Effect of antibiotics on redox transformations of arsenic and diversity of arsenite-oxidizing bacteria in sediment microbial communities

Effects of water environmental factors and antibiotics on bacterial community in urban landscape lakes

The presence of antibiotics can affect the natural microbial community and exert selective pressure on the environment's microorganisms. This study focused on three types of urban landscape lakes in Xi'an that were closely related to human activities. By combining basic water quality indicators, antibiotic occurrence status, bacterial communities and their potential metabolic functions, Spearman correlation coefficient and redundancy analysis were used to explore the relationship between them, and further explore the impact mechanism of environmental factors and antibiotics on bacterial community structure. The results showed that ofloxacin, erythromycin, and roxithromycin were the main types of antibiotics in the three landscape lakes, with low ecological risks, and there was a clear clustering of antibiotic occurrence. Proteobacteria was the most abundant bacterial phylum, and each lake had its own unique dominant bacteria, which indicates that they are influenced by varying water sources, pollution, and other nearby environments. Statistical analysis showed that pH and nitrogen nutrients were the most critical environmental factors affecting bacterial communities (P<0.01), while tetracyclines and lincomycins were the antibiotics that had a significant impact on bacterial communities (P<0.05). Antibiotics mainly promote defense- and signal transduction-related functions, and inhibit the metabolic activity of bacterial communities. However, the impact of antibiotics on bacterial diversity, community structure, and potential metabolic function in the three urban lakes was less than that of environmental factors. These results help to clarify the mechanism and degree of impact of different interference factors (environmental factors, conventional pollutants, and antibiotics) on bacterial communities in the water environment and are important for the management of urban landscape lake water environments.

Sub-inhibitory concentrations of ampicillin affect microbial Fe(III) oxide reduction

Antibiotics are ubiquitous in the iron-rich environments but their roles in microbial reduction of Fe(III) oxides are still unclear. Using ampicillin and Geobacter soli, this study investigated the underlying mechanism by which antibiotic regulated microbial reduction of Fe(III) oxides. Results showed that sub-minimal inhibitory concentrations (sub-MIC) of ampicillin significantly affected ferrihydrite reduction by G. soli, with a stimulatory effect at 1/64 and 1/32 MIC and an inhibitory effect at 1/8 MIC. Increasing ampicillin concentration resulted in increasing cell length and decreasing bacterial zeta potential that were beneficial for ferrihydrite reduction, and decreasing outer membrane permeability that was unfavorable for ferrihydrite reduction. The respiratory metabolism ability was enhanced by 1/64 and 1/32 MIC ampicillin and reduced by 1/8 MIC ampicillin, which was also responsible for regulation of ferrihydrite reduction by ampicillin. The ferrihydrite reduction showed a positive correlation with the redox activity of extracellular polymeric substances (EPS) which was tied to the cytochrome/polysaccharide ratio and the content of α-helices and β-sheet in EPS. These results suggested that ampicillin regulated microbial Fe(III) oxide reduction through modulating the bacterial morphology, metabolism activity and extracellular electron transfer ability. Our findings provide new insights into the environmental factors regulating biogeochemical cycling of iron.

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.

Ecological Responses of Maize Rhizosphere to Antibiotics Entering the Agricultural System in an Area with High Arsenicals Geological Background

Metal(loid)s can promote the spread and enrichment of antibiotic resistance in the environmental ecosystem through a co-selection effect. Little is known about the ecological effects of entering antibiotics into the environment with long-term metal(loid)s' resistance profiles. Here, cow manure containing oxytetracycline (OTC) or sulfadiazine (SA) at four concentrations (0 (as control), 1, 10, and 100 mg/kg) was loaded to a maize cropping system in an area with high a arsenicals geological background. Results showed that exogenous antibiotics entering significantly changed the nutrient conditions, such as the concentration of nitrate nitrogen, ammonium nitrogen, and available phosphorus in the maize rhizosphere soil, while total arsenic and metals did not display any differences in antibiotic treatments compared with control. Antibiotics exposure significantly influenced nitrate and nitrite reductase activities to reflect the inhibition of denitrification rates but did not affect the soil urease and acid phosphatase activities. OTC treatment also did not change soil dehydrogenase activities, while SA treatment posed promotion effects, showing a tendency to increase with exposure concentration. Both the tested antibiotics (OTC and SA) decreased the concentration of arsenite and arsenate in rhizosphere soil, but the inhibition effects of the former were higher than that of the latter. Moreover, antibiotic treatment impacted arsenite and arsenate levels in maize root tissue, with positive effects on arsenite and negative effects on arsenate. As a result, both OTC and SA treatments significantly increased bioconcentration factors and showed a tendency to first increase and then decrease with increasing concentration. In addition, the treatments decreased translocation capacity of arsenic from roots to shoots and showed a tendency to increase translocation factors with increasing concentration. Microbial communities with arsenic-resistance profiles may also be resistant to antibiotics entering.

An Overview of Antibiotic Resistance and Abiotic Stresses Affecting Antimicrobial Resistance in Agricultural Soils

Excessive use of antibiotics in the healthcare sector and livestock farming has amplified antimicrobial resistance (AMR) as a major environmental threat in recent years. Abiotic stresses, including soil salinity and water pollutants, can affect AMR in soils, which in turn reduces the yield and quality of agricultural products. The objective of this study was to investigate the effects of antibiotic resistance and abiotic stresses on antimicrobial resistance in agricultural soils. A systematic review of the peer-reviewed published literature showed that soil contaminants derived from organic and chemical fertilizers, heavy metals, hydrocarbons, and untreated sewage sludge can significantly develop AMR through increasing the abundance of antibiotic resistance genes (ARGs) and antibiotic-resistant bacteria (ARBs) in agricultural soils. Among effective technologies developed to minimize AMR's negative effects, salinity and heat were found to be more influential in lowering ARGs and subsequently AMR. Several strategies to mitigate AMR in agricultural soils and future directions for research on AMR have been discussed, including integrated control of antibiotic usage and primary sources of ARGs. Knowledge of the factors affecting AMR has the potential to develop effective policies and technologies to minimize its adverse impacts.

Influence of tetracycline on arsenic mobilization and biotransformation in flooded soils

This study examined the effect of tetracycline addition on arsenic (As) mobilization and biotransformation in two contrasting soils (upland soil and paddy soil) under flooded conditions. The soils with added tetracycline (0-50 mg kg^-1) were incubated for 30 days, and soil properties and microbial functional genes over time were quantified. Tetracycline significantly promoted As reduction and As release into porewater in both soils. The enhancement had resulted from an increase in the concentration of dissolved organic carbon and a decrease in soil redox potential. Tetracycline also increased the abundances of As-reducing genes (arsC and arrA) and the relative abundances of As-reducing bacteria Streptomyces, Bacillus, Burkholderia, Clostridium and Rhodococcus, all of which have been found resistant to tetracycline. These genera play a key part in stimulating As reduction in the presence of tetracycline. The study indicated the significance of tetracycline in the biochemical behavior of As in flooded soils and provided new insights into the potential effects of tetracycline on the quality and safety of agricultural products in the future.

Profiles of antibiotic resistance genes in an inland salt-lake Ebinur Lake, Xinjiang, China: The relationship with antibiotics, environmental factors, and microbial communities

Lakes in arid northwestern China, as the main pollutant-holding water bodies in the typical ecologically fragile areas, are facing the unknown risk of exposure to antibiotics and antibiotic resistance genes (ARGs). In this study, five ARGs and one mobile genetic element (intI1) and their relation with antibiotics, microbial communities and water quality were investigated in Ebinur Lake Basin, a typical salt-lake of China. Quantitative PCR analysis indicated that ARGs decreasing order in both surface water and sediment was sul1 >sul2 >tetW>ermB>qnrS, which means sulfonamide resistance genes were the main pollution ARGs. Macrolide antibiotics were the predominant antibiotics in the surface water and sediment in winter, while sulfonamides and quinolones accounted for a high proportion in summer. There was a non-corresponding relationship between ARGs and antibiotics. Moreover, the relationship between ARGs and microbial communities were defined. Sulfonamide resistance genes were carried by a greater diversity of potential host bacteria (76 genera) than other ARGs (9 genera). And their positive correlation with intI1 (p < 0.05) which promotes their migration and provides possibility of their co-occurrence in bacterial populations (e.g., Nitrospira). Bacterial genera were the main driver of ARGs distribution pattern in highly saline lake sediment. Environmental factors like salinity, total nitrogen and organic matter could have a certain influence on the occurrence of ARGs by affecting microorganisms. The results systematically show the distribution and propagation characteristics of ARGs in typical inland salt-lakes in China, and preliminarily explored the relationship between ARGs and antibiotics, resistance genes and microorganisms in lakes in ecologically fragile areas.

A systematic review of antibiotics and antibiotic resistance genes in estuarine and coastal environments

Antibiotics and antibiotic resistance genes (ARGs) are prevalent in estuarine and coastal environments due to substantial terrestrial input, aquaculture effluent, and sewage discharge. In this article, based on peer-reviewed papers, the sources, spatial patterns, driving factors, and environmental implications of antibiotics and ARGs in global estuarine and coastal environments are discussed. Riverine runoff, WWTPs, sewage discharge, and aquaculture, are responsible for the prevalence of antibiotics and ARGs. Geographically, pollution due to antibiotics in low- and middle-income countries is higher than that in high-income countries, and ARGs show remarkable latitudinal variations. The distribution of antibiotics is driven by antibiotic usage and environmental variables (heavy metals, nutrients, organic pollutants, etc.), while ARGs are affected by antibiotics residues, environmental variables, microbial communities, and mobile genetic elements (MGEs). Antibiotics and ARGs alter microbial communities and biogeochemical cycles, as well as pose threats to marine organisms and human health. Our results provide comprehensive insights into the transport and environmental behaviors of antibiotics and ARGs in global estuarine and coastal environments.

Effects of Arsenic and Iron on the Community and Abundance of Arsenite-Oxidizing Bacteria in an Arsenic-Affected Groundwater Aquifer

Arsenic (As) contamination of groundwater aquifers is a global environmental problem, especially in South and Southeast Asian regions, and poses a risk to human health. Arsenite-oxidizing bacteria that transform As(III) to less toxic As(V) can be potentially used as a groundwater As remediation strategy. This study aimed to examine the community and abundance of arsenite-oxidizing bacteria in groundwater with various As concentrations from Rayong Province, Thailand using PCR-cloning-sequencing and quantitative PCR (qPCR) of catalytic subunit of arsenite oxidase gene (aioA). Key factors influencing their community and abundance were also identified. The results demonstrated that arsenite-oxidizing bacteria retrieved from groundwater were phylogenetically related to Betaproteobacteria and Alphaproteobacteria. The aioA gene abundances ranged from 8.6 × 10¹ to 1.1 × 10⁴ copies per ng of genomic DNA, accounting for 0.16-1.37% of the total 16S rRNA bacterial gene copies. Although the abundance of arsenite-oxidizing bacteria in groundwater was low, groundwater with As(III) dominance likely promoted their abundance which possibly played an important role in chemolithoautotrophic oxidation of As(III) to As(V). Fe and As(III) were the major environmental factors influencing the community and abundance of arsenite-oxidizing bacteria. The knowledge gained from this study can be used to further contribute to the development of bioremediation strategies for As removal from groundwater resources.

Effects of antibiotic norfloxacin on the degradation and enantioselectivity of the herbicides in aquatic environment

Antibiotics are currently extensively used in human medicine, animal farming, agriculture and aquaculture, and their residue has become a global environmental problem. However, the effects of antibiotic on other pollutants in aquatic environment are still poorly understood. In this study, the influences of norfloxacin on the residue, degradation and distribution of the herbicides (simazine, atrazine, terbuthylazine, acetochlor and metolachlor) and the enantioselectivity of acetochlor in sediment and water-sediment microcosm system were investigated. Sediment was spiked with norfloxacin and water was contaminated by herbicides to simulate environmental pollution. The amounts of herbicides in water and sediment samples were analyzed within 30 days of cultivation. The results showed that norfloxacin could significantly inhibit the dissipation, lengthen the half-lives and enhance the residues of herbicides in sediment. Take simazine as an example, its half-life significantly increased from 16.1 days to 19.3 days and its residual percentage grew from 24.2% to 30.4% when sediment was contaminated with 5 mg·kg^-1 norfloxacin. However, only acetochlor degradation was significantly inhibited by norfloxacin in water-sediment microcosm and the distribution of the herbicides were not affected. Enantioselective degradation of acetochlor was observed both in control and norfloxacin-treated water-sediment system, with R-acetochlor preferential elimination, suggesting the co-existence of norfloxacin had very limited influence on the enantioselectivity. The findings indicated that co-contamination with norfloxacin could increase the persistence of herbicides in aquatic environment, thus increasing the environmental risks to aquatic organisms.

Microbial community structure in aquifers associated with arsenic: analysis of 16S rRNA and arsenite oxidase genes

The microbiomes of deep and shallow aquifers located in an agricultural area, impacted by an old tin mine, were explored to understand spatial variation in microbial community structures and identify environmental factors influencing microbial distribution patterns through the analysis of 16S rRNA and aioA genes. Although Proteobacteria, Cyanobacteria, Actinobacteria, Patescibacteria, Bacteroidetes, and Epsilonbacteraeota were widespread across the analyzed aquifers, the dominant taxa found in each aquifer were unique. The co-dominance of Burkholderiaceae and Gallionellaceae potentially controlled arsenic immobilization in the aquifers. Analysis of the aioA gene suggested that arsenite-oxidizing bacteria phylogenetically associated with Alpha-, Beta-, and Gamma proteobacteria were present at low abundance (0.85 to 37.13%) and were more prevalent in shallow aquifers and surface water. The concentrations of dissolved oxygen and total phosphorus significantly governed the microbiomes analyzed in this study, while the combination of NO₃ ^--N concentration and oxidation-reduction potential significantly influenced the diversity and abundance of arsenite-oxidizing bacteria in the aquifers. The knowledge of microbial community structures and functions in relation to deep and shallow aquifers is required for further development of sustainable aquifer management.

Microbial reactions and environmental factors affecting the dissolution and release of arsenic in the severely contaminated soils under anaerobic or aerobic conditions

The soils near the abandoned Shimen Realgar Mine are characterized by containing extremely high contents of total and soluble arsenic. To determine the microbial reactions and environmental factors affecting the mobilization and release of arsenic from soils phase into pore water, we collected 24 soil samples from the representative points around the abandoned Shimen Realgar Mine. They contained 8310.84 mg/kg total arsenic and 703.21 mg/kg soluble arsenic in average. The soluble arsenic in the soils shows significant positive and negative correlations with environmental SO₄^2-/TOC/pH/PO₄^3-, and Fe/Mn, respectively. We found that diverse dissimilatory As(V)-respiring prokaryotes (DARPs) and As(III)-oxidizing bacteria (AOB) exist in all the examined soil samples. The activities of DARPs led to 65-1275% increase of soluble As(III) in the examined soils after 21.0 days of anaerobic incubation, and the microbial dissolution and releases of arsenic show significant positive and negative correlations with the environmental pH/TN and NH₄⁺/PO₄^3-, respectively. In comparison, the activities of AOB led to 24-346% inhibition of the dissolved oxygen-mediated dissolution of arsenic in the soils, and the AOB-mediated releases of As(V) show significant positive and negative correlations with the environmental SO₄^2- and pH/NH₄⁺, respectively. The microbial communities of 24 samples contain 54 phyla of bacteria that show extremely high diversities. Total arsenic, TOC, NO₃^- and pH are the key environmental factors that indirectly controlled the mobilization and release of arsenic via influencing the structures of the microbial communities in the soils. This work gained new insights into the mechanism for how microbial communities catalyze the dissolution and releases of arsenic from the soils with extremely high contents of arsenic.

Soil Microbial Communities Involved in Reductive Dissolution of Arsenic from Arsenate-Laden Minerals with Different Carbon Sources

The natural microbial communities involved in arsenic (As) extraction under biostimulated conditions are still unclear. In this study, soil slurry was incubated with arsenate [As(V)]-laden Fe(III) or Al (hydr)oxides with lactate or acetate. After 40 d, dissolved As released from As(V)-laden Fe(III) accounted for 54% of the initial solid-phase As in lactate-amended slurries, while much less As was released from acetate-amended slurries. As was released more rapidly from As(V)-laden Al, but the total release was relatively low (45%). High-throughput Illumina sequencing of 16S rRNA genes revealed that dissimilatory metal(loid) reducers such as Desulfitobacterium became predominant in lactate-amended slurries. Moreover, anaerobic fermenters in the Sporomusaceae family were predominant. Interestingly, a Sporomusaceae bacterial strain isolated from the slurry was capable of releasing As from both As(V)-laden (hydr)oxides in the presence of lactate. The strain first released As as As(V) and subsequently reduced it to As(III) in the aqueous phase. These results suggest that lactate is a suitable carbon source for As extraction by natural microbial communities, and that both dissimilatory metal(loid) reducers and certain anaerobic fermenters play significant roles in As extraction. Microbial reductive dissolution of As may be expected to be a cost-effective restoration technique for As-contaminated soils.

Effects of arsenic on the biofilm formations of arsenite-oxidizing bacteria

Arsenite-oxidizing bacteria (AOB) play a key role in the biogeochemical cycle of arsenic in the environment, and are used for the bioremediation of As contaminated groundwater; however, it is not yet known about how arsenic affects biofilm formations of AOB, and how biofilm formations affect bacterial arsenite-oxidizing activities. To address these issues, we isolated seven novel AOB strains from the arsenic-contaminated soils. They can completely oxidize 1.0 mM As(III) in 22-60 h. Their arsenite oxidase sequences show 43-99% identities to those of other known AOB. Strains Cug1, Cug2, Cug3, Cug4, and Cug6 are able to form biofilms with thickness of 15-95 µm, whereas Cug8 and Cug9 cannot form biofilms. It is interesting to see that arsenite inhibited the biofilm formations of heterotrophic AOB strains, but promoted the biofilm formations of autotrophic strains in a concentration-dependent manner. The arsenite-oxidizing rates of Cug1 and Cug4 biofilms are 31.6% and 27.6% lower than those of their suspension cultures, whereas the biofilm activities of other strains are similar to those of their suspension cultures. The biofilm formation significantly promoted the bacterial resistance to arsenic. This work is the first report on the complex correlations among environmental arsenic, bacterial biofilm formations and bacterial arsenite-oxidizing activities. The data highlight the diverse lifestyle of different AOB under arsenic stress, and provide essential knowledge for the screening of efficient AOB strains used for constructions of bioreactors.

Sedimentary record of antibiotic accumulation in Minnesota Lakes

The widespread detection of antibiotics in the environment is concerning because antibiotics are designed to be effective at small doses. The objective of this work was to quantify the accumulation rates of antibiotics used by humans and animals, spanning several major antibiotic classes (sulfonamides, tetracyclines, fluoroquinolones, and macrolides), in Minnesota lake-sediment cores. Our goal was to determine temporal trends, the major anthropogenic source to these lacustrine systems, and the importance of natural production. A historical record of usage trends for ten human and/or animal-use antibiotics (four sulfonamides, three fluoroquinolones, one macrolide, trimethoprim, and lincomycin) was faithfully captured in the sediment cores. Nine other antibiotics were not detected. Ofloxacin, trimethoprim, sulfapyridine, and sulfamethazine were detected in all of the anthropogenically-impacted studied lakes. Maximum sediment fluxes reached 20.5ngcm^-2yr^-1 (concentration 66.1ng/g) for ofloxacin, 1.2ngcm^-2yr^-1 (1.2ng/g) for trimethoprim, 3.3ngcm^-2yr^-1 (11.3ng/g) for sulfapyridine, and 1.0ngcm^-2yr^-1 (1.6ng/g) for sulfamethazine, respectively. Natural production of lincomycin may have occurred in one lake at fluxes ranging from 0.4 to 1.8ngcm^-2yr^-1 (0.1 to 5.8ng/g). Wastewater effluent appears to be the primary source of antibiotics in the studied lakes, with lesser inputs from agricultural activities.

Effect of extracellular electron shuttles on arsenic-mobilizing activities in soil microbial communities

Microbially mediated arsenate (As(V)) and Fe(III) reduction play important roles in arsenic (As) cycling in nature. Extracellular electron shuttles can impact microbial Fe(III) reduction, yet little is known about their effects on microbial As mobilization in soils. In this study, microcosm experiments consisting of an As-contaminated soil and microbial communities obtained from several pristine soils were conducted, and the effects of electron shuttles on As mobilization were determined. Anthraquinone-2,6-disulfonate (AQDS) and riboflavin (RF) were chosen as common exogenous and biogenic electron shuttles, respectively, and both compounds significantly enhanced reductive dissolution of As and Fe. Accumulation of Fe(II)-bearing minerals was also observed, which may lead to re-immobilization of As after prolonged incubation. Interestingly, Firmicutes-related bacteria became predominant in all microcosms, but their compositions at the lower taxonomic level were different in each microcosm. Putative respiratory As(V) reductase gene (arrA) analysis revealed that bacteria closely related to a Clostridia group, especially those including the genera Desulfitobacterium and Desulfosporosinus, might play significant roles in As mobilization. These results indicate that the natural soil microbial community can use electron shuttles for enhanced mobilization of As; the use of this type of system is potentially advantageous for bioremediation of As-contaminated soils.

Investigation of Arsenotrophic Microbiome in Arsenic-Affected Bangladesh Groundwater

Arsenotrophic bacteria contribute to the nutrient cycling in arsenic (As) affected groundwater. This study employed a culture-independent and -dependent investigation of arsenotrophic microbiomes in As affected groundwater samples collected from Madhabpur, Sonatengra, and Union Porishod in Singair Upazila, Manikganj, Bangladesh. Total As contents, detected by Atomic Absorption Spectrophotometry (AAS) of the samples, were 47 µg/L (Madhabpur, SNGW-1), 53 µg/L (Sonatengra, SNGW-2), and 12 µg/L (Union porishod, SNGW-3), whereas the control well (SNGW-4; depths >150 m) showed As content of 6 µg/L. Denaturing Gradient Gel Electrophoresis (DGGE) analysis of the amplified 16S rRNA gene from As-affected groundwater samples revealed the dominance of aerobic bacteria Pseudomonas within heterogeneous bacterial populations. DGGE of heterotrophic enrichments supplemented with arsenite [As (III)] for 4 weeks showed the dominance of Chryseobacterium, Flavobacterium, and Aquabacterium, whereas the dominant genera in that of autotrophic enrichments were Aeromonas, Acinetobacter, and Pseudomonas. Cultured bacteria retrieved from both autotrophic and heterotrophic enrichments were distinguished into nine genotypes belonging to Chryseobacterium, Acinetobacter, Escherichia, Pseudomonas, Stenotrophomonas, Janibacter, Staphylococcus, and Bacillus. They exhibited varying range of As(III) tolerance from 4 to 27 mM. As(III) transformation potential was confirmed within the isolates with oxidation rate as high as 0.143 mM/h for Pseudomonas sp. Sn 28. The arsenotrophic microbiome specifies their potential role in groundwater As-cycling and their genetic information provide the scientific basis for As-bioremediation.

Bioavailability and risk assessment of arsenic in surface sediments of the Yangtze River estuary

The bioavailability and risk assessment of As were studied in sediments of the Yangtze River estuary (YRE). Results showed that residual fractions dominated the As partition (>85%), which attenuated overall bioavailability. After the residual fraction, As mainly partitioned into the Fe-Mn oxides fraction (3.16-4.22%). Arsenic bound to Fe-Mn oxides was higher in wet seasons. The carbonate fraction was minimal, which may result from the negative state presence of As in sediments. According to the risk assessment code, the YRE was classified as low risk. Additionally, the reduction of As(V) to As(III) may occur due to the reducing condition in wet seasons. Considering As(III) is more toxic and mobile, As bound to the exchangeable and Fe-Mn oxides fractions may have more potential ecological risk. Thus, the speciation and fraction should be both considered on the ecological risk of As in sediments of the YRE.

Antibacterial Compounds-Macrolactin Alters the Soil Bacterial Community and Abundance of the Gene Encoding PKS

Macrolactin produced by many soil microbes has been shown to be an efficient antibacterial agent against many bacterial pathogens. However, studies examining the effect of macrolactin on both the soil bacterial community and the intrinsic bacterial species that harbor genes responsible for the production of this antibiotic have not been conducted so far. In this study, a mixture of macrolactin was isolated from the liquid culture of Bacillus amyloliquefaciens NJN-6, and applied to the soil once a week for four weeks. 16S rRNA Illumina MiSeq sequencing showed that continuous application of macrolactin reduced the α-diversity of the soil bacterial community and thereby changed the relative abundance of microbes at both the phylum and genus level. The relative abundance of Proteobacteria and Firmicutes was significantly increased along with a significant decrease in the relative abundance of Acidobacteria. However, the application of macrolactins had an insignificant effect on the total numbers of bacteria. Further, the native gene responsible for the production of macrolactin, the gene encoding polyketide synthase was reduced in copy number after the application of macrolactin. The results of this study suggested that a bactericide from a microbial source could decrease the diversity of the soil bacterial community and change the bacterial community structure. Moreover, the populations of the intrinsic bacterial species which harbor genes responsible for macrolactin production were inhibited when the external source antibiotic was applied.

Do antibiotics have environmental side-effects? Impact of synthetic antibiotics on biogeochemical processes

Antibiotic use in the early 1900 vastly improved human health but at the same time started an arms race of antibiotic resistance. The widespread use of antibiotics has resulted in ubiquitous trace concentrations of many antibiotics in most environments. Little is known about the impact of these antibiotics on microbial processes or "non-target" organisms. This mini-review summarizes our knowledge of the effect of synthetically produced antibiotics on microorganisms involved in biogeochemical cycling. We found only 31 articles that dealt with the effects of antibiotics on such processes in soil, sediment, or freshwater. We compare the processes, antibiotics, concentration range, source, environment, and experimental approach of these studies. Examining the effects of antibiotics on biogeochemical processes should involve environmentally relevant concentrations (instead of therapeutic), chronic exposure (versus acute), and monitoring of the administered antibiotics. Furthermore, the lack of standardized tests hinders generalizations regarding the effects of antibiotics on biogeochemical processes. We investigated the effects of antibiotics on biogeochemical N cycling, specifically nitrification, denitrification, and anammox. We found that environmentally relevant concentrations of fluoroquinolones and sulfonamides could partially inhibit denitrification. So far, the only documented effects of antibiotic inhibitions were at therapeutic doses on anammox activities. The most studied and inhibited was nitrification (25-100 %) mainly at therapeutic doses and rarely environmentally relevant. We recommend that firm conclusions regarding inhibition of antibiotics at environmentally relevant concentrations remain difficult due to the lack of studies testing low concentrations at chronic exposure. There is thus a need to test the effects of these environmental concentrations on biogeochemical processes to further establish the possible effects on ecosystem functioning.

Microbial community composition of a household sand filter used for arsenic, iron, and manganese removal from groundwater in Vietnam

Household sand filters are used in rural areas of Vietnam to remove As, Fe, and Mn from groundwater for drinking water purposes. Currently, it is unknown what role microbial processes play in mineral oxide formation and As removal during water filtration. We performed most probable number counts to quantify the abundance of physiological groups of microorganisms capable of catalyzing Fe- and Mn-redox transformation processes in a household sand filter. We found up to 10(4) cells g(-1) dry sand of nitrate-reducing Fe(II)-oxidizing bacteria and Fe(III)-reducing bacteria, and no microaerophilic Fe(II)-oxidizing bacteria, but up to 10(6) cells g(-1) dry sand Mn-oxidizing bacteria. 16S rRNA gene amplicon sequencing confirmed MPN counts insofar as only low abundances of known taxa capable of performing Fe- and Mn-redox transformations were detected. Instead the microbial community on the sand filter was dominated by nitrifying microorganisms, e.g. Nitrospira, Nitrosomonadales, and an archaeal OTU affiliated to Candidatus Nitrososphaera. Quantitative PCR for Nitrospira and ammonia monooxygenase genes agreed with DNA sequencing results underlining the numerical importance of nitrifiers in the sand filter. Based on our analysis of the microbial community composition and previous studies on the solid phase chemistry of sand filters we conclude that abiotic Fe(II) oxidation processes prevail over biotic Fe(II) oxidation on the filter. Yet, Mn-oxidizing bacteria play an important role for Mn(II) oxidation and Mn(III/IV) oxide precipitation in a distinct layer of the sand filter. The formation of Mn(III/IV) oxides contributes to abiotic As(III) oxidation and immobilization of As(V) by sorption to Fe(III) (oxyhydr)oxides.

Simultaneous removal of tetracycline hydrochloride and As(III) using poorly-crystalline manganese dioxide

Simultaneous removal of antibiotic tetracycline hydrochloride (TC) and As(III) by poorly-crystalline Mn dioxide was investigated. TC and As(III) can be effectively oxidized and removed by MnO2. High concentrations of TC and As(III) competed with each other for oxidation or adsorption sites on MnO2 and thus affected their removal efficiency. The intermediates and products of TC after reaction with poorly-crystalline manganese dioxide were identified by LC-ESI-MS (liquid chromatography-electrospray ionization-mass spectrometry), and the decomposition pathways of TC by MnO2 were proposed. This study is helpful for understanding the importance of environmental Mn dioxides in the decontamination of combined pollution by organic pollutants and metal(loid)s.

Diversity and abundance of arsenic biotransformation genes in paddy soils from southern China

Microbe-mediated arsenic (As) biotransformation in paddy soils determines the fate of As in soils and its availability to rice plants, yet little is known about the microbial communities involved in As biotransformation. Here, we revealed wide distribution, high diversity, and abundance of arsenite (As(III)) oxidase genes (aioA), respiratory arsenate (As(V)) reductase genes (arrA), As(V) reductase genes (arsC), and As(III) S-adenosylmethionine methyltransferase genes (arsM) in 13 paddy soils collected across Southern China. Sequences grouped with As biotransformation genes are mainly from rice rhizosphere bacteria, such as some Proteobacteria, Gemmatimonadales, and Firmicutes. A significant correlation of gene abundance between arsC and arsM suggests that the two genes coexist well in the microbial As resistance system. Redundancy analysis (RDA) indicated that soil pH, EC, total C, N, As, and Fe, C/N ratio, SO4(2-)-S, NO3(-)-N, and NH4(+)-N were the key factors driving diverse microbial community compositions. This study for the first time provides an overall picture of microbial communities involved in As biotransformation in paddy soils, and considering the wide distribution of paddy fields in the world, it also provides insights into the critical role of paddy fields in the As biogeochemical cycle.

Microbiology of inorganic arsenic: From metabolism to bioremediation

Arsenic (As) contamination of drinking water and soils poses a threat to a large number of people worldwide, especially in Southeast Asia. The predominant forms of As in soils and aquifers are inorganic arsenate [As(V)] and arsenite [As(III)], with the latter being more mobile and toxic. Thus, redox transformations of As are of great importance to predict its fate in the environment, as well as to achieve remediation of As-contaminated water and soils. Although As has been recognized as a toxic element, a wide variety of microorganisms, mainly bacteria, can use it as an electron donor for autotrophic growth or as an electron acceptor for anaerobic respiration. In addition, As detoxification systems in which As is oxidized to the less toxic form or reduced for subsequent excretion are distributed widely in microorganisms. This review describes current development of physiology, biochemistry, and genomics of arsenic-transforming bacteria. Potential application of such bacteria to removal of As from soils and water is also highlighted.