Overlooked Risks of Biochars: Persistent Free Radicals trigger Neurotoxicity in Caenorhabditis elegans

In recent years, biochars have gained increasing interest in mitigating climate changes and revitalizing contaminated or drained soil. Studies determining their impact on the ecosystem, especially on soil invertebrates, however, are still scarce and the neurotoxic potential of biochars has never been evaluated before. Using the model organism Caenorhabditis elegans we determined the neurotoxic effect of biochar produced from rice straw by pyrolysis at 500 °C at concentrations ranging from 0 to 2000 mg C·L^-1. Biochar had a hormetic effect on locomotion behavior. Furthermore, high concentrations impaired defecation as well as the recognition and response to a chemical attractant. None of the potential toxic chemicals in the biochar had sufficient high concentrations to explain the detected neurotoxic effect. Using electron paramagnetic resonance (EPR) spectroscopy, we detected free radicals in the biochar. Detrimental reaction of free radicals with biotic macromolecules can induce oxidative stress responses and are a potential reason for the evaluated neurotoxic effect of biochar. Overall, we were able to prove that biochars have the potential to act as weak neurotoxins to soil organisms and effects of persistent free radicals should be investigated further.