Metal contaminated soil leachates from an art glass factory elicit stress response, alter fatty acid metabolism and reduce lifespan in Caenorhabditis elegans

Highlights

• Soil and soil leachates showed high levels of metals such as As, Cd, Fe and Pb.

• Leachates altered C. elegans stress gene expression in short and long exposures.

• Although the soil leachates were not lethal, they shortened C. elegans lifespan.

• Alteration of fat metabolism in C. elegans correlated with the shortened lifespan.

Abstract

The present study evaluated the toxicity of metal contamination in soils from an art glass factory in Småland Sweden using a Caenorhabditis elegans nematode model. The aim of the study was to chemically analyze the soil samples and study the biological effects of water-soluble leachates on the nematodes using different physiological endpoints. The total metal content showed that As, Cd and Pb were at levels above the guideline values for soils in areas around the factory. Less than 10% of the total metal content in the soil was found in the water-soluble leachates, however, Al, As, Fe and Pb remained higher than the guideline values for safe drinking water. Exposure of C. elegans to the water-soluble leachates, at both post-hatching larvae stage (L1-young adult) for 48 h and at the young adult stage (L4) for 6 h, showed significant gene alteration. Although the nematodes did not exhibit acute lethality, lifespan was significantly reduced upon exposure. C. elegans also showed altered gene expression associated with stress response and fat metabolism, as well as enhanced accumulation of body fat. The study highlighted the significance of assessing environmental samples using a combination of gene expression analysis, fatty acid metabolism and lifespan for providing valuable insight into the negative impact of metals. The altered fat metabolism and reduced lifespan on exposure to soil leachates motivates further studies to explore the mechanism of the toxicity associated with the metals present in the environment.

Introduction

Contamination and degradation of soil quality has increased due to human activities such as farming, industrialization and inefficient waste management (European Commission, 2013). Glass manufacturing, particularly art glass production, has contributed to environmental contamination with heavy metals. The global use of glass in households and industries has been ongoing for centuries and the pollution from glass manufacturers has led to health concerns. The hazardous waste from glass production is due to metals such as chromium (Cr), cobalt (Co), iron (Fe), manganese (Mn), lead (Pb) and uranium (U) that are often used to impart colors to the glass. This potential source of metal pollution is multi-facetted since it involves both chemicals that are discarded during glass production as well as glass particles. It is however, unclear how much metal is leached into the environment from the glass matrix and becomes bioavailable to impart toxic effects (Pant and Singh, 2014). Since, the total concentration of metals present in the contaminated site does not provide information about the bioavailability, selective chemical extractions using solvents such as water or ammonium acetate are used to estimate the water-soluble and exchangeable fractions from the soil (Schultz et al., 2004). Moreover, living organisms are sensitive to the bioavailable fraction and not the total content present in contaminated sites (Garcia-Lorenzo et al., 2009). Hence for better understanding of the environmental hazards associated with metal contamination, chemical analysis of the soil and the water-soluble fractions should be complemented with biological assays.

Elements such as arsenic (As), cadmium (Cd), Cr, mercury (Hg) and Pb in soil can be mobilized, taken up by plants, accumulate and pose a serious health hazard to humans and animals consuming them (Peralta-Videa et al., 2009). However, the effect of these metals on a test organism such as C. elegans will vary depending on the physicochemical state (speciation) of the metals, therefore the total metal content is a poor estimator for biological impact (Boyd and Williams, 2003). Soil samples from a private garden in the vicinity of a glass factory had similar levels of metals as the soil collected from directly outside the glass factory. Moreover, vegetables grown in the contaminated soils also showed high levels of metals, which suggest that metals can accumulate in vegetation and can pose a potential risk to higher trophic levels (Uddh-Soderberg et al., 2015). Recent findings also indicate that residents living near the glass factory have higher risk of developing different types of cancer such as colon, rectal, pancreatic and prostate cancers (Nyqvist et al., 2017). Metals such as As, Cd, Hg, Mn, Pb, and zinc (Zn) have been shown to act as endocrine disruptors in animals and humans (Iavicoli et al., 2009). Epidemiological studies have implicated the association of Co, Ni and Zn with altered blood glucose levels among workers in a metal manufacturing company, suggesting a role of these metals in the development of diabetes (Yang et al., 2017). A recent study also reported high levels of Hg and Pb in the breast milk of lactating mothers and thus exposing infants to these metals (Park et al., 2018). These observations highlight the health risks associated with metal contamination and the significance of analyzing the associated toxicity.

There are various biological tests that are recommended for assessment of toxicity associated with soil contamination (Karjalainen et al., 2009). In earlier studies both plants and bacteria have been used as representatives of primary producers and decomposers in the ecosystem to assess toxicity of metal (such as Cd, Pb and Zn) contaminated soils (Garcia-Lorenzo et al., 2009; Kahru et al., 2005). Among other key organisms, enchytraeids (potworms) have been considered to be sensitive to environmental stresses and their presence and species composition have therefore been considered as indicators of metal toxicity (Kapusta and Sobczyk, 2015). However, none of the above-mentioned species have been used in a comprehensive toxicity study of metal contaminated soils from art glass manufacturing using different physiological endpoints.

Caenorhabditis elegans is a free-living bacterivorous nematode found in soils enriched with decomposing organic matter and is a good model for screening environmental pollutants in soils (Choi, 2008; Leung et al., 2008). The nematode is genetically, physiologically and developmentally characterized and most of its basic stress response pathways are highly conserved with higher organisms (Kaletta and Hengartner, 2006). They share 60–80% homology with humans, thus providing a relevant model for analyzing element toxicity in the soil environment (Rodriguez et al., 2013). Earlier studies have used C. elegans to assess toxicity of soils (Baderna et al., 2014), water (Ju et al., 2014; Kumar et al., 2015), particulate matter (Sun et al., 2015), sewage sludge (McLaggan et al., 2012) and river sediments (Wolfram et al., 2012) using different endpoints. Hence, toxicological studies using C. elegans as model system can predict possible toxic impact of metal to higher organisms.

The aim of the present study was to evaluate the biological effects of metal contaminated soils from an art glass-manufacturing site in Southern Sweden. The objectives were to analyze the toxicity of the metals in the water-soluble fraction of the soil leachates in a C. elegans nematode model using gene expression, fatty acid staining and lifespan assays. A more comprehensive analysis of the toxicity of soluble metal contamination in the environmental contributes towards improved risk analysis.