Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic north pacific ocean

Resolution of the nitrogen (N) cycle in the marine environment requires an accurate assessment of dinitrogen (N₂) fixation. We present here an update on progress in conducting field measurements of acetylene reduction (AR) and 15_N2 tracer assimilation in the oligotrophic North Pacific Subtropical Gyre (NPSG). The AR assay was conducted on discrete seawater samples using a headspace analysis system, followed by quantification of ethylene (C₂H₄) with a reducing compound photodetector. The rates of C₂H₄ production were measurable for nonconcentrated seawater samples after an incubation period of 3 to 4 h. The 15_N2 tracer measurements compared the addition of 15_N2 as a gas bubble and dissolved as 15_N2 enriched seawater. On all sampling occasions and at all depths, a 2- to 6-fold increase in the rate of 15N₂ assimilation was measured when 15_N2-enriched seawater was added to the seawater sample compared to the addition of 15N₂ as a gas bubble. In addition, we show that the 15N₂-enriched seawater can be prepared prior to its use with no detectable loss (< 1.7%) of dissolved 15_N2 during 4 weeks of storage, facilitating its use in the field. The ratio of C2H4 production to 15N₂ assimilation varied from 7 to 27 when measured simultaneously in surface seawater samples. Collectively, the modifications to the AR assay and the 15_N2 assimilation technique present opportunities for more accurate and high frequency measurements (e.g., diel scale) of N₂ fixation, providing further insight into the contribution of different groups of diazotrophs to the input of N in the global oceans.