Nutrient controls on nitrogen uptake and metabolism by natural populations and cultures of Trichodesmium (Cyanobacteria)

The effects of inorganic nutrient (ammonium [NH4+] and nitrate [NO3-]) and amino acid (glutamate [glu] and glutamine [gln]) additions on rates of N-2 fixation, N uptake, glutamine synthetase (GS) activity, and concentrations of intracellular pools of gln and glu were examined in natural and cultured populations of Trichodesmium. Additions of 1 muM glu, gln, NO3-, or NH4+ did not affect short-term rates of N-2 fixation. This may be an important factor that allows for continued N-2 fixation in oligotrophic areas where recycling processes are active. N-2 fixation rates decreased when nutrients were supplied at higher concentrations (e.g. 10 muM). Uptake of combined N (NH4+, NO3-, and amino acids) by Trichodesmium was stimulated by increased concentrations. For NO3-, proportional increases in NO3- uptake and decreases in N-2 fixation were observed when additions were made to cultures before the onset of the light period. GS activity did not change much in response to the addition of NH4+, NO3-, glu, or gln. GS is necessary for N metabolism, and the bulk of this enzyme pool may be conserved. Intracellular pools of glu and gln varied in response to 10 muM additions of NH4+, glu, or gln. Cells incubated with NH4+ became depleted in intracellular glu and enriched with intracellular gln. The increase in the gln-/glu ratio corresponded to a decrease in the rate of N-2 fixation. Although the gln/glu ratio decreased in cells exposed to the amino acids, there was only a corresponding decrease in N-2 fixation after the gln addition. The results presented here suggest that combined N concentrations on the order of 1 muM do not affect rates of N-2 fixation and metabolism, although higher concentrations (e.g. 10 muM) can. Moreover, these effects are exerted through products of NH4+ assimilation rather than exogenous N, as has been suggested for other species. These results may help explain how cultures of Trichodesmium are able to simultaneously fix N-2 and take up NH4+ and how natural populations continue to fix N-2 once combined N concentrations increase within a bloom.