• nitrogen assimilation;
  • organic nitrogen;
  • Thalassiosira spp;
  • urea;
  • urease assay

An in vitro urease enzyme assay was developed for the marine diatoms Thalassiosira pseudonana Hasle et Heimdal (clone 3H) and T. weissflogii (Grunow) Fryxell et Hasle (clone Actin). This assay involves the colorimetric measurement of ammonium following the hydrolysis of urea in crude cell homogenates and it is the first assay to account for the rate of nitrogen assimilation in both species grown on urea as the sole nitrogen source. Urease activity was found to be present regardless of nitrogen source, although activities showed distinctly different patterns depending on the species examined and form of nitrogen supplied. Under nitrogen-replete conditions, urease activity in T. pseudonana was present constitutively when grown on NH4+ and upregulated when grown on NO3 or urea. In nitrogen-replete T. weissflogii, urease activity was present at high constitutive levels regardless of the nitrogen source and showed no upregulation. Nitrogen starvation did not upregulate activity in either species.