Enhancement of heavy metal tolerance and lipid accumulation in Euglena gracilis under Cd stress by γ-aminobutyric acid: A physiological, biochemical, and metabolomic study

Abstract

Euglena gracilis (E. gracilis) is a unicellular eukaryotic photosynthetic protist without cell wall. As a model organism, it has been extensively studied in endosymbiosis, chloroplast development, environmental bioremediation, and high-value product synthesis. Heavy metal pollution, such as cadmium (Cd) stress, is prevalent and induces high levels of reactive oxygen species (ROS) in microalgae, leading to oxidative stress, reduced photosynthetic efficiency, and inhibited growth. This study explored the application of γ-aminobutyric acid (GABA), a plant hormone-like substance, to enhance heavy metal tolerance in E. gracilis under Cd stress, aiming to increase the accumulation of high-value by-products such as lipids. As previously studied, our findings also showed that Cd stress significantly inhibited E. gracilis growth and pigment accumulation. However, it is worth noting that our results showed that exogenous GABA addition improved cell counts, lipid content, protein content and pigment content. GABA also significantly increased glutathione (GSH) content. Notably, Metabolomic analysis revealed that Cd stress shifted E. gracilis energy storage from carbohydrates to lipids and proteins, and exogenous GABA addition further enhanced this trend. These results suggest that GABA play a significant role in increasing lipid accumulation and enhancing Cd tolerance in E. gracilis, providing an economical and eco-friendly strategy for lipid production and Cd adsorption.