Time-Dependent Effect of Graphene on the Microbial Activity of the Soil Under Single and Repeated Exposures

Graphene (GR) has huge industrial and biomedical potential, and its adverse effect on soil microorganisms has been evaluated in ecotoxicological studies. These studies focused on single exposure to GR, but repeated exposures with low concentration are more likely to occur under actual exposure scenario. In this study, we compared the impact of single and repeated exposures of GR on structure, abundance and function of soil bacterial community based on soil enzyme activity and high-throughput sequencing. The results displayed that after 4 days of incubation the activities of urease and fluorescein diacetate esterase increased by approximately 12% and 4% upon repeated exposure to GR (biweekly 15 mg doses GR for the two-exposure experiment, 10 mg doses GR for the three-exposure experiment, resulting in the same final concentration of 300 mg/kg), respectively. Instead, the activities of urease and fluorescein diacetate esterase decreased by 13% and 6% upon single exposure (30 mg for the single-exposure experiment), respectively. As the incubation time increased to 60 days, these activities showed little difference. The alpha diversity of soil bacterial community under repeated exposures increased more than that under single exposure, demonstrating that a low concentration chronic exposure to GR increase the diversity within a specific bacterial community. The PCoA and sample level clustering tree showed single exposure to GR after 4 days alter the soil bacterial community to some extent. During the entire incubation process, no matter what kind of exposure scenarios to GR, the majority of bacterial phylotypes at the phylum level had no great change except for Proteobacteria and Actinobacteria according to the relative abundance of phylotypes. These results elucidated the repeated exposures to GR increased the metabolic activity and diversity of the soil microbial community as compared with single exposure. This study can provide a new perspective on the impact of carbon nanomaterials on soil microbial community.