Uptake and toxicity of engineered copper nanoparticles in Chlamydomonas reinhardtii
Engineered copper nanoparticles (CuNPs) are being used in a wide range in commercial products due to their unique optical, magnetic, and electronic properties. However, the fate, bioavailability, and effects of these nanoparticles are not well understood. In this work particle size distribution, dissolution, toxicity and bioaccumulation of two carbon coated copper nanoparticle batches (denoted as 1st and 2nd CuNP batch) were investigated in short term experiments up to 2 hours. Test organism was the green alga Chlamydomonas reinhardtii. The used exposure medium was MOPS 10-2 M with a constant pH of 7.5. Toxicity and bioaccumulation experiments were also carried out with CuSO4 in order to compare data from CuNPs with CuSO4 based on the ionic Cu concentration. The 1st and 2nd CuNP batches were 7.5 and 6 nm of size, and their carbon fraction was determined to be 35 % and 50 %, respectively. Size distribution experiments with DLS and Nanosight technique showed a fast aggregation and agglomeration, and a high polydispersity of the CuNPs. The measured dissolved Cu after 1 or 2 hours of the 1st and 2nd CuNP batch performed with ultrafiltration technique was up to 2 % and less than 1 %, respectively, with a rather lower relative dissolved concentration at higher CuNP concentration. For toxicity experiments the PSII maximum quantum yield was taken as the toxic endpoint and values were expressed as percentage of an algae control sample. With respect to the total Cu concentration EC50 were about two orders of magnitudes higher for CuNPs than for CuSO4 (i.e. ≈ 100 μM for the 1st CuNP batch, ≈ 200 μM for the 2nd CuNP batch, and ≈ 1 μM for CuSO4 after 1 hour of exposure). However, based on the Cu2+ exposure concentration, a distinct higher effect occurs from CuNPs compared to CuSO4. It is supposed that this higher CuNP vs. CuSO4 toxicity can be explained by a particle induced reduced light availability (i.e. shading) or an enhanced CuNP dissolution due to the presence of algae, as the occurrence of these two mechanisms are supported by several indications made in this work. Intracellular accumulated Cu was measured to be 3·10-7 nmol per cell to an exposure concentration of 1 μM CuSO4, and was equal to the total accumulated Cu. With a higher Cu exposure concentration, the fraction of intracellular accumulated Cu decreased. For CuNPs no reliable accumulation data were obtained as the particles were retained on the filters. It is supposed that uptake of CuNPs will only occur indirectly by dissolution with followed Cu2+ uptake. Mostly effects are assumed to occur from Cu2+, but some effects might also occur from the particles.