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NITROGEN UTILIZATION AND METABOLISM RELATIVE TO PATTERNS OF N 2 FIXATION IN CULTURES OF TRICHODESMIUM NIBB1067
Author(s) -
Mulholland Margaret R.,
Ohki Kaori,
Capone Douglas G.
Publication year - 1999
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1046/j.1529-8817.1999.3550977.x
Subject(s) - biology , glutamine , metabolism , glutamine synthetase , urea , trichodesmium , ammonium , biochemistry , acclimatization , nitrogen fixation , zoology , botany , amino acid , diazotroph , chemistry , bacteria , genetics , organic chemistry
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 NO, or with excess urea. Of the combined N sources tested, Trichodesmium NIBB1067 had the highest affinity for NH; high uptake capacities for NH, urea, and glu; and little capacity for NO uptake. In cultures grown on medium without added N, NH accumulated in the medium during growth, resulting in high NH 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 NO or urea, uptake of these compounds supplied the majority of the daily N demand, although some N 2 fixation occurred during the light period. NO uptake rates were reduced when N 2 ‐fixation rates were high. In all of the cultures, the highest 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 NH 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.