The effect of riverine dissolved organic matter and other nitrogen forms on the growth and physiology of the dinoflagellate Prorocentrum minimum (Pavillard) Schiller
The effect of various nitrogen (N) sources, including riverine dissolved organic matter (DOM), nitrate, ammonium, and urea, on the growth and physiology of the dinoflagellate Prorocentrum minimum was compared in a batch culture experiment. P. minimum grew equally well in the presence of identical amounts of nitrate, ammonium, and urea. Approximately 18 to 20% of organic N bound to the DOM was bioavailable. Although the available N added in the DOM treatment was only 1/3 of the amount of any other N sources, the cell densities of P. minimum in the DOM treatment increased to 61 ~ 65% of those in the nitrate, ammonium or urea treatment. The maximum specific growth rates did not differ significantly between the treatments with the highest in the ammonium treatment (0.55 ± 0.13 d- 1) and the lowest in the urea treatment (0.39 ± 0.04 d- 1). P. minimum assimilated the available DOM-bound N in a short period (fewer than 5 days), which was faster than utilizing urea. The increase in the cellular N:P ratios of P. minimum showed the alleviation of N stress in all the treatments after the addition of various N forms. The densities and cellular compositions of P. minimum stabilizing in all the treatments for the whole stationary phase indicated that P. minimum has adaptive physiology under sub-optimal conditions and is a competitive bloom species. We suggest that P. minimum cells utilize DOM-bound N for their growth, and the efficiency in utilizing the available DOM-bound N for growth is comparable to when P. minimum utilizes nitrate, ammonium or urea.