AME 52:149-157 (2008)  -  DOI: https://doi.org/10.3354/ame01213

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 cryptophyte 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 different 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 dinoflagellates may be better adapted to utilize urea than other species based on rates of high urease activity 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.

KEY WORDS: Phytoplankton · Urea · Urease · Harmful algae · Dinoflagellates · Enzyme activity