MEPS 292:147-157 (2005)  -  doi:10.3354/meps292147

ABSTRACT: The uptake, storage and reduction of nitrate (NO₃^–) by Enteromorpha sp. during a NO₃^– pulse was studied in the laboratory using algae collected from the Mobile Bay estuary, Alabama, USA. Peak uptake occurred when Enteromorpha sp. was initially exposed to NO₃^–. As the internal NO₃^– pool filled, NO₃^– uptake declined. After 6 h of exposure to a 30 µM NO₃^– pulse, the tissue NO₃^– pool was filled and NO₃^– uptake decreased to 1.64 ± 2.63 µmol NO₃^– g^–1 fresh wt h^–1. While tissue NO₃^– increased when NO₃^– became available, the size of the internal nitrite (NO₂^–), ammonium (NH₄⁺), and free amino acid (FAA) pools remained roughly constant, suggesting that reduction of NO₃^– to nitrite (NO₂^–) by nitrate reductase (NR) was the rate-limiting step in NO₃^– assimilation. Meanwhile, there was a lag time (2 to 3 h) between exposure to NO₃^– and peak NR activity (0.80 ± 0.23 µmol NO₂^– g^–1 fresh wt h^–1). These characteristics, combined with a peak NR activity that was 11-fold less than the peak NO₃^– uptake rate, suggest that during a NO₃^– pulse the internal NO₃^– pool quickly fills and uptake becomes limited by the rate at which the internal NO₃^– pool is reduced by NR. Predicted levels of tissue NO₃^– calculated from initial tissue NO₃^–, NO₃^– uptake and NR activity agreed with the measured tissue NO₃^–.

KEY WORDS: Nitrate pulse · Nitrate uptake · Nitrate reductase · Enteromorpha sp. · Mobile Bay estuary · Ammonium · Free amino acids