Nitrogen repletion favors cellular metabolism and improves eicosapentaenoic acid production in the marine microalga Isochrysis sp. CASA CC 101

Abstract

Isochrysis sp. is a marine microalga and is a potential source of eicosapentaenoic acid and docosahexaenoic acid, which possess several health benefits. In general, Isochrysis sp. grow well in nitrate-rich culture medium which influences fatty acid biosynthesis; therefore, this study focused on how nitrate influences cellular metabolism in the marine microalga Isochrysis sp. CASA CC 101. Initially, response surface methodology was performed to optimize the level of major nutrients: nitrate and phosphate. The results showed that Isochrysis sp. when grown with nitrate at a concentration of 593 mg.L−1 produced higher eicosapentaenoic acid (7.3 mg.L1), whereas Isochrysis sp. when grown with higher concentration of phosphate did not produce higher levels of eicosapentaenoic acid. The influence of nitrate on cellular metabolism was further confirmed by culturing Isochrysis sp. at higher nitrate conditions. The cellular metabolic activity was analyzed by reactive oxygen species staining assay and the results showed that the cells exhibited reduced oxidative stress with lower green fluorescence under nitrate-repleted conditions. Furthermore, live/dead staining and cell viability assay revealed that live cell population was higher in nitrate-repleted conditions. Nile red and scanning electron microscopy analysis revealed that the cell size and intracellular lipid droplets of microalgal cells grown under nitrate-repleted cells were low, whereas the cell size and intracellular lipid droplets of the microalgal cells grown under nitrogen-depleted conditions were high. Further, the fatty acid profile of Isochrysis sp. grown under nitrogen-repleted conditions showed an increased eicosapentaenoic acid (22.52 mg.g−1 biomass). The mRNA expression of polyunsaturated fatty acid biosynthetic genes such as Δ5-desaturase (Δ5Des) was significantly elevated in nitrate-repleted conditions, whereas the expression of the Δ6-desaturase (Δ6Des) remained unaltered under all the conditions. This is the first study to demonstrate the correlation between nitrate repletion and cellular metabolic activity in terms of eicosapentaenoic acid production in Isochrysis sp. CASA CC 101.