Impact of tetracycline on the toxic effects of titanium dioxide (TiO2) nanoparticles towards the freshwater algal species, Scenedesmus obliquus

Titanium dioxide nanoparticles (TiO2 NPs) are the most risk assessed nanoparticles in the aquatic environment due to their increased usage in the various sectors from electronics to consumer products. The natural aquatic system also comprises of numerous toxicants like antibiotics, whose impact on the toxicity of nanoparticles are less assessed. Hence, it is essential to determine the effect of other toxicants on the TiO2 NP toxicity. In the current study, the impact of antibiotic (tetracycline, TC) on the toxic effect of TiO2 NPs was studied on a freshwater alga, Scenedesmus obliquus. The median effective concentrations (EC50) of TiO2 NPs and TC were noted to be 136.88 +/- 2.30 mu M and 0.63 +/- 0.02 mu M, respectively. Based on the EC50 obtained, three different concentrations of TC, such as 0.34, 0.68, and 1.36 mu M have been selected to evaluate their effect on the toxicity of 18.75, 37.5, and 75 mu M of TiO2 NPs. Existence of TC provoked the growth inhibition of TiO2 NPs at their lower concentrations. In contrast, a reduction in the growth inhibition was noted as the concentrations of TC and TiO2 NPs were increased. Abbott modeling confirmed the additive and antagonistic effects noted. The stability profile of TiO2 NPs elucidated the aggregation of NPs with an increase in time. Even though a similar trend has been followed for TiO2 NPs + TC, a significant difference in the aggregation has not been observed in most cases when compared with TiO2 NPs alone. The presence of TC lowered the Ti uptake by the algal cells, which portrayed the dominance of TC in the toxic effect of TiO2 NPs to be either additive or antagonistic. The SEM images of the algal cells upon exposure to TiO2 NPs, TC, and their mixture elucidated the aggregation of algal cells, cellular deformations like compromised cell membrane, and vacuole formation, etc. In addition, the release of algal exudates was also noticed as a protective layer over the cells to counteract the stress. EPS secretion in response to TiO2 NPs along with TC is found to be in corroboration with the toxicity patterns observed.