Expression of the Vitreoscilla hemoglobin gene in Nannochloropsis oceanica regulates intracellular oxygen balance under high-light.

Nannochloropsis oceanica is widely used as a model photosynthetic chassis to produce fatty acids and carotenoid pigments. However, intense light typically causes excessive generation of reactive oxygen species (ROS) and photorespiration in microalgal cells, which results in decreased cell growth rate and unsaturated fatty acid content. In this study, the Vitreoscilla hemoglobin gene (vgb) was introduced into N. oceanica cells and expressed by using the light-harvesting complex promoter and its signal peptide. Compared with wild type (WT), the growth rate of transformants increased by 7.4%-18.5%, and the eicosapentaenoic acid content in an optimal transformant increased by 21.0%. Correspondingly, the intracellular ROS levels decreased by 56.9%-70.0%, and the catalase content in transformants was about 1.8 times that of WT. The photorespiration level of transformants was reduced by the measurement and calculation of the dissolved oxygen concentration under the condition of light-dark transition. The expression level of the key genes related to the photorespiration pathway in transformants was more than 80% lower than that in WT. These results indicated that Vitreoscilla hemoglobin could improve microalgal growth by reducing ROS damage and modulating photorespiration under stress conditions.