Size Dependent and Reactive Oxygen Species Related Nanosilver Toxicity to Nitrifying Bacteria

TLDR

Nitrifying bacteria grow slowly and are highly sensitive to environmental perturbations, making them vulnerable to toxicants, while nanoparticles are of concern because their small size and high catalytic properties can affect ecosystems. This study aimed to determine how Ag nanoparticle size influences inhibition of nitrifying bacteria and to assess the link between this inhibition and reactive oxygen species production. Ag nanoparticles (9–21 nm) were synthesized by adjusting BH4⁻/Ag⁺ ratios, and their ROS generation was quantified with and without bacteria, while inhibition was inferred from specific oxygen uptake rates measured by respirometry. Inhibition of nitrification correlated with the fraction of Ag nanoparticles smaller than 5 nm and with intracellular ROS levels, but not with photocatalytic ROS, indicating that very small particles are most toxic and that additional factors beyond ROS influence nanosilver toxicity.

Abstract

The intrinsic slow growth of nitrifying bacteria and their high sensitivity to environmental perturbations often result in cell growth inhibition by toxicants. Nanoparticles are of great concern to the environment because of their small size and high catalytic properties. This work sought to determine size-dependent inhibition by Ag nanoparticles and evaluate the relationship between the inhibition and reactive oxygen species (ROS). Nanoparticles with an average size range of 9−21 nm were synthesized by varying the molar ratios of BH4− /Ag+in the solution. The resulting ROS generation was quantified in the presence and absence of the bacteria while the degree of inhibition was inferred from specific oxygen uptake rate measurements, determined by extant respirometry. By examining the correlation between nanoparticle size distribution, photocatalytic ROS generation, intracellular ROS accumulation, and nitrification inhibition, we observed that inhibition to nitrifying organisms correlated with the fraction of Ag nanoparticles less than 5 nm in the suspension. It appeared that these size nanoparticles could be more toxic to bacteria than any other fractions of nanoparticles or their counterpart bulk species. Furthermore, inhibition by Ag nanoparticles as well as other forms of silver (AgCl colloid and Ag+ ion) correlated well with the intracellular ROS concentrations, but not with the photocatalytic ROS fractions. The ROS correlations were different for the different forms of silver, indicating that factors other than ROS are also important in determining nanosilver toxicity.