Structure and function response of bacterial communities towards antibiotic contamination in hyporheic zone sediments.

Bacterial communities are crucial for processing and degrading contaminants in hyporheic zones (HZ). However, the effects of antibiotics on HZ bacterial communities have seldom been addressed. Here, using MiSeq 16S amplicon sequencing technology, the effects of acute exposure to Enrofloxacin, Sulfathiazole, Tetracycline hydrochloride, and Penicillin V potassium on HZ bacterial communities were investigated. Results revealed that HZ sediment communities responded differently to different classes of antibiotics, reflecting the distinct selection stress of antibiotics on HZ bacterial communities. Besides, HZ communities from the locations with more severe antibiotic contamination backgrounds (∼150 µg kg-1) were more resistant towards antibiotic treatment. Compared with small/non-significant changes in HZ community diversity and composition treated with ng L-1∼ug L-1 level antibiotics compared to the control group, treatments with antibiotics over mg L-1 level significantly reduced the diversity and changed the structures of HZ bacterial communities, and enhanced the resistance of the community to antibiotics by enriching antibiotic resistant bacteria. The exposure to mg L-1 level antibiotics also changed community functions by restricting the growth of functional bacteria, such as ammonia oxidizing bacteria (AOB) Nitrosomonas, resulting in ammonia accumulation in sediments. The results implied that at field-relevant concentrations, there was no or minor effect of antibiotics on HZ bacterial community structure and functions, and only those areas with high antibiotic concentrations would have effects.

Effects of chronic exposure of antibiotics on microbial community structure and functions in hyporheic zone sediments.

Microbial communities in hyporheic zones (HZ) provide vital biogeochemical functions such as contaminant degradation for river ecosystems. Antibiotics are contaminants that have been increasingly detected in HZ sediments. In this study, sediments from different HZ locations in a contaminated river, Maozhou river, China were sampled and analyzed using qPCR and high-throughput sequencing to investigate the effect of antibiotic contamination on microbial community structures and functions in HZ sediments. Results indicated that types and concentrations of antibiotics in HZ sediments were heterogeneously distributed that were largely consistent with the distribution of antibiotic sources. Sediments near animal farm and hospital contained higher antibiotic concentrations compared with those from mainstream. The distribution of ARGs was well correlated with antibiotics. Bacterial indicator genera indicating differences between mainstream area and other sampling areas were positively correlated with antibiotics, suggesting the influences of antibiotics on reshaping microbial community structures. PICRUSt revealed positive relationships between antibiotics and predicted functional genes involved in defense, signal transduction, and recombination and repair. This imply the defensive response of microbial communities on antibiotic attack. These results indicated that antibiotic contamination in the watershed posed a potential risk on HZ microbial community structures and functions, which may further threaten river ecosystem functions.

River restoration changes distributions of antibiotics, antibiotic resistance genes, and microbial community.

Although river restoration has been increasingly implemented to restore water quality in ecosystems, its effect on the removal of emerging pollutant antibiotics, and their resultant influence on microbial community structure and functions in river water is still unclear. This study investigated the changes of antibiotics, antibiotic resistant genes (ARGs), microbial communities, and their spatial distributions in a megacity river before and after river restoration. Results indicated that although the restoration activities including riverbed dredging, riverbank hardening, sewage and storm water separation and re-pipelining improved water quality such as by decreasing total phosphorus (TP) content from 4.60 ± 6.38 mg/L in 2018 to 0.98 ± 0.44 mg/L in 2020, the antibiotic concentrations in river water increased. Total antibiotic concentrations in the water samples were higher in 2020 (506.89-6952.50 ng/L) than those in 2018 (137.93-1751.51 ng/L), likely caused by increased usage of antibiotics in 2020 for COVID-19 treatment. The spatial distributions of antibiotics were less varied likely as a result of less retardation and fast mixing during antibiotic transport. The result also found that the abundance of Actinobacteria and Proteobacteria, and their correlations with ARGs increased. The spatial distributions of ARGs and microbial communities became less varied in the river water, consistent with the antibiotic variations before and after river restoration. Physicochemical changes such as decreased TP and dissolved organic carbon content may also be a factor. The results indicated that the current river restoration efforts were not effective in removing antibiotics, and implied that further studies are needed to investigate their subsequent transformation and transport, and to assess their risks to the health of ecosystems.

Transport and retention of Shewanella oneidensis strain MR1 in water-saturated porous media with different grain-surface properties.

Bacterium Shewanella oneidensis strain (MR1), a facultative microorganism that plays critical role in contaminant transformation and degradation, was used as an example to decipher the transport and retention of microorganisms in water-saturated porous media with different grain-surface properties and under different ionic compositions (i.e. Na+, Ca2+, and Mg2+). Dolomite and quartz sands, which contained different surface charge properties, were used as the representative minerals. Dolomite was selected because its surface charges are significantly affected by solution composition. The mobility of MR1 in the dolomite column was lower than that in the quartz column, because the lower energy barrier between MR1 and dolomite than that between MR1 and quartz, resulting in the larger retention of MR1 in the dolomite column. The breakthrough curves were well simulated by the two sites kinetic model with HYDRUS-1D. The maximum concentration of attached bacteria (Smax) were positively correlated to the ionic strength regardless of mineral types. The values of Smax were about 1.1-4.0 times larger in the MR1-dolomite system than that in the MR1-quartz system under different ionic strength conditions. The retention of the MR1 on dolomite surfaces in the presence of divalent cations Ca2+ is significantly higher than that on quartz surfaces primarily due to the larger electrostatic attraction energy between the MR1 and dolomite grains. The findings demonstrate that the porous media with the lower negative charge has the higher capacity for the retention and deposition of MR1, potentially affecting the transport of MR1 and other bacteria in the subsurface.