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 (N-2) 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 N-2 fixation rates were compared in simultaneous incubations using N-15(2) uptake and acetylene reduction, respectively. The difference between net and gross N-2 fixation is assumed to be an approximation of the rate of N release. Results demonstrate that Trichodesmium in the Gulf of Mexico are fixing N-2 at high rates and that an average of 52% of this recently fixed N-2 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 N-15(2) 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 Dichodesmium vary among studies and environments. Comparing gross N-2 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.