Every living thing needs nitrogen, and the world uses a significant portion of its energy making nitrogen fertilizer for agriculture. Studying microorganisms that naturally capture atmospheric nitrogen - a process called nitrogen fixation - can inspire new sustainable methods to produce fertilizers, saving energy and reducing water pollution.
In a new study, published in Proceedings of the National Academy of Sciences, researchers at Lawrence Livermore National Laboratory (LLNL), University of California Berkeley and Northern Arizona University investigated a California river ecosystem and found a nitrogen-fixing bacterium that acts like a proto-organelle, which could provide a roadmap for harnessing nitrogen fixation for agriculture.
"About two percent of global energy use goes toward making nitrogen fertilizer," said LLNL scientist and author Peter Weber. "If we can mimic nature's approach and build a nitrogen-fixing organelle, then we can potentially get nitrogen on demand."
The river ecosystem studied in this work contained three key members: (1) green macroalgae (similar to seaweed), which appear on riverbed rocks in spring and, by summer, grow long filamentous streamers that move with the flow of water, (2) diatoms, or unicellular, golden-brown microalgae, that colonize the surface of the streamers, and (3) nitrogen-fixing bacteria that live symbiotically inside the diatoms as if they were proto-organelles - the precursors to subcellular structures.
To investigate the nutrient exchanges among these ecosystem members, the scientists collected river streamer samples in water and injected heavy isotopes of carbon and nitrogen in the lab. They used nanoscale secondary ion mass spectrometry (NanoSIMS) to visualize the distribution of the newly acquired carbon and nitrogen among the three members. Much like a medical PET scan illuminates organs, these techniques allowed the team to trace the allocation of these essential nutrients in the algae system down to the organelles and proto-organelles.
"Because we have this equipment that's able to measure the concentration of the isotopes at a very high spatial resolution, you can see where they go at the subcellular level, including in the symbiotic bacteria," said author and LLNL scientist Ty Samo.
NanoSIMS showed that nitrogen-fixing bacteria get first dibs and the largest serving of the newly acquired nitrogen. They also had some of the highest amounts of newly acquired carbon, alongside the algal chloroplasts where photosynthesis occurs. The bacteria transform atmospheric nitrogen gas into useable, organic nitrogen that is a building block for cell growth - and they likely use the carbon to power that process.
"The macroalgae in river streamers pull carbon dioxide out of the atmosphere through photosynthesis. The diatoms that live on top of the macroalgae are doing the same thing. The unique thing is that the symbiotic bacteria - which live inside the diatoms - are also fixing nitrogen and helping to support the diatom's needs," said author and LLNL scientist Jennifer Pett-Ridge. "These types of bacteria are the only ones that can take nitrogen gas out of the air and turn it into organic nitrogen, a critical resource that all cells and ecosystems need. Their activity eventually supports a whole food web - from frog tadpoles, to snails, aquatic invertebrates and eventually salmon and other fish."
These miniscule bacteria provide the nitrogen for the entire river food web, from the diatoms to caddisflies that eat the diatom-coated algae streamers to the endangered salmon that eat the caddisflies.
"The bacteria are so tiny, but in aggregate, they have this massive, massive impact," said Samo.
By studying the symbiosis in more detail, the team hopes to be able to transfer the nitrogen-fixing abilities to other cells that are applicable to bioenergy, agriculture and biomaterials.
