氮源赋存形态与组成对拟柱孢藻生长和细胞内氮磷比的影响

Cylindrospermopsis spp. have bloomed in some tropical and subtropic reservoirs of southern China. Cylindrospermopsis spp. are generally considered tropical species, and have low demand of phosphorus, showing competitive advantage in high nitrogen-phosphorus ratio water bodies. These species are able to utilize different species of nitrogen Therefore, we hypothesize that Cylindrospermopsis spp. have a high intracellular ratio of nitrogen to phosphorus (much higher than 16:1), they prefer to utilize dissolved organic nitrogen (DON) that tends to increase the intracellular N/P ratio. Two strains of Cylindrospermopsis spp. have been isolated from our reservoirs. With the two strains, we will apply 15N stable isotopes method and continuously culturing it using chemostat under different nitrogen species, water temperature and luminous intensity. We will observe the growth and morphology of the experimental strains, measure the intracellular N/P, cytology parameters and evaluate features of inorganic nitrogen utilization. In the enclosure experiments, we add different nitrogen species to reveal the contribution of nitrogen in the dominance of Cylindrospermopsis spp.. Temporal and spatial changes of nitrogen species and Cylindrospermopsis populations also will be monitored in the two reservoirs where the Cylindrospermopsis strain were collected, and then statistically analyze the relationship between the intracellular ratio of N/P and different nitrogen sources. This study is helpful to elucidate the critical mechanisms of nitrogen utilization under the dominance of Cylindrospermopsis spp..

近年来，我国南亚热带地区多个水库发生了较大规模的拟柱孢藻水华。已有研究表明，拟柱孢藻属热带种类，可在低磷水体占优势，能够利用不同的氮源。结合我们已完成的野外观测与培养实验，提出假设：拟柱孢藻细胞内的氮磷原子比大于藻类平均值16，有利于其在高氮磷比水体中形成竞争优势；当水体中无机氮被消耗后，溶解有机氮可供其利用。本项目以形成水华的拟柱孢藻为材料，运用15N稳定同位素技术，通过室内恒化连续培养实验研究不同温度和光强、不同氮源组成条件下，拟柱孢藻生长特性、细胞学特征、形态变化（异形胞是否出现及其比例）与胞内氮磷比；水库围隔添加不同氮源，研究不同光强下拟柱孢藻对不同氮源的利用效率及其在优势形成过程中的作用；结合对氮源和拟柱孢藻生物量及形态特征的现场观测，分析不同氮源与拟柱孢藻细胞内氮磷比之间的相关性，为阐明拟柱孢藻水华过程中利用氮源的机制提供基础。

拟柱孢藻在南亚热带水库广泛分布，发生拟柱孢藻水华的水库超过5座。本项目研究了南亚热带水库拟柱孢藻对不同氮源的利用特性，发现拟柱孢藻能有效吸收利用溶解有机氮，但利用效率低于硝氮和铵氮，在氮磷比约为90时，拟柱孢藻生长状况最佳，推测高氮磷比是拟柱孢藻在南亚热带水库形成竞争优势的重要基础。当氮源缺乏时，拟柱孢藻能形成异形胞固氮，其固氮浓度阈值约为0.3mg/L。对拟柱孢藻长期长期占优的镇海水库进行了为期2年的野外观测，发现拟柱孢藻生物量与水体中无机氮呈显著负相关，且与氮磷比显著正相关，表明氮源特别是有机氮在拟柱孢藻形成竞争优势过程中起到重要驱动作用。