Methodological underestimation of oceanic nitrogen fixation rates

• Published in: PloS one 2010-09, Vol.5 (9), p.e12583-e12583
• Main Authors: Mohr, Wiebke, Grosskopf, Tobias, Wallace, Douglas W R, LaRoche, Julie
• Format: Article
• Language: English
• Subjects: Acetylene; Acetylene reduction; Biomass; Bubbles; Chemical analysis; Circadian rhythm; Cyanobacteria; Cyanobacteria - chemistry; Cyanobacteria - metabolism; Equilibrium; Excretion; Experiments; Gene expression; Growth rate; Incubation; Isotope Labeling - methods; Kinetics; Light; Marine and Aquatic Sciences/Biogeochemistry; Marine and Aquatic Sciences/Biological Oceanography; Marine and Aquatic Sciences/Microbiology; Mathematical analysis; Metabolism; Nitrogen; Nitrogen (Chemical element); Nitrogen - chemistry; Nitrogen - metabolism; Nitrogen budget; Nitrogen Fixation; Nitrogen Isotopes - chemistry; Nitrogen Isotopes - metabolism; Oceans and Seas; Seawater; Seawater - chemistry; Seawater - microbiology; Tracers (Biology); Water analysis
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0012583

The two commonly applied methods to assess dinitrogen (N(2)) fixation rates are the (15)N(2)-tracer addition and the acetylene reduction assay (ARA). Discrepancies between the two methods as well as inconsistencies between N(2) fixation rates and biomass/growth rates in culture experiments have been attributed to variable excretion of recently fixed N(2). Here we demonstrate that the (15)N(2)-tracer addition method underestimates N(2) fixation rates significantly when the (15)N(2) tracer is introduced as a gas bubble. The injected (15)N(2) gas bubble does not attain equilibrium with the surrounding water leading to a (15)N(2) concentration lower than assumed by the method used to calculate (15)N(2)-fixation rates. The resulting magnitude of underestimation varies with the incubation time, to a lesser extent on the amount of injected gas and is sensitive to the timing of the bubble injection relative to diel N(2) fixation patterns. Here, we propose and test a modified (15)N(2) tracer method based on the addition of (15)N(2)-enriched seawater that provides an instantaneous, constant enrichment and allows more accurate calculation of N(2) fixation rates for both field and laboratory studies. We hypothesise that application of N(2) fixation measurements using this modified method will significantly reduce the apparent imbalances in the oceanic fixed-nitrogen budget.