Tartrate fermentation with H2 production by a new member of Sporomusaceae enriched from rice paddy soil.

In rice paddies, soil and plant-derived organic matter are degraded anaerobically to methane (CH4), a powerful greenhouse gas. The highest rate of methane emission occurs during the reproductive stage of the plant when mostly dicarboxylic acids are exudated by the roots. The emission of methane at this stage depends largely on the cooperative interaction between dicarboxylic acid-fermenting bacteria and methanogenic archaea in the rhizosphere. The fermentation of tartrate, one of the major acids exudated, has been scarcely explored in rice paddy soils. In this work, we characterized an anaerobic consortium from rice paddy soil composed of four bacterial strains, whose principal member (LT8) can ferment tartrate, producing H2 and acetate. Tartrate fermentation was accelerated by co-inoculation with a hydrogenotrophic methanogen. The assembled genome of LT8 possesses a Na+-dependent oxaloacetate decarboxylase and shows that this bacterium likely invests part of the H2 produced to reduce NAD(P)+ to assimilate C from tartrate. The phylogenetic analysis of the 16S rRNA gene, the genome-based classification as well as the average amino acid identity (AAI) indicated that LT8 belongs to a new genus within the Sporomusaceae family. LT8 shares a few common features with its closest relatives, for which tartrate degradation has not been described. LT8 is limited to a few environments but is more common in rice paddy soils, where it might contribute to methane emissions from root exudates.IMPORTANCEThis is the first report of the metabolic characterization of a new anaerobic bacterium able to degrade tartrate, a compound frequently associated with plants, but rare as a microbial metabolite. Tartrate fermentation by this bacterium can be coupled to methanogenesis in the rice rhizosphere where tartrate is mainly produced at the reproductive stage of the plant, when the maximum methane rate emission occurs. The interaction between secondary fermentative bacteria, such as LT8, and methanogens could represent a fundamental step in exploring mitigation strategies for methane emissions from rice fields. Possible strategies could include controlling the activity of these secondary fermentative bacteria or selecting plants whose exudates are more difficult to ferment.

Genome-centric metagenomic insights into the role of Chloroflexi in anammox, activated sludge and methanogenic reactors.

BACKGROUND: The phylum Chloroflexi is highly abundant in a wide variety of wastewater treatment bioreactors. It has been suggested that they play relevant roles in these ecosystems, particularly in degrading carbon compounds and on structuring flocs or granules. Nevertheless, their function is not yet well understood as most species have not been isolated in axenic cultures. Here we used a metagenomic approach to investigate Chloroflexi diversity and their metabolic potential in three environmentally different bioreactors: a methanogenic full-scale reactor, a full-scale activated sludge reactor and a lab scale anammox reactor. RESULTS: Differential coverage binning approach was used to assemble the genomes of 17 new Chloroflexi species, two of which are proposed as new Candidatus genus. In addition, we recovered the first representative genome belonging to the genus 'Ca. Villigracilis'. Even though samples analyzed were collected from bioreactors operating under different environmental conditions, the assembled genomes share several metabolic features: anaerobic metabolism, fermentative pathways and several genes coding for hydrolytic enzymes. Interestingly, genome analysis from the anammox reactor indicated a putative role of Chloroflexi in nitrogen conversion. Genes related to adhesiveness and exopolysaccharides production were also detected. Complementing sequencing analysis, filamentous morphology was detected by Fluorescent in situ hybridization. CONCLUSION: Our results suggest that Chloroflexi participate in organic matter degradation, nitrogen removal and biofilm aggregation, playing different roles according to the environmental conditions.

Microbiomes and glyphosate biodegradation in edaphic and aquatic environments: recent issues and trends.

Glyphosate (N-(phosphonomethyl)glycine) has emerged as the top-selling herbicide worldwide because of its versatility in controlling annual and perennial weeds and the extensive use of glyphosate-resistant crops. Concerns related to the widespread use of glyphosate and its ubiquitous presence in the environment has led to a large number of studies and reviews, which examined the toxicity and fate of glyphosate and its major metabolite, aminomethylphosphonic acid (AMPA) in the environment. Because the biological breakdown of glyphosate is most likely the main elimination process, the biodegradation of glyphosate has also been the object of abundant experimental work. Importantly, glyphosate biodegradation in aquatic and soil ecosystems is affected not only by the composition and the activity of microbial communities, but also by the physical environment. However, the interplay between microbiomes and glyphosate biodegradation in edaphic and aquatic environments has rarely been considered before. The proposed minireview aims at filling this gap. We summarize the most recent work exploring glyphosate biodegradation in natural aquatic biofilms, the biological, chemical and physical factors and processes playing on the adsorption, transport and biodegradation of glyphosate at different levels of soil organization and under different agricultural managements, and its impact on soil microbial communities.

Glyphosate Biodegradation Potential in Soil Based on Glycine Oxidase Gene (thiO) from Bradyrhizobium.

Despite the intensive use of glyphosate (GP) and its ubiquitous presence in the environment, studies addressing the presence of microbial genes involved in glyphosate degradation in natural conditions are scarce. Based on the agronomical importance of Bradyrhizobium genus and its metabolic versatility, we tested the hypothesis that species or genotypes of Bradyrhizobium could be a proxy for GP degrader potential in soil. A quantitative PCR assay was designed to target a specific region of the glycine oxidase gene (thiO), involved in the oxidation of glyphosate to AMPA, from known sequences of Bradyrhizobium species. The abundance of the thiO gene was determined in response to herbicide application in soils with different GP exposure history both under field and microcosm conditions. The gene coding for RNA polymerase subunitB (rpoB) was used as a reference for the abundance of total Bradyrhizobia. The assay using the designed primers was linear over a very large concentration range of the target and showed high efficiency and specificity. In a field experiment, there was a differential response related to the history of glyphosate use and the native Bradyrhizobium genotypes. In a soil without previous exposure to herbicides, thiO gene increased over time after glyphosate application with most genotypes belonging to the B. jicamae and B. elkanni supergroups. Conversely, in an agricultural soil with more than 10 years of continuous glyphosate application, the abundance of thiO gene decreased and most genotypes belonged to B. japonicum supergroup. In a microcosm assay, the amount of herbicide degraded after a single application was positively correlated to the number of thiO copies in different agricultural soils from the Pampean Region. Our results suggest that Bradyrhizobium species are differently involved in glyphosate degradation, denoting the existence of metabolically versatile microorganisms which can be explored for sustainable agriculture practices. The relationship between the abundance of thiO gene and the GP degraded in soil point to the use of thiO gene as a proxy for GP degradation in soil.

Microbiome network analysis of co-occurrence patterns in anaerobic co-digestion of sewage sludge and food waste.

Addition of food waste (FW) as a co-substrate in anaerobic digesters of wastewater treatment plants is a desirable strategy towards achievement of the potential of wastewater treatment plants to become energy-neutral, diverting at the same time organic waste from landfills. Because substrate type is a driver of variations in phylogenetic structure of digester microbiomes, it is critical to understand how microbial communities respond to changes in substrate composition and concentration. In this work, high throughput sequencing was used to monitor the dynamics of microbiome changes in four parallel laboratory-scale anaerobic digesters treating sewage sludge during acclimation to an increasing amount of food waste. A co-occurrence network was constructed using data from 49 metagenomes sampled over the 161 days of the digesters' operation. More than half of the nodes in the network were clustered in two major modules, i.e. groups of highly interconnected taxa that had much fewer connections with taxa outside the group. The dynamics of co-occurrence networks evidenced shifts that occurred within microbial communities due to the addition of food waste in the co-digestion process. A diverse and reproducible group of hydrolytic and fermentative bacteria, syntrophic bacteria and methanogenic archaea appeared to grow in a concerted fashion to allow stable performance of anaerobic co-digestion at high FW.

Shotgun Metagenomic Profiles Have a High Capacity To Discriminate Samples of Activated Sludge According to Wastewater Type.

UNLABELLED: This study was conducted to investigate whether functions encoded in the metagenome could improve our ability to understand the link between microbial community structures and functions in activated sludge. By analyzing data sets from six industrial and six municipal wastewater treatment plants (WWTPs), covering different configurations, operational conditions, and geographic regions, we found that wastewater influent composition was an overriding factor shaping the metagenomic composition of the activated sludge samples. Community GC content profiles were conserved within treatment plants on a time scale of years and between treatment plants with similar influent wastewater types. Interestingly, GC contents of the represented phyla covaried with the average GC contents of the corresponding WWTP metagenome. This suggests that the factors influencing nucleotide composition act similarly across taxa and thus the variation in nucleotide contents is driven by environmental differences between WWTPs. While taxonomic richness and functional richness were correlated, shotgun metagenomics complemented taxon-based analyses in the task of classifying microbial communities involved in wastewater treatment systems. The observed taxonomic dissimilarity between full-scale WWTPs receiving influent types with varied compositions, as well as the inferred taxonomic and functional assignment of recovered genomes from each metagenome, were consistent with underlying differences in the abundance of distinctive sets of functional categories. These conclusions were robust with respect to plant configuration, operational and environmental conditions, and even differences in laboratory protocols. IMPORTANCE: This work contributes to the elucidation of drivers of microbial community assembly in wastewater treatment systems. Our results are significant because they provide clear evidence that bacterial communities in WWTPs assemble mainly according to influent wastewater characteristics. Differences in bacterial community structures between WWTPs were consistent with differences in the abundance of distinctive sets of functional categories, which were related to the metabolic potential that would be expected according to the source of the wastewater.

Crop monoculture rather than agriculture reduces the spatial turnover of soil bacterial communities at a regional scale.

The goal of this study was to investigate the spatial turnover of soil bacterial communities in response to environmental changes introduced by the practices of soybean monoculture or crop rotations, relative to grassland soils. Amplicon sequencing of the 16S rRNA gene was used to analyse bacterial diversity in producer fields through three successive cropping cycles within one and a half years, across a regional scale of the Argentinean Pampas. Unlike local diversity, which was not significantly affected by land use type, agricultural management had a strong influence on ß-diversity patterns. Distributions of pairwise distances between all soils samples under soybean monoculture had significantly lower ß-diversity and narrower breadth compared with distributions of pairwise distances between soils managed with crop rotation. Interestingly, good agricultural practices had similar degree of ß-diversity as natural grasslands. The higher phylogenetic relatedness of bacterial communities in soils under monoculture across the region was likely determined by the observed loss of endemic species, and affected mostly to phyla with low regional diversity, such as Acidobacteria, Verrucomicrobia and the candidates phyla SPAM and WS3. These results suggest that the implementation of good agricultural practices, including crop rotation, may be critical for the long-term conservation of soil biodiversity.

Expression of stress-related proteins in Sediminibacterium sp. growing under planktonic conditions.

Aggregation is a common trait of bacteria in natural and engineered biological systems. Microbial aggregates, such as flocs, granules, and biofilms, are spatially heterogeneous environments. It is generally observed that by growing under aggregated conditions bacteria respond and adapt to environmental stress better than free-swimming bacteria of the same species. We performed a proteomic analysis of a strain of Sediminibacterium, isolated from activated sludge, which grew planktonically in diluted culture media and in an aggregated form in media containing a high concentration of organic substrate. Auto-aggregation was also observed in the presence of pyruvate in dilute media. Expression of a number of stress-related proteins significantly increased under planktonic growth in comparison to aggregate growth. The upregulated proteins, identified by MALDI-TOF mass spectrometry, were two isoforms of a protein belonging to the universal stress family (UspA), a thioredoxin-disulfide reductase, the Campylobacter jejuni orthologue transcriptional regulator (Cj1172c), and the CocE/NonD hydrolase. We conclude that Sediminibaterium sp. C3 growth is stressed under planktonic conditions and that aggregation induced by pyruvate protects the bacteria against oxidative stress.

Biotic and abiotic factors acting on community assembly in parallel anaerobic digestion systems from a brewery wastewater treatment plant.

Anaerobic digestion is a complex microbial process that mediates the transformation of organic waste into biogas. The performance and stability of anaerobic digesters relies on the structure and function of the microbial community. In this study, we asked whether the deterministic effect of wastewater composition outweighs the effect of reactor configuration on the structure and dynamics of anaerobic digester archaeal and bacterial communities. Biotic and abiotic factors acting on microbial community assembly in two parallel anaerobic digestion systems, an upflow anaerobic sludge blanket digestor (UASB) and a closed digester tank with a solid recycling system (CDSR), from a brewery WWTP were analysed utilizing 16S rDNA and mcrA amplicon sequencing and genome-centric metagenomics. This study confirmed the deterministic effect of the wastewater composition on bacterial community structure, while the archaeal community composition resulted better explained by organic loading rate (ORL) and volatile free acids (VFA). According to the functions assigned to the differentially abundant metagenome-assembled genomes (MAGs) between reactors, CDSR was enriched in genes related to methanol and methylamines methanogenesis, protein degradation, and sulphate and alcohol utilization. Conversely, the UASB reactor was enriched in genes associated with carbohydrate and lipid degradation, as well as amino acid, fatty acid, and propionate fermentation. By comparing interactions derived from the co-occurrence network with predicted metabolic interactions of the prokaryotic communities in both anaerobic digesters, we conclude that the overall community structure is mainly determined by habitat filtering.

MiDAS 5: Global diversity of bacteria and archaea in anaerobic digesters.

Anaerobic digestion of organic waste into methane and carbon dioxide (biogas) is carried out by complex microbial communities. Here, we use full-length 16S rRNA gene sequencing of 285 full-scale anaerobic digesters (ADs) to expand our knowledge about diversity and function of the bacteria and archaea in ADs worldwide. The sequences are processed into full-length 16S rRNA amplicon sequence variants (FL-ASVs) and are used to expand the MiDAS 4 database for bacteria and archaea in wastewater treatment systems, creating MiDAS 5. The expansion of the MiDAS database increases the coverage for bacteria and archaea in ADs worldwide, leading to improved genus- and species-level classification. Using MiDAS 5, we carry out an amplicon-based, global-scale microbial community profiling of the sampled ADs using three common sets of primers targeting different regions of the 16S rRNA gene in bacteria and/or archaea. We reveal how environmental conditions and biogeography shape the AD microbiota. We also identify core and conditionally rare or abundant taxa, encompassing 692 genera and 1013 species. These represent 84-99% and 18-61% of the accumulated read abundance, respectively, across samples depending on the amplicon primers used. Finally, we examine the global diversity of functional groups with known importance for the anaerobic digestion process.

Datathons: fostering equitability in data reuse in ecology.

Approaches to rapidly collecting global biodiversity data are increasingly important, but biodiversity blind spots persist. We organized a three-day Datathon event to improve the openness of local biodiversity data and facilitate data reuse by local researchers. The first Datathon, organized among microbial ecologists in Uruguay and Argentina assembled the largest microbiome dataset in the region to date and formed collaborative consortia for microbiome data synthesis.

Extracellular hydrolytic potential drives microbiome shifts during anaerobic co-digestion of sewage sludge and food waste.

Bacterial community structure and dynamics in anaerobic digesters are primarily influenced by feedstock composition. It is therefore important to unveil microbial traits that explain microbiome variations in response to substrate changes. Here, gene and genome-centric metagenomics were used to examine microbiome dynamics in four laboratory-scale reactors, in which sewage sludge was co-digested with increasing amounts of food waste. A co-occurrence network revealed microbiome shifts in response to changes in substrate composition and concentration. Food waste concentration correlated with extracellular enzymes and metagenome-assembled genomes (MAGs) involved in the degradation of complex carbohydrates commonly found in fruits and plant cell walls as well as with the abundance of hydrolytic MAGs. A key role was attributed to Proteiniphillum for being the only bacteria that encoded the complete pectin degradation pathway. These results suggest that changes of feedstock composition establish new microbial niches for bacteria with the capacity to degrade newly added substrates.

Balance of neutral and deterministic components in the dynamics of activated sludge floc assembly.

Understanding the processes that generate patterns of community structure is a central focus of ecological research. With that aim, we manipulated the structure of bacterial activated sludge to test the influence of the species richness and composition of bacterial communities on the dynamics of activated sludge floc assembly in lab-scale bioreactors. Bacterial community structure was analyzed using denaturing gradient gel electrophoresis of RT-PCR amplified 16S rRNA. Fingerprinting of four parallel reactors, started with the same source communities added in different proportions, converged to patterns that were more similar than expected by chance, suggesting a deterministic selection in floc development. Evidence for neutral dynamics was suggested by the dependence of the rate of replacement of species (bacterial taxa-time relationships) on the number of available species in the source community. Further indication of stochastic dynamics was obtained by the application of the Sloan neutral model for prokaryotes. The fitting of the observed data to the model predictions revealed that the importance of the stochastic component increased with the size of the reservoir of species richness from which the community is drawn. Taken together, the results illustrate how both neutral and deterministic dynamics operate simultaneously in the assembly of the bacterial floc and show that the balance of the two depends on the richness of the source community.

[Impact of the recent advances in the analysis of microbial communities on the control of the wastewater treatment process]. / Impacto de los recientes avances en el análisis de comunidades microbianas sobre el control del proceso de tratamiento de efluentes.

One of the main functions of environmental biotechnology is to address the study of microbial communities that provide essential services to society. Beyond the similarities with industrial and agricultural microbiology, the unique features exhibited by environmental biotechnology, such as process objectives, biomass characteristics and type and mode of feeding (substrates), allow a clear distinction from the other related disciplines. Recent advances in microbial ecology, ecophysiology, genomics and process engineering are herein reviewed to illustrate how the integration of the new knowledge can help overcome the shortcomings of classic microbiological analyses to understand, predict and optimize the performance of wastewater treatment.

Long-run bacteria-phage coexistence dynamics under natural habitat conditions in an environmental biotechnology system.

Bacterial viruses are widespread and abundant across natural and engineered habitats. They influence ecosystem functioning through interactions with their hosts. Laboratory studies of phage-host pairs have advanced our understanding of phenotypic and genetic diversification in bacteria and phages. However, the dynamics of phage-host interactions have been seldom recorded in complex natural environments. We conducted an observational metagenomic study of the dynamics of interaction between Gordonia and their phages using a three-year data series of samples collected from a full-scale wastewater treatment plant. The aim was to obtain a comprehensive picture of the coevolution dynamics in naturally evolving populations at relatively high time resolution. Coevolution was followed by monitoring changes over time in the CRISPR loci of Gordonia metagenome-assembled genome, and reciprocal changes in the viral genome. Genome-wide analysis indicated low strain variability of Gordonia, and almost clonal conservation of the trailer end of the CRISPR loci. Incorporation of newer spacers gave rise to multiple coexisting bacterial populations. The host population carrying a shorter CRISPR locus that contain only ancestral spacers, which has not acquired newer spacers against the coexisting phages, accounted for more than half of the total host abundance in the majority of samples. Phages genome co-evolved by introducing directional changes, with no preference for mutations within the protospacer and PAM regions. Metagenomic reconstruction of time-resolved variants of host and viral genomes revealed how the complexity at the population level has important consequences for bacteria-phage coexistence.

Impacts of switching tillage to no-tillage and vice versa on soil structure, enzyme activities and prokaryotic community profiles in Argentinean semi-arid soils.

The effects of tillage on soil structure, physiology and microbiota structure were studied in a long-term field experiment, with side-to-side plots, established to compare effects of conventional tillage (CT) vs no-till (NT) agriculture. After 27 years, part of the field under CT was switched to NT and vice versa. Soil texture, soil enzymatic profiles and the prokaryotic community structure (16S rRNA genes amplicon sequencing) were analyzed at two soil depths (0-5 and 5-10 cm) in samples taken 6, 18 and 30 months after switching tillage practices. Soil enzymatic activities were higher in NT than CT, and enzymatic profiles responded to the changes much earlier than the overall prokaryotic community structure. Beta diversity measurements of the prokaryotic community indicated that the levels of stratification observed in long-term NT soils were already recovered in the new NT soils 30 months after switching from CT to NT. Bacteria and Archaea OTUs that responded to NT were associated with coarse soil fraction, soil organic carbon and C cycle enzymes, while CT responders were related to fine soil fractions and S cycle enzymes. This study showed the potential of managing the soil prokaryotic community and soil health through changes in agricultural management practices.

A rapid and simple protocol for concentration of SARS-CoV-2 from sewage.

The aim of this study was to set up a simple protocol to concentrate SARS-CoV-2 from sewage, which can be implemented in laboratories with minimal equipment resources. The method avoids the need for extensive purification steps and reduces the concentration of potential inhibitors of RT-qPCR contained in sewage. The concentration method consists of a single step, in which a small volume (40 mL) of sewage sample is incubated with polyaluminum chloride (PAC)(0.00045 N Al3+ final concentration). Virus particles adsorbed to the precipitate are collected by low-speed centrifugation, after which the recovered pellet is resuspended with a saline buffer. PAC-concentrated samples are stable for at least one week at 4 °C. Therefore, they may be sent refrigerated to a diagnosis center for RNA extraction and RT-qPCR for SARS-CoV-2 RNA detection if the lab does not have such capabilities. The PAC concentration method produced an average shift of 4.5-units in quantification cycle (Cq) values compared to non-concentrated samples, indicating a 25-fold increase in detection sensitivity. The lower detection limit corresponded approximately to 100 viral copies per ml. Kappa index indicated substantial agreement between PAC and polyethylene glycol (PEG) precipitation protocols (k = 0.688, CI 0.457-0.919). This low-cost concentration protocol could be useful to aid in the monitoring of community circulation of SARS-CoV-2, especially in low- and middle-income countries, which do not have massive access to support from specialized labs for sewage surveillance.

Nonrandom assembly of bacterial populations in activated sludge flocs.

The aim of this work was to investigate the dynamics of assembly of bacterial populations in activated sludge flocs. We approached this question by following the development of active bacterial populations during floc development in four replicated lab-scale activated sludge reactors, in which solid retention time (SRT) was set at 4 days. The null hypothesis was that the similarities in community composition could be accounted for by the probability that the same organisms occur in more than one replicated reactor. Microscopic imaging showed that the size of flocs in reactors with biomass retention increased during the first few days until a steady-state size was reached. The diversity and community structure of the sludge in all reactors were analyzed during a period of up to ten SRT, using denaturing gradient gel electrophoresis (DGGE) of reverse-transcription polymerase chain reaction-amplified 16S rRNA. High rates of change in DGGE profiles from consecutive sampling points suggested a high level of dynamics in all reactors. This conclusion was confirmed by the application of the Raup and Crick probability-based similarity index (S(RC)) for the comparison of rRNA-based fingerprinting patterns, which indicated that bacterial communities within reactors were not significantly similar after three SRT (0.05 < S(RC) < 0.95) and became significantly dissimilar after five SRT (S(RC) < 0.05). More importantly, significant similarity between replicate reactors was observed at all times analyzed (S(RC) > 0.95). The fact that the patterns between replicates were more reproducible than expected by chance under highly dynamic conditions allowed us to reject the null hypothesis that activated sludge floc communities assemble randomly from the available source pool of bacteria. We suggest that communities progressively recruit from the available pool of bacterial species, each with particular ecological requirements that determine their time of emergence into the community.

Time Series Genome-Centric Analysis Unveils Bacterial Response to Operational Disturbance in Activated Sludge.

Understanding ecosystem response to disturbances and identifying the most critical traits for the maintenance of ecosystem functioning are important goals for microbial community ecology. In this study, we used 16S rRNA amplicon sequencing and metagenomics to investigate the assembly of bacterial populations in a full-scale municipal activated sludge wastewater treatment plant over a period of 3 years, including a 9-month period of disturbance characterized by short-term plant shutdowns. Following the reconstruction of 173 metagenome-assembled genomes, we assessed the functional potential, the number of rRNA gene operons, and the in situ growth rate of microorganisms present throughout the time series. Operational disturbances caused a significant decrease in bacteria with a single copy of the rRNA (rrn) operon. Despite moderate differences in resource availability, replication rates were distributed uniformly throughout time, with no differences between disturbed and stable periods. We suggest that the length of the growth lag phase, rather than the growth rate, is the primary driver of selection under disturbed conditions. Thus, the system could maintain its function in the face of disturbance by recruiting bacteria with the capacity to rapidly resume growth under unsteady operating conditions.IMPORTANCE Disturbance is a key determinant of community assembly and dynamics in natural and engineered ecosystems. Microbiome response to disturbance is thought to be influenced by bacterial growth traits and life history strategies. In this time series observational study, the response to disturbance of microbial communities in a full-scale activated sludge wastewater treatment plant was assessed by computing specific cellular traits of genomes retrieved from metagenomes. It was found that the genomes observed in disturbed periods have more copies of the rRNA operon than genomes observed in stable periods, whereas the in situ mean relative growth rates of bacteria present during stable and disturbed periods were indistinguishable. From these intriguing observations, we infer that the length of the lag phase might be a growth trait that affects the microbial response to disturbance. Further exploration of this hypothesis could contribute to better understanding of the adaptive response of microbiomes to unsteady environmental conditions.

Soil microbial communities and glyphosate decay in soils with different herbicide application history.

This study evaluates the glyphosate dissipation under field conditions in three types of soil, and aims to determine the importance of the following factors in the environmental persistence of herbicide: i) soil bacterial communities, ii) soil physicochemical properties, iii) previous exposure to the herbicide. A soil without previous record of GP application (P0) and two agricultural soils, with 5 and >10years of GP exposure (A5 and A10) were subjected to the application of glyphosate at doses of 3mg·kg-1. The concentration of GP and AMPA was determined over time and the dynamics of soil bacterial communities was evaluated using 16S ARN ribosomal gene amplicon-sequencing. The GP exposure history affected the rate but not the extent of GP biodegradation. The herbicide was degraded rapidly, but P0 soil showed a dissipation rate significantly lower than soils with agricultural history. In P0 soil, a significant increase in the relative abundance of Bacteroidetes was observed in response to herbicide application. More generally, all soils displayed shifts in bacterial community structure, which nevertheless could not be clearly associated to glyphosate dissipation, suggesting the presence of redundant bacteria populations of potential degraders. Yet the application of the herbicide prompted a partial disruption of the bacterial association network of unexposed soil. On the other hand, higher values of linear (Kd) and nonlinear (Kf) sorption coefficient in P0 point to the relevance of cation exchange capacity (CEC), clay and organic matter to the capacity of soil to adsorb the herbicide, suggesting that bioavailability was a key factor for the persistence of GP and AMPA. These results contribute to understand the relationship between bacterial taxa exposed to the herbicide, and the importance of soil properties as predictors of the possible rate of degradation and persistence of glyphosate in soil.