颗石藻Pleurochrysis carterae抗捕食特征

颗石藻Pleurochrysis carterae是沿海水域中常见钙化微藻,易形成高密度水华,也是养殖环境致害种之一。抗捕食防御能力可能是其种群增殖优势的一个重要原因。以卤虫作为捕食者,分析了颗石藻P.carterae抗捕食现象,以及在捕食压力下的重要生理生化响应特征,以期为颗石藻P.carterea抗捕食机制研究及其高密度增殖机理提供参考。研究结果显示:(1)当颗石藻P.carterae比例增加时,卤虫对微藻的摄食率显著降低,且存活率显著下降,显示该藻具抗捕食能力。(2)以卤虫饵料微藻球等鞭金藻(Isochrysis galbana)为对照,比较研究发现,相同的捕食压力下,饵料金藻的叶绿素荧光参数(电子传递速率ETR和最大量子产率Fv/Fm)显著降低,但颗石藻P.carterae的ETR和Fv/Fm没有显著变化,显示颗石藻P.carterae对卤虫抗捕食作用。(3)相对于没有捕食压力的对照组,捕食压力下,饵料金藻I.galbana的脂类组成没有显著差异。但是,颗石藻P.carterae的脂类组成则发生了显著变化,主要表现在对细胞叶绿体有重要作用的单半乳糖甘油二酯(MGDG),双半乳糖甘油二酯(DGDG),磷脂酰甘油二酯(PG)含量上升,与促细胞分裂相关的二酰甘油(DAG)和磷脂酰肌醇(PI)也上升。这些脂类代谢物的变化可能在其种群水平上抵抗捕食并实现种群增殖中发挥作用。(4)培养介质中磷的状态对颗石藻P.carterae细胞二甲基巯基丙酸(Dimethyl sulfonio propionate,DMSP)含量有显著影响,且影响颗石藻P.carterae对卤虫的致害效应:缺磷条件下生长的颗石藻P.carterae首先使卤虫受害。当培养液中仅以ATP为磷源时,颗石藻P.carterae的卤虫致害效应则降低。研究证明,颗石藻P.carterae具有抗捕食能力,细胞的脂类代谢物质以及DMSP可能在抗捕食防御中发挥作用。

Characterization of anti-predator abilities of Pleurochrysis carterae

The coccolithophorid species Pleurochrysis carterae is a common coastal calcifying phytoplankton, which is also able to form dense blooms that harm marine aquaculture. The anti-predator ability of P. carterae is supposed to be one reason why it is a dominant species. In this study, this hypothesis is tested by using brine shrimp Artemia salina as the model predator. Another Prymnesiophyceae species, Isochrysis galbana, which is considered as the benign live food source for A. salina, was used as prey. Changes to the grazing rate or survival rate of A. salina were analyzed when P. carterae was present in the prey mixture. Algal chlorophyll fluorescence efficiency and lipid changes in these two algal species under predation pressure were also studied. Some very interesting P. carterae features were observed, which may provide insights into its anti-predator abilities. These were as follows:(1) When the P. carterae cell numbers increases in the microalgal prey mixture, the brine shrimp A. salina has a lower predation capacity, which is shown by the gradient decrease of chlorophyll content in the brine shrimp intestines. The survival rate is also lower as the number of P. carterae cells increase. Therefore, these results clearly indicated the anti-predator ability of P. carterae.(2) In comparison, the chlorophyll fluorescence(ETR and Fv/Fm) for the two microalgae facing the same predation pressure decreased significantly from 0.75 to 0.60 for Fv/Fm, and from 88% to 65% for ETR, respectively, for I. galbana, but were almost unchanged for P. carterae. This means that photosynthesis of P.carterae is virtually unaffected by the presence of the predator. Such unconstrained ability against the predator was very prominent.(3) Lipidomic analyses, with without the predator, between the two microalgal species indicated that I. galbana did not show any significant changes, whereas P. carterae did significant increase its monogalactosyl diacylglycerol(MGDG), digalactosyldiacylglycerol(DGDG), phosphatidylglycerol(PG), diacylglycerol(DAG), and phosphatidylinositol(PI) contents. Since MGDG, DGDG, and PG are the major components of the chloroplast membrane, the increase in these compounds can have protective effects on the microalgal cell, and both DAG and PI are considered important signals for cell division, so their increase can stimulate the population growth. Hence, the lipid changes clearly provide the biochemical evidence for the physiologically positive changes to P. carterae, in the presence of the predator.(4) Dimethyl sulfoniopro pionate(DMSP) is supposed to contribute in the anti-predator abilities of microalgae. We further studied whether the mortality of A. salina was affected by the algal cellular DMSP content. Cultures containing different cellular DMSP contents were set up by adjusting the phosphorus chemical states in the f/2 culture medium, namely phosphate deficiency, normal f/2, and f/2 medium with ATP as the phosphate supplier. The results showed that, the ambient phosphorus chemical states significantly affect the intracellular DMSP level in P. carterae. After three days of exponential growth, the phosphate deficiency group increased DMSP from 2.35 to 2.42 mg/g lyophilized cells, whereas the normal f/2 medium group reduced DMSP from 2.35 to 2.26 mg/g lyophilized cells, and the ATP enriched group showed the largest decrease in DMSP from 2.35 to 1.90 mg/g lyophilized cells. This indicated that the phosphorus level and chemical state had negative effects on the accumulation of DMSP. Furthermore,, the phosphorus state can affect the preying behavior and mortality of A.salina. Phosphorus deficiency in the culture medium can increase the algal cell''s fatal effects on the predator, while ATP, as the sole phosphorus source, may decrease the fatal effects. We conclude that P. carterae contains positive biochemical mechanisms that underlie its anti-predator abilities, possibly through cell membrane component protective reconstruction and DMSP metabolism. Its active anti-preying abilities mean that P. carterae can reach a sustainable population size at the ecological level.