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