Effect of Various Concentrations of Zinc as Soil Health Indicator on Earthworm (Pheretima posthuma) and Some Soil Bacteria

Abstract

Metal toxicity has become a critical environmental and health concern, as it can adversely affect entire organisms or specific components such as cells or organs. Zinc, in particular, is a trace element essential for the proper physiological functioning of organisms. As a vital micronutrient, zinc is required for growth, development, and differentiation across a wide range of biota, including bacteria. It also serves as an essential cofactor for numerous classes of enzymes. However, excessive zinc concentrations can be detrimental and induce toxic effects. Most studies on zinc toxicity have focused on organisms at higher trophic levels. Nonetheless, elevated zinc levels can also disrupt food webs at lower trophic levels. Interestingly, bacterial isolates from natural environments often exhibit zinc tolerance, reflecting adaptive mechanisms within soil microbial communities. Soil itself is a rich source of both bacteria and earthworms, both of which serve as reliable indicators of soil quality and ecological health. The present study demonstrated that even a relatively low zinc concentration (0.50 mg/mL) inhibited the growth of soil bacteria. In contrast, earthworms remained unaffected at zinc concentrations as high as 0.75 mg/mL (ZnSO₄), highlighting the differential sensitivity of soil biota to zinc exposure. These findings underscore the importance of considering both microbial and macrofaunal responses when assessing the ecological impact of metal contamination.