Nitrogen fixation (acetylene reduction) associated with macroalgae in a coral-reef community in the Bahamas

N₂ fixation (C₂H₂ reduction) was associated with several species of macroalgae on a coral reef near Grand Bahama Island. The highest rates were associated with Microdictyon sp. (Chlorophyceae) and Dictyota sp. (Phaeophyceae). Extensive mats of filamentous blue-green algae, not heterotrophic bacteria, were the N₂ fixing agents: in experiments with samples of Microdictyon sp., the activity was lightdependent and not stimulated by organic compounds under either aerobic or anaerobic conditions. Assays in situ, at 20 m depth, and on shipboard, gave similar rates of N₂ fixation; the cyanophytes presumably have pigment adaptations to function in blue light. The maximum rate of N₂ fixation, associated with Microdictyon sp., was 3.8 g N fixed g dry weight^-1 h^-1. Coral-reef communities flourish in nutrientimpoverished waters, and therefore any input of nitrogen is probably important in stabilizing such ecosystems.