中国科学院海洋研究所机构知识库

     本研究分别以球形棕囊藻（Phaeocystis globosa）和米氏凯伦藻（Karenia mikimotoi）为研究对象，通过室内培养的方法，考察了处于氮饥饿状态的两种典型赤潮藻对不同形态氮源（NaNO₃、NH₄Cl和尿素）的吸收利用和生理生态响应情况。

    球形棕囊藻和米氏凯伦藻对不同氮源（NaNO₃、NH₄Cl和尿素）的吸收利用及生长结果表明，两种微藻均能分别利用三种氮源维系生长。在氮源初始浓度相同条件下，以尿素（urea）为氮源时球形棕囊藻的藻密度以及叶绿素a浓度显著高于以NH₄Cl或NaNO₃为氮源时藻密度以及叶绿素a浓度（P<0.01），表明尿素是更适合球形棕囊藻生长的氮源。分析不同氮源条件米凯伦藻的生长情况显示，各实验组的生长曲线间无显著性差异（P>0.05），说明米氏凯伦藻在三种氮源下的生长无明显差异。藻细胞对氮源的吸收结果显示，处于氮饥饿状态的两种微藻对各氮源的吸收速率在初始阶段最高，吸收速率随培养液内氮浓度降低而降低，吸收速率与培养液内氮源浓度呈正相关关系，表明经过饥饿处理后的藻细胞对氮的吸收速率受其生长环境中氮源浓度调控；此外，培养于不同氮源中的两种微藻在初始阶段的比生长率均较低，这可能是由于在实验初始阶段藻细胞处于氮饥饿状态，添加氮源后藻细胞将吸收的大部分氮源用于填补细胞内氮库的亏空，因此在初始阶段两种微藻的氮吸收速率较高而比生长率较低。

    两种微藻对不同氮源生理生态响应结果显示，球形棕囊藻与米氏凯伦藻实验组单位藻细胞内可溶性蛋白的含量与培养液中氮源浓度呈极显著的负相关关系（P<0.01），藻细胞内可溶性蛋白含量在实验初始阶段呈升高趋势，待培养液内氮源几近耗尽后其含量开始降低，说明藻细胞吸收的氮源一部分被同化为蛋白质储存在细胞内，待外界氮源耗尽后可利用其维持细胞生长。球形棕囊藻的硝酸盐还原酶和脲酶活性受氮源形态调控，分别在硝酸盐和尿素作为氮源时表达出最大活性，而且球形棕囊藻细胞的脲酶活性显著高于硝酸盐还原酶活性（P<0.01），这可能是球形棕囊藻在以尿素为氮源条件下达到更高藻密度的主要原因。米氏凯伦藻的脲酶在以尿素为氮源时表达出最大活性，且浓度高的尿素添加组的藻细胞脲酶活性比低浓度组更强；进一步分析发现该藻藻细胞的谷氨酰胺合成酶（GS）活性与各组培养液内氮源的浓度呈极显著正相关关系（P<0.01），GS活性随着培养液内氮源浓度的降低而降低，且各组单位藻细胞内的GS活性大小相近，此结果说明米氏凯伦藻将硝酸盐和尿素转化为铵盐的速率相近，这可能是米氏凯伦藻在三种不同氮源条件下生长情况无显著差异的主要原因。

    本文的研究结果显示，处于氮饥饿状态的球形棕囊藻可更好的吸收利用尿素维持其生长繁殖，而处于氮饥饿状态的米氏凯伦藻在三种氮源条件下藻密度无明显差异，这主要是与两种微藻藻细胞内氮同化吸收相关酶的酶活性，以及藻细胞对不同形态氮源的利用方式存在差异相关。

    In this study, Phaeocystis globosa and Karenia mikimotoi were studied using indoor culture methods. The nutrient utilization and physiological and ecological responses of the two nitrogen starved algal in different nitrogen forms (NaNO₃, NH₄Cl, and urea) were investigated.

    Both microalgae could use the three kinds of nitrogen sources to maintain their growth. Under the same initial nitrogen source concentration, the P. globosa grown with urea has a significantly higher cell density and chlorophyll-a than grown with NH₄Cl or NaNO₃ (P<0.01), indicating that urea is a more suitable nitrogen source for the growth of P. globosa. The results of Karenia mikimotoi grown with different nitrogen sources showed that there was no significant difference between the growth curves of the experimental groups (P>0.05), indicating that K. mikimotoi has no significant difference in the growth under these three nitrogen sources. The results of nitrogen utilization showed that the uptake rate of each nitrogen source by the nitrogen-starved microalgae was the highest at the initial stage, and the uptake rate decreased with the decrease of the nitrogen concentration in the culture fluid. There was a positive correlation between the source concentrations, indicating that the rate of nitrogen uptake by the starved algae cells was regulated by the nitrogen source concentration in their growth ambient; in addition, the two microalgae cultured in different nitrogen sources had a lower specific growth rates at the initial stage, this may be due to the fact that the algal cells were starved at the initial, after the addition of the nitrogen source, most of the absorbed nitrogen sources were used to fill the deficits of the nitrogen pool in the cells. Therefore, the two kinds of microalgae has a higher nitrogen absorption rate but lower specific growth rate at the initial stage.

     The results of physio-ecological responses of two microalgae to different nitrogen sources showed that the content of soluble protein in the cells of P. globosa and K. mikinotoi was extremely negatively correlated with the concentration of nitrogen in the culture medium (P<0.01), the content of soluble protein in the algal cells increased at the initial, and decreased when the ambient nitrogen almost exhausted, indicating that part of the nitrogen source absorbed by the algae cells was assimilated into protein and stored in the cells. Internally, it can be used to maintain cell growth when the outside nitrogen source was depleted. The nitrate reductase and urease activities of P. globosa were regulated by nitrogen sources and expressed maximum activity when nitrate and urea were used as nitrogen sources respectively. Moreover, the urease activity of the cells was significantly higher than that of nitrate reductase. (P<0.01). This may be the main reason for the higher algal density of P. globosa grown with urea. The urease activity of K. mikimotoi expressed the maximum activity when urea was used as the nitrogen source, and the urease activity of the alga cells in the high concentration urea supplement group was stronger than that in the low concentration group; further analysis revealed that the glutamine synthesis of the algae and algae cells. The activity of enzymes (GS) was significantly positively correlated with the concentration of nitrogen sources in each culture medium (P<0.01). The GS activity decreased with the decrease of the nitrogen concentration in the culture medium, and the GS activity in the cells of each group were almost similar. This result indicated that the rate of conversion of nitrate and urea to ammonium at a similar rate in K. mikimotoi. This may be the main reason for the insignificant difference in the growth of K. mikimotoi under three different nitrogen sources.

    The results of this study showed that the P. globose, which in nitrogen starvation state, could better use urea to maintain its growth. However, there is no significant difference in the algae density in the nitrogen starved state of K. mikimotoi under three nitrogen sources. This is mainly related to the enzymatic activity of nitrogen-assisted absorption of related enzymes in the algae cells of the two microalgae and the differences in the use of different nitrogen sources by the algae cells.