MEPS 188:33-49 (1999)  -  doi:10.3354/meps188033

ABSTRACT: Uptake rates of several combined N sources, N₂ 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/α-ketoglutarate (gln/αkg) ratios were higher when N₂ 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/αkg ratio over the diel cycle. High gln/glu and gln/ αkg 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 N₂ fixation were highest and accounted for the majority of the total measured N utilization during mid-day, rates of NH₄⁺ uptake exceeded rates of N₂ fixation throughout much of the diel cycle. In exponentially growing cultures, only 23% of the total daily N utilization was due to N₂ fixation while NH₄⁺ uptake accounted for more than 70%. Based on N₂ 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 N₂ fixation was responsible for net growth. Our results contrast with the view that natural populations of Trichodesmium spp. acquire their cell N exclusively through N₂ fixation. C productivity may overestimate N demand for net production if regenerated production is significant in these populations.

KEY WORDS: Trichodesmium · Nitrogen metabolism · N₂ fixation · Amino acid pools · Nitrogen uptake