Dinitrogen fixation and release of ammonium and dissolved organic nitrogen by Trichodesmium IMS101

Abstract

Two methods used to measure dinitrogen (N 2 ) fixation (acetylene reduction and 15 N 2 uptake) often result in different N 2 fixation rates. Part of the discrepancy may arise from the observation that Trichodesmium can release a fraction of their recently fixed N 2 as dissolved organic nitrogen (DON) and/or ammonium (NH 4 + ). To resolve outstanding issues regarding N 2 fixation and the production of dissolved combined nitrogen (N) by Trichodesmium, we conducted a comprehensive analysis of N 2 fixation and the production of DON and NH 4 + in cultures of Trichodesmium IMS101. We performed 15 N 2 uptake experiments in parallel with acetylene (C 2 H 2 ) reduction assays, and measured production of 15 NH 4 + and DO 15 N from 15 N 2 , and 15 NH 4 + uptake and regeneration by isotope dilution. Four main results are highlighted. First, 15 N 2 uptake appears to provide a better approximation of net N-specific growth rates than N 2 fixation estimates made using C 2 H 2 reduction. Second, the C 2 H 2 reduction method provides a closer approximation of gross N 2 fixation. Third, simultaneous measurements of relevant N pools and pathways by several methods enabled us to rigorously evaluate deviations from theoretical conversion factors and to interpret the basis for those deviations. Our results suggest that a conversion ratio (mol C 2 H 2 reduced: mol N 2 reduced to PON, ammonium and DON) of 4:1 may be more appropriate for total N 2 fixation. Fourth, the difference between estimates of gross N 2 fixation, made using the C 2 H 2 reduction technique, and net 15 N 2 uptake into particulate N may be a good indicator of N release from N 2 fixation.