Marine snow is made up of various debris from the water column. Picture of marine snow taken from the Sargasso Sea Credit: L. Riemann
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 (N 2 ) into ammonia, 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 N 2 fixation. However, this theory was never proved. This is the case until 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.
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 N 2 fixation.
The researchers were able to show depth distributions of N2 fixation in the marine water column using their model. The size, density, and sinking speed for marine snow particles are all factors that affect N 2 fixation, according to the researchers. They also demonstrated that the modeled rates were comparable with actual rates in marine waters.
Sampler of marine water
Lasse Riemann (Professor at the Department of Biology) says that "this comparison gave us confidence in our model." He said, "We are proud of our study because it provides the first explanation as to how marine-snow-associated N 2 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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Further information: Subhendu Chkraborty and colleagues, Quantifying nitrogen fixation in sinking marine particles by heterotrophic bacteria, Nature Communications (2021). Information from Nature Communications Subhendu Chakraborty and colleagues, Quantifying nitrogen fixation in sinking marine particles by heterotrophic bacteria, (2021). DOI: 10.1038/s41467-23875-6