Nitrate reductase and urease enzyme activity in the marine diatom Thalassiosira weissflogii (Bacillariophyceae): interactions among nitrogen substrates

Abstract

The dissolved nitrogen pool in aquatic systems is comprised of many different nitrogen forms, both inorganic and organic. Interaction among these nitrogen forms at the level of uptake and enzyme activity is, with the exception of NH4 + and NO3 −, not completely understood. Nitrate reductase (NR) and urease (UA) activities in the marine diatom Thalassiosira weissflogii (Grunow) Fryxell et Hasle were measured in NO3 −, NH4 +, and urea-sufficient cultures before and after challenge additions of NH4 +, NO3 −, and urea in a factorial design. NR and UA were constitutively expressed during growth on NO3 −, NH4 +, and urea. Growth on NH4 + or urea resulted in NR activities that were <10% of the activity observed in the NO3 −-grown culture, while growth on NO3 − resulted in UA values that were ~35% of the activities during growth on either NH4 + or urea. The addition of NH4 + or urea to NO3 −-grown cultures resulted in an immediate decrease in cellular NO3 − uptake rate, which was not mirrored by an immediate repression of in vitro NR activity; however, the diel peak in NR was suppressed in these challenge experiments. The addition of NO3 − or NH4 + to urea-grown cultures resulted in non-significant decreases in the urea uptake rate. UA was not impacted by NO3 − addition, but NH4 + addition significantly decreased UA throughout the experiment. These studies demonstrate that the uptake and assimilation of NO3 − and urea may not be subject to the same internal feedback mechanism when challenged with other nitrogen substrates.