Virginia Institute of Marine Science
Aquatic Microbial Ecology
Two methods used to measure dinitrogen (N-2) fixation (acetylene reduction and N-15(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 (NH4+). 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 NH4+ in cultures of Trichodesmium IMS101. We performed N-15(2) uptake experiments in parallel with acetylene (C2H2) reduction assays, and measured production of (NH4+)-N-15 and DO N-15 from N-15(2), and (NH4+)-N-15 uptake and regeneration by isotope dilution. Four main results are highlighted. First, N-15(2) uptake appears to provide a better approximation of net N-specific growth rates than N-2 fixation estimates made using C2H2 reduction. Second, the C2H2 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 C2H2 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 C2H2 reduction technique, and net N-15(2) Uptake into particulate N may be a good indicator of N release from N-2 fixation.
Mulholland, MR; Bronk, DA; and Capone, DG, Dinitrogen fixation and release of ammonium and dissolved organic nitrogen by Trichodesmium IMS101 (2004). Aquatic Microbial Ecology, 37(1), 85-94.