| The growth of different typical red tide microalgaes were studied under different nitrogen (N) and phosphorus (P) concentrations as well as various organic nutrient sources under the levels of typical cultural area of Daya Bay, South China Sea. The activities of alkaline phosphatase (APA) were examined under P shortage and various organic P conditions. The purpose of this study is to understand the abilities of different groups of microalgae to utilize various nutrient sources especially for dissolved organic nutrients, and to demonstrate enzyme responses of microalgal species to P depleted conditions. The results of this paper will provide basic data for evaluation phytoplankton community changes in eutrophic coastal waters, and play important roles in marine environmental protection and the balance of coastal ecosystem.Results showed that nitrogen limitation influenced the growth of Skeletonema costatum, Scrippsiella trochoidea and Chattonella marina significantly, while phosphorus limitation exerted no significant effects on growth. Interspecies competition inhibited the growth of algal cells under nutrient limited conditions. In the co-cultures of two algal species, S. costatum took up the quantitative predominance in the competition of the other two speices through its high growth rate. However, in the co-culture of three algal species, S. trochoidea and C. marina produced synergistic inhibitory effects on the growth of S. costatum. S. trochoidea and C. marina showed comparable competition capacity under N limited conditions, while C. marina was more competitive in P limited cultures. Cyst formation of S. trochoidea moved forward under nutrient limitation and in co-culture systems. The final cyst formation rates in N and P limited cultures were between 51.7% and 92.2%, and the relative formation rates in co-culture systems were up to 112.4-170.4%.Both C. marina and Heterosigma akashiwo had the ability to utilize various forms of organic nitrogen. C. marina could use all of the seven kinds of test organic nitrogen sources such as L-serine (Ser), glycine (Gly), alanine (Ala), L-threonine (Thr), glutamic acid (Glu), urea and uric acid, while H. akashiwo could efficiently utilize most of amino acids and urea but could not use L-serine and uric acid. The four test flagellate species, C. marina, H. akashiwo, Prorocentrum micans and Alexandrium tamarense grew well by using various kinds of organic phosphorus as sole P sources including ATP, AMP, CMP, GMP, UMP, Glucose-6-Phosphate (G6P), sodium glycerophosphate(GYP),4-nitrophenylphosphate(NPP),triethyl phosphate (TEP). Within the four species, C. marina and P. micans grew well in most organic P cultures, which were even better than those in inorganic P cultures. The growth of H. akashiwo and A. tamarense in organic P cultures were comparable with those in inorganic P cultures. However, the diatom species S. costatum could only use ATP and GMP.When faced with P depletion stresses, C. marina and H. akashiwo switched on the mechanism of cell intrinsic enzyme, and the alkaline phosphatase activity increased quickly, meanwhile the amount of the cellular phosphorus increased. The response of C. marina to P stress was more quickly and stronger, and the capacity of P storage was higher as well. Available organic P (GMP) enhanced alkaline phosphatase activity in phosphorus starvation cells, but inorganic P and unavailable organic P (TEP) did not enhance the enzyme's activity.
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