Nitrogen fixation in marine diazotrophs
Primary producers are organisms and plants that harness sunlight to take up CO2 and generate the food that fuels ecosystems. To do this, they need nitrogen. In the open ocean, marine microorganisms called diazotrophs make nitrogen available to primary producers through nitrogen fixation. Photosynthetic diazotrophs use sunlight to fix nitrogen, whereas non-photosynthetic diazotrophs can also fix nitrogen deeper in the ocean, out of reach of sunlight. Researchers recently found that in the open ocean, non-photosynthetic diazotrophs outnumber photosynthetic diazotrophs, which were previously thought to be the primary marine nitrogen fixers. Scientists from the EU-funded DIADOM (Interactions between nitrogen-fixers (diazotrophs) and dissolved organic matter in the ocean) initiative worked to build on this finding. They studied the relationship between the nitrogen fixation of non-photosynthetic diazotrophs and the organic matter they rely on for nutrition. To do this, the researchers embarked on scientific cruises in the southwest Pacific Ocean and the Mediterranean Sea to collect samples and conduct experiments. They measured nitrogen fixation, assessed the composition of oceanic organic matter and did genetic tests. The scientists found that rates of nitrogen fixation in the dark ocean were comparable to those in the surface ocean. Fixation rates were also higher in locations where more organic matter was available. This shows that non-photosynthetic diazotrophs make a significant contribution to the amount of fixed nitrogen available for marine life. Using genetic techniques, the scientists also found a diverse community of diazotrophs living in the dark ocean. This research expands our knowledge of where nitrogen fixation is thought to be possible. This impacts our understanding of the marine nitrogen available globally, and thus the capacity of oceans to take up CO2 and mitigate global warming.
Keywords
Nitrogen fixation, diazotrophs, ocean, global warming, marine organisms, CO2, DIADOM
