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Aquatic Microbial Ecology

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

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

Margaret R. Mulholland1,*, Deborah A. Bronk2, Douglas G. Capone3

1Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Avenue, Norfolk, Virginia 23529-0276, USA
2Department of Physical Science, Virginia Institute of Marine Science, The College of William and Mary, Greate Rd., Gloucester Point, Virginia 23062, USA
3Wrigley Institute of Environmental Studies and Department of Biological Sciences, University of Southern California, Los Angeles, California 90089, USA

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

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