Nitrogen uptake and assimilation kinetics in Alexandrium minutum (Dynophyceae):: Effect of N-limited growth rate on nitrate and ammonium interactions

Uptake and assimilation kinetics of nitrate and ammonium were investigated along with inhibition of nitrate uptake by ammonium in the harmful dinoflagellate Alexandrium minutum Halim at different nitrogen (N)-limited growth rates. Alexandrium minutum had a strong affinity for nitrate and ammonium (K(s)=0.26 +/- 0.03 and 0.31 +/- 0.04 mu mol.L(-1), respectively) whatever the degree of N deficiency of the cells. Ammonium was always the preferred form of nitrogen taken up (rho(NO3)/rho(NH4) = 0.42-0.50). In the presence of both forms, nitrate uptake was inhibited by ammonium, and inhibition was particularly marked in N-sufficient cells (I(max)similar to 0.9 and K(i)=0.31-0.56 mu mol.L(-1)). In the case of N assimilation, ammonium was also the preferred form in N-deficient cells (rho(assim NO3) /rho(assim NH4) = 0.54-0.72), whereas in N-sufficient cells, both N sources were equally preferred (rho(assim NO3) /rho(assim NH4) =0.90-1.00). The comparison of uptake and assimilation rates highlighted the ability of A. minutum to significantly store in 1 h nitrate and ammonium in amounts sufficient to supply twice the daily N requirements of the slowest-growing N-deficient cells. Nitrogen uptake kinetic parameters of A. minutum and their ecological implications are discussed.