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Diamond’s new principal beamline scientist on the B24 beamline, Valentina Loconte, was recently awarded the AAAS Newcomb Cleveland Prize for work that identified a new organelle within single-celled algae that converts nitrogen gas into ammonia.
Valentina was a co-author of Nitrogen-fixing organelle in a marine alga, which was published in Science in April 2024. In her previous position at the Berkeley Lab and University of California at San Francisco, she worked alongside researchers at the University of California, Santa Cruz, Lawrence Berkeley National Laboratory, Kochi University and National Taiwan Ocean University. The paper was also a runner-up for the 2024 Science Breakthrough of the Year.
Deputy Director of Life Sciences, Martin Walsh, said: “We are delighted to welcome Valentina who will lead B24 at Diamond which provides correlative Cryo-Imaging of biological samples, combining data from our full-field cryo-soft X-ray microscope and our bespoke super resolution structured illumination microscope (CryoSIM). Valentina brings a wealth of experience in the field of cryo-bioimaging, cementing the capabilities of the Biological Cryo-Imaging group at Diamond that encompasses B24 and the UK’s national CryoEM centre, eBIC.
It has long been understood that only bacteria can perform nitrogen fixation, the process of turning nitrogen gas from the atmosphere into a usable form like ammonia. Plants that fix nitrogen, like legumes, can only do so by harbouring symbiotic bacteria in their roots, and not on their own. But the team’s groundbreaking discovery has challenged this idea by identifying a marine alga that performs nitrogen fixation on its own. The algae contain something they have called a nitroplast, a nitrogen-fixing organelle, a former bacterium that has become part of the cell.
The single-celled alga, Braarudosphaera bigelowii, can do what no other plant can: convert nitrogen on its own into a form the plant can use. Because of the nitroplasts, the cells essentially produce their own fertiliser.
The research that discovered the nitroplast has been a long time in the making – almost two decades. It started in the late 1990s, when Jon Zehr, emeritus professor and microbial ecologist at UC Santa Cruz, kept finding the same fragment of DNA in the ocean while doing research cruises. When his team tried to find the organism it came from, no one ever managed to see it. Samples under the microscope revealed nothing even though the DNA was found in all parts of the ocean.
A further puzzlement was that whatever organism has produced the DNA fragment was missing essential genes, including ones that would help it be visualised under microscopes.
A conclusion was made that for this organism to survive, it had to be getting help from something else. As Zehr said, “This would explain how it could be missing so many genes because it lives with somebody else who can provide those things.”
The next step was to try to grow the algae in a lab which proved to be a challenge. Kochi University’s Kyoko Hagino spent over a decade on this endeavour. It took six years and more than 300 sampling trips to find the healthiest cells and it took another six years to successfully culture them in her lab.
However, another massive hurdle was on the horizon. Just days after Kyoko had given Zehr’s lab the carefully grown samples, the university shut down due to Covid. UC Santa Cruz graduate student Esther Mak managed to keep the cultures going while the lab was shut down, something co-author of the paper, Kendra Turk-Kubo, noted was amazing and meant there was no halt to the research.
The team from UC Santa Cruz then collaborated with UC San Francisco structural biologist Carolyn Larabell and her then postdoctoral researcher, Valentina, who visualised the algae with soft X-ray tomography.
The X-ray tomography conducted by Valentina was crucial in identifying the organelle. What Valentina saw showed how the nitroplasts duplicated during the cell cycle.
Valentina said:
Inside the alga I found a little cell. We couldn’t see any of the original alga that didn’t have a little cell inside. It meant that the two organisms are really living together.
When she looked at how the cells divided, they were synchronised. Tyler Coale, a phytoplankton biologist at UC Santa Cruz, analysed the proteins of both cells, and realised they were interdependent. The smaller cell’s missing genes were part of the bigger cell; and the bigger cell received nitrogen from the little one. This co-dependency was the proof they needed to assert that they had discovered a single organism, with the nitroplast (the smaller cell) a component of the bigger algal cell.
Valentina said:
It was an honour and a pleasure to contribute to the discovery. In addition to its significant implications for agriculture, the discovery demonstrates how X-ray tomography enabled the identification of multiple stages of cells in their native state, in a high-throughput manner, without the need for slicing or staining the cell. Integrating hundreds of tomograms with proteomic data also marked a significant step forward, showing that data integration at multiple scales is key to fully understand the dynamics, function, and structure of a whole cell.
This revelation was huge: the process of endosymbiosis, where two organisms live inside another, is known to have caused the ancient events that gave rise to mitochondria and chloroplasts, but now the scientists have shown that the process created the nitroplast as well.
The discovery has the potential to change agriculture. Natural nitrogen fixation in agriculture would reduce the use of synthesised ammonia fertilisers which creates about 1.4% of global carbon monoxide emissions. However, this possibility is a long way off, but this incredible discovery has shown that organisms have already found a way to do it.
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