Application of genetic disruption of a Nannochloropsis oceanica cell wall synthesizing gene to n-3 HUFA enrichment of Brachionus plicatilis

Abstract

It is believed that microalgae of the genus Nannochloropsis can replace fish oil as an n-3 highly unsaturated fatty acid (n-3 HUFA) source in live feed for sustainable marine fish larviculture. This is due to the fact that these microalgae are rich in eicosapentaenoic acid (EPA). On the other hand, the cell wall of microalgae is an obstacle to enriching live feed with n-3 HUFA. Manipulation of the cellulose synthase gene has the potential to facilitate degradation of the cell wall structure, although little is known how effective the genetically modified microalga would be in n-3 HUFA enrichment of live feed. This study aimed to evaluate the significance of the cellulose synthase gene-knockout strain (ΔCESA) of N. oceanica as an EPA enrichment diet for Brachionus plicatilis. The transgenic microalga was established via homologous recombination, and cultured under the same conditions as the wild-type strain (WT). According to the cellular ultrastructure, ΔCESA had a thinner cell wall than WT and its outer layer was unnaturally peeled back. Biochemical analysis revealed that with the exception of carbohydrates, WT and ΔCESA had comparable levels of proteins, lipids, and EPA. Carbohydrates were lower in ΔCESA than WT, which can cause changes in the cell wall structure. Digestive enzymatic and mechanical treatments of microalga showed ΔCESA to have higher accessibility of intracellular fatty acids. When microalga was used as the diet in the long-term cultivation of rotifers (1 week), rotifers fed ΔCESA presented higher growth rates than those fed WT. In short-term cultivation for rotifer EPA enrichment (≤24 h), the percentage of EPA in neutral lipids and phospholipids increased more rapidly in rotifers fed ΔCESA than those fed WT. As a result, rotifers fed ΔCESA for only 12 h exhibited comparable EPA levels to those fed WT for 24 h. The results of this study suggest that the efficiency of n-3 HUFA enrichment of rotifers is enhanced by feeding cellulose synthase gene-disrupted N. oceanica, due to the high vulnerability of its cell wall and unchanged nutritional value. Our findings provide evidence that gene modification technology can be applied to improve the targeted biological characteristics of microalgae used as the source of n-3 HUFA in marine larviculture.