| Water samples of sea surface microlayer and subsurface water were collected from Daya Bay, Guanghai Bay, and Guishan Island of Guangdong Province between May 2014 and December 2014. Concentrations of different forms of nitrogen(N) were analyzed to understand N nutrient structure and N contamination in typical coastal sea areas. The growth of three bloom causative microalgal species, Skeletonema costatum(Bacillariophyceae, diatom), Prorocentrum micans(Dinophyceae, dinoflagellate), and Chattonella marina(Raphidophyceae), response to different inorganic and organic sources of N, N concentrations and different proportions of nitrate(NO3-) and urea were measured under nutrient levels of natural sea water of Daya Bay. Furthermore, concentrations of dissolved inorganic nitrogen(DIN), dissolved organic nitrogen(DON), particle nitrogen(PN), and total nitrogen(TN) were analyzed in cultures during the experiment. The purpose of this study is to understand the capacity of different groups of microalgae for N utilization. The results of this study will provide some basis data for further understanding the changes of phytoplankton community structure in eutrophic sea waters, and also for stabilization and protection of marine ecosystems and environments.Following results are drown from the study:1) Nitrogen concentrations in all of the three sea areas show a significant increase compared to previous reports. Concentrations of DIN in the Daya Bay and Guishan Island sea area were over the second class sea water quality of China, while those in Guanghai Bay were higher than the third class sea water quality. However, there were no significant enrichment of all forms of nitrogen in sea surface microlayer.2) Significant changes occurred in N structure as well. Concentrations of DON were far more than those of DIN in the aquaculture areas of Daya Bay and Guishan Island. DON accounted for more than 80% of DTN, and some even over 90%. The results suggest that DON has become the main available nitrogen source for phytoplankton in aquaculture waters. Although DON concentrations were relatively low in the Guanghai Bay, high DIN concentrations would provide abundant nitrogen source for the growth of phytoplankton.3) Skeletonema costatum has the ability to utilize both DIN(NO3-) and wide kinds of DON sources, such as urea, almandine, glycine, threonine, serine and aspartic acid. This capacity provides S. costatum a competitive advantage in phytoplankton competition in eutrophic coastal waters, and thus would be the important reason for the wide distribution and predominance of this species in worldwide coastal and estuarine environments. P. micans has the ability to use NO3- and urea, but it cannot grow in cultures when using the five amino acids as the sole N sources. C. marina grow well in cultures of NO3-, urea and some amino acid such as glycine and serine, and has the ability to survive in N limited condition, and thus has the advantage in DIN depleted environments.4) All of the three microalgal species, S. costatum, P. micans and C. marina, grew well in cultures with both with NO3- and urea. Furthermore S. costatum and C. marina grew better when using both NO3- and urea as nitrogen sources compared to those with only NO3- or urea.5) The half-saturation constant(Ks) for NO3- of S. costatum, P. micans and C. marina were 25.0μmol/L, 2.29μmol/L and 4.98μmol/L, respectively. The result indicates the low affinity of S. costatum to nitrogen. However, high N concentration in the coastal sea water provides luxurious N for the growth of S. costatum. P. micans and C. marina has high affinities to N, indicating that they have high competitive advantages under the low N environments. Therefore, these harmful flagellate species could take over diatoms in phytoplankton community, and usually form blooms during the temporary decrease of N level after the blooms of diatoms. |
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