Urease activity in five phytoplankton species

Abstract

Growth rates, internal nitrogen (N) pools of NH4 + and urea, and urease, the enzyme responsible for the catabolism of urea to NH4 + and CO2, were investigated in laboratory cultures of 5 species of estuarine phytoplankton grown on NO3 - , NH4 + , and urea. The phytoplankton used were the dinoflagellates Prorocentrum minimum, Karlodinium veneficum, Heterocapsa triquetra, the cryp- tophyte Storeatula major, and the haptophyte Isochrysis sp. All dinoflagellates had larger internal pools of NH4 + than did the other species. With the exception of H. triquetra grown on NO3 - , this was also true for internal pools of urea. Two of the harmful dinoflagellates, P. minimum and K. veneficum, had significantly higher urease activity rates on both a per cell basis and a per cell volume basis than the other species. These dinoflagellates had different rates of urease activity when grown on differ- ent N sources, suggesting that urease may be down-regulated by NH4 + or up-regulated by urea and/or NO3 - . In all dinoflagellates, the intracellular urea concentrations were greater than the half- saturation constant for enzyme activity, suggesting that in vivo urease activity rates were nearer to maximal in those species. Results from this study and previous studies suggest that harmful dinofla- gellates may be better adapted to utilize urea than other species based on rates of high urease activ- ity and large intracellular urea pools, providing some insight as to why these species may proliferate when urea is a significant N source in the environment.