A globally important microbial process hidden on marine particles

All life on Earth depends on nitrogen. However, nitrogen is very scarce in the oceans, so it is critical to ensure that marine life thrives. Some bacteria that is found in marine waters can convert N2 gas (N2) into ammonia, thereby supplying nitrogen to the marine food web. What on Earth is it? For years scientists have been puzzled by the question of how bacteria that lives from dissolved organic material in marine waters can fix N2. The combination of high oxygen levels and low amounts of dissolved organic material in marine waters would have prevented anaerobic, energy-consuming N2 fixation. In the 1980s, it was suggested that aggregates (or "marine snow particles") could be suitable locations for N2 fixation. However, this theory was never proved. Bis now. Researchers from the University of Copenhagen have demonstrated, using mathematical models, that marine plankton aggregates can be used to fix nitrogen by microbes. This study was published in Nature Communications. Advertisement Marine snow Marine snow is made up of various organisms that have been dissolved in the water column. Picture of marine snow taken from the Sargasso Sea Photo by L. Riemann Our work took nearly two years but was well worth it. The results were quite remarkable. We were able to develop a model that mimics the conditions of marine snow particles in close collaboration with our collaborators at DTU Aqua's Center for Ocean Life. This model shows that marine particles can be densely colonized with bacteria. Subhendu Chakraborty, first author and postdoc at University of Copenhagen's Department of Biology, explains how this bacteria growth leads to extensive respiration, which eventually allows for anaerobic N2 fixation. The researchers were able to show depth distributions of N2 fixation within the marine water column using their model. The researchers discovered that N2 fixation depends on the size, density, and sinking speed marine snow particles. They also demonstrated that the rates they calculated were comparable to those measured in marine water. Sampler of marine water As seen here, marine water samples are often collected with bottles attached to a rosette. Photo by L.asse Riemann Lasse Riemann (Professor at the Department of Biology) says that "this comparison gave us confidence with the model." He said, "We are proud of our study because it provides the first explanation as to how marine-snow-associated N2 fixing can occur." The results also indicate that this process is crucial for global marine nitrogen cycling, and thus for plankton growth as well as productivity. Because of its apparent pivotal role in the cycle of many nutrients in oceans, the researchers hope that their research will inspire future research on marine microbial life.

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