Nitrogen fixation, uptake and metabolism in natural and cultured populations of Trichodesmium spp.

Uptake rates of several combined N sources, N-2 fixation, intracellular glu tamate (glu) and glutamine (gln) pools, and glutamine synthetase (GS) activ ity were measured in natural populations and a culture of Trichodesmium IMS 101 grown on seawater medium without added N. In cultured populations, the ratio of GS transferase/biosynthetic activity (an index of the proportion o f the GS pool that is active) was lower, and intracellular pools of glu and gln and the ratios of gln/glu and gln/alpha-ketoglutarate (gln/alpha kg) r atios were higher when N-2 fixation was highest (mid-day). There was an exc ess capacity for NH4+ assimilation via GS, indicating that this was not the rate-limiting step in N utilization. In natural populations of Trichodesmi um spp., the gln/glu ratio closely approximated the gln/alpha kg ratio over the diel cycle. High gln/glu and gln/alpha kg ratios were noted in near-su rface populations. These ratios decreased in samples collected from greater depths. Natural populations of Trichodesmium spp, showed a high capacity f or the uptake of NH4+, glu, and mixed amino acids (AA). Rates of NO3- and u rea uptake were low. NH4+ accumulated in the culture medium during growth a nd rates of NH4+ uptake showed a positive relationship with the NH4+ concen tration in the medium. Although rates of N-2 fixation were highest and acco unted for the majority of the total measured N utilization during mid-day, rates of NH4+ uptake exceeded rates of N-2 fixation throughout much of the diel cycle. In exponentially growing cultures, only 23 % of the total daily N utilization was due to N-2 fixation while NH4+ uptake accounted for more than 70 %. Based on N-2 fixation alone, the N turnover time for this cultu re during exponential growth was on the order of 9 d. This is consistent wi th the observed chlorophyll-based growth rates for these cultures suggestin g that N-2 fixation was responsible for net growth. Our results contrast wi th the view that natural populations of Trichodesmium spp. acquire their ce ll N exclusively through N-2 fixation. C productivity may overestimate N de mand for net production if regenerated production is significant in these p opulations.