Evaluation of cobalt hyperaccumulation and tolerance potential of the duckweed (Lemna minor L.)

doi:10.1016/j.ecoenv.2019.04.058

Ecotoxicology and Environmental Safety

Volume 179, 15 September 2019, Pages 79-87

https://doi.org/10.1016/j.ecoenv.2019.04.058 Get rights and content

Highlights

•
Aquatic plant Lemna minor was investigated for cobalt hyperaccumulation and tolerance.
•
Carboxyl, carbonyl, and hydroxyl groups play a role in Co adsorption.
•
Enzymatic and non-enzymatic antioxidants are highly implicated in the detoxification process.
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The modulation of organic and amino acid concentrations is important for Co tolerance in L. minor.
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CS, MDH, PEPC, and ICDH related to organic acids metabolism was evaluated in L. minor.

Abstract:

Lemna minor could tolerate and accumulate more than 5,000 μg g⁻¹ DW of cobalt (Co) without foliar symptoms, indicating it is a Co hyperaccumulator. However, the physiological and metabolomics mechanisms that are responsible for Co accumulation and tolerance are largely unknown. In the present study, Fourier transform infrared spectroscopy suggested that C double bond O, C single bond H, and OH groups are involved in Co biosorption. The activation of antioxidant enzymes, such as superoxide dismutase, guiacol peroxidase, catalase, and glutathione reductase, as well as ascorbic acid and glutathione might be involved in capturing reactive oxygen species as evidenced by decreased malondialdehyde in fronds treated with Co. Metabolomics analysis revealed that Co stress significantly increased the production of several amino acids (except aspartic acid and cysteine at 200 μM) and organic acids (with the exception of succinic acid). In particular, an approximate 15-fold increase was noted in the citric acid concentration. Upon exposure to Co, increases were observed in citrate synthase, malate dehydrogenase, and phosphoenolpyruvate carboxylase activities, and a decrease was observed in isocitrate dehydrogenase related to the metabolism of organic acids. Overall, the increase in concentration of organic and amino acids and antioxidants support their effective involvement in improving Co tolerance and accumulation in L. minor.

Introduction

Despite its beneficial effects in small amounts (Palit et al., 1994; Tewari et al., 2002), cobalt (Co) has not been recognized as an essential element for higher plants (Tappero et al., 2007). In contrast, ecotoxicological data has indicated that excess Co may induce chlorosis, inhibit growth and photosynthesis, decrease the water potential and transpiration rate, and disrupt iron (Fe) homeostasis (Chatterjee and Chatterjee, 2000, 2003). In addition, toxicity of Co is also linked to oxidative stress (Tewari et al., 2002; Karuppanapandian and Kim, 2013; Begović et al., 2016).

Although excess Co is toxic to most plants, Co hyperaccumulators can accumulate more than 0.1% Co in dry shoots without any sign of toxicity (Baker et al., 1999). These plants have evolved several strategies that can activate a number of defense systems. For example, citric acid and cysteine are involved in Co ion complexation in suspension cells of the Co hyperaccumulator Crotalaria cobalticola (Oven et al., 2002). Plants that hyperaccumulate Co have likely evolved adjustments of Fe homeostatic mechanisms (Lange et al., 2017). The hyperaccumulator Alyssum murale could alleviate Co toxicity via an exocellular sequestration mechanism (Tappero et al., 2007). However, the metabolic profile of organic acids and amino acids in plants treated with Co has not yet been extensively investigated (Lange et al., 2017; Ilunga Kabeya et al., 2018).

High concentrations of cobalt are often found in industrial wastewater due to its application in the steel industry, for diamond polishing, and in the production of drying agents, pigments, and catalysts (Barceloux, 1999). Lemna minor L. (common duckweed) is one of the most popular taxa to use in various ecotoxicological studies (Üçüncü et al., 2014; Begović et al., 2016). A previous study (Sree et al., 2015) has shown that duckweed is morphologically unaffected, showing no toxicity symptoms, despite the hyperaccumulation of Co, indicating that efficient internal mechanisms detoxifying excess Co inside the cell might exist in this free-floating aquatic macrophyte. However, the physiological, biochemical, and molecular fundamentals that are responsible for Co accumulation and tolerance are still unknown. Therefore, the current study was designed to assess the involvement of antioxidative capacity and the accumulation of major organic and amino acids in response to Co in L. minor. Fourier transform infrared spectroscopy (FTIR) was employed to identify the involvement of functional groups in the biosorption process of Co. We also evaluated the effect of Co on the metabolism of organic acids in L. minor. Our goal was to assess the possible implications of antioxidants, organic acids, and amino acids in Co detoxification in L. minor. The results of this study would contribute to the advance of our understanding of the physiological mechanisms that are associated with Co accumulation and stress adaption in duckweed.

Section snippets

Plant cultures and treatments

Lemna minor was obtained from Nanjing, China and pre-cultivated under laboratory conditions (a day/night cycle of 16/8 h, at 25/22 °C, 160 μmol m⁻² s⁻¹ light irradiance) in 1/10 Hoagland solution. Healthy and equally sized fronds (about 5 g) were exposed to 0, 100, and 200 μM CoCl₂·6 H₂O. The concentrations of Co used in this work were set according to the results presented by Sree et al. (2015) and Begović et al. (2016). The solutions were changed every 2 days. Three biological replicates were

Co content

As shown in Fig. 1a, significant differences in Co accumulation were observed between control and treated fronds and between the two Co concentrations (100 and 200 μM). Co content in the fronds of L. minor increased in a concentration-dependent manner. Maximum accumulation of Co was recorded in the fronds (approximately 5426 μg g⁻¹ DW) exposed to 200 μM Co for 7 days.

Growth, proteins and photosynthetic pigments

Co exposure reduced L. minor growth. Significant reductions in the relative growth rates were found for all Co concentrations

Discussion

Duckweed species are known to uptake ions directly from the aquatic medium through the entire lower surface of the frond, which is in permanent contact with the surface of the liquid (Cedergreen and Madsen, 2002; Sree et al., 2015). In the present study, the analyses of total Co content clearly showed that L. minor is a hyperaccumulator (Oven et al., 2002). Under all Co-concentrations tested (Fig. 3a), the results from pigments (Fig. 1d–f) and protein content (Fig. 1c) analyses suggested that

Conclusions

The present investigation further confirmed the previous finding that L. minor possesses the ability to hyperaccumulate Co. FTIR spectroscopy suggests that carboxyl, carbonyl, and hydroxyl groups might be implicated in Co adsorption. The plant also displayed strong tolerance to Co toxicity, growing healthily after accumulating 5,426 μg g⁻¹ DW in the fronds. Lemna minor is able to carry out a cellular strategy involving the activation of various enzymatic and non-enzymatic antioxidants to cope

Acknowledgments

This research was supported by the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD). FTIR samples were analyzed by Nanjing Normal University Center for Analysis and Testing. The organic acids were analyzed for three times by Qingdao Sci-tech Innovation Quality Testing Co., Ltd. We thank International Science Editing (http://www.internationalscience editing.com) for editing this manuscript.

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Evaluation of cobalt hyperaccumulation and tolerance potential of the duckweed (Lemna minor L.)

Highlights

Abstract:

Introduction

Section snippets

Plant cultures and treatments

Co content

Growth, proteins and photosynthetic pigments

Discussion

Conclusions

Acknowledgments

Methods Enzymol.

J. Biol. Chem.

Aquat. Toxicol.

Anal. Biochem.

Plant Sci.

Environ. Pollut.

Plant Sci.

Aquat. Toxicol.

Ecotoxicol. Environ. Saf.

Environ. Exp. Bot.

Methods Enzymol.

J. Hazard Mater.

Phytochemistry

Polyhedron

Chem. Eng. J.

Food Chem.

Chemosphere

Methods Enzymol.

Plant Sci.

Chemosphere

Water Res.

Involvement of polyamines in plant response to abiotic stress

Biotechnol. Lett.

Metal hyperaccumulator plants: a review of the ecology and physiology of a biological resource for phytoremediation of metal polluted soils