Nitrogen utilization and metabolism relative to patterns of N2 fixation in cultures of Trichodesmium NIBB1067

We measured uptake kinetics for four combined N sources, ambient rates of N uptake and N-2 fixation, glutamine synthetase activity (transferase and biosynthetic), and concentrations of intracellular pools of glutamate (glu) and glutamine (gln) in cultures of Trichodesmium NIBB1067. N dynamics and metabolism were examined to assess the relative importance of N-2 fixation and N uptake to Trichodesmium nutrition, Comparisons were made between cultures grown on medium without added N, with excess NO3-, or with excess urea. Of the combined N sources tested, Trichodesmium NIBB1067 had the highest affinity for NH4+; high uptake capacities for NH4+, urea, and glu; and little capacity for NO3- uptake. In cultures grown on medium without added N, NH4+ accumulated in the medium during growth, resulting in high NH4+ uptake rates relative to rates of N-2 fixation, Glu uptake rates were low but consistent throughout the diel period. In cultures grown on excess NO3- or urea, uptake of these compounds supplied the majority of the daily N demand, although some N-2 fixation occurred during the light period. NO3- uptake rates were reduced when N-2-fixation rates were high, In all of the cultures, the big-best gln/glu ratios and the lowest glutamine synthetase transferase/biosynthetic ratios were observed during the period when rates of total N uptake were highest. In cultures growing exponentially on medium without added N, N-2 fixation accounted for 14%-16% of the total daily N uptake, Uptake of NH4+ and glu, presumably regenerated within the culture vessels, represented 84%-86% of the daily N uptake, Because these systems were closed, net growth was constrained by the rate at which N-2 could be fixed into the system, However, total daily N turnover was greater than that necessary to accommodate the observed increase in culture biomass, The rapid N turnover rates observed in these cultures may support gross productivity and balance the high rates of C fixation observed in natural populations of Trichodesmium.