Earthworm bioassay and bacterial community structure as bioindicators of metal contamination: Iron smelters' soil impact

Anthropogenic activities such as mining and agricultural practices often have adverse effects on soil health. This study aimed to assess the impact of iron smelting plants on the surrounding soil utilising earthworm responses and bacterial community structure as bioindicators. Soil samples were collected at different distances (ranging from 100 to 10,000 m) away from three smelters along the prevailing wind direction. Metal concentrations (accumulation and pollution indices) in soil and metal bioaccumulation by earthworms were determined. Earthworm endpoints determined included mortality, reproduction and avoidance-behaviour. Bacterial community structure was determined through metabarcoding of the V4-V5 region of 16S rRNA amplicons. The results revealed varying degrees of metal contamination across the sites and distances, which corresponded with earthworms and soil bacterial community responses. As expected, Fe had the highest concentration among the metal deposits in soil across all the three smelting sites followed by Al and Mn. Further, bioaccumulation of metals in the earthworms' tissue was observed, with distinct patterns for different metals and sites. The five most abundant phyla common to all sites includes Actinobacteria, Proteobacteria, Acidobacteria, Firmicutes and Chloroflexi. There are observed degrees of similarity among the bacterial communities across the smelting sites, with patches of small fractions of the community unique to each site. It was concluded that a combination of multiple bioindicators, i.e. earthworms and bacterial community structure, can be used successfully to assess soil degradation caused by iron smelting activities.