The Study On Physiological Characteristics Of Urease Activities Of Five Phytoplankton

Urea concentrations in coastal and estuarine water showed elevated trends in the recent years. Some toxic and harmful algal blooms(HABs) events have been connected with the occurrence of high concentrations of urea in the water. Urea might be an important nitrogen source and thus play an important role in inter-specific competition among phytoplankton. Urease is a hydrolytic enzyme which exists in cellular cytoplasm and plays an important role in catabolizing urea inside cells.In this study, batch culture experiments in lab were used to study the physiological characteristics of urease activities of five HAB species(diatom Skeletonema costatum, dinoflagellate Karenia mikimoto, Prorocentrum donghaiense,haptophyte Phaeocystis globosa, cyanophyte Synechococus sp.) in the coastal waters of China. Urease activities were also investigated in field from the coastal water of Taiwan Strait to the estuary of the Yangtze River in April and May, 2013 during the bloom period of P. donghaiense. The objectives of the thesis were to understand mechanisms of different species in hydrolyzing urea and the role of urea in the growth and competition between species. Furthermore, the study would provide some theoretical foundation for the mechanism of urea catabolization during the occurrence and persistence of algal blooms in coastal waters of China. Results were as follows:The five studied phytoplankton species all expressed urease activities when grown on NO3–, NH4+ or urea which indicated that at least some urease is constitutive.Urease activity was regulated by nitrogen sources in the media. For P. donghaiense,urease activities were up-regulated when urea existed. The cellular nitrogen pools of five species were mainly composed by NH4+ which indicated urease play a role in hydrolyzing urea inside cells. Urease activity seemed to be influenced by growth rates of species. High urease activities were expressed in the earlier and middle stage of the exponenial gowth phase. Urease activities of the studied species except Synechococus sp. showed diurnal variations with high values during 12:00 ~ 18:00 pm during the day period. Urease activities per cell or per volume of the dinoflagellates P.donghaiense and K. mikimotoi were much higher than that of the diatom S. costatum which means that these two dinoflagellates might be more competitive in hydrolyzing urea when compared with the diatom S. costatum. P. donghaiense was sensititve in the response of urease expression to external urea and showed a stronger capability in hydrolyzing urea which might be an advatange of this species in utilizing urea for growth.During the spring of 2013, high urease activities mostly located in the transections of ZE, FA, FB where a bloom of P. donghaiense occurred. During the early stage of P. donghaiense bloom, urease activities were relatively low. Urease activities were significantly elevated along with the development of the bloom.Urease activities maintained at a relatively high level in the later stage of the bloom.Temperature showed a positive correlation with urease activities. Higher levels of urease activities showed that urea could be an important nitrogen source for phytoplankton growth, in particularly during the later period of the bloom. Urea, to a certain extent, may promote the growth and maintenance of P. donghaiense bloom.Through the field investigation and lab-based incubation experiments, results showed that the physiological characteristics of urease of the five studied species were different. These inter-specific differences might directly affect their ability in utilizing urea as a nitrogen source. P. donghaiense showed stronger capability in urease activities, and urea might play a role in the maintainance of the bloom of P.donghaiense.