Nitrogen fixation and release of fixed nitrogen by Trichodesmium spp. in the Gulf of Mexico

During a 3-yr study in the Gulf of Mexico, we measured dinitrogen (N2) fixation and nitrogen (N) release by Trichodesmium and compared these rates with water column N demand and the estimated N necessary to support blooms of Karenia brevis, a toxic dinoflagellate that severely affects the West Florida shelf. Net and gross N2 fixation rates were compared in simultaneous incubations using d15N2 uptake and acetylene reduction, respectively. The difference between net and gross N2 fixation is assumed to be an approximation of the rate of N release. Results demonstrate that Trichodesmium in the Gulf of Mexico are fixing N2 at high rates and that an average of 52% of this recently fixed N2 is rapidly released. Calculations suggest that observed densities of Trichodesmium can provide enough N to support moderately sized K. brevis blooms. Based on other studies where 15N2 and acetylene reduction were compared directly, it appears that N release from Trichodesmium is common but varies in magnitude among environments. In addition, carbon (C) and N-based doubling times for Trichodesmium vary among studies and environments. Comparing gross N2 fixation and C fixation directly, C-based doubling times exceeded N-based doubling times, which suggests an imbalance in elemental turnover or a failure to fully quantify Trichodesmium N use