Soil Ecotoxicology Needs Robust Biomarkers: A Meta‐Analysis Approach to Test the Robustness of Gene Expression‐Based Biomarkers for Measuring Chemical Exposure Effects in Soil Invertebrates

Gene expression‐based biomarkers are regularly proposed as rapid, sensitive, and mechanistically informative tools to identify whether soil invertebrates experience adverse effects due to chemical exposure. However, before biomarkers could be deployed within diagnostic studies, systematic evidence of the robustness of such biomarkers to detect effects is needed. In our study, we present an approach for conducting a meta‐analysis of the robustness of gene expression‐based biomarkers in soil invertebrates. The approach was developed and trialed for two measurements of gene expression commonly proposed as biomarkers in soil ecotoxicology: earthworm metallothionein (MT) gene expression for metals and earthworm heat shock protein 70 (HSP70) gene expression for organic chemicals. We collected 294 unique gene expression data points from the literature and used linear mixed‐effect models to assess concentration, exposure duration, and species effects on the quantified response. The meta‐analysis showed that the expression of earthworm MT was strongly metal concentration dependent, stable over time and species independent. The metal concentration‐dependent response was strongest for cadmium, indicating that this gene is a suitable biomarker for this metal. For copper, no clear concentration‐dependent response of MT gene expression in earthworms was found, indicating MT is not a reliable biomarker for this metal. For HSP70, overall marginal up‐regulation and lack of a concentration‐dependent response indicated that this gene is not suitable as a biomarker for organic pollutant effects in earthworms. The present study demonstrates how meta‐analysis can be used to assess the status of biomarkers. We encourage colleagues to apply this open‐access approach to other biomarkers, as such quantitative assessment is a prerequisite to ensuring that the suitability and limitations of proposed biomarkers are known and stated. Environ Toxicol Chem 2022;41:2124–2138. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.