Coupling between ammonium uptake and incorporation in a marine diatom: Experiments with the short‐lived radioisotope 13N

The coupling between ammonium transport and incorporation in the neritic marine diatom Thalassiosira pseudonana (3H) was investigated with the short-lived radioisotope 13N (& = 9.96 min). In ammonium-limited continuous cultures, growing at rates from 0.2 to 3.2 d-l, no trend in uptake rates per ccl1 was apparent, but the rate of incorporation of radiolabeled ammonium into trichloroacetic acid (TCA)-insoluble material, following a saturating pulse of ammonium, was linearly correlated with growth rate (r2 = 0.93). Calculated growth rates overestimated the measured rates. The ratio of calculated growth requirements (&) to growth requirements based on incorporation of 13N into TCA-insoluble material (pq) was nonlinearly related to growth rate. The discrepancy between & and pq was explained by isotope dilution in the internal free amino acid pool. The ratio of uptake to incorporation, corrected for isotope dilution in the amino acid pool, showed that uptake and growth were uncoupled at low growth rates but were coupled at P~,,~~. The ratio of short-term uptake to incorporation was found to be a direct measure of enhanced uptake and decreased with increasing growth rate. This ratio may be a useful parameter for determining growth rates and nitrogen deficiency in natural phytoplankton communities.

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