Interactions between nitrate uptake and nitrogen fixation in continuous cultures of the marine diazotroph Trichodesmium (Cyanobacteria)

Diazotrophic cyanobacteria can take up combined nitrogen ( nitrate, ammonium, amino acids, dissolved organic nitrogen) from solution, but the interaction between N(2) fixation and uptake of combined nitrogen is not well understood. We studied the effects of combined nitrogen (NO(3)(-)) additions on N(2) fixation rates in the cyanobacterium Trichodesmium erythraeum (IMS-101) maintained in continuous culture in an N-free medium (YBCII) and a 12:12-h light: dark cycle. We measured acetylene reduction rates, nutrient concentrations, and biomass throughout the 12 h of illumination after the addition of nitrate (0.5-20 mu M) at the start of the light period. Compared with unamended controls, Trichodesmium showed strong inhibition of acetylene reduction (up to 70%) in the presence of NO(3)(-), with apparent saturation of the inhibition effect at an initial NO(3)(-) concentration of approximately 10 mu M. The inhibition of acetylene reduction persisted through much of the light period as NO(3)(-) concentration in the culture vessel decreased. Recovery of N(2) fixation was observed late in the light period in cultures amended with low concentrations of NO(3)(-) (<5 mu M) when ambient NO(3)(-) concentrations had decreased to 0.3-0.4 mu M in the culture vessel. Nitrate uptake accounted for as much as 86% of total N uptake and, at the higher treatment concentrations, more than made up for the observed decrease in N(2) fixation rates. We conclude that Trichodesmium can obtain significant quantities of N through uptake of nitrate and does so in preference to N(2) fixation when sufficient NO(3)(-) is available.