Developing novel tools to screen for belowground chemical contaminants: Insect-induced plant galls and their application in phytoscreening organic and inorganic pollutants

In polluted urban settings, plants have been used to detect belowground chemical contaminants in a process referred to as ‘phytoscreening’. However, the concentration of contaminants detected varies among plant tissue type (e.g. stems, fruit, leaves) and plant species, making decisions about which locations to prioritize for remediation difficult. During their growth phase, insect-induced plant galls are sinks for exceedingly high concentrations of photoassimiliates and nutrients. We hypothesize that galls may also accumulate belowground chemical contaminants in similarly high concentrations, regardless of the plant species on which they form. In December 2019, contaminated groundwater was found spewing onto Interstate Highway 696 approximately 50-km north of metro Detroit, Michigan, USA. The contamination originated from a privately owned electroplating facility adjacent to the highway that was illegally dumping waste, for decades, into a hole dug through the foundation of the basement. To test our hypothesis, we screened for and compared the concentration of inorganic (e.g., heavy or semi-metals) and organic (volatile organic compounds) electroplating waste in multiple plant tissues, including galls, sampled from multiple plant species along a 2-km² area surrounding the facility. Regardless of plant species, on average, galls detected contaminates including chromium, arsenic, benzene, toluene in higher concentrations than shoots, fruit and tree cores. Moreover, the concentration of chemical contaminants generally decreased in gall tissue as the distance from electroplating facility increased, corresponding with the concentrations of other plant tissues. Overall, our results imply that galls may serve as a superior phytoscreening strategy for belowground chemical contaminants.