Toxic effects of nickel oxide (NiO) nanoparticles on the freshwater alga Pseudokirchneriella subcapitata
Introduction
Nanoparticles (NPs) have exceptional physical and chemical properties compared to their bulk counterparts mainly due to their nanosize (<100 nm) and large specific surface area. The number of consumer products containing NPs has increased and their value is expected to grow from a global market of $1.6 billion in 2016 to $5.3 billion in 2021, representing an annual growth rate of 26.7% from 2016 to 2021 (Research, 2017). Among the different nanomaterials, metal oxide NPs have been subject to significant interest due to their easy synthesis and extensive usage. Nickel oxide (NiO), is used in ceramic materials, chemical catalysts, printing inks, electronic components, biosensors and water purification methods (Ravindhranath and Ramamoorty, 2017; Srivastava et al., 2014; Zhou et al., 2017; Zhu et al., 2012).
The widespread use of NPs has inevitably increased their unintended introduction into aquatic and terrestrial environments. Thus, concerns have been raised regarding the potential adverse effects of NPs on biota and human health (Batley et al., 2013; Beaudrie et al., 2013). In fact, the characteristics that give NPs their exceptional chemical properties can also provide them with intrinsic toxicity. Various negative impacts of NiO NPs on aquatic organisms have been described, including bioluminescence inhibition in the bacterium Vibrio fischeri (Nogueira et al., 2015), growth inhibition in the algae Chlorella vulgaris and Pseudokirchneriella subcapitata (Gong et al., 2011; Nogueira et al., 2015; Oukarroum et al., 2017), toxicity to zebrafish (Danio rerio) (Kovriznych et al., 2014), and oxidative stress in the crustacean Artemia salina (Ates et al., 2016) and the aquatic plant Lemna gibba (Oukarroum et al., 2015). Nevertheless, NiO NPs did not cause a significant increase of mortality in the estuarine amphipod Leptocheirus plumulosus (Hanna et al., 2013).
As primary producers (trophic level 1), microalgae form the basis of the food chain in aquatic systems. This means that any significant change in this trophic level will have a strong impact at higher trophic levels. Among the microalgae, P. subcapitata has been considered to be an important organism, being recommended by international agencies (such as the OECD and U.S. EPA) (OECD, 2011; US-EPA, 2002) as a toxicity bioindicator of freshwater environments due to its ecological relevance, ubiquitous distribution and high sensitivity to a wide range of hazardous substances, including heavy metals and organic compounds (Geis et al., 2000; Rojickova-Padrtova and Marsalek, 1999).
Although there is a general consensus from the scientific community and intergovernmental organizations (such as the OECD) about the urgency and importance of knowledge related to the potential environmental adverse effects of NPs (Hunt et al., 2013), there are no safe guidelines regarding their release into fresh or salt water (Baker et al., 2014). Taking into account that aquatic environments are considered to be an important environmental sink for NPs, their environmental fate and corresponding potential toxic effects deserve further investigation. In fact, the increasing use of NPs requires an improved understanding of their potential impacts on the environment. However, the possible environmental hazards of NiO NPs have been poorly studied, and only a few studies are available on their mechanisms of action.
The present work aimed to investigate the possible adverse effects of NiO NPs on aquatic systems using the alga P. subcapitata as a test organism by means of a mechanism-based approach. To achieve this objective, the impacts of NiO NPs on algal growth, plasma membrane integrity, metabolism (esterase activity and reactive oxygen species accumulation), photosynthesis (pigment production and photosynthetic activity) and morphology, and on the algal cell cycle was evaluated.
Section snippets
Preparation of the NiO NPs stock suspension
Nickel oxide nanoparticles (NiO NPs) with a particle size of <50 nm and a purity of 99.8% (trace metal basis) were obtained from Sigma-Aldrich (catalogue number 637130). The stock suspensions (0.5 g L−1 NiO NPs) were prepared in deionized water. The suspensions were shaken, sonicated for 1 h in an ultrasonic bath (80–160 W, Bandelin Sonorex RK 100) and sterilized under an ultraviolet lamp for 30 min. The NP stock suspensions were stored in the dark at 4 °C (for up to one month) and were shaken
Characterization of the NiO NP suspensions in OECD medium
According to the manufacturer, the primary (nominal) size of individual NiO NPs is <50 nm. This size was confirmed by transmission electron microscopy micrographs in a previous work (Sousa et al., 2018). After suspension (zero time) in OECD medium, the NiO NPs presented a Z-average diameter (mean size), determined using DLS, of ∼800 nm (Table S2), which indicates the almost immediate agglomeration of the NPs. The NP agglomerations increased in size over time and presented a Z-average diameter
Discussion
The characterization of NiO NPs in an aqueous suspension is very important to understand their potential ecotoxicity and thus to identify their potential environmental and toxicological risks. Therefore, knowledge regarding NiO NP agglomeration, surface charge, and stability (dissolution of NPs) is essential for understanding their bioavailability and mobility (Wilkinson, 2013). The characterization of NiO in OECD medium showed that NPs are not stable (low negative zeta potential values) (Table
Conclusions
NiO NPs are unstable in aqueous solution and form agglomerates. In aqueous suspensions, these NPs release Ni2+, which is likely the main contributor to the toxicity observed in P. subcapitata. NiO NP toxicity was assessed using the growth inhibition assay with the freshwater alga P. subcapitata. NiO NPs presented a 72 h-EC50 of 1.6 mg L−1, allowing this NP to be classified as toxic. Algal cells exposed for 72 h to 4 mg L−1 NiO (72 h-EC90) presented a loss of metabolic activity, photosynthetic
Funding source declaration
This work was performed in the framework of the financing by FCT under the scope of the strategic funding of UID/BIO/04469/2013 unit, COMPETE 2020 (POCI-01-0145-FEDER-006684), and BioTecNorte operation (NORTE-01-0145-FEDER-000004) funded by the European Regional Development Fund under the scope of Norte2020—Programa Operacional Regional do Norte and LAQV (UID/ QUI/50006/2013—POCI/01/0145/FEDER/007265) with financial support from FCT/MEC through national funds and co-financed by FEDER, under the
Declaration of interest
The authors declare that they have no conflict of interest.
Acknowledgments
Cátia A. Sousa gratefully acknowledges the doctoral grant (SFRH/BD/101452/2014) from Portuguese Foundation for Science and Technology (FCT).
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