Organocatalysts have received a great deal of attention over the last decade due to their low cost, availability, high yield, short reaction time, simplicity of product isolation, clean reaction profile, environmental benignity, recyclability, and reusability [1]. Having a strong hydrogen bonding capability, organic-based catalysts can significantly promote chemical reactions in environmental studies [2]. Urea has been recently applied as an organic base for organocatalysts used in wastewater management and air pollution to remove heavy metals [3]. Thiourea catalyst is generated from magnetic nanoparticles because of their high surface area, low toxicity and superparamagnetic behavior, and potential applications in many fields [4]. Many metals have been used as the metallic base of Thiourea catalyst, including Al, Fe, Cu, Si, and Zn; however, functionalized iron nanoparticles (Fe3O4) are more interesting due to their easily separable, reusable, non-toxic, low cost, and flexible design [5]. In organic chemistry, Fe3O4 catalyst coated with thiourea is used for the Knoevenagel reaction where formation of C-C double bonds and synthesis of α, β-unsaturated carbonyl compounds from active methylene and carbonyl compounds may occur [7]. Knoevenagel reaction is mainly used in the chemical processes to mitigate the human interference and secure operators from dangerous and carcinogenic solvents [6]. Fe3O4 nanomaterials play an essential role as a functionalized base for Thiourea catalyst, and is greatly used in such reactions in numerous industrial activities [8]. Such iron-based catalyst can release into the aquatic ecosystems and affect the organisms like microalgae as primary producers in these environments [9].
The environmental concentration of iron-based materials has been increased in the biota, including soil, air, and water [10]. Literature have reported the fate, biotransformation, bioaccumulation, and toxicity of such elements in various ecosystems [11, 12]. In particular, aquatic environments are the most important destinations for iron-based compounds through wastewater drainage and landfills in populated areas. Fe3O4, in general, possess various behaviors when enter to the water systems, and undergo some processes, including aggregation, dissolution, redox reaction, and interaction with potential macromolecules [13]. These iron-based products can act as the best iron resource for microalgae, and in highly-contaminated systems lead to eutrophication and algal bloom [14]. Furthermore, iron-based materials in overloaded-conditions cause bioaccumulation and bioconcentration in the aquatic species, and thus, oxidative stress could result in through reactive oxygen species (ROS) production [15]. Aquatic flora and fauna are more sensitive to toxic materials compared with terrestrial species, and related compounds of iron can easily pass the cell membrane and cause severe damages to enzymes, proteins, and DNA integrity of organisms[16]. Lei et al. (2016) examined the toxicity of iron-based NPs to green algae in terms of fate, particle size, and oxidation effects [17]. Chlorella vulgaris (C. vulgaris) was used as an aquatic biological model to investigate iron-based oxidative stress [18]. The toxicity effects of iron oxide nanoparticles were assessed using Zebrafish (Danio rerio) in early life stages, that mortality, hatching delay, and malformation occurred in exposing to these nanomaterials [19]. The toxicity of nanoparticles to aquatic species has been investigated in numerous studies as Behzadi Tayemeh et al. (2020) examined the toxic effects of silver nanoparticles and ions on C. vulgaris biological responses [20]. Sayadi et al. (2020) used blackfish (Capoeta fusca) to study exposure effects of iron oxide nanoparticles and iron salts in causing toxicity, bioaccumulation, and tissue histopathology [21].
Microalga play a vital role in forming the primary production and energy base for all species inhabited in aquatic ecosystems[22, 19]. Such valuable photosynthetic species ranged from microscopic to macro sizes and are the main cause of the food and oxygen production in both freshwater and marine systems. Over the past decade, microalgae have gained significant attention among environmentalists due to their advantageous properties in aquatic ecological balance [23]. Many studies have been conducted concerning the toxic effects of metal contaminants on the biological aspects of microalgae, including growth rate, yield rate, pigment content, reproduction, and nutrient content [24]. Copper nanoparticles caused growth inhibition in Skeletonema costatum in exposing to microplastics [23]. Nutritional characteristics of microalgae examined through exposing to metals and metallic NPs, and pigments, biological macromolecules, and phenolic compounds reduced in the presence of these deleterious materials [26]. For this, microalgae have been always considered as ideal candidates and indicators in biological and ecological monitoring of the aquatic biota [27]. Moreover, phytoplanktons are being consumed as functional foods and reliable supplementary ingredients in human diet [26, 27, 29]. Having the richest source of unsaturated fatty acids, antioxidants, proteins, and pigments, microalgae have played a pivotal role in supplying aquatic-based bioactive compounds for consumers [30].
C. vulgaris is classified as unicellular green microalgae and known as an essential functional food in the world [19]. C. vulgaris is being sold in many countries like Japan, China, Germany, and India as the most important algal nutrient in human health and aquaculture activities[31]. This freshwater microalga, in addition, is known for its anticancer, anti-inflammatory, antioxidant, and antibacterial merits, which is widely used in pharmaceutical and food industries [32]. C. vulgaris has a great potential of biosorption in binding with toxic materials and metals dissolved in the water[33, 34]. Having intracellular metal binding proteins such as metallothioneins, C. vulgaris has been identified as a viable microalga in alleviating the adverse effects of xenobiotics in aquatic ecosystems[35]. In addition, C. vulgaris has been an ideal biological model for primary producers to investigate the toxicological effects of metals and nanoparticles released into the aquatic environments. Therefore, this study intended to investigate the effect of iron-based nanoparticles released from synthesized Thiourea catalyst to C. vulgaris, as a pivotal primary producer in aquatic systems, in terms of bioaccumulation, bioconcentration, cell growth, pigment content, and risk assessment for potential human consumptions.