Abstract
Biological dinitrogen fixation (BNF) represents a major input of reduced nitrogen (N) to the oceans. Accurate direct measurements of BNF rates are crucial for reliably determining the biogeochemical significance of diazotrophy at local and global scales. Traditionally, borosilicate glass fiber filters (GF/F, Whatman) with a nominal pore size of 0.7 µm are used to collect suspended particles by filtration after incubations with added 15N2 tracer. We carried out BNF experiments in the Baltic Sea, Danish coastal waters, and the Pacific Ocean comparing the retentive characteristics of precombusted GF/F filters with newer Advantec glass fiber filters which have a smaller nominal pore size of 0.3 µm. Where BNF was detected, rates were nearly always higher, and sometimes even exclusively detectable, when using Advantec filters. In the majority of samples across tested habitats, significantly more cells were lost to GF/F filtrate (average = 51 %, range = 10 – 70 % of cells) than to Advantec filtrate (average = 40 %, range = 10 – 54 %). Using Illumina sequencing of nitrogenase (nifH) gene amplicons, we show that diazotroph communities can markedly differ between bulk water and filtrates from GF/F and Advantec filtrations, suggesting that different diazotrophs can pass through the filter types. In order to reduce the potential underestimations of BNF due to filtration loss of diazotrophs, we recommend using Advantec filters or alternatively silver membranes with 0.2 µm pore size, especially in waters expected to be inhabited by relatively small, unicellular diazotrophs.
Highlights
Biological nitrogen (N2) fixation (BNF) in the pelagic ocean amounts to up to 200 × 1012 g of nitrogen (N) globally per year, and thereby greatly influences oceanic primary production and CO2 sequestration (Capone et al, 2005; Gruber and Galloway, 2008; Canfield et al, 2010)
The present study shows that glass fiber filters do not catch the entire diazotroph community and might commonly lead to underestimates of Biological dinitrogen fixation (BNF) rates
The experiments carried out in this study document the significant loss of bacterial cells to filtrates of both GF/F and Advantec filters, but with the latter performing clearly better in retaining cells. While such losses have been described previously, with little or no effect on productivity or pigment measurements, we show that better retention by Advantec filters can at times result in higher BNF rates compared to GF/F filters
Summary
Biological nitrogen (N2) fixation (BNF) in the pelagic ocean amounts to up to 200 × 1012 g of nitrogen (N) globally per year, and thereby greatly influences oceanic primary production and CO2 sequestration (Capone et al, 2005; Gruber and Galloway, 2008; Canfield et al, 2010). Nitrogen Fixation Filter Bias 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. The ecology and actual contribution of heterotrophic diazotrophs to oceanic BNF is still largely elusive (Bombar et al, 2016)
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