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

Uptake rates of several combined N sources, N-2 fixation, intracellular glutamate (glu) and glutamine (gln) pools, and glutamine synthetase (GS) activity were measured in natural populations and a culture of Trichodesmium IMS101 grown on seawater medium without added N. In cultured populations, the ratio of GS transferase/biosynthetic activity (an index of the proportion of 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) ratios were higher when N-2 fixation was highest (mid-day). There was an excess capacity for NH4+ assimilation via GS, indicating that this was not the rate-limiting step in N utilization. In natural populations of Trichodesmium 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-surface populations. These ratios decreased in samples collected from greater depths. Natural populations of Trichodesmium spp, showed a high capacity for the uptake of NH4+, glu, and mixed amino acids (AA). Rates of NO3- and urea uptake were low. NH4+ accumulated in the culture medium during growth and rates of NH4+ uptake showed a positive relationship with the NH4+ concentration in the medium. Although rates of N-2 fixation were highest and accounted 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 culture during exponential growth was on the order of 9 d. This is consistent with the observed chlorophyll-based growth rates for these cultures suggesting that N-2 fixation was responsible for net growth. Our results contrast with the view that natural populations of Trichodesmium spp. acquire their cell N exclusively through N-2 fixation. C productivity may overestimate N demand for net production if regenerated production is significant in these populations.