氮饥饿与磷饥饿促使缺刻缘绿藻花生四烯酸含量增加的比较

Comparison of increased arachidonic acid content in Myrmecia incisa cultured during the course of nitrogen or phosphorus starvation

以富含花生四烯酸(AA)的缺刻缘绿藻H4301为研究对象,探讨了不同光照强度条件下氮饥饿与磷饥饿对藻生物量、AA及脂肪酸含量变化的影响。发现氮饥饿与磷饥饿均降低了藻类的生长速率与生物量,当在60μmol photons/(m2.s)的低光照强度下,磷饥饿时的藻类平均生长速率最低[0.025 g/(d.L)],不足BG-11完全培养基中该藻生长速率的一半;氮饥饿与磷饥饿均能提高藻细胞总脂肪酸及AA的含量,但在低光照强度下磷饥饿的促进效果比较差;无论是完全培养基中还是饥饿处理时,200μmol photons/(m2.s)的高光照强度都不利于藻细胞AA及多不饱和脂肪酸的合成与积累;随着饥饿时间的不断持续,AA占总脂肪酸的百分含量逐渐增加,而亚油酸的百分含量逐渐降低,但在磷饥饿时,油酸的百分含量也增加,特别在高光照强度下,以油酸为主的单不饱和脂肪酸含量在第27天时占细胞干重的5.28%,以致AA含量的增加没有氮饥饿时的显著。从脂肪酸成分的变化来分析,该藻在氮或磷饥饿过程中主要是从亚油酸到γ-亚麻酸再到20∶3ω6这个途径来合成并积累AA,其中Δ6去饱和酶是限速酶,而ω3去饱和酶催化步骤受饥饿处理的负调控对确保AA的合成与积累有较大的积极作用。氮饥饿使藻细胞蛋白质合成受阻以及磷饥饿使核酸合成、糖类与能量代谢产生障碍,从而阻止藻类的生长并迫使细胞代谢流转向不含氮和磷的脂肪酸合成代谢,以提高藻细胞总脂肪酸及AA含量。

The growth rate and biomass,contents of arachidonic acid（AA）and other fatty acids in Myrmecia incisa Reisigl H4301,a freshwater green microalga rich in AA,were comparatively studied while cultured between nitrogen and phosphorus starvation under different light intensities.It was found that either nitrogen or phosphorus starvation could cause a reduced growth rate of this microalga as well as a resultant biomass.If grown under a lower light intensity[60 μmol photons/（m2·s）],the lowest average growth rate[0.025 g/（d·L）]of this microalga resulted from phosphorus starvation,which was nearly half of the growth rate while grown in a complete BG-11 medium.Both of nitrogen and phosphorus starvation could increase the contents of total fatty acids and AA,however,when this alga was exposed to low light intensity the positive effect of phosphorus starvation was limited.High light intensity[200 μmol photons/（m2·s）]was not beneficial to the synthesis and accumulation of AA no matter how this microalga was cultured in the complete or nitrogen-free or phosphorus-free media.The AA content gradually increased accompanying with a decrease of linoleic acid content during the starvation.However,the oleic acid percentage also gradually increased which limited more significantly the accumulation of AA in this microalga grown under phosphorus starvation than nitrogen one.Based on the composition changes of every species of fatty acids,the main synthesis metabolic pathway of AA suggested that it proceeded from linoleic acid to γ-linolenic acid and then to 20∶ 3ω6 under nitrogen or phosphate starvation,and that the Δ6 desaturase was a rate-limiting enzyme.The step in which ω3 desaturase played was down regulated by nitrogen or phosphorus starvation,thus ensuring the synthesis and accumulation of AA positively.Nitrogen starvation blocked the protein synthesis,and phosphorus starvation caused the disorders of nucleic acid,carbohydrate and energy metabolism,which resulted in the slow growth rate and the synthetic conversion of compounds into ones without nitrogen or phosphorus as components.In such a case of nutrient starvation,therefore,the increase of total fatty acids and AA contents in this microalga was enhanced.This research will lay a foundation for the further understanding of the promotion mechanism of fatty acid contents especially of AA in M.incisa by a nutrient stress,and for manipulation and optimization of AA commercial production on a large scale.