Determination of total N <sub>2</sub> fixation rates in the ocean taking into account both the particulate and filtrate fractions

Abstract

Abstract. Using the 15 N 2 tracer method and high-sensitivity δ 15 N analytical systems, we determined N 2 fixation rates for ocean samples by dividing them into particulate (>0.7 µm) and filtrate (<0.7 µm) fractions. While N 2 fixation in the filtrate fraction had been ignored in previous studies, we found a significant N 2 fixation rates in the filtrate fraction in our study. The areal N 2 fixation rates in the western North Pacific Ocean estimated from the particulate fraction varied from <1 to 160 µmol N m −2 d −1 , and those rates estimated from the filtrate fraction ranged from <0.5 to 54 µmol N m −2 d −1 . Thus, N 2 fixation in the filtrate fraction accounts for on average 50% (ranging from <10% to 84%) of the total N 2 fixation rates. If these results are confirmed generally in the ocean, the new total N 2 fixation flux, which includes fixation in the filtrate fraction, possibly doubles the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.

Effect of light on N&lt;sub&gt;2&lt;/sub&gt; fixation and net nitrogen release of &lt;i&gt;Trichodesmium&lt;/i&gt; in a field study

“…5). The overall range agrees well with previous field studies (Glibert and Bronk, 1994;Mulholland et al, 2006;Bonnet et al, 2016a;Konno et al, 2010;Benavides et al, 2013;Berthelot et al, 2015). Our data revealed that the fraction of DDN release to gross NF increased as light decreased (all p values < 0.05).…”

“…During the NF process, Trichodesmium release 10 to 50 % of the DD 15 N in the dissolved pool (Glibert and Bronk, 1994;Konno et al, 2010), primarily as dissolved organic N (DON, such as dissolved free amino acid) and NH + 4 (Capone et al, 1994;Mulholland et al, 2004). High DON and NH + 4 concentrations are often measured within Trichodesmium blooms (Karl et al, 1992;Lenes et al, 2001), being supportive of DDN release.…”

“…However, it has to be noted that some of the NF rate measurements available in this global database might be questionable due to previously unrealized technical problems, e.g., incomplete 15 N 2 dissolution in the 15 N 2 bubble tracer method (Mohr et al, 2010), bioavailable 15 N forms contamination in some commercial 15 N 2 gas (Dabundo et al, 2014) and inconsideration of diazotroph-derived N (DDN) release in the filtrate fraction (Konno et al, 2010). Nevertheless, the abovementioned experiments and global analysis support the idea of a light control on NF activity, CF and oxygen evolution of Trichodesmium; however, limited field experiments have been conducted on studying the effect of light on CF and NF of bulk seawater, particularly during naturally occurring Trichodesmium blooms.…”

Biogeochemical and biological impacts of diazotroph blooms in a low-nutrient, low-chlorophyll ecosystem: synthesis from the VAHINE mesocosm experiment (New Caledonia)

“…blooms in the Indian (Devassy et al, 1978(Devassy et al, , 1979Glibert and O'Neil, 1999), Pacific (Karl et al, 1992(Karl et al, , 1997b, and Atlantic (Lenes et al, 2001) oceans. Subsequent culture (Hutchins et al, 2007;Karl et al, 1992Karl et al, , 1997a) and field studies (Benavides et al, 2013b;Konno et al, 2010;Mulholland and Bernhardt, 2005) have quantified that diazotrophs release ∼ 50 % of the total fixed N 2 to the dissolved pool. Most of these studies were performed on the conspicuous Trichodesmium spp.…”

Biogeochemical fluxes and fate of diazotroph-derived nitrogen in the food web after a phosphate enrichment: modeling of the VAHINE mesocosms experiment

“…Diazotrophs release part of the recently fixed N 2 as dissolved organic N (DON) and ammonium (NH + 4 ) in the dissolved pool (Glibert and Bronk, 1994;Mulholland et al, 2006). The magnitude of this release (10 to 80 %) is still under debate in the scientific community (Glibert and Bronk, 1994;Konno et al, 2010;Benavides et al, 2013b, a) and seems to depend on the physiological state of the cells (Berthelot et al, 2015a) as well as on exogenous factors such as viral lysis (Hewson et al, 2004) or sloppy feeding (O'Neil et al, 1996). Nevertheless, recent methods coupling 15 N 2 isotopic labeling, cell sorting by flow cytometry and high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) analyses enable the quantification of the DDN transfer from diazotrophs to specific groups of nondiazotrophic phytoplankton and bacteria, indicating that the DDN released in the dissolved pool is available and actively used by surrounding non-diazotrophic communities (Bonnet et al, 2016a).…”