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

[摘要] 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 thatTrichodesmium 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 ofdissolved 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 performed¹⁵N₂ uptake experiments in parallel with acetylene (C₂H₂) reduction assays, and measured production of ¹⁵NH₄⁺ and DO¹⁵N from ¹⁵N₂, and¹⁵NH₄⁺ 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 N₂fixation 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 pathwaysby 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 N₂reduced 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 net¹⁵N₂ uptake into particulate N may be a good indicator of N release from N₂ fixation.

[学科分类] 生物科学（综合）

[关键词] Nitrogen fixation;Nitrogen regeneration;Trichodesmium;Ammonium uptake;Ammonium regeneration