Methanotroph-based microbial remediation for heavy metal polluted soil
Aerobic methanotrophs (MOB) possess efficient capabilities for heavy metal detoxification and passivation. Copper ions, by significantly affecting the composition and activity of MOB, can regulate the transformation of other heavy metals. By setting up a gradient of copper ion concentrations, MOB were enriched and cultured, then inoculated into sterilized soil to explore the effect and pathway of methane oxidation coupled with copper and zinc transformation under different copper pollution levels.Results of bacterial solution enrichment verified the uptake and utilization of copper by MOB: the methane oxidation efficiency of the microbial community gradually increased with the rise of copper ion concentration in the nutrient solution. As soil copper pollution intensified, methane oxidation efficiency decreased progressively, and the stable fractions of copper (F3 and F4) also declined, which showed a positive correlation with protein-based tightly bound extracellular polymeric substances (TB-EPS). This suggests that microbial secretion of protein-based EPS is a key process for copper detoxification and passivation.Although straw biochar significantly enhanced MOB metabolism, it reduced the passivated fractions of copper, presumably promoting the conversion of copper to reduced fractions through adsorption. After the addition of zinc ions, soil methane oxidation was significantly inhibited. In the control group without copper addition, copper was further passivated, whereas in the high-copper addition group, it was significantly activated. Meanwhile, all groups showed an increase in polysaccharide-based soluble EPS (S-EPS) and a decrease in protein-based TB-EPS.In contrast, the passivation effect of zinc in high-copper soil was better than that in the control and low-copper groups, accompanied by an increase in heavy metal-oxidizing microorganisms unique to the high-copper group and functional genes related to detoxification. The results revealed that the coupled transformation process of copper and zinc in MOB-dominated soil systems depends on the level of copper ion concentration in the soil. Additionally, straw biochar is beneficial for zinc passivation when soil copper pollution is mild, but promotes the simultaneous activation of copper and zinc when copper pollution is severe.