ReviewMicroalgal ecotoxicity of nanoparticles: An updated review
Introduction
In recent decades, achievements in nanotechnology have drawn the attention of scientists. Almost all aspects of high-tech products nowadays have utilized nanomaterials, such as electronics, cosmetics, environmental treatments, food, textiles, and many other industrial sectors (Kook et al., 2019; Vance et al., 2015). Following that is the dramatic increase in the production of nanomaterials entailing their potential release via product degradation and making the aquatic environments a reservoir of NP pollution (Lead et al., 2018; Shevlin et al., 2018; Z. Y. Wang et al., 2016a, 2016b, 2016c). Nanomaterials, with dimensions at less than 100 nm, are capable of being released into the environment at an unmanageable rate. Accordingly, this small size, coupled with its high operability, the low mass weight in a complex background makes accurate detection and estimation of aquatic NPs difficult (Zhang et al., 2019). Not only that, just by short exposure time, the NPs can also interact a lot with aquatic organisms and more dangerous to be absorbed by them (Espinasse et al., 2018). These NP-consumed organisms can lead to human exposure if ingested (Shevlin et al., 2018). Therefore, it creates scientific concerns on the toxic effects of these nanomaterials themselves. In aquatic environments, NPs are capable of going through various physio-chemical shifts such as aggregation/dissolution, accumulation/dispersion or penetration/transformation/reactions (Shevlin et al., 2018). These are significantly influenced by not only the inherent properties of NPs such as size, shape, concentration, or their coating layers, but also other environmental exposure factors such as dissolved organic matter (i.e. NOM), pH values, or ionic strength (C. Taylor et al., 2016a, 2016b; Gao et al., 2018; Lei et al., 2016; Ma et al., 2015; Nolte et al., 2017; Sendra et al., 2017a; Zamani et al., 2014).
Algae are one of the basic single-celled organisms that underlie the food chain in aquatic environments and participate in the aquatic nutrient cycle. They have been used as a model to evaluate the ecological impact of NPs which exhibits high susceptibility through low values of 50% effective concentration (EC50) (Oukarroum et al., 2017; Schiavo et al., 2016; Y. X. Wang et al., 2016; Zhao et al., 2016). Therefore, the ecotoxicity of these NPs to algae has also attracted the attention of scientists (Bundschuh et al., 2016; Bhuvaneshwari et al., 2018; Dunphy Guzmán et al., 2006; Tang et al., 2018). For different types of NPs, they have different toxicity behaviors. They are identified to cause growth inhibition and further algal deaths at the NP-dependent concentration and by their inherent properties (Li et al., 2015).
The physio-chemical stresses of NPs have been shown different effects on algal cells, forcing them to respond to these stresses. Accordingly, algae exhibit their defense behaviors via the stimulation of the antioxidative system to remove NP-induced reactive oxygen species (ROS) (Pikula et al., 2019; Qian et al., 2016; Rezayian et al., 2019). In addition, they use the intracellular metabolism, as well as forming a protective coating by secreting extracellular polymeric substances (EPS), to reduce the amount of NP intrusion (Chiu et al., 2017; Gao et al., 2018; K. Zhou et al., 2016). Currently, understanding the toxic mechanisms of NPs on algae is essential to have an effective strategy for a safe use of NPs. Some review papers have mainly summarized the NP behaviors in the aquatic environment (Bundschuh et al., 2016) and the interactions between NPs and the organic matter (Goswami et al., 2017; Z. Y. Wang et al., 2016), or other organisms (plants, fungi and algae) (Navarro et al., 2008). The algal toxicity of NPs was highlighted in several reviews (Libralato et al., 2017; Minetto et al., 2016). The activities of the antioxidant systems of algae were also considered (Pikula et al., 2019; Rezayian et al., 2019). Methods of algal nano-toxicology assay were mentioned (Moreno-Garrido et al., 2015; Skjolding et al., 2016). This review attempts to provide the basic mechanisms of algal toxicity of NPs with a systematic investigation. Accordingly, this study emphasizes the influence of some affecting factors on the algal toxicity of NPs (i.e. particle properties, solubility, and EPS layer) and provides an understanding of extra- and intracellular damages of algal cells. In addition, an overview of the toxic effects of some commonly-used NPs on algae is presented. On this basis, the perspectives are discussed and suggested for further studies about the applications of NPs in controlling algal growths.
Our review collects nanomaterial-applied studies according to a flow of two parts, from the viewpoint of NPs to the viewpoint of microalgae. As a result, the review is structured in order of the individual sections as follows: Section 2.1 deals with a brief and essential discussion of the factors that contribute to the toxicity level of NPs (such as particle properties and extracellular polymeric substances formation). Section 2.2 will give an updated summary of the toxicity data gathered from literature and systematize it, as to specific aspects (such as NPs types, NPs doses, algal species, or endpoints of effects). A brief overview of the NP toxicity that causes changes in algal cells is discussed (Section 2.3). Section 3 will discuss perspectives and future directions for algal toxicity testing of NPs.
Section snippets
Effects of particle properties on NPs toxicity
The algal toxicity of NPs has been carefully evaluated over the years, based on the relationship between ecotoxicity and the properties of NPs. However, identifying the particle properties that affect algal toxicity of NPs is also a challenge (von Moos and Slaveykova, 2014; Xia et al., 2014). The NP's inherent properties, such as particle size, physical impact, aggregation/dispersion state, ions-release capacity, concentration, surface chemistry, and photo-chemical properties, affect the
Conclusions and perspectives
The production of many entirely new nanomaterials is exploding energetically. Therefore, their appearances in the environment accompanied by ecotoxicity are indispensable problems. The current urgent issues in evaluating the environmental impacts of these materials are their movement, transformation, and bioavailability. With a broad range of existing studies, the level of damage of NPs is typically assessed in terms of direct interaction with microalgae, their surface characteristics, the
Author contributions
M.K.N.: Conceptualization, Methodology, Validation, Investigation, Formal analysis, and Writing - original draft. M.K.N.; J.-Y.M.; Y.-C.L.: Writing - review & editing. M.K.N.: Visualization. Y.-C.L.: Supervision. M.K.N.; J.-Y.M.; Y.-C.L.: Final approval of manuscript.
Statement of informed consent, human/animal rights
No conflicts, informed consent, human or animal rights applicable.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2017R1D1A1A09000642).
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