EFFECT OF MICROPLASTIC AND NANOPLASTIC ON THE TOXICITY OF COPPER IN MICROALGAE

Abstract

Plastics discarded into freshwater and marine environment degrade into smaller microscopic particles called microplastics (<5 mm) and nanoplastics (<1 μm). These microscopic particles have been documented to cause various adverse effects on organisms such as microalgae, zooplankton, mussels, fish and birds. Concurrently, heavy metals pollution is a serious cause for concern. As such, this study was undertaken to investigate the effect of micro- and nanoplastics (MP and NP) on copper (Cu) toxicity on selected freshwater microalgae as they are the primary producers of the food chain. Adverse effects on microalgae could potentially affect other trophic levels. Two freshwater microalgae, Chlorella sp. TJ6-5 and Pseudokirchneriella subcapitata, were exposed to Cu (CuSO4) for 96 h and polystyrene MP (10 μm) and polystyrene NP (500 nm) for 16 days, individually. The EC50 and EC10 at 96 h were calculated for cultures exposed to Cu, MP and NP alone. In combined toxicity experiments, the microalgae were exposed to a combination of MP/NP and Cu at EC10 iii and EC50 for 16 days. The growth response of microalgae in terms of chlorophyll-a (chl-a), and their pigment ratios, reactive oxygen species (ROS) production, lipid peroxidation and morphological changes were investigated. In addition, shading effect of MP/NP on the microalgae, the adsorption of Cu on MP/NP, as well as zeta potential of MP/NP were also investigated. The results showed that Chlorella sp. was more sensitive to Cu (EC10 = 88.5 μM; EC50 = 110.3 μM) compared to Pseudokirchneriella subcapitata (EC10 = 92.4 μM; EC50 = 116.6 μM). Polystyrene MP and NP were non-toxic to Chlorella sp. and Pseudokirchneriella subcapitata. Microplastic (MP) and NP increased the toxicity of Cu at EC50 on Chlorella sp. and Pseudokirchneriella subcapitata. Combined treatment of MP/NP and Cu induced a decrease in chlorophyll-a to chlorophyll-b ratio (chl-a:chl-b) of Chlorella sp., but not in Pseudokirchneriella subcapitata, whereas it increased the carotenoid to chlorophyll-a ratio (car:chl-a) of both microalgae. Exposure to Cu and MP/NP in single and combined treatment induced morphological and ultrastructural changes in the microalgae compared to the control cultures. There was higher production of ROS and lipid peroxidation in Chlorella sp. and Pseudokirchneriella subcapitata when treated with Cu, in the presence or absence of MP and NP on day 4 compared to day 16. Overall, there was no significant shading effect of MP and NP despite the opacity of MP and NP suspensions. The adsorption of Cu to MP and NP was low (0.23 – 14.9%), with most of the Cu ions present in free ionic form (81.6 – 105.8%). Microplastic (MP) and NP were electronegative as determined by zeta-potential, in the presence or absence of Cu. The results of this study showed that although MP and NP were non-toxic to the tested microalgae they may potentially increase the toxicity of Cu in the microalgae. iv The findings may have significant implication as microalgae are likely to be exposed to a mixture of MP/NP and Cu in the environment. This is because in agriculture practice, plastic equipment used may degrade into MP and NP with time, and Cu-containing products such pesticides, herbicides and fungicides are widely used.