The regulation by environmental factors of two enzymes involved with urea utilization - urea transport and urease - in estuarine phytoplankton and bacteria was studied in cultures of five phytoplankton species, in Chesapeake Bay and Choptank River assemblages, and in bioassay and mesocosm experiments. In these experiments, temperature and nitrogen availability (NO sub 3 minus, NH sub 4 plus, and urea) were found to regulate urea uptake and urease activity. However, regulation by these environmental factors was dependent on the composition of the plankton community.

Dinoflagellates were found to have the highest urease activity in culture among five phytoplankton species (Prorocentrum minimum, Karlodinium veneficum, Heterocapsa triquetra, Storeatula major, and Isochrysis sp.) in culture on a per cell and per cell volume basis with an optimized method to measure urease activity. Urease activity was also lower when the dinoflagellates were grown on NH sub 4 plus than when grown on NO sub 3 minus or urea, suggesting repression by NH sub 4 plus.

Higher rates of urea uptake and urease activity in Chesapeake Bay and the Choptank River were often associated with the presence of dinoflagellates and cyanobacteria during the warmer months. Rates were also higher under N-limitation when these phytoplankton were present than under P-limitation when diatoms were present. Rates of urea uptake and urease activity in natural assemblages were repressed when NO sub 3 minus and NH sub 4 plus concentrations exceeded 40 and 5 ug at N l to the negative 1, respectively.

Rates of urea uptake and urease activity decreased in response to additions of NH sub 4 plus in bioassay and mesocosm experiments. In these experiments, dinoflagellates had the highest urea uptake and urease activity on a per cell basis while cyanobacteria had the highest urea uptake and urease activity on a per cell volume or per chlorophyll a basis. The difference in regulation of urea uptake and urease activity among the diatoms, dinoflagellates and cyanobacteria provide some biochemical explanantions about how they utilize urea under contrasting environmental conditions.