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Marine Ecology Progress Series

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

Response of Enteromorpha sp. (Chlorophyceae) to a nitrate pulse: nitrate uptake, inorganic nitrogen storage and nitrate reductase activity

Julien Lartigue1,2,4,*, Timothy D. Sherman3

1Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA
2Department of Marine Sciences, University of South Alabama, LSCB-25, Mobile, Alabama 36688, USA
3Department of Biology, University of South Alabama, LSCB-124, Mobile, Alabama 36688, USA
4Present address: Marine Science Institute, University of Texas at Austin, 750 Channel View Drive, Port Aransas,Texas 78373, USA

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

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