I've seen things you wouldn't believe, like an atom about to photosynthesize

Paper details previously unknown step in process of converting light energy to chemical energy

Photosynthesis – the process by which plants and some other organisms convert sunlight to food – is complex, and scientists don't fully understand how it works. But a team of researchers led by the US Department of Energy's SLAC National Accelerator Laboratory reckon they're closer to solving the mystery – they captured an image of the atoms inside cyanobacteria undergoing photosynthesis just as the tiny organism released oxygen.

X-ray lasers at SLAC's Linac Coherent Light Source at Stanford University, and the SPring-8 Angstrom Compact free electron LAser (SACLA) at a synchrotron radiation facility in Japan's SPring-8 lab, were directed at a specific protein used to catalyze the chemical reaction. The protein complex, dubbed Photosystem II, is used by organisms like cyanobacteria or algae to photosynthesize. 

Photosystem II carries a molecule, made up of four manganese (Mn) atoms and one calcium (Ca) atom connected by oxygen atoms, and splits a water molecule apart to release oxygen. The research team managed to image the different steps in the reaction, and discovered a previously unknown step in the process. 

"Many of the steps in the reaction involve very small changes in the position of individual atoms and therefore we need to get very clear atomic pictures of the Photosystem II at the different times during the reaction," the team told The Register. Zapping the cyanobacteria with the laser and measuring the diffraction patterns of the reflected beam allows researchers to build up an image of the positions of each of the atoms involved in photosynthesis.

Tens or even hundreds of thousands of these snapshots are required to see how the atoms move over time. Capturing them is a tedious process that constantly requires fresh cyanobacteria samples, since each one gets destroyed after being exposed to the powerful X-ray pulse. The experimental setup to facilitate the research took six years to build, as the authors explained in a study published by Nature on Wednesday. 

"The reaction cycle in Photosystem II involves four main steps, each triggered by the absorption of one photon. In each of the steps at the site of action in Photosystem II, a group of four Mn atoms and one Ca atom acts like a battery that gets more and more charged with each step," they told us. After four photons have been absorbed, Photosystem II strips protons from water molecules and brings them close enough that a new chemical bond is forged between the oxygen molecules to produce gas. 

"In this last step there are at least four different events happening and in this new study it was possible to visualize some of these events for the first time" they explained. 

"Other experts argued that this is something that could never be captured," Uwe Bergmann, professor in ultrafast X-ray science at the University of Wisconsin-Madison and co-author of the paper, added. "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. It's the closest anyone has ever come to capturing this final step and showing how this process works with actual structural data."

Although the data gives scientists a better understanding of photosynthesis, the full process remains unknown and is the subject of ongoing debate. The decades-long effort to figure out how a few simple ingredients create the food we eat and the air we breathe is, however, well and truly worth it. Scientists could potentially reproduce the chemical reaction to create new fuels that use sunlight to make clean energy. 

The researchers remain committed to their research. "This is a unique capability in nature. We focus our research on this protein because of the exciting implications for clean energy production and sustainability, and because Photosystem II has produced essentially all of the oxygen in our atmosphere, enabling the evolution of complex lifeforms that depend on oxygen respiration. It has been doing this reaction, which profoundly shaped our planet, for more than three billion years." ®

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