Characterization of heavy metal toxicity in some plants and microorganisms—A preliminary approach for environmental bioremediation

Highlights

• Heavy metal toxicity in plants and microorganisms was investigated.

• Phytotoxic effects of Cr(VI) and Cd(II) were tested on L. sativum.

• Ecotoxicity studies were developed for bacteria Azotobacter sp. and fungi Pichia sp.

• L. sativum plant, Azotobacter sp. and Pichia sp. can be used in bioremediation.

Abstract

In situ bioremediation processes are important for control of pollution and clean-up of contaminated sites. The study and implementation of such processes can be designed through investigations on natural mechanisms of absorption, biotransformation, bioaccumulation and toxicity of pollutants in plants and microorganisms. Here, the phytotoxic effects of Cr(VI) and Cd(II) on seed germination and plant growth of Lepidium sativum have been examined at various concentrations (30−300 mg/L) in single ion solutions. The studies also addressed the ecotoxicity of metal ions on Azotobacter chroococcum and Pichia sp. isolated from soil. Microbial growth was estimated by weighing the dry biomass and determining the enzymatic activities of dehydrogenase and catalase. The results showed that Cr(VI) and Cd(II) can inhibit L. sativum seed germination and root development, depending on the metal ion and its concentration. The phytotoxic effect of heavy metals was also confirmed by the reduced amounts of dried biomass. Toxicity assays demonstrated the adverse effect of Cr(VI) and Cd(II) on growth of Azotobacter sp. and Pichia sp., manifested by a biomass decrease of more than 50 % at heavy metal concentrations of 150−300 mg/L. The results confirmed close links between phytotoxicity of metals and their bioavailability for phytoextraction. Studies on the bioremediation potential of soils contaminated with Cr(VI) and Cd(II) using microbial strains focusing on Azotobacter sp. and Pichia sp. showed that the microbes can only tolerate heavy metal stress at low concentrations. These investigations on plants and microorganisms revealed their ability to withstand metal toxicity and develop tolerance to heavy metals.

Introduction

Industrialization and modern agriculture have increased environmental contamination with heavy metals, which can accumulate in living organisms, causing the emergence of toxicity symptoms [[1], [2], [3], [4], [5], [6]]. Although several heavy metals (Co, Cu, Cr, Ni, Fe, Mn, Zn etc.) play a vital role in the metabolic processes in enzymatic reactions and in providing osmotic balance [[7], [8], [9]], some others (Cr, Cd, Hg, Ni, Pb) are toxic, inhibiting growth to different extents, even at very low concentrations [1,3,[8], [9], [10], [11], [12]]. An excessive accumulation of heavy metals in aquatic and soil environments can induce adverse phytotoxic effects, such as growth inhibition, photosynthesis disturbance, biomass decrease, and nutrient uptake deficiency [[13], [14], [15]].

Plants growing on soils contaminated with heavy metals are able to absorb significant amounts of metal ions, which thus enter the food chain affecting human health [[16], [17], [18], [19], [20]]. Likewise, some microorganisms living in the soil can accumulate heavy metals, since they are able to initiate and develop various mechanisms for metal mobilization or immobilization (e.g. biosorption, bioprecipitation), depending on soil properties (pH, type, salinity etc.) [21]. Hence, the cleanup of contaminated soils using plants, microorganisms or other biological systems, within the limits of their tolerance for heavy metals and under controlled conditions, remains a constant challenge for researchers and for regulatory authorities [1,3,13,[21], [22], [23]].

Cr and Cd were selected for this study since they are recognized as frequently-encountered toxic heavy metals and were categorized as human carcinogens by the International Agency for Research on Cancer in 1993 [[24], [25], [26], [27]]. They are very toxic to both plants and microorganisms, whose response to any stress generated by heavy metals depends on the heavy metal concentration, type and speciation, but also on environmental factors and organism species [[28], [29], [30], [31], [32], [33], [34]]. Cd can play the role of cofactor for oxidative reactions that disrupt and damage living tissues, and can increase the oxidative capacity in the generation of reactive-oxygen species (ROS), lipid peroxidation and depleting glutathione, enhancing and linking protein sulfhydryl groups [[35], [36], [37]]. The reduction/oxidation of Cr from Cr(VI) to Cr(III) is possible from a thermodynamic point of view in certain physiological conditions. Cr(VI) is the most toxic form of Cr, often found as oxyanions associated with oxygen, as chromate (CrO₄²) or dichromate (Cr₂O₇²⁻) [17]. It is acknowledged that Cr(III) is indispensable for sustaining the glucose metabolism of lipids and proteins. In addition, Cr(III) can stabilize the tertiary structure of proteins, RNA and DNA conformation. On the other hand, the compounds of Cr(VI) are toxic. Interactions between bacteria, algae, fungi and plants, with Cr and its compounds have been thoroughly reviewed in the literature [12,15,17,38,39].

Here, the phytotoxicity of Cr(VI) and Cd(II) has been tested by investigating their effects on the efficiency of seed germination and growth process of the plant Lepidium sativum. Studies on the ecotoxicity of Cr(VI) and Cd(II) using bacteria (Azotobacter sp.) and fungi (Pichia sp.) isolated from the soil were also developed.