A Coupled Biogeochemical-Genomic Approach to Answer ñ Have We Underestimated Sediment N Fixation?

Objectives:To directly test the impact of the acetylene reduction assay (ARA) on sediment N fixation using a coupled biogeochemical-genomic approach. Methodology:Briefly, sediment cores will be collected for measurement of direct N2 fluxes using the N2/Ar technique and a membrane inlet mass spectrometer (MIMS). Sediments will also be measured with the ARA technique. Before and after the ARA treatment I will collect samples for genomic analysis. Specifically I will identify and follow the presence and expression of functional genes for N fixation (nifH). If ARA inhibits N fixation then we will see a decrease in nifH expression after treatment. This would be clear evidence that we have underestimated N fixation in marine sediments.Rationale:Recently, there have been several lines of evidence showing that sulfate reducing bacteria are not only actively fixing nitrogen in marine sediments but may be the dominant microbial community responsible for N fixation in these environments. However, the most common technique used to measure sediment N fixation (acetylene reduction assay or ARA) has been to inhibit the growth of these bacteria. If the most common technique we use to measure N fixation inhibits these bacteria then we, as a community, have grossly underestimated rates of sediment N fixation. This has profound impacts for our understanding of global marine N cycling.