Details

Autor(en) / Beteiligte

• Mulholland, M. R.
• Bernhardt, P. W.
• Widner, B. N.
• Selden, C. R.
• Chappell, P. D.
• Clayton, S.
• Mannino, A.
• Hyde, K.

Titel

High Rates of N2 Fixation in Temperate, Western North Atlantic Coastal Waters Expand the Realm of Marine Diazotrophy

Ist Teil von

• Global biogeochemical cycles, 2019-07, Vol.33 (7), p.826-840

Ort / Verlag

Washington: Blackwell Publishing Ltd

Erscheinungsjahr

2019

Quelle

Wiley Online Library - AutoHoldings Journals

Beschreibungen/Notizen

• Dinitrogen (N2) fixation can alleviate N limitation of primary productivity by introducing fixed nitrogen (N) to the world's oceans. Although measurements of pelagic marine N2 fixation are predominantly from oligotrophic oceanic regions, where N limitation is thought to favor growth of diazotrophic microbes, here we report high rates of N2 fixation from seven cruises spanning four seasons in temperate, western North Atlantic coastal waters along the North American continental shelf between Cape Hatteras and Nova Scotia, an area representing 6.4% of the North Atlantic continental shelf area. Integrating average areal rates of N2 fixation during each season and for each domain in the study area, the estimated N input from N2 fixation to this temperate shelf system is 0.02 Tmol N/year, an amount equivalent to that previously estimated for the entire North Atlantic continental shelf. Unicellular group A cyanobacteria (UCYN‐A) were most often the dominant diazotrophic group expressing nifH, a gene encoding the nitrogenase enzyme, throughout the study area during all seasons. This expands the domain of these diazotrophs to include coastal waters where dissolved N concentrations are not always depleted. Further, the high rates of N2 fixation and diazotroph diversity along the western North Atlantic continental shelf underscore the need to reexamine the biogeography and the activity of diazotrophs along continental margins. Accounting for this substantial but previously overlooked source of new N to marine systems necessitates revisions to global marine N budgets. Plain Language Summary Measurements suggest that at present, the marine nitrogen (N) budget is not balanced, and that rates of N losses exceed rates of N inputs in the world's oceans. Identifying quantitatively significant sources of new N inputs via marine dinitrogen (N2) fixation could potentially offset this imbalance. Here we provide an unprecedentedly large data set showing high rates of seasonally and interannually averaged N2 fixation rates over a large swath of western North Atlantic Ocean coastal waters along the continental shelf of North America, an area where N2 fixation was previously thought to be negligible. If marine N2 fixation has also been seriously underestimated in other coastal systems, global estimates of N inputs from N2 fixation need to be revised upward, offsetting the current marine N budget imbalance. Key Points High rates of N2 fixation were observed in temperate, western North Atlantic coastal waters Diverse diazotrophic groups were identified from samples collected in western North Atlantic coastal waters Global estimates of new N inputs from N2 fixation should be revised to include coastal waters