Natural variability and correlations in the metabolic profile of healthy Eisenia fetida earthworms observed using 1H NMR metabolomics

1H NMR metabolomics can be used to assess the sub-lethal toxicity of contaminants to earthworms by identifying alterations in the metabolic profiles of contaminant- exposed earthworms in contrast to those of healthy (control) individuals. In support of this method this study sought to better characterize the baseline metabolic profile of healthy, mature earthworms of the species, Eisenia fetida, which is recommended for both acute and sub-lethal toxicity testing for soil contaminants. Profiles of D2O-buffer extracted metabolites were determined using 1H NMR spectroscopy and both inter-individual metabolic variability and pair-wise metabolic correlations were assessed. The control earthworm extracts exhibited low overall inter-individual metabolic variability, with a spectrum-wide median relative standard deviation (%RSD = standard deviation/mean × 100) of 14%, which suggests that the metabolic profile of E. fetida earthworms is well controlled in laboratory conditions and supports further use of this organism in environmental metabolomics research. In addition, strong positive correlations were detected between the levels of maltose, betaine, glycine, and glutamate as well as between the levels of lactate, valine, leucine, alanine, lysine, tyrosine, and phenylalanine which had not previously been reported. Since comparison of pair-wise metabolic correlations between control and treated organisms can reveal changes in the underlying pattern of biochemical relationships between the metabolites, identification of these significant metabolic correlations in control earthworms provides an additional characteristic that may be applied to delineate between control and treated earthworms in future NMR-based metabolomic studies.

Research highlights
► Low inter-individual metabolic variability in Eisenia fetida under laboratory conditions.
► Spectrum-wide median relative standard deviation of 14%.
► Two groups significant pair-wise metabolic correlations in healthy E. fetida.
►  +correlations: maltose, betaine, glycine, glutamate.
►  +correlations: lactate, valine, leucine, alanine, lysine, tyrosine, phenylalanine