Silver nanoparticles (AgNPs) produced, transported and applicated by humans are ultimately released into the environment and are thus potentially toxic to environmental organisms (Dale et al., 2015). Besides, in natural environment, Ag+ can be reduced into nano silver by dehydrogenase or reducing sugar in microorganisms or plants (Mo et al., 2020). AgNPs is highly chemically active and can easily interact with environmental medium (e.g., physical, chemical and biological reaction), which leads to the migration and transformation of AgNPs, eventually affecting their toxic effects (Wang et al. 2020). Although much work was gone into focusing on the biological effects of AgNPs, which systematically revealed action mechanisms and dose-effect relationship of AgNPs with single morphology (e.g., spherical, rodlike, and hexagonal) to various organisms, the shape-dependent toxicity was less explored (Ji et al. 2010; Oukarroum et al. 2012). Noticeably, there is an internal relationship between the structure of pollution and their toxic effect, that is, the structure-activity relationship. Different morphologies of nanoparticles possess different crystal planes and quantum structures, leading to the unique physical and chemical properties (Shen et al. 2015). Several studies have evaluated the ecotoxicity of AgNPs with various morphologies to bacterial, algae and fish. Mayer et al. demonstrated that AgNPs morphologies had no significant effect on the cytotoxicity (2016), while Babak et al. compared the toxicities of silver nanoplates (AgPLs) and silver nanospheres (AgNSs) and found that AgNSs were highly toxic to Staphylococcus aureus and Escherichia coli (E. coli), yet less toxic than AgPLs (2012). In contrast, some studies reported that the toxicities of AgPLs and AgNSs in P. aeruginosa and E. coli were opposite (Muhammad et al. 2016). Therefore, there is a dispute related to shape-dependent toxicity of AgNPs. Thus, it is of great importance to explore the toxic differences of AgNPs with various morphologies on organisms in ecosystem, providing direct evidence for the risk assessment of AgNPs with different structure.
As a main primary producer in aquatic systems, algae play a crucial role in the environmental homeostasis of water body. The toxicity of AgNPs to microalgae are known to be related to photosynthetic efficiency inhibition, reactive oxygen species (ROS) generation, metabolism interference, and organelles damage (He et al. 2017; Dorobantu et al. 2015). Whereas, the research concerning the toxic effects of AgNPs with multi-morphologies on algae were still limited. The investigation of structure-activity relationship of between AgNPs and algae is of great scientific value to the comprehensive and in-depth understanding of AgNPs biological toxicity in aquatic environment and the evaluation of safety of water body.
Herein, we compared the toxicities of three AgNPs of various morphologies (AgNSs, AgNCs, and AgPLs) on an alga, Chlorella vulgaris. We determined the growth condition, the chlorophyll-a content, antioxidant enzyme activity, lipid peroxidation degree, and cell apoptosis. The experimental results of study could provide valuable information about the toxicity of AgNPs with various microstructure morphologies to aquatic organisms, which might be useful for assessing ecological risk of AgNPs.