| Eutrophication, caused by the enrichment of the essential macronutrients:nitrogen and phosphorus, is one of the globally environmental problems, and is suggested to be the causative factor resulting in the occurrence of harmful algal blooms (HABs). This unusual explosive blooming phenomenon poses the most serious threat to whole marine environment worldwide, can not only destroy the balance of aquatic eco-system, but also alter the primary structures and finally damage the whole marine ecosystem. Nitrogen (N) and phosphorus (P) are important regulators of phytoplankton growth, and the availability of various concentrations and sources of N and P serve to affect the function and structure of phytoplankton community. Recently, more attentions are played on the kinetics characteristics of harmful algae and their responses to the changes of ambient nutrient.Karenia mikimotoi, known as one of the causative harmful bloom-forming species of East China Sea, is thus selected as the target organisms in this study. The main objective of the present study is to study the growth dynamics, nutrient uptake and cellular physiologic characteristics of K. mikimotoi when exposed to different nutrient conditions, including various nitrate and phosphorous concentrations and sources in mono-culture under controlled laboratory conditions. The primary studies on compensatory growth under nitrogen nutrient limitation conditions are conducted simultaneously. Results showed that:1. Effects of different concentration of N and P on population growth and nutrient uptake characteristic of K. mikimotoi(1) Various concentration of N and P show different effects on the carrying capability (K), the maximal growth rates (r), the time of entering exponential phase and stationary phase, and the reflection point's time (Tp) (P<0.05). The carrying capability (K) is in order of controlled group>P limitation>N limitation>P depletion>N depletion; the maximal growth rates (r) is in order of N depletion>P depletion>N limitation>P limitation>controlled group. The time entering exponential phase is similar in 5 treated groups, and the time for entering stationary phase and the reflection point's time are both in order of controlled group>P limitation>N limitation>P depletion>N depletion.(2) Temporary cysts formation is observed when cultured in N depletion condition, and is suggested to be a response to the stressful environmental conditions of nutrient limitation.(3) Nutrient uptake of K. mikimotoi could be well fitted by Monod models. Compared with some other harmful microalgae, K. mikimotoi has relatively high half saturation constants (Ks) of nitrate and low one of phosphate. In this case, K. mikimotoi need relatively great more nitrogen, but little concentration of phosphate to maintain common growth condition.(4) The study of nitrate reductase (NR) and alkaline phosphatase (AP) activity under different nutrient conditions can indicates:the ability of NR and AP can be obviously affected by the concentration of N and P. With increasing concentration of nitrate, the activity of NR become relatively higher; on the contrary, AP activity has the decreasing trend according to the increasing ambient phosphate.(5) Generally speaking, according to the study mentioned above, K. mikimotoi has shown a very high preference to the nitrogen, as well as a high sensitivity to the nutrient limitation environment.2. Compensatory growth of K. mikimotoi in response to the stress of nitrogen limitation(1) During the nitrate limitation stage, the growth conditions and some macromolecular content of K. mikimotoi were negatively affected in various degrees. With decreasing initial concentration of nitrate, cell density, intracellular chlorophyll a and protein content of K. mikimotoi all present an obviously declining trend.(2) During the nitrate recovery stage, the 6 treated groups of K. mikimotoi showed better growth condition and higher cell density. The difference of cell density at the end of K. mikimotoi growth among treatments and the control was not significant (P>0.05), which means that K. mikimotoi showed a strong ability of exact compensation in the course of the culture after stresses removal; In addition, content of intracellular chlorophyll a and protein in 6 treatments have relatively higher level than the treatments in nutrient limitation stage.3. The effects of different nitrogen sources on growth and fatty acid production of K. mikimotoi(1) K. mikimotoi was showed to grow well under dissolved inorganic nitrogen: NH4Cl, NaNO3 and NaNO2, as well as dissolved organic nitrogen urea. However, the growth condition and maximal cell density of K. mikimotoi are different according to the various nitrogen sources condition. The carrying capability (K) and the maximal growth rates (r) of are both in order of NH4Cl>NaNO3>NaNO2>urea, which indicates that the ability for using nitrogen was in order of NH4Cl>NaNO3>NaNO2 >urea.(2) The kinds and contents of cellular organic compounds of K. mikimotoi under 4 different nitrogen sources were examined by GC-MS. The major class among the materials that been examined was free fatty acid (FFA; 40.61%-84.31%), with low levels of long chain alkane and olefin (13.65%-38.72%), other organic compounds (1.31%-16.61%), and steroid ketones and specific sterols (0.66%-4.06%). The class and content of the free fatty acid (FFA) that been examined are in order of NH4Cl |
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