Abstract
Due to their abundance and persistence, microplastics pose significant risks in soil ecosystems, threatening edible crop plants integral for future food security. The synergistic interaction between microplastics, plasticizers and absorbed pollutants impacts plant performance and crop yields worldwide, while simultaneously providing a gateway for xenobiotics to enter the wider food web. This study used polyethylene microspheres in the size range of 10 - 45 μm to investigate two critical impacts of microplastics on plant health and human health. Firstly, independent trials were run where mung bean (Vigna radiata) or wheat (Triticum aestivum) seedlings were cultivated in a two-layered sand matrix, consisting of a layer of microplastic contaminated sand, and a layer of microplastic free sand. Microplastics were added at low (1000 mg/kg), medium (5000 mg/kg) and high (25000 mg/kg) concentrations, where they were applied to either the top layer of sand, referred to as direct contact, as particles were directly in contact with the root system. Alternatively they were applied to the bottom layer of sand, referred to as indirect exposure, where microplastics shared no physical contact to the root system. Postharvest growth parameters and physiological measurements were taken. Biochemical measurements were conducted for enzymatic antioxidant activities (SOD, CAT, APOX, DHAR, MDHAR, GR and GPOX) and markers of oxidative damage (PC, and LPOX). Results identified a novel indirect exposure toxicity effect where what was leached from the high dose microplastics into the sand matrix could induce oxidative damage and hinder growth in mung bean and wheat seedlings. Importantly, these findings suggest that leachates from microplastics, and not just the particles themselves, are responsible for phytotoxicity, highlighting a potential risk to the broader food chain. Secondly, human digestion models were simulated in vitro using standardized INFOGEST methods on human epithelial colorectal adenocarcinoma (Caco-2) cells. Microplastics were added to culture medium or culture medium plus mung bean or wheat digests at 4 different doses: control (0 μg/mL), low (100 μg/mL), medium (500 μg/mL) and high (2500 μg/mL). Various assays were conducted to measure cell viability (MTT assay), membrane leakage (LDH assay), inflammation (NO assay) and enzymatic antioxidant activities (SOD, CAT, and GPOX). Results found that the ingestion of microplastics alone, at a high dose, significantly induced cytotoxicity in human Caco-2 cells. However, a novel digest- dependant effect was found, where the latter toxicity was mitigated by the addition of plant digests. Conclusively, results demonstrate that seedlings exposed to high concentrations of microplastics, either via direct contact or indirect exposure, experience toxicity. Moreover, while microplastics can induce cytotoxicity in Caco-2 cells, plant digests can offer a protective mechanism. This study highlights the need to implement risk management strategies for both plant health and human health.