Study on the Influence of Polystyrene Nanoplastics on the Toxicity of Glyphosate in Eisenia fetida

Abstract

Background: The widespread presence of microplastics/nanoplastics (MPs/NPs) in both aquatic and terrestrial ecosystems, along with their associated ecological consequences, has gained worldwide attention. Nanoplastics (NPs) have a broad distribution across diverse environments, including soil, and are recognized for their harmful effects on soil organisms. Nanoplastics have garnered worldwide attention owing to their extensive presence in the environment and their capacity to penetrate biological barriers. Glyphosate (GLY) stands out as the most extensively utilized organophosphorus herbicide in agriculture. The simultaneous presence of microplastics/nanoplastics alongside other environmental pollutants has prompted an increased emphasis on their co-occurrence and interactive toxicity. Nanoplastics and glyphosate often coexist within the soil environment. The primary objective of this study was to investigate the extensive toxicological impacts of NPs and GLY on earthworms. Methodology: This study attempted to explore the chronic impacts arising from the separate and combined exposures of polystyrene nanoplastics (PSNPs) concentration (12.5 mg/kg) and glyphosate (GLY) concentration (26 mg/kg) on the earthworms over a period of 28 days. Earthworms are highly important for soil health and functioning. In a 28 day chronic toxicity test, 300 earthworms were exposed to different concentrations of nanoplastics and glyphosate over 7, 14, 21, and 28 days, divided into three treatment groups and a control group. Samples were collected at 7-day intervals, with 11 earthworms sacrificed per treatment. Tissues from the brain, intestine, and other organs were stored for analysis, and histopathology was examined using formalin-preserved intestinal tissues. The experiment was conducted in triplicates using natural soil as the substrate. Biochemical parameters, including AChE activity, ROS, MDA, antioxidant enzyme levels, DNA damage, and histopathological changes, were evaluated. Results: The activity of acetylcholinesterase (AChE) in earthworms was suppressed by both nanoplatics and glyphosate, resulting in an excessive accumulation of acetylcholine within the synaptic space ultimately inducing neurotoxic effects. The combined treatment resulted in a greater inhibition of AChE compared to the individual treatments. Exposure to nanoplatics and glyphosate can initiate a process where the production of reactive oxygen species (ROS) triggers the release of more ROS within the earthworm, and increased malondialdehyde content (MDA). The combined exposure to nanoplatics and glyphosate led to increased levels of ROS and MDA content at all sampling intervals compared to individual exposures. Antioxidant enzymes (SOD and CAT) activity decreased significantly with increasing exposure time. The viii combined exposure to nanoplatics and glyphosate significantly reduced the activity of SOD and CAT compared to the individual groups. Nanoplatics and glyphosate caused DNA damage by a substantial increase in tail moment and tail length at all sampling intervals by combined exposure of glyphosate and nanoplatics as compared to individual treatments. Histopathological alterations in intestine were most significant and caused irregular cell shape, enlarge nucleus and cell lysis. The combined exposure to nanoplatics and glyphosate results in more pronounced alterations than the individual treatments. Conclusion: These pollutants disrupt the integrity of the intestinal cell membrane in earthworms. This study revealed that earthworms exposed to specified doses of nanoplastics and glyphosate exhibit synergistic effects. Furthermore, this study contributes toxicity data relevant to soil invertebrates and nontarget terrestrial organisms. Keywords: Earthworms, Polystyrene nanoplastics, Glyphosate, Co-occurrence, Coexposure, Oxidative stress, DNA damage