Effects of ZnWO4 nanoparticles on growth, photosynthesis, and biochemical parameters of the green microalga Raphidocelis subcapitata.

Nanoparticles have applications in many sectors in the society. ZnWO4 nanoparticles (ZnWO4-NPs) have potential in the fabrication of sensors, lasers, and batteries, and in environmental remediation. Thus, these NPs may reach aquatic ecosystems. However, we still do not know their effects on aquatic biota and, to our knowledge, this is the first study that evaluates the toxicity of ZnWO4-NPs in a eukaryotic organism. We evaluated the toxicity of ZnWO4-NPs on the green microalga Raphidocelis subcapitata for 96 h, in terms of growth, cell parameters, photosynthesis, and biochemical analysis. Results show that most of Zn was presented in its particulate form, with low amounts of Zn2+, resulting in toxicity at higher levels. The growth was affected from 8.4 mg L-1, with 96h-IC50 of 23.34 mg L-1. The chlorophyll a (Chl a) content increased at 30.2 mg L-1, while the fluorescence of Chl a (FL3-H) decreased at 15.2 mg L-1. We observed increased ROS levels at 44.4 mg L-1. Regarding photosynthesis, the NPs affected the oxygen evolving complex (OEC) and the efficiency of the photosystem II at 22.9 mg L-1. At 44.4 mg L-1 the qP decreased, indicating closure of reaction centers, probably affecting carbon assimilation, which explains the decay of carbohydrates. There was a decrease of qN (non-regulated energy dissipation, not used in photosynthesis), NPQ (regulated energy dissipation) and Y(NPQ) (regulated energy dissipation via heat), indicating damage to the photoprotection system; and an increase in Y(NO), which is the non-regulated energy dissipation via heat and fluorescence. The results showed that ZnWO4-NPs can affect the growth and physiological and biochemical parameters of the chlorophycean R. subcapitata. Microalgae are the base of aquatic food chains, the toxicity of emerging contaminants on microalgae can affect entire ecosystems. Therefore, our study can provide some help for better protection of aquatic ecosystems.

Toxicity of α-Ag2WO4 microcrystals to freshwater microalga Raphidocelis subcapitata at cellular and population levels.

Silver-based materials have microbicidal action, photocatalytic activity and electronic properties. The increase in manufacturing and consumption of these compounds, given their wide functionality and application, is a source of contamination to freshwater ecosystems and causes toxicity to aquatic biota. Therefore, for the first time, we evaluated the toxicity of the silver tungstate (α-Ag2WO4), in different morphologies (cube and rod), for the microalga Raphidocelis subcapitata. To investigate the toxicity, we evaluated the growth rate, cell complexity and size, reactive oxygen species (ROS) production and chlorophyll a (Chl a) fluorescence. The α-Ag2WO4 - R (rod) was 1.7 times more toxic than α-Ag2WO4-C (cube), with IC10 and IC50 values of, respectively, 8.68 ± 0.91 µg L-1 and 13.72 ± 1.48 µg L-1 for α-Ag2WO4 - R and 18.60 ± 1.61 µg L-1 and 23.47 ± 1.16 µg L-1 for α-Ag2WO4-C. The release of silver ions was quantified and indicated that the silver ions dissolution from the α-Ag2WO4 - R ranged from 34 to 71%, while the Ag ions from the α-Ag2WO4-C varied from 35 to 97%. The α-Ag2WO4-C induced, after 24 h exposure, the increase of ROS at the lowest concentrations (8.81 and 19.32 µg L-1), whereas the α-Ag2WO4 - R significantly induced ROS production at 96 h at the highest concentration (31.76 µg L-1). Both microcrystal shapes significantly altered the cellular complexity and decreased the Chl a fluorescence at all tested concentrations. We conclude that the different morphologies of α-Ag2WO4 negatively affect the microalga and are important sources of silver ions leading to harmful consequences to the aquatic ecosystem.

Using multiple endpoints to assess the toxicity of cadmium and cobalt for chlorophycean Raphidocelis subcapitata.

Metals may cause damage to the biota of contaminated environments. Moreover, using multiple endpoints in ecotoxicological studies is useful to better elucidate the mechanisms of toxicity of these compounds. Therefore, this study aimed to evaluate the effects of cadmium (Cd) and cobalt (Co) on growth, biochemical and photosynthetic parameters of the microalgae Raphidocelis subcapitata, through quantification of lipid classes composition, chlorophyll a (Chl a) content, maximum (ΦM) and effective (Φ'M) quantum yields and efficiency of the oxygen-evolving complex (OEC). Both metals affected the algal population growth, with an IC50-96h of 0.67 and 1.53 µM of Cd and Co, respectively. Moreover, the metals led to an increase in the total lipid content and reduced efficiency of OEC and ΦM. Cell density was the most sensitive endpoint to detect Cd toxicity after 96 h of treatment. Regarding Co, the photosynthetic parameters were the most affected and the total lipid content was the most sensitive endpoint as it was altered by the exposure to this metal in all concentrations. Cd led to increased contents of the lipid class wax esters (0.89 µM) and phospholipids (PL - at 0.89 and 1.11 µM) and decreased values of triglycerides (at 0.22 µM) and acetone-mobile polar lipids (AMPL - at 0.44 and 1.11 µM). The percentage of free fatty acids (FFA) and PL of microalgae exposed to Co increased, whereas AMPL decreased in all concentrations tested. We were able to detect differences between the toxicity mechanisms of each metal, especially how Co interferes in the microalgae at a biochemical level. Furthermore, to the best of our knowledge, this is the first study reporting Co effects in lipid classes of a freshwater Chlorophyceae. The damage caused by Cd and Co may reach higher trophic levels, causing potential damage to the aquatic communities as microalgae are primary producers and the base of the food chain.