Trichodesmium spp. are filamentous, non-heterocystous, cyanobacteria that have a nearly ubiquitous distribution in the euphotic zone of tropical and sub-tropical oligotrophic seas. These species can dominate the chlorophyll biomass seasonally and alleviate nitrogen (N) limitation by fixing dinitrogen gas (N2). Uptake rates of combined N sources were measured concurrently with rates of N2 fixation in order to estimate the contribution of Trichodesmium spp. to new and recycled N production in oligotrophic marine systems. High rates of ammonium (NH4+) uptake were found in natural populations of Trichodesmium spp., and cultured populations of Trichodesmium IMS101 and Trichodesmium NIBB1067 grown on media without added N substrates. Of all N substrates tested, Trichodesmium spp. had the highest capacity for NH4+ uptake. Uptake of NH4+ accounted for 70 to 80% of the total daily N turnover while N2 fixation accounted for 15 to 20%. Glutamine synthetase (GS) activity varied by about 20% over a diel cycle. The GS transferase/ biosynthetic activity ratio was lowest and intracellular concentrations of glutamine highest when N2 fixation rates and N utilization were high. Rates of nitrate (NO3-) uptake, glutamine/glutamate (gln/glu) ratios and GS transferase/biosynthetic activity varied on a diel basis in cultures of Trichodesmium NIBB1067 grown on nitrate-enriched media but not in cells grown on urea-enriched media.

Additions of NH4+ and organic N stimulated N uptake by Trichodesmium spp. N additions did not inhibit N2 fixation in the short term but did over time. Total GS transferase and biosynthetic activity decreased over time after N additions and there was an increase in the transferase/biosynthetic ratio. Intracellular glutamate concentrations decreased in response to N additions, but changes in glutamine pools affected the ratios of gln/glu more. Colonies and filaments appear to be capable of maintaining their capacity for N2 fixation while utilizing combined N sources supplied at environmentally relevant concentrations. Inhibition of GS and GOGAT resulted in decreases and increases, respectively in the gln/glu ratio. Decreases in gln/glu stimulated N2 fixation. Increases in gln/glu resulted in reduced N2 fixation. These results are consistent with the idea N2 fixation and assimilation are regulated by some product(s) of N metabolism.