两种水华藻——球形棕囊藻和铜绿微囊藻的脂肪酸组成特征与水华形成机制

THE FATTY ACID COMPOSITION OF TWO BLOOM-CAUSING ALGAL SPECIES: PHAEOCYSTIS GLOBOSA AND MICROCYSTIS AERUGINOSA

浮游植物所含的不饱和脂肪酸是测定其作为食物质量的指标,并在浮游植物向浮游动物及其它动物能量转化过程中起着关键的作用,必需不饱和脂肪酸的缺乏有利于水华的形成.球形棕囊藻(Phaeocystis globosa)和铜绿微囊藻(Microcystis aeruginosa)分别是常见的海洋和淡水水华藻类,该文分析了它们在不同生长期的脂肪酸组成,探讨了这两种藻类的脂肪酸组成特征.球形棕囊藻和铜绿微囊藻的脂肪酸碳链长为14～20个碳原子,脂肪酸种类组成都比较简单,以饱和脂肪酸为主,未检测到二十碳五烯酸(Eicosapentaenoic acid,EPA)和二十二碳六烯酸(Docosahexaenoic acid,DHA)等动物的必需脂肪酸.球形棕囊藻的总脂肪酸含量在247.294～735.44μg·g-1干重之间,在对数期和延滞期含量最高的脂肪酸分别是C14:0和C16:0;而两株铜绿微囊藻的总脂肪酸在1405.095～6 087.617μg·g-1干重之间,以C16:0含量最高.两株铜绿微囊藻的脂肪酸含量在对数期和延滞期差异明显(p＜0.05),但球形棕囊藻的脂肪酸含量在不同生长期差别不大.由于缺乏必需脂肪酸EPA和DHA,球形棕囊藻和铜绿微囊藻不能为高营养级的生物提供必需的不饱和脂肪酸,不是浮游动物等生物的良好食物.因此球形棕囊藻和铜绿微囊藻作为浮游动物的食物质量较低,浮游动物对它们的捕食压力也较小,可能是这两种藻容易暴发水华的重要原因.

Fatty acids, being energy-rich compounds, are useful for understanding the transfer of energy from primary production to secondary production. One of the factors affecting the energy value of phytoplankton as a food source in aquatic food chains is a species of polyunsaturated fatty acid (PUFA) content. Polyunsaturated fatty acids, especially eicosapentaesnoic acid (EPA), play a key role in the transfer of energy in aquatic ecosystems. A low EPA content results in a low efficiency of energy transfer between phytoplankton and zooplankton which can contribute to the formation of algal blooms. Phaeocystis globosa and Microcystis aeruginosa are two common species that cause blooms in marine and freshwater systems. The fatty acid composition of cultured P. globosa HK and two strains of M. aeruginosa WN1 and WN2, were measured during their logarithmic and stationary growth phases by gas chromatography-mass spectrophotometry. These three strains of algae have simple fatty acid compositions, and their carbon chain lengths of fatty acids varied from 14 to 20. Saturated fatty acids are the dominant class, but the two essential fatty acids, EPA and docosahexaenoic acid (DHA), were not detected in the three strains. The total fatty acid content varied from 247.294 μg·g -1 DW to 735.44 μg·g -1 DW in P. globosa, C14∶0 and C16∶0 were dominant in the logarithmic and stationary phases, respectively. The total fatty acid content was much higher in M. aeruginosa WN1 and WN2, ranging from 1 405.095 μg·g -1 DW to 6 087.617 μg·g -1 DW weight, and C16∶0 dominated both growth phases for the two strains. There was a significant difference in the content of fatty acid on a dry weight basis between the two growth phases for both strains of M. aeruginosa. This indicated that the composition of algal fatty acid changed with growth phase. However, this difference did not occur in P. globosa. Due to the absence of EPA and DHA in these two species, both P. globosa and M. aeruginosa cannot provide the essential fatty acids to grazers at higher tropic levels, such as Daphnia. Because of the poor food quality, these two species are not grazed by zooplankton, which most likely contributes to their noxious quality of forming algal blooms.