Uptake of urea carbon and urea nitrogen by the coastal marine diatom thalassiosira pseudonana
Price, N.M.; Harrison, P.J.
Limnology and Oceanography 33(4 Part 1): 528-537
Urea uptake rates of Thalassiosira pseudonana (clone 3H) were determined using [14C]urea, [15N]urea, and by measuring disappearance of dissolved urea from the medium after adding 10 .mu.g-atoms urea-N liter-1. In nitrate-sufficient cultures, the average [14C]urea uptake rate was 60% of the urea disappearance rate. Nitrate uptake continued in the presence of urea at a reduced rate, and only 15% of the urea N taken up was retained by the phytoplankton. The increase in PON during incubation was roughly equal to the total NO3- and urea N taken up. Average data uptake rates measured by all methods in 24-h nitrate-starved cultures were in excellent agreement with the rate of increase in PON during a 1-h incubation. Uptake rate of [14C]urea and disappearance rate of urea were constant and equal. In neither nitrate-sufficient nor nitrate-starved cultures were [15N]urea uptake rates constant, with maximal rates measured 5-15 min after the addition of urea. Ammonium was released by T. pseudonana following uptake of urea and was then taken up. A model of urea uptake and assimilation by T. pseudonana that involves efflux of urea N as NH3 and its rapid reabsorption is proposed. These results can explain previous observations and have implications for utilization and cycling of urea N by phytoplankton in nature.