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

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.