Interaction between graphene oxide and nitrogen-fixing bacterium Azotobacter chroococcum: Transformation, toxicity and nitrogen fixation

Abstract

Nitrogen is an essential element and 80% of nitrogen fixation from atmosphere is achieved by biological nitrogen fixation. When graphene enters the environment, it would inevitably interact with nitrogen-fixing bacteria and might disturb the nitrogen cycle. Herein, the interaction between graphene oxide (GO) and nitrogen-fixing bacterium Azotobacter chroococcum was studied to reveal the potential impact of graphene materials on biological nitrogen fixation. After incubation with A. chroococcum, GO was separated, washed and characterized by different techniques. The toxicity of GO to A. chroococcum was monitored by colony-forming unit (CFU) counting, growth curve, live/dead staining and ultrastructural observations. The nitrogen fixation activity of A. chroococcum was measured by acetylene reduction assay and the soil nitrogen contents were measured. GO was immediately reduced by cell secretions and kept stable thereafter. GO stimulated A. chroococcum growth at low concentrations and showed inhibitive effect at high concentrations. GO induced cell death and cell wall break at high concentration. The toxicological mechanism was assigned to membrane damage and oxidative stress. In the presence of soil, GO showed similar concentration-dependent but alleviated toxicity, while the soil nitrogen contents slightly increased at high concentrations. Our results collectively indicated that GO was bio-reduced and toxic to nitrogen-fixation bacteria.