Preliminary estimates of the contribution of Arctic nitrogen fixation to the global nitrogen budget

Abstract

Dinitrogen (N 2 ) fixation is the source of all biologically available nitrogen on earth, and its presence or absence impacts net primary production and global biogeochemical cycles. Here, we report rates of 3.5-17.2 nmol N L 21 d 21 in the ice-free coastal Alaskan Arctic to show that N 2 fixation in the Arctic Ocean may be an important source of nitrogen to a seasonally nitrogen-limited system. If widespread in surface waters over ice-free shelves throughout the Arctic, N 2 fixation could contribute up to 3.5 Tg N yr 21 to the Arctic nitrogen budget. At these rates, N 2 fixation occurring in ice-free summer waters would offset up to 27.1% of the Arctic denitrification deficit and contribute an additional 2.7% to N 2 fixation globally, making it an important consideration in the current debate of whether nitrogen in the global ocean is in steady state. Additional investigations of high-latitude marine diazotrophic physiology are required to refine these N 2 fixation estimates.

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

“…Planktonic N 2 fixation rates by some marine cyanobacteria are thought to be limited by temperature (Carpenter & Capone, 2008). N 2 fixation by cyanobacteria was previously thought to be constrained to surface waters with temperatures >20°C (e.g., Carpenter & Capone, 2008), but more recently, active N 2 fixation has been detected in higher-latitude temperate regions (Cassar et al, 2018;Moisander et al, 2010) and even coastal Arctic Seas (Blais et al, 2012;Harding et al, 2018;Sipler et al, 2017). However, linear regression analyses suggest no relationship between water temperature and N 2 fixation rates either in the pooled data (R 2 = 0.10) or in surface waters (R 2 = 0.08).…”

“…We now know that marine diazotrophs are diverse and include unicellular cyanobacteria as well as noncyanobacterial diazotrophs that occupy a wider range of marine habitats than previously thought (Berthelot et al, 2017;Bombar et al, 2016;Messer et al, 2016;Moisander et al, 2010;Rees et al, 2009;Zehr & Turner, 2001), but we know little about their physiological capabilities and environmental controls on their biogeography. High N 2 fixation rates have now been measured during summer in temperate northwestern Atlantic and Pacific coastal waters when dissolved N concentrations in surface water were seasonally depleted (Mulholland et al, 2012;Shiozaki et al, 2015), in temperate and tropical coastal systems (Cassar et al, 2018;Chen et al, 2014;Grosse et al, 2010;Larsson et al, 2001;Moisander et al, 2010;Mulholland et al, 2012;Rees et al, 2009;Shiozaki et al, 2015;Voss et al, 2006;Zhang et al, 2012), in nutrient-rich coastal upwelling systems (Voss et al, 2004;Wen et al, 2017), and in coastal Arctic Seas (Blais et al, 2012;Harding et al, 2018;Sipler et al, 2017), broadening the latitudinal range and the diversity of habitats supporting diazotrophy. Based on their phylogenetic affiliations and the lack of quantifiable N 2 fixation rates, the high nifH gene diversity and presence of diverse bacterial diazotroph groups in coastal systems were attributed to microorganisms being transported there from terrestrial systems or sediments, rather than to autochthonous populations of active planktonic diazotrophs (Jenkins et al, 2004;Zehr et al, 2003).…”

Machine Learning Estimates of Global Marine Nitrogen Fixation

“…This is in line with recent observations that suggest that N 2 fixation may not be confined to warm and nutrient‐poor waters (Blais et al, ; Shiozaki et al, ). UCYN‐A has recently been found in cold waters (Harding et al, ; Shiozaki et al, ), and N 2 fixation could provide as much as 3.5 Tg N/year to the Arctic Ocean (Sipler et al, ). Differences between model simulations may be attributed to the parameterization of the types of diazotrophs in the models.…”

Variations in Rates of Biological Production in the Beaufort Gyre as the Arctic Changes: Rates From 2011 to 2016

“…A similar process may be happening here-recent work showed prevalent submesoscale dynamics in the early fall in the Beaufort Gyre (Mensa et al, 2018). Additionally, some of the required nitrogen could be supplied by nitrogen fixation, a process that was recently reported to occur in the Arctic Ocean (Harding et al, 2018;Sipler et al, 2017) or by urea (Varela et al, 2013).…”