AME 37:85-94 (2004)  -  doi:10.3354/ame037085

ABSTRACT: Two methods used to measure dinitrogen (N₂) fixation (acetylene reduction and ¹⁵N₂ uptake) often result in different N₂ fixation rates. Part of the discrepancy may arise from the observation that Trichodesmium can release a fraction of their recently fixed N₂ as dissolved organic nitrogen (DON) and/or ammonium (NH₄⁺). To resolve outstanding issues regarding N₂ fixation and the production of dissolved combined nitrogen (N) by Trichodesmium, we conducted a comprehensive analysis of N₂ fixation and the production of DON and NH₄⁺ in cultures of Trichodesmium IMS101. We performed ¹⁵N₂ uptake experiments in parallel with acetylene (C₂H₂) reduction assays, and measured production of ¹⁵NH₄⁺ and DO¹⁵N from ¹⁵N₂, and ¹⁵NH₄⁺ uptake and regeneration by isotope dilution. Four main results are highlighted. First, ¹⁵N₂ uptake appears to provide a better approximation of net N-specific growth rates than N₂ fixation estimates made using C₂H₂ reduction. Second, the C₂H₂ reduction method provides a closer approximation of gross N₂ 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 C₂H₂ reduced: mol N₂ reduced to PON, ammonium and DON) of 4:1 may be more appropriate for total N₂ fixation. Fourth, the difference between estimates of gross N₂ fixation, made using the C₂H₂ reduction technique, and net ¹⁵N₂ uptake into particulate N may be a good indicator of N release from N₂ fixation.

KEY WORDS: Nitrogen fixation · Nitrogen regeneration · Trichodesmium · Ammonium uptake · Ammonium regeneration