Nitrogen Fixation Hotspots in Atlantic Seaweed
Published:01 Sep.2023    Source:University of North Carolina at Chapel Hill

A new study by researchers at the University of North Carolina at Chapel Hill examined nitrogen fixation among diazotrophs -- microorganisms that can convert nitrogen into usable form for other plants and animals -- living among sargassum. Sargassum, a brown macroalgae in the seaweed family, floats on the surface of the open ocean and provides habitat for a colorful array of marine life such as small fish, brine shrimp and other microorganisms. Previous studies have overlooked diazotrophs associated with sargassum, which could mean a historical underestimation of nitrogen fixation in the Atlantic nitrogen budget. The study, published today in PLOS ONE, found that nitrogen fixation in sargassum communities was significant.

Nitrogen is critical for life. Plants and animals need it for growth. More nitrogen in the ocean means greater biological productivity and growth. Sargassum mats provide an important habitat for organisms to perform nitrogen fixation, but few studies have measured it in sargassum communities. The team compared the nitrogen fixation rate with other marine sources, including commonly studied nitrogen fixing organisms such as planktonic diazotrophs and coastal epiphytes -- plants that grow on other plants -- and found the sargassum communities outpaced them -- contributing significantly to the marine nitrogen cycle and potentially to sargassum blooms.
Collecting this type of data can be difficult, but the team's proximity to the Gulf Stream from their lab at the Coastal Studies Institute on East Carolina University's Outer Banks campus on Roanoke Island made it possible for them to make day-long trips to collect samples seasonally and process them quickly. The team was able to collect whole fronds of the seaweed and process them with minimal handling, better keeping the microorganisms intact for the study. Each piece was carefully managed in large tubs at the laboratory, where they collected data on nitrogen fixation rates. The team collected samples over a six-year period. Sargassum is typically abundant in the Caribbean Sea, Gulf of Mexico, Gulf Stream and Sargasso Sea. The team's work in North Carolina provides a comprehensive view of how nitrogen fixation rates vary over time, but understanding how they could vary across a wider geographic range could be a next step for further research.