Mercury is a rare element found in the Earth’s crust. The presence of mercury in lower concentrations is not significantly toxic. But, if present in higher concentrations like 200–800 mg/kg [Ye et al, 2016], it leads to impairment of growth, phytotoxicity, and abiotic stress. In 2003, Agency for Toxic Substances and Disease Registry (ATSDR) declared mercury as a “Heavy Metal” based on its: i) toxicity ii) frequency of occurrence, and iii) exposure potential to flora and fauna. Among all the existential forms of mercury [elemental form (Hg0), inorganic form (Hg2+), associated with ions (HgS, ClHg2, Hg2Cl2), and organic form (CH3-Hg)], the ionic form (Hg2+) is found in abundance in agricultural soil [Tiwari and Lata, 2018].
Stress caused by the presence of mercury leads to ROS (Reactive Oxygen Species) generation, which can lead to rapid cell death with necrotic morphology [Minina et al, 2013]. Reactive oxygen species (ROS) [superoxide anion (\({\text{O}}_{2}^{\text{⦁}-})\), hydroxyl radical (OH⦁), hydrogen peroxide (H2O2), singlet oxygen (1O2)] is beneficial when present in low or moderate levels, but becomes toxic to life when present in excessive concentration. Therefore, the balance between ROS generation and decomposition plays a vital role in the maintenance of normal physiology. ROS scavenging enzymes present in plants are i) Superoxide Dismutase (SOD) ii) Catalase (CAT) and Glutathione peroxidase (GPx), and Peroxidase (POD). Oxidative stress caused by nanoparticles can be better understood by studying the activities of these enzymes along with the concentration of superoxide anions and H2O2 [Vinothkumar et al, 2018; Wu et al, 2017; Mitra et al, 2022]. Scientists have been looking for solutions to mitigate abiotic stress in plants so that they can survive in adverse conditions. Several other nanoparticles like Fe3O4, TiO2, ZnO, Al, Cu, and Ag have been in prior use for bypassing stress [Nile et al, 2022]. Cerium oxide nanoparticles are known for their high redox potential, a property that can be exploited to relieve abiotic stress.
Cerium falls under the category of lanthanide series [Wu and Ta, 2021]. Oxide of cerium forms a non-stoichiometric oxide CeO2 − x (0 ≤ x ≤ 0.5), where the value of x depends on gas phase composition and temperature [Schilling et al, 2017]. Depending on synthesis parameters and the chemical environment, cerium in nanostructured cerium oxide has the unique property of existing in two reversible oxidation states of Ce3+ and Ce4+ [Vinothkumar et al, 2018]. When 2Ce4+ ions are reduced to 2Ce3+, an oxygen vacancy is generated, which is represented by the Kröger-Vink notation as given below [Vinothkumar et al, 2018; Anandkumar et al, 2015]:
$${O}_{o}^{x}+{2Ce}_{Ce}^{x}\to {V}_{o}^{"}+{2Ce}_{Ce}^{{\prime }}+\left(1/2\right){O}_{2}$$
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The biological function of ceria depends on the co-existence of + 3/+4 states and oxygen vacancy concentration. Ce4+ serves as a nitric oxide radical scavenger and Ce3+ (higher oxygen vacancy) serves as a hydroxyl and superoxide radical scavenger. Ceria has both anti-oxidant and pro-oxidant properties depending on the Ce3+/ Ce4+ ratio [Vinothkumar et al, 2018]. Though the influence of nanoceria has been investigated through various aforementioned studies, its influence on plant and bacterial growth modulators is yet to be investigated. Further, here, we have synthesized CeO2 with a low Ce3+/ Ce4+ ratio, expecting for anti-oxidant nature of CeO2, and investigated the effect of dosage dependency on mitigating the stress. The hypothesis of this study is the ability of cerium oxide nanoparticles to enhance plant growth by reducing oxidative stress, alleviating the stress caused in Vigna radiata and Bacillus coagulans by heavy metals, and enhancing the growth of both gram-positive and gram-negative bacteria.
Vigna radiata, being the model dicot leguminous plant was selected as a potential candidate for conducting the experiment [Kozlov et al, 2020]. The role of nanoceria on morphology and oxidative stress of Vigna was observed both in the presence and absence of heavy metal.
Bacillus coagulans, a gram-positive, probiotic, soil bacteria, which is also found in the digestive system of animals as a symbiotic organism facilitating digestion, was selected as a potential candidate for experimenting. Besides, nitrogen fixation and enhancing soil fertility, Bacillus helps in improving human health. Therefore, the effect of nanoceria on the growth of Bacillus was assessed in the absence and presence of Hg to observe the biocompatibility of the particles and also to find whether it can rescue the bacteria from heavy metal stress.
E.coli, the model gram-negative organism was selected as a potential candidate for the experiment. It is present in the soil as well as in the gut of animals as a disease-causing pathogen. Therefore, it would be interesting to find out whether this nanoparticle is safe for both gram-positive and gram-negative bacteria irrespective of pathogenicity.
The application of nanoceria applied in ppm dosage and considering cost-effectiveness may have a wider impact in agriculture.