Abstract
Heavy metal pollutants in the environment are increasing exponentially due to various anthropogenic factors including mining, industrial and agricultural wastes. Living organisms exposed to heavy metals above a certain threshold level induces deleterious effects in these organisms. To live in such severe environments, microbes have developed a range of tolerance mechanisms which include upregulation of stress-responsive genes and/or antioxidant enzymes to detoxify the metal stress. Single cell eukaryotic microorganisms, i.e., ciliates, are highly sensitive to environmental pollutants mainly due to the absence of cell wall, which make them suitable candidates for conducting ecotoxicological studies. Therefore, the present investigation describes the effects of heavy metals (cadmium and copper) on freshwater ciliate, Euplotes aediculatus. The activities of antioxidant enzymes, i.e., catalase and glutathione peroxidase in E. aediculatus were determined under heavy metal exposure. Besides, the expression of stress-responsive genes, namely, heat-shock protein 70 (hsp70) and catalase (cat), has also been determined in this freshwater ciliate species under metal stress. The present study showed that the enzyme activity and the expression of these genes increased with an increase in the heavy metal concentration and with the duration of metal exposure. Also, these stress-responsive genes were sequenced and characterized to comprehend their role in cell rescue.
Highlights
Heavy metal contamination is increasing exponentially and rapidly in the environment especially in the aquatic ecosystems due to their high rate of solubility (Gheorghe et al 2017; Hameed et al 2020)
Similar kind of result has been reported in Tetmemena sp. where the activity of CAT increased with increase in heavy metal concentration but decrease in the enzyme activity has been reported at higher concentration (Somasundaram et al 2019)
Similar type of result was observed in Tambaqui fish (Colossoma macropomum) where expression of hsp70 gene was highest after 3 h of Cu treatment whereas for Cd treatment, hsp70 expression was upregulated after 1 h and lowered after 3 h (Casanova et al 2013)
Summary
Heavy metal contamination is increasing exponentially and rapidly in the environment especially in the aquatic ecosystems due to their high rate of solubility (Gheorghe et al 2017; Hameed et al 2020). Anthropogenic activities such as mining and industrial activities are the major source of heavy metal pollution (Gutiérrez et al 2008; Jin et al 2018; Malidareh et al 2014; Nriagu and Pacyna 1988; Osman 2014; Peñuels and Filella 2002; Wong et al 2016). Some of the mechanisms to combat heavy metal toxicity are activation of antioxidant enzymes and bioaccumulation of heavy metals by metal-binding proteins such as metallothionein (Abraham et al 2017; Emamverdian et al 2015; Ghori et al 2019; Somasundaram et al 2019; Toteja et al 2017)
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