Nano-Titanium Dioxide Exposure Impacts Nitrogen Metabolism Pathways in Cyanobacteria

Abstract

The wide and increasing use of nano-titanium dioxide (nTiO2) has led to its release in the environment that will likely impact aquatic eco-relevant biota. In this study, we investigated the impact of sublethal concentrations of nTiO2 on the nitrogen (N) metabolism of the primary producer, nitrogen-fixing cyanobacteria Anabaena PCC 7120, using transcriptional-level information of biomarker genes involved in global N regulation, N fixation-, N assimilation-, and N storage-specific pathways. The results showed that both the circadian patterns of cyanobacterial metabolism and nTiO2 intrinsic properties distinctively governed the toxicity responses during light and dark exposure to nTiO2. During illuminated conditions, the majority of genes linked to cellular nitrogen status exhibited a clear upregulation in the dose range of 6–60 mg/L·h, whereas overall gene downregulation was mainly observed at the end of dark cycle, characterized by low cell metabolism and energy (ATP) levels. The nTiO2 dose-dependent production of intracellular metabolites (amino acids) involved in both the GS-GOGAT pathway of N assimilation and the intracellular N storage pathway suggests that pathways involving amino acid biosynthesis or degradation might be activated or repressed. This, possibly, contributes to the increase of newly synthesized proteins needed for detoxification in response to the cellular stresses induced by nTiO2 treatment. These findings suggest that under environmental perturbations generated by nTiO2, cyanobacteria are likely to modify their intracellular carbon and nitrogen balance, thus impact, at a larger scale, ecological trophic interactions and food web dynamics within complex ecological systems.