Nitrogen fixation and identification of potential diazotrophs in the Canadian Arctic

Abstract

[1] Global gaseous nitrogen (N 2 ) fixation rates may be underestimated and data is lacking from many regions without conspicuous diazotrophic cyanobacteria, such as cold oceans. We estimated N 2 fixation rates at diverse sites in the Canadian Arctic, including the mouth of the Mackenzie River, the offshore Beaufort Sea, Lancaster Sound, Baffin Bay and a river influenced fjord. We also identified potential diazotrophic communities using a targeted survey of the nifH gene. Nitrogen fixation rates ranged from 0.02 nmol N L À1 d À1in Baffin Bay to 4.45 nmol N L À1 d À1 in the Mackenzie River plume. Sequences recovered from the nifH gene survey belonged mainly to Cluster III, a group of nifH sequences associated with diverse microorganisms, with some a-and g-proteobacteria nifH genes at most sites. Cyanobacteria nifH genes with best matches to Nostocales, which are common in Arctic freshwaters, were recovered from the marine Beaufort Sea. The geographic pattern of N 2 fixation rates and nifH gene identities suggest that the Mackenzie River is the source of a diazotrophic community that contributes new nitrogen to the nitrogen-depleted surface waters of the Beaufort Sea. This first record of N 2 fixation at high latitudes refines our understanding of the global nitrogen budget.

Early response of the northeast subarctic Pacific plankton assemblage to volcanic ash fertilization

“…The filters were placed in open cryovials and allowed to dry in an oven at 60uC for 48 h. The cryovials were then capped and kept in Drierite until onshore analysis. The 13 C enrichment of the organic material on dried filters was measured in the laboratory using an isotope-ratio mass spectrometer (Delta V Advantage, Thermo Electron) coupled to an elemental analyzer (Costech ECS-4010) following the procedures described in Blais et al (2012).…”

Nutritive and photosynthetic ecology of subsurface chlorophyll maxima in Canadian Arctic waters

“…All filters were desiccated at 60 • C and stored dry for post-cruise analysis. An elemental analyzer (ECS 4010, Costech Analytical Technologies Inc.) coupled to a mass spectrometer (Delta V Advantage, ThermoFinnigan) was used to determine isotopic enrichment and particulate organic carbon and nitrogen using a modified Dumas method (for details see Blais et al, 2012). Specific C and N uptake were calculated using Eq.…”

Filtration via Conventional Glass Fiber Filters in 15N2 Tracer Assays Fails to Capture All Nitrogen-Fixing Prokaryotes

“…However, nitrogenase (nifH) gene sequencing has uncovered an almost ubiquitous distribution of diverse and presumably heterotrophic diazotrophs (Zehr et al, 1998;Farnelid et al, 2011). Notably, these organisms are even found in high-latitude, deep, cold, or coastal waters where cyanobacteria are few or absent (e.g., Fernandez et al, 2011;Blais et al, 2012;Bonnet et al, 2013) and their lifestyles and how they impact biogeochemical element cycling seems fundamentally different from photoautotrophic diazotrophs (Rahav et al, 2013;Farnelid et al, 2014;Moisander et al, 2014;Benavides et al, 2015;Bentzon-Tilia et al, 2015a). This has inspired many investigators to hypothesize that global N inputs by BNF could potentially be much higher than previously thought.…”