NMR- and HRMS-based untargeted metabolomic study of metal-stressed Euglena gracilis cells

Abstract

Environmental contamination is an increasing problem and one of the main threats to human health. While euglenoids have been shown to tolerate high levels of metals, little is known about the metabolic mechanisms at play. To address this gap, Euglena gracilis cells in the mixotrophic exponential phase were exposed to high levels of cadmium (Cd), cobalt (Co), or copper (Cu) (2.7–190.7 mg L−1) and the cellular metabolic profiles were explored using a multi-pronged untargeted approach. The combined application of proton nuclear magnetic resonance (1H NMR) and high-resolution mass spectrometry (HRMS) expanded the coverage of detected metabolites by 38–58 % using the computational platform Pathos and the metabolite databases KEGG, Metlin, and Chenomx. Only 3 % of the identified metabolites were found to be common to 1H NMR and HRMS. This low overlap indicates that the two methods complement each other, with each method uncovering a unique set of metabolites. Although four metabolic pathways including purine metabolism, alanine, aspartate, and glutamate metabolism, and the citrate cycle were common to all metal treatments, sixteen metabolic pathways were metal-specific. Notably, the metabolic alterations induced by Cd and Co differed from each other and the control, while the metabolic profile of Cu-treated cells resembled the control more closely. Unlike Co which showed a broad impact on Euglena gracilis metabolism, the stress induced by Cd and Cu exposure seems to target more the amino sugar production and the chlorophyll machinery, respectively. The observed metabolic alterations in response to various metals highlight a potential correlation with the generation of reactive oxygen species (ROS). Together these results contribute to a better understanding of cellular toxicity and the impacts of metal-induced oxidative stress in Euglena gracillis cells.