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

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 NO 3-. 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 NO 3- 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 NO 3- 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-. © Inter-Research 2005.