Exposure concentration ratios and biological responses play a critical role in determining the joint toxicity of TiO2 nanoparticles and As(V) to the organism: The case study in marine algae Phaeodactylum tricornutum

Environmental risks of manufactured nanomaterials (MNMs) have been widely investigated while the understanding for joint toxicity mechanism of MNMs with other contaminants is still limited. This limitation may be attributed to variations in the concentration ratios of MNMs and co-existing contaminants in the real environment. To better assess the joint toxicity and clarify its underlying mechanisms, this study exposed Phaeodactylum tricornutum to different concentration combinations of nano-sized titanium dioxide (nTiO2) and As(V) at toxic unit (TU) ratios of 1:4,1:1, and 4:1. The results demonstrated that the joint toxicity modes of nTiO2 and As(V) varied with the TU ratios exhibiting synergism for 1:4, partially addition for 1:1, and antagonism for 4:1. Specifically, at low TU ratio of 1:4, the adsorption of As(V) by nTiO2 together with the subsequent internalization of nTiO2 promoted a significant enrichment of As in algae. Simultaneously, the up-regulation of pst (phosphate transporter) genes in charge of the As(V) transport molecular further exacerbated the enrichment of inorganic As in algae, while the down-regulation of ArsM (arsenite S-adenosylmethionine methyltransferases) genes in charge of the As metabolism inhibited As biotransformation from toxic inorganic to nontoxic organic, causing the aggravated accumulation of toxic inorganic As in algae. At higher TU ratios of 1:1 and 4:1, the accumulation of As decreased in algae due to the higher sedimentation of nTiO2 and thus the lower internalization of As-adsorbed nTiO2, as well as the down-regulation of pst genes restricting the transportation of As(V) into algal cells, which jointly accelerated the As biotransformation from toxic inorganic to nontoxic organic. Our results suggest that more attention should be paid to exposure concentration ratios of MNMs and co-existing contaminants and biological responses including bioavailability, bioaccumulation, biotransformation, which would play a critical role in determining the joint toxicity to the organism.