and carbon-based fuels.

Jan Kern, a scientist at Berkeley Lab and co-author of the study, explained the potential implications of the findings: “The more we learn about how nature does it, the closer we get to using those same principles in human-made processes, including ideas for artificial photosynthesis as a clean and sustainable energy source.” 

Fellow co-author and Berkeley Lab scientist Junko Yano added, “Photosystem II is giving us the blueprint for how to optimize our clean energy sources and avoid dead ends and dangerous side products that damage the system. What we once thought was just fundamental science could become a promising avenue to improving our energy technologies.”

Using a baseball analogy to explain photosynthesis

During photosynthesis, the oxygen-evolving center of Photosystem II – a cluster comprising four manganese atoms, one calcium atom, and connecting oxygen atoms – facilitates a series of complex chemical reactions responsible for splitting water molecules and releasing molecular oxygen. 

The center cycles through four stable oxidation states (S0 through S3) when exposed to sunlight. This process can be likened to a baseball game, where S0 represents a player on home base prepared to bat, while S1-S3 correspond to players on first, second, and third bases.

As a player progresses from one base to the next with each successful hit, the oxygen-evolving center similarly advances through the oxidation states with each absorbed photon of sunlight. Once the fourth ball is hit and the player reaches home, a run is scored; in the case of Photosystem II, a molecule of breathable oxygen is released. 

The scientists probed the atomic structure of a cluster in cyanobacteria samples. They excited the samples with optical light and used ultrafast X-ray pulses from the Linac Coherent Light Source (LCLS) and the SPring-8 Angstrom Compact free electron LAser (SACLA) to collect data.

This technique allowed the researchers to image, for the first time, the transient state (S4) where two oxygen atoms bond and release an oxygen molecule. The data also revealed additional, previously unseen steps in this reaction. 

“It’s really going to change the way we think about Photosystem II. Although we can’t say we have a unique mechanism based on the data yet, we can exclude some models and ideas people have proposed over the last few decades,” said Uwe Bergmann, a co-author of the study and a scientist and professor at the University of Wisconsin-Madison.

What comes next

The study is part of a series conducted by the team over the past decade, with earlier work focusing on observing the photosynthetic cycle at the temperature at which it occurs in nature. Vittal Yachandra, a co-author and scientist at Berkeley Lab, explained the complexity of Photosystem II, stating, “There are several things happening at different parts of Photosystem II and they all have to come together in the end for the reaction to succeed.”

To further their understanding, the researchers plan to capture more snapshots of the process. Jan Kern acknowledged that there are still aspects of the process that remain elusive, emphasizing the need for additional snapshots. 

Overcoming the challenges posed by faint X-ray signals and low repetition rates of existing X-ray lasers like LCLS and SACLA required years of effort and data collection.

However, with the LCLS-II upgrade scheduled to come online later this year, the repetition rate will increase dramatically, from 120 pulses per second to up to a million per second. 

Bergmann expressed enthusiasm for the future of the research, saying, “With these upgrades, we will be able to collect several days’ worth of data in just a few hours. We will also be able to use soft X-rays to further understand the chemical changes happening in the system. These new capabilities will continue to drive this research forward and shed new light on photosynthesis.”

This groundbreaking research not only demystifies the complex process of photosynthesis but also offers promising avenues for developing sustainable energy solutions.

More about photosynthesis

Photosynthesis is a fundamental biological process that occurs in plants, algae, and certain types of bacteria, such as cyanobacteria. It plays a critical role in maintaining Earth’s ecosystems and regulating the planet’s climate by converting sunlight, carbon dioxide (CO2), and water (H2O) into glucose and oxygen.

In this process, plants absorb sunlight through a pigment called chlorophyll, which is found in specialized organelles called chloroplasts. The energy from sunlight drives a series of chemical reactions that convert carbon dioxide and water into glucose, a type of sugar that plants use for energy and growth. Oxygen is released as a byproduct of these reactions, which is vital for the survival of many living organisms, including humans and animals.

Photosynthesis serves several essential functions in Earth’s ecosystems:

Oxygen production

The oxygen released during photosynthesis is crucial for the survival of aerobic organisms, including humans and animals, who rely on it for respiration. The process is responsible for producing the majority of the planet’s atmospheric oxygen.

Carbon sequestration

Photosynthesis helps reduce the amount of carbon dioxide in the atmosphere by using it to produce glucose. This process is essential in mitigating the greenhouse effect, as increased levels of CO2 can contribute to global warming.

Energy source for food chains

Photosynthesis enables plants to produce glucose, which serves as a primary energy source for many organisms. Herbivores consume plants to obtain energy, and carnivores, in turn, consume herbivores. As such, photosynthesis forms the foundation of most food chains and supports the survival and growth of countless species.

Soil formation and nutrient recycling

Dead plant material decomposes and returns nutrients to the soil, enriching it and allowing new plants to grow. Photosynthesis, therefore, plays a vital role in maintaining soil fertility and promoting nutrient cycling in ecosystems.

Climate regulation

Photosynthesis helps regulate Earth’s climate by removing carbon dioxide from the atmosphere and releasing oxygen. This process also contributes to the formation of clouds through the process of transpiration, where plants release water vapor, which eventually condenses to form clouds. Clouds play a crucial role in reflecting sunlight back into space, helping to regulate Earth’s temperature.

In summary, photosynthesis is a crucial process that supports life on Earth by producing oxygen, sequestering carbon dioxide, forming the basis of food chains, contributing to soil formation and nutrient recycling, and helping regulate the planet’s climate.

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