MiDAS 2.0: an ecosystem-specific taxonomy and online database for the organisms of wastewater treatment systems expanded for anaerobic digester groups

Eat seldom is better than eat frequently: Pharmaceuticals degradation kinetics, enantiomeric profiling and microorganisms in moving bed biofilm reactors are affected by feast famine cycle times

Feast-famine (FF) regimes improved the removal of recalcitrant pharmaceuticals in moving bed biofilm reactors (MBBRs), but the optimal FF cycle remained unresolved. The effects of FF cycle time on the removal of bulk substrates (organic carbon and nitrogen) and trace pharmaceuticals by MBBR are systematically evaluated in this study. The feast to famine ratio was fixed to 1:2 to keep the same loading rate, but the time for the FF cycles varied from 18 h to 288 h. The MBBR adapted to the longest FF cycle time (288 h equaling 48 × HRT) resulted in significantly higher degradation rates (up to +183%) for 12 out of 28 pharmaceuticals than a continuously fed (non-FF) reactor. However, other FF cycle times (18, 36, 72 and 144 h) only showed a significant up-regulation for 2-3 pharmaceuticals compared to the non-FF reactor. Enantioselective degradation of metoprolol and propranolol occurred in the second phase of a two phase degradation, which was different for the longer FF cycle time. N-oxidation and N-demethylation pathways of tramadol and venlafaxine differed across the FF cycle time suggestin the FF cycle time varied the predominant transformation pathways of pharmaceuticals. The abundance of bacteria in the biofilms varied considerably between different FF cycle times, which possibly caused the biofilm to remove more recalcitrant bulk organic C and pharmaceuticals under long cycle times.

Amplicon-guided isolation and cultivation of previously uncultured microbial species from activated sludge

IMPORTANCE

Biological wastewater treatment relies on complex microbial communities that assimilate nutrients and break down pollutants in the wastewater. Knowledge about the physiology and metabolism of bacteria in wastewater treatment plants (WWTPs) may therefore be used to improve the efficacy and economy of wastewater treatment. Our current knowledge is largely based on 16S rRNA gene amplicon profiling, fluorescence in situ hybridization studies, and predictions based on metagenome-assembled genomes. Bacterial isolates are often required to validate genome-based predictions as they allow researchers to analyze a specific species without interference from other bacteria and with simple bulk measurements. Unfortunately, there are currently very few pure cultures representing the microbes commonly found in WWTPs. To address this, we introduce an isolation strategy that takes advantage of state-of-the-art microbial profiling techniques to uncover suitable growth conditions for key WWTP microbes. We furthermore demonstrate that this information can be used to isolate key organisms representing global WWTPs.

A comprehensive overview of the Chloroflexota community in wastewater treatment plants worldwide

IMPORTANCE

Chloroflexota are often abundant members of the biomass in wastewater treatment plants (WWTPs) worldwide, typically with a filamentous morphology, forming the backbones of the activated sludge floc. However, their overgrowth can often cause operational issues connected to poor settling or foaming, impairing effluent quality and increasing operational costs. Despite their importance, few Chloroflexota genera have been characterized so far. Here, we present a comprehensive overview of Chloroflexota abundant in WWTPs worldwide and an in-depth characterization of their morphology, phylogeny, and ecophysiology, obtaining a broad understanding of their ecological role in activated sludge.

Functional Redundancy Secures Resilience of Chain Elongation Communities upon pH Shifts in Closed Bioreactor Ecosystems

For anaerobic mixed cultures performing microbial chain elongation, it is unclear how pH alterations affect the abundance of key players, microbial interactions, and community functioning in terms of medium-chain carboxylate yields. We explored pH effects on mixed cultures enriched in continuous anaerobic bioreactors representing closed model ecosystems. Gradual pH increase from 5.5 to 6.5 induced dramatic shifts in community composition, whereas product range and yields returned to previous states after transient fluctuations. To understand community responses to pH perturbations over long-term reactor operation, we applied Aitchison PCA clustering, linear mixed-effects models, and random forest classification on 16S rRNA gene amplicon sequencing and process data. Different pH preferences of two key chain elongation species─one Clostridium IV species related to Ruminococcaceae bacterium CPB6 and one Clostridium sensu stricto species related to Clostridium luticellarii─were determined. Network analysis revealed positive correlations of Clostridium IV with lactic acid bacteria, which switched from Olsenella to Lactobacillus along the pH increase, illustrating the plasticity of the food web in chain elongation communities. Despite long-term cultivation in closed systems over the pH shift experiment, the communities retained functional redundancy in fermentation pathways, reflected by the emergence of rare species and concomitant recovery of chain elongation functions.

Evolving understanding of rumen methanogen ecophysiology

Production of methane by methanogenic archaea, or methanogens, in the rumen of ruminants is a thermodynamic necessity for microbial conversion of feed to volatile fatty acids, which are essential nutrients for the animals. On the other hand, methane is a greenhouse gas and its production causes energy loss for the animal. Accordingly, there are ongoing efforts toward developing effective strategies for mitigating methane emissions from ruminant livestock that require a detailed understanding of the diversity and ecophysiology of rumen methanogens. Rumen methanogens evolved from free-living autotrophic ancestors through genome streamlining involving gene loss and acquisition. The process yielded an oligotrophic lifestyle, and metabolically efficient and ecologically adapted descendants. This specialization poses serious challenges to the efforts of obtaining axenic cultures of rumen methanogens, and consequently, the information on their physiological properties remains in most part inferred from those of their non-rumen representatives. This review presents the current knowledge of rumen methanogens and their metabolic contributions to enteric methane production. It also identifies the respective critical gaps that need to be filled for aiding the efforts to mitigate methane emission from livestock operations and at the same time increasing the productivity in this critical agriculture sector.

Spatiotemporal succession of phosphorous accumulating biofilms during the first year of establishment

Many wastewater treatment plants are dependent on the utilization of microorganisms in biofilms. Our knowledge about the establishment of these biofilms is limited, particular with respect to biofilms involved in enhanced biological phosphorus removal (EBPR). These biofilms rely on polyphosphate-accumulating organisms (PAOs), requiring alternating oxic and anaerobic conditions for phosphorous uptake. This challenge has been solved using the Hias process, which combines moving-bed biofilm-reactor (MBBR) technology with physical transfer of biofilm-carriers from oxic to anaerobic zones. We combined biofilm fractionation with temporal analyses to unveil the establishment in the Hias process. A stable phosphorous removal efficiency of >95% was reached within 16 weeks of operation. Phosphorus removal, however, was not correlated with the establishment of known PAOs. The biofilms seemed associated with an outer microbiota layer with rapid turnover and an inner layer with a slow expansion. The inner layer showed an overrepresentation of known PAOs. In conclusion, our spatiotemporal analyses of phosphorous accumulating biofilm establishment lead to a new model for biofilm growth, while the mechanisms for phosphorous removal remain largely unresolved.

Process stability in expanded granular sludge bed bioreactors enhances resistance to organic load shocks

Environmental perturbations such as changes in organic loading rate (OLR) can have deleterious effects on the anaerobic digestion process, leading to VFA accumulation and process failure. However, the operational history of a reactor, such as prior exposure to VFA build up, can impact a reactor's resistance to shock loads. In the present study, the effects of long term (>100 days) bioreactor (un)stability on OLR shock resistance were assessed. Three 4 L EGSB bioreactors were subjected to varying levels of process stability. Operational conditions such as OLR, temperature and pH were maintained stable in R1; R2 was subjected to a series of minor OLR perturbations and R3 was subjected to a series of non-OLR perturbations, including ammonium, temperature, pH and sulfide. The effect of these different operational histories on each reactor's resistance to a sudden 8-fold increase in OLR were assessed by monitoring COD removal efficiency and biogas production. The microbial communities of each reactor were monitored using 16S rRNA gene sequencing to understand the relationship between microbial diversity and reactor stability. It was determined that the stable (un-perturbed) reactor performed best in terms of its resistance to a large OLR shock, despite its lower microbial community diversity.

Enhanced process control of trickle-bed reactors for biomethanation by vertical profiling directed by hydrogen microsensor monitoring

Biomethanation is an emerging Power-to-X technology enabling CO₂ valorisation to produce biomethane using renewable H₂. A promising reactor for facilitating biomethanation is the trickle bed reactor (TBR), however, these bioreactors are conventionally operated with a black-box approach, where the system is solely described by the input and output characteristics. This study employed a novel approach for process surveillance of internal dynamics in TBRs by installing multiple H₂ microsensors along its vertical axis. The H₂ microsensor monitoring was demonstrated for 135 days in a TBR integrated into a full-scale biogas plant. Despite achieving an overall CH₄ productivity of 12.6 L L^-1 d^-1, the vertical positioning of microsensors revealed a clear zonation with CH₄ productivity zones reaching 54.8 L L^-1 d^-1 and enabled early warning detection of deteriorating process performance days before detecting it in the product gas. Thus, vertically positioned microsensors present a promising solution for securing process stability.

Ecosystem-specific microbiota and microbiome databases in the era of big data

The rapid development of sequencing methods over the past decades has accelerated both the potential scope and depth of microbiota and microbiome studies. Recent developments in the field have been marked by an expansion away from purely categorical studies towards a greater investigation of community functionality. As in-depth genomic and environmental coverage is often distributed unequally across major taxa and ecosystems, it can be difficult to identify or substantiate relationships within microbial communities. Generic databases containing datasets from diverse ecosystems have opened a new era of data accessibility despite costs in terms of data quality and heterogeneity. This challenge is readily embodied in the integration of meta-omics data alongside habitat-specific standards which help contextualise datasets both in terms of sample processing and background within the ecosystem. A special case of large genomic repositories, ecosystem-specific databases (ES-DB's), have emerged to consolidate and better standardise sample processing and analysis protocols around individual ecosystems under study, allowing independent studies to produce comparable datasets. Here, we provide a comprehensive review of this emerging tool for microbial community analysis in relation to current trends in the field. We focus on the factors leading to the formation of ES-DB's, their comparison to traditional microbial databases, the potential for ES-DB integration with meta-omics platforms, as well as inherent limitations in the applicability of ES-DB's.

Unravelling gradient layers of microbial communities, proteins, and chemical structure in aerobic granules

Most bacteria live in microbial assemblages like biofilms and granules, and each layer of these assemblages provides a niche for certain bacteria with specific metabolic functions. In this study, a gentle (non-destructive) extraction approach based on a cation exchange resin and defined shear was employed to gradually disintegrate biomass and collect single layers of aerobic granules from a full-scale municipal wastewater treatment plant. The microbial community composition of granule layers was characterized using next-generation sequencing (NGS) targeting the 16S rRNA gene, and protein composition was investigated using metaproteomics. The chemical composition of eroded layers was explored using Fourier Transformed Infrared Spectroscopy. On the surface of the granules, the microbial structure (flocculation-supporting Nannocystis sp.) as well as composition of extracellular polymers (extracellular DNA) and proteome (chaperonins and binding proteins) favored microbial aggregation. Extracellular polymeric substances in the granules were composed of mostly proteins and EPS-producers, such as Tetrasphaera sp. and Zoogloea sp., were evenly distributed throughout the granule structure. The interior of the granules harbored several denitrifiers (e.g., Thauera sp.), phosphate-accumulating denitrifiers (Candidatus Accumulibacter, Dechloromonas sp.) and nitrifiers (Candidatus Nitrotoga). Proteins associated with glycolytic activity were identified in the outer and middle granule layers, and proteins associated with phosphorus conversions, in the deeper layers. In conclusion, the use of an existing cation-exchange resin for gradual biomass disintegration, combined with NGS and metaproteomic analysis was demonstrated as a promising approach for simultaneously investigating the identity and functions of microbes in multilayered biofilm structures.

Tracking bacterial pollution at a marine wastewater outfall site - A case study from Norway

Coastal marine environments are increasingly affected by anthropogenic impacts, such as the release of sewage at outfall sites and agricultural run-off. Fecal pollution introduced to the sea through these activities poses risks of spreading microbial diseases and disseminating antibiotic resistant bacteria and their genes. The study area of this research, Bore beach, is situated between two such point sources, an outfall site where treated sewage is released 1 km off the coast and a stream that carries run-off from an agricultural area to the northern end of the beach. In order to investigate whether and to what extent fecal contamination from the sewage outfall reached the beach, we used microbial source tracking, based on whole community analysis. Samples were collected from sea water at varying distances from the sewage outfall site and along the beach, as well as from the sewage effluent and the stream. Amplicon sequencing of 16S rRNA genes from all the collected samples was carried out at two time points (June and September). In addition, the seawater at the sewage outfall site and the sewage effluent were subject to shotgun metagenomics. To estimate the contribution of the sewage effluent and the stream to the microbial communities at Bore beach, we employed SourceTracker2, a program that uses a Bayesian algorithm to perform such quantification. The SourceTracker2 results suggested that the sewage effluent is likely to spread fecal contamination towards the beach to a greater extent than anticipated based on the prevailing sea current. The estimated mixing proportions of sewage at the near-beach site (P4) were 0.22 and 0.035% in June and September, respectively. This was somewhat below that stream's contribution in June (0.028%) and 10-fold higher than the stream's contribution in September (0.004%). Our analysis identified a sewage signal in all the tested seawater samples.

Reevaluation of the Phylogenetic Diversity and Global Distribution of the Genus "Candidatus Accumulibacter"

“Candidatus Accumulibacter” was the first microorganism identified as a polyphosphate-accumulating organism (PAO) important for phosphorus removal from wastewater. Members of this genus are diverse, and the current phylogeny and taxonomic framework appear complicated, with most publicly available genomes classified as “Candidatus Accumulibacter phosphatis,” despite notable phylogenetic divergence. The ppk1 marker gene allows for a finer-scale differentiation into different “types” and “clades”; nevertheless, taxonomic assignments remain inconsistent across studies. Therefore, a comprehensive reevaluation is needed to establish a common understanding of this genus, in terms of both naming and basic conserved physiological traits. Here, we provide this reassessment using a comparison of genome, ppk1, and 16S rRNA gene-based approaches from comprehensive data sets. We identified 15 novel species, along with “Candidatus Accumulibacter phosphatis,” “Candidatus Accumulibacter delftensis,” and “Candidatus Accumulibacter aalborgensis.” To compare the species in situ, we designed new species-specific fluorescence in situ hybridization (FISH) probes and revealed their morphology and arrangement in activated sludge. Based on the MiDAS global survey, “Ca. Accumulibacter” species were widespread in wastewater treatment plants (WWTPs) with phosphorus removal, indicating process design as a major driver for their abundance. Genome mining for PAO-related pathways and FISH-Raman microspectroscopy confirmed the potential for PAO metabolism in all “Ca. Accumulibacter” species, with detection in situ of the typical PAO storage polymers. Genome annotation further revealed differences in the nitrate/nitrite reduction pathways. This provides insights into the niche differentiation of these lineages, potentially explaining their coexistence in the same ecosystem while contributing to overall phosphorus and nitrogen removal.

IMPORTANCE “Candidatus Accumulibacter” is the most studied PAO, with a primary role in biological nutrient removal. However, the species-level taxonomy of this lineage is convoluted due to the use of different phylogenetic markers or genome sequencing approaches. Here, we redefined the phylogeny of these organisms, proposing a comprehensive approach which could be used to address the classification of other diverse and uncultivated lineages. Using genome-resolved phylogeny, compared to phylogeny based on the 16S rRNA gene and other phylogenetic markers, we obtained a higher-resolution taxonomy and established a common understanding of this genus. Furthermore, genome mining of genes and pathways of interest, validated in situ by application of a new set of FISH probes and Raman microspectroscopy, provided additional high-resolution metabolic insights into these organisms.

Nitrate Removal from Groundwater by Heterotrophic and Electro-Autotrophic Denitrification

A heterotrophic and autotrophic denitrification (HAD) system shows satisfactory performance for groundwater with nitrate contamination. In this study, an HAD system combining solid-phase heterotrophic denitrification and electrochemical hydrogen autotrophic denitrification (SHD-EHD) was developed for the treatment of nitrate-contaminated groundwater, in which polycaprolactone (PCL) was used as the carbon source to enhance the nitrate removal performance and prevent secondary pollution of the electrochemical hydrogen autotrophic denitrification (EHD) system. The denitrification performance, microbial community structure and nitrogen metabolism were investigated. The results showed that a high nitrate removal rate of 99.04% was achieved with an influent nitrate concentration of 40 mg/L, a current of 40 mA and a hydraulic retention time (HRT) of 4 h. By comparing the performance with the EHD system, it was found that the HAD system with PCL promoted the complete denitrification and reduced the accumulation of NO2−-N. Analysis of the microbial community structure identified the key denitrifying bacteria: Dechloromonas, Thauera and Hydrogenophaga. A comparison of microbial communities from SHD-EHD and solid-phase heterotrophic denitrification (SHD) demonstrated that electrical stimulation promoted the abundance of the dominant denitrifying bacteria and the electroactive bacteria. Analysis of the nitrogen metabolic pathway revealed that the conversion of NO to N2O was the rate-limiting step in the overall denitrification pathway. The SHD-EHD developed in this study showed great potential for groundwater nitrate removal.

Alpine constructed wetlands: A metagenomic analysis reveals microbial complementary structure

Constructed wetlands (CWs) are used to water treatment worldwide, however their application at high-altitude is poorly studied. In order to survive mountain winters, CWs rely on native flora and associated microbial communities. However, the choice of plant-microbes pairs more suitable for water treatment is challenging in alpine environments. Using a metagenomic approach, we investigated the composition of prokaryotes and fungal communities, through extensive sampling inside a constructed wetland in the SW-Alps. Best performing plant species were searched among those hosting the most diverse and resilient microbial communities and to this goal, we analysed them in the natural environment also. Our results showed that microbial communities were less diverse in the CW than at natural conditions, and they differed from plant to plant, revealing a clear variation in taxonomic composition between forbs and gramineous plants. Carex rostrata, Deschampsia caespitosa and Rumex alpinus hosted bacteria very active in N-cycles. Moreover, fungal and prokaryotic communities associated to R. alpinus (Polygonaceae) turned to be the richest and stable among the studied species. In our opinion, this species should be prioritized in CWs at high elevations, also in consideration of its low maintenance requirements.

Quantification of Biologically and Chemically Bound Phosphorus in Activated Sludge from Full-Scale Plants with Biological P-Removal

Phosphorus (P) is present in activated sludge from wastewater treatment plants in the form of metal salt precipitates, extracellular polymeric substances, or bound into the biomass, for example, as intracellular polyphosphate (poly-P). Several methods for a reliable quantification of the different P-fractions have recently been developed, and this study combines them to obtain a comprehensive P mass-balance of activated sludge from four enhanced biological phosphate removal (EBPR) plants. Chemical characterization by ICP-OES and sequential P fractionation showed that chemically bound P constituted 38-69% of total P, most likely in the form of Fe, Mg, or Al minerals. Raman microspectroscopy, solution state ³¹P NMR, and ³¹P MAS NMR spectroscopy applied before and after anaerobic P-release experiments, were used to quantify poly-P, which constituted 22-54% of total P and was found in approximately 25% of all bacterial cells. Raman microspectroscopy in combination with fluorescence in situ hybridization was used to quantify poly-P in known polyphosphate-accumulating organisms (PAO) (Tetrasphaera, Candidatus Accumulibacter, and Dechloromonas) and other microorganisms known to possess high level of poly-P, such as the filamentous Ca. Microthrix. Interestingly, only 1-13% of total P was stored by unidentified PAO, highlighting that most PAOs in the full-scale EBPR plants investigated are known.

MiDAS 4: A global catalogue of full-length 16S rRNA gene sequences and taxonomy for studies of bacterial communities in wastewater treatment plants

Microbial communities are responsible for biological wastewater treatment, but our knowledge of their diversity and function is still poor. Here, we sequence more than 5 million high-quality, full-length 16S rRNA gene sequences from 740 wastewater treatment plants (WWTPs) across the world and use the sequences to construct the ‘MiDAS 4’ database. MiDAS 4 is an amplicon sequence variant resolved, full-length 16S rRNA gene reference database with a comprehensive taxonomy from domain to species level for all sequences. We use an independent dataset (269 WWTPs) to show that MiDAS 4, compared to commonly used universal reference databases, provides a better coverage for WWTP bacteria and an improved rate of genus and species level classification. Taking advantage of MiDAS 4, we carry out an amplicon-based, global-scale microbial community profiling of activated sludge plants using two common sets of primers targeting regions of the 16S rRNA gene, revealing how environmental conditions and biogeography shape the activated sludge microbiota. We also identify core and conditionally rare or abundant taxa, encompassing 966 genera and 1530 species that represent approximately 80% and 50% of the accumulated read abundance, respectively. Finally, we show that for well-studied functional guilds, such as nitrifiers or polyphosphate-accumulating organisms, the same genera are prevalent worldwide, with only a few abundant species in each genus.

Microbial communities are responsible for biological wastewater treatment. Here, Dueholm et al. generate more than 5 million high-quality, full-length 16S rRNA gene sequences from wastewater treatment plants across the world to construct a database with a comprehensive taxonomy, providing insights into diversity and function of these microbial communities.

Machine learning-assisted identification of bioindicators predicts medium-chain carboxylate production performance of an anaerobic mixed culture

BACKGROUND

The ability to quantitatively predict ecophysiological functions of microbial communities provides an important step to engineer microbiota for desired functions related to specific biochemical conversions. Here, we present the quantitative prediction of medium-chain carboxylate production in two continuous anaerobic bioreactors from 16S rRNA gene dynamics in enriched communities.

RESULTS

By progressively shortening the hydraulic retention time (HRT) from 8 to 2 days with different temporal schemes in two bioreactors operated for 211 days, we achieved higher productivities and yields of the target products n-caproate and n-caprylate. The datasets generated from each bioreactor were applied independently for training and testing machine learning algorithms using 16S rRNA genes to predict n-caproate and n-caprylate productivities. Our dataset consisted of 14 and 40 samples from HRT of 8 and 2 days, respectively. Because of the size and balance of our dataset, we compared linear regression, support vector machine and random forest regression algorithms using the original and balanced datasets generated using synthetic minority oversampling. Further, we performed cross-validation to estimate model stability. The random forest regression was the best algorithm producing more consistent results with median of error rates below 8%. More than 90% accuracy in the prediction of n-caproate and n-caprylate productivities was achieved. Four inferred bioindicators belonging to the genera Olsenella, Lactobacillus, Syntrophococcus and Clostridium IV suggest their relevance to the higher carboxylate productivity at shorter HRT. The recovery of metagenome-assembled genomes of these bioindicators confirmed their genetic potential to perform key steps of medium-chain carboxylate production.

CONCLUSIONS

Shortening the hydraulic retention time of the continuous bioreactor systems allows to shape the communities with desired chain elongation functions. Using machine learning, we demonstrated that 16S rRNA amplicon sequencing data can be used to predict bioreactor process performance quantitatively and accurately. Characterizing and harnessing bioindicators holds promise to manage reactor microbiota towards selection of the target processes. Our mathematical framework is transferrable to other ecosystem processes and microbial systems where community dynamics is linked to key functions. The general methodology used here can be adapted to data types of other functional categories such as genes, transcripts, proteins or metabolites. Video Abstract.

Disentangle genus microdiversity within a complex microbial community by using a multi-distance long-read binning method: example of Candidatus Accumulibacter

Complete genomes can be recovered from metagenomes by assembling and binning DNA sequences into metagenome assembled genomes (MAGs). Yet, the presence of microdiversity can hamper the assembly and binning processes, possibly yielding chimeric, highly fragmented and incomplete genomes. Here, the metagenomes of four samples of aerobic granular sludge bioreactors containing Candidatus (Ca.) Accumulibacter, a phosphate-accumulating organism of interest for wastewater treatment, were sequenced with both PacBio and Illumina. Different strategies of genome assembly and binning were investigated, including published protocols and a binning procedure adapted to the binning of long contigs (MuLoBiSC). Multiple criteria were considered to select the best strategy for Ca. Accumulibacter, whose multiple strains in every sample represent a challenging microdiversity. In this case, the best strategy relies on long-read only assembly and a custom binning procedure including MuLoBiSC in metaWRAP. Several high-quality Ca. Accumulibacter MAGs, including a novel species, were obtained independently from different samples. Comparative genomic analysis showed that MAGs retrieved in different samples harbour genomic rearrangements in addition to accumulation of point mutations. The microdiversity of Ca. Accumulibacter, likely driven by mobile genetic elements, causes major difficulties in recovering MAGs, but it is also a hallmark of the panmictic lifestyle of these bacteria.

Dynamics of Microbial Communities in Phototrophic Polyhydroxyalkanoate Accumulating Cultures

Phototrophic mixed cultures (PMC) are versatile systems which can be applied for waste streams, valorisation and production of added-value compounds, such as polyhydroxyalkanoates (PHA). This work evaluates the influence of different operational conditions on the bacterial communities reported in PMC systems with PHA production capabilities. Eleven PMCs, fed either with acetate or fermented wastewater, and selected under either feast and famine (FF) or permanent feast (PF) regimes, were evaluated. Overall, results identified Chromatiaceae members as the main phototrophic PHA producers, along with Rhodopseudomonas, Rhodobacter and Rhizobium. The findings show that Chromatiaceae were favoured under operating conditions with high carbon concentrations, and particularly under the PF regime. In FF systems fed with fermented wastewater, the results indicate that increasing the organic loading rate enriches for Rhodopseudomonas, Rhizobium and Hyphomicrobiaceae, which together with Rhodobacter and Chromatiaceae, were likely responsible for PHA storage. In addition, high-sugar feedstock impairs PHA production under PF conditions (fermentative bacteria dominance), which does not occur under FF. This characterization of the communities responsible for PHA accumulation helps to define improved operational strategies for PHA production with PMC.

Recent advances in understanding the ecology of the filamentous bacteria responsible for activated sludge bulking

Activated sludge bulking caused by filamentous bacteria is still a problem in wastewater treatment plants around the world. Bulking is a microbiological problem, and so its solution on species-specific basis is likely to be reached only after their ecology, physiology and metabolism is better understood. Culture-independent molecular methods have provided much useful information about this group of organisms, and in this review, the methods employed and the information they provide are critically assessed. Their application to understanding bulking caused by the most frequently seen filament in Japan, 'Ca. Kouleothrix', is used here as an example of how these techniques might be used to develop control strategies. Whole genome sequences are now available for some of filamentous bacteria responsible for bulking, and so it is possible to understand why these filaments might thrive in activated sludge plants, and provide clues as to how eventually they might be controlled specifically.

"Candidatus Dechloromonas phosphoritropha" and "Ca. D. phosphorivorans", novel polyphosphate accumulating organisms abundant in wastewater treatment systems

Members of the genus Dechloromonas are often abundant in enhanced biological phosphorus removal (EBPR) systems and are recognized putative polyphosphate accumulating organisms (PAOs), but their role in phosphate removal is still unclear. Here, we used 16S rRNA gene sequencing and fluorescence in situ hybridization (FISH) to investigate the abundance and distribution of Dechloromonas spp. in Danish and global wastewater treatment plants. The two most abundant species worldwide revealed in situ dynamics of important intracellular storage polymers, measured by FISH-Raman in activated sludge from four full-scale EBPR plants and from a lab-scale reactor fed with different substrates. Moreover, seven distinct Dechloromonas species were determined from a set of ten high-quality metagenome-assembled genomes (MAGs) from Danish EBPR plants, each encoding the potential for polyphosphate (poly-P), glycogen, and polyhydroxyalkanoates (PHA) accumulation. The two species exhibited an in situ phenotype in complete accordance with the metabolic information retrieved by the MAGs, with dynamic levels of poly-P, glycogen, and PHA during feast-famine anaerobic-aerobic cycling, legitimately placing these microorganisms among the important PAOs. They are potentially involved in denitrification showing niche partitioning within the genus and with other important PAOs. As no isolates are available for the two species, we propose the names Candidatus Dechloromonas phosphoritropha and Candidatus Dechloromonas phosphorivorans.

Faecal microbiota transplantation from patients with depression or healthy individuals into rats modulates mood-related behaviour

Differences in gut microbiota composition have been observed in patients with major depressive disorder (MDD) compared to healthy individuals. Here, we investigated if faecal microbiota transplantation (FMT) from patients with MDD into rats could induce a depressive-like phenotype. We performed FMT from patients with MDD (FMT-MDD) and healthy individuals (FMT-Healthy) into male Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats and assessed depressive-like behaviour. No behavioural differences were observed in the FSL rats. In FRL rats, the FMT-Healthy group displayed significantly less depressive-like behaviour than the FMT-MDD group. However, there was no difference in behaviour between FMT-MDD FRL rats and negative controls, indicating that FMT-Healthy FRL rats received beneficial bacteria. We additionally found different taxa between the FMT-MDD and the FMT-Healthy FRL rats, which could be traced to the donors. Four taxa, three belonging to the family Ruminococcaceae and the genus Lachnospira, were significantly elevated in relative abundance in FMT-MDD rats, while the genus Coprococcus was depleted. In this study, the FMT-MDD group was different from the FMT-Healthy group based on behaviour and intestinal taxa.

Linkage of community composition and function over short response time in anaerobic digestion systems with food fermentation wastewater

We investigated the short-term dynamics of microbial composition and function in bioreactors with inocula collected from full-scale and laboratory-based anaerobic digestion (AD) systems. The Bray-Curtis dissimilarity of both inocula was approximately 10% of the predicted Kyoto Encyclopedia of Genes and Genomes pathway and 40% of the taxonomic composition and yet resulted in a similar performance in methane production, implying that the variation of community composition may be decoupled from performance. However, the significant correlation of volatile fatty acids with taxonomic variation suggested that the pathways of AD could be different because of the varying genus. The predicted function of the significantly varying genus was mostly related to fermentation, which strengthened the conclusion that most microbial variation occurred within the fermentative species and led to alternative routes to result in similar methane production in methanogenic bioreactors. This finding sheds some light on the understanding of AD community regulation, which depends on the aims to recover intermediates or methane.

Prokaryotic viruses impact functional microorganisms in nutrient removal and carbon cycle in wastewater treatment plants

As one of the largest biotechnological applications, activated sludge (AS) systems in wastewater treatment plants (WWTPs) harbor enormous viruses, with 10-1,000-fold higher concentrations than in natural environments. However, the compositional variation and host-connections of AS viruses remain poorly explored. Here, we report a catalogue of ~50,000 prokaryotic viruses from six WWTPs, increasing the number of described viral species of AS by 23-fold, and showing the very high viral diversity which is largely unknown (98.4-99.6% of total viral contigs). Most viral genera are represented in more than one AS system with 53 identified across all. Viral infection widely spans 8 archaeal and 58 bacterial phyla, linking viruses with aerobic/anaerobic heterotrophs, and other functional microorganisms controlling nitrogen/phosphorous removal. Notably, Mycobacterium, notorious for causing AS foaming, is associated with 402 viral genera. Our findings expand the current AS virus catalogue and provide reference for the phage treatment to control undesired microorganisms in WWTPs.

Evaluating the nitrogen-contaminated groundwater treatment by a denitrifying granular sludge bioreactor: effect of organic matter loading

A sequential bed granular bioreactor was adapted to treat nitrate-polluted synthetic groundwater under anaerobic conditions and agitation with denitrification gas, achieving very efficient performance in total nitrogen removal at influent organic carbon concentrations of 1 g L^-1 (80-90%) and 0.5 g L^-1 (70-80%) sodium acetate, but concentrations below 0.5 g L^-1 caused accumulation of nitrite and nitrate and led to system failure (30-40% removal). Biomass size and settling velocity were higher above 0.5 g L^-1 sodium acetate. Trichosporonaceae dominated the fungal populations at all times, while a dominance of terrestrial group Thaumarchaeota and Acidovorax at 1 and 0.5 g L^-1 passed to a domination of Methanobrevibacter and an unclassified Comamonadaceae clone for NaAc lower than 0.5 g L^-1. The results obtained pointed out that the denitrifying granular sludge technology is a feasible solution for the treatment of nitrogen-contaminated groundwater, and that influent organic matter plays an important role on the conformation of microbial communities within it and, therefore, on the overall efficiency of the system.

Dynamics of Microbial Community During Nitrification Biofilter Acclimation with Low and High Ammonia

The acclimation of a nitrifying biofilter is a crucial and time-consuming task for setting up a recirculating aquaculture system (RAS). Gaining a better understanding of the dynamics of the microbial community during the acclimation period in the system could be useful for the development of mature nitrifying biofilters. In this study, high-throughput DNA sequencing was applied to monitor the microbial communities on a biofilter during the acclimation period (7 weeks) in high (100 mg N/L) and low (5 mg N/L) total ammonia nitrogen (TAN) treatments. Both treatments were successful for developing a mature nitrifying biofilter, dominated by Proteobacteria, Bacteroidetes, and Nitrospirae. Complete nitrification was found after 7 days of biofilter acclimation as indicated by decreasing TAN concentration, increasing nitrate concentration, and high abundances of the nitrifying bacteria, Nitrosomonadaceae and Nitrospiraceae. The beta diversity analysis of microbial communities showed different clustering of the samples between high and low TAN treatment groups. A greater abundance of nitrifying bacteria was found in the high TAN treatments (27-51%) than in the low TAN treatment (15-29%). The bacterial diversity in biofilters acclimated at high TAN concentration (Shannon's index 5.40-6.15) were lower than those found at low TAN treatment levels (Shannon's index 6.40-7.01). The higher diversity in biofilters acclimated at low TAN concentrations, consisting of Planctomycetes and Archaea, might benefit the nutrient recycling in the system. Although nitrification activity was observed from the first week of the acclimation period, the acclimation period should be taken as at least 6 weeks for full development of nitrifying biofilm. Moreover, the reduction of potentially pathogenic Vibrio on biofilters was found at that period.

Distinct growth stages controlled by the interplay of deterministic and stochastic processes in functional anammox biofilms

Mainstream anaerobic ammonium oxidation (anammox) represents one of the most promising energy-efficient mechanisms of fixed nitrogen elimination from wastewaters. However, little is known about the exact processes and drivers of microbial community assembly within the complex microbial biofilms that support anammox in engineered ecosystems. Here, we followed anammox biofilm development on fresh carriers in an established 8m³ mainstream anammox reactor that is exposed to seasonal temperature changes (~25-12°C) and varying NH₄⁺ concentrations (5-25 mg/L). We use fluorescence in situ hybridization and 16S rRNA gene sequencing to show that three distinct stages of biofilm development emerge naturally from microbial community composition and biofilm structure. Neutral modelling and network analysis are employed to elucidate the relative importance of stochastic versus deterministic processes and synergistic and antagonistic interactions in the biofilms during their development. We find that the different phases are characterized by a dynamic succession and an interplay of both stochastic and deterministic processes. The observed growth stages (Colonization, Succession and Maturation) appear to be the prerequisite for the anticipated growth of anammox bacteria and for reaching a biofilm community structure that supports the desired metabolic and functional capacities observed for biofilm carriers already present in the system (~100g_NH4-N m³ d^-1). We discuss the relevance of this improved understanding of anammox-community ecology and biofilm development in the context of its practical application in the start-up, configuration, and optimization of anammox biofilm reactors.

Prokaryotic taxonomy and nomenclature in the age of big sequence data

The classification of life forms into a hierarchical system (taxonomy) and the application of names to this hierarchy (nomenclature) is at a turning point in microbiology. The unprecedented availability of genome sequences means that a taxonomy can be built upon a comprehensive evolutionary framework, a longstanding goal of taxonomists. However, there is resistance to adopting a single framework to preserve taxonomic freedom, and ever increasing numbers of genomes derived from uncultured prokaryotes threaten to overwhelm current nomenclatural practices, which are based on characterised isolates. The challenge ahead then is to reach a consensus on the taxonomic framework and to adapt and scale the existing nomenclatural code, or create a new code, to systematically incorporate uncultured taxa into the chosen framework.

Cellulolytic and Xylanolytic Microbial Communities Associated With Lignocellulose-Rich Wheat Straw Degradation in Anaerobic Digestion

The enzymatic hydrolysis of lignocellulosic polymers is generally considered the rate-limiting step to methane production in anaerobic digestion of lignocellulosic biomass. The present study aimed to investigate how the hydrolytic microbial communities of three different types of anaerobic digesters adapted to lignocellulose-rich wheat straw in continuous stirred tank reactors operated for 134 days. Cellulase and xylanase activities were monitored weekly using fluorescently-labeled model substrates and the enzymatic profiles were correlated with changes in microbial community compositions based on 16S rRNA gene amplicon sequencing to identify key species involved in lignocellulose degradation. The enzymatic activity profiles and microbial community changes revealed reactor-specific adaption of phylogenetically different hydrolytic communities. The enzymatic activities correlated significantly with changes in specific taxonomic groups, including representatives of Ruminiclostridium, Caldicoprobacter, Ruminofilibacter, Ruminococcaceae, Treponema, and Clostridia order MBA03, all of which have been linked to cellulolytic and xylanolytic activity in the literature. By identifying microorganisms with similar development as the cellulase and xylanase activities, the proposed correlation method constitutes a promising approach for deciphering essential cellulolytic and xylanolytic microbial groups for anaerobic digestion of lignocellulosic biomass.

Performance and microbial community structure of a full-scale ANITATMMox bioreactor for treating reject water located in Finland

The aim of this work was to study the operational performance and the microbial community dynamics during the start-up of ANITA^TMMox technology implemented at full-scale wastewater treatment plant in Finland to treat reject water from anaerobic digesters. The average ammonium removal in the studied setup reached around 90%, withstanding ammonium loads up to 0.13 g N m^-2h^-1. The nitrite concentration in the effluent did not exceed 10 mg L^-1, and there was a slight accumulation of NO₃^--N during the operation which was controlled. Thus, the result showed a robust success to high ammonium loading in presence of organic matter. The sequencing showed a heterogeneous microbial population where Methanosaeta, WCHA1-57 genus, Sphingobacteriia, Chlorobia and diverse unknown fungi were found as dominant phylotypes. Moreover, members of the Brocadiaceae family were dominant in the adhered biomass, mostly represented by Candidatus Scalindua, rarely reported in WWTPs. Overall, the results demonstrated a drastic effect of region-specific operational conditions on carrier biofilm microbial communities as it was demonstrated by the microbial studies.

Total and Metabolically Active Microbial Community of Aerobic Granular Sludge Systems Operated in Sequential Batch Reactors: Effect of Pharmaceutical Compounds

Two aerobic granular sludge (AGS) sequential batch reactors were operated at a mild (15 °C) temperature for 180 days. One of those bioreactors was exposed to a mixture of diclofenac, naproxen, trimethoprim, and carbamazepine. The AGS system, operating under pressure from emerging contaminants, showed a decrease in COD, BOD5, and TN removal capacity, mainly observed during the first 100 days, in comparison with the removal ratios detected in the control bioreactor. After an acclimatisation period, the removal reached high-quality effluent for COD and TN, close to 95% and 90%, respectively. In the steady-state period, trimethoprim and diclofenac were successfully removed with values around 50%, while carbamazepine and naproxen were more recalcitrant. The dominant bacterial OTUs were affected by the presence of a mixture of pharmaceutical compounds, under which the dominant phylotypes changed to OTUs classified among the Pseudomonas, Gemmobacter, and Comamonadaceae. The RT-qPCR and qPCR results showed the deep effects of pharmaceutical compounds on the number of copies of target genes. Statistical analyses allowed for linking the total and active microbial communities with the physico-chemical performance, describing the effects of pharmaceutical compounds in pollution degradation, as well as the successful adaptation of the system to treat wastewater in the presence of toxic compounds.

The war using microbes: A sustainable approach for wastewater management

Anthropogenic activities and population growth have resulted in a reduced availability of drinking water. To ensure consistency in the existence of drinking water, it is inevitable to establish wastewater treatment plants (WWTPs). 70% of India's rural population was found to be without WWTP, waste disposal, and good sanitation. Wastewater has emerged from kitchens, washrooms, etc., with industry activities. This scenario caused severe damage to water resources, leading to degradation of water quality and pathogenic insects. Thus, it is a need of an hour to prompt for better WWTPs for both rural and urban areas. Many parts of the world have started to face severe water shortages in recent years, and wastewater reuse methods need to be updated. Clean water supply is not enough to satisfy the needs of the planet as a whole, and the majority of freshwater in the polar regions takes the form of ice and snow. The increasing population requires clean water for drinks, hygiene, irrigation, and various other applications. Lack of water and contamination of water result from human activities. 90% of wastewater is released to water systems without treatment in developing countries. Studies show that about 730 megatons of waste are annually discharged into water from sewages and other effluents. The sustenance of water resources, applying wastewater treatment technologies, and calling down the percentage of potable water has to be strictly guided by mankind. This review compares the treatment of domestic sewage to its working conditions, energy efficiency, etc. In this review, several treatment methods with different mechanisms involved in waste treatment, industrial effluents, recovery/recycling were discussed. The feasibility of bioaugmentation should eventually be tested through data from field implementation as an important technological challenge, and this analysis identifies many promising areas to be explored in the future.

BEExact: a Metataxonomic Database Tool for High-Resolution Inference of Bee-Associated Microbial Communities

High-throughput 16S rRNA gene sequencing technologies have robust potential to improve our understanding of bee (Hymenoptera: Apoidea)-associated microbial communities and their impact on hive health and disease. Despite recent computation algorithms now permitting exact inferencing of high-resolution exact amplicon sequence variants (ASVs), the taxonomic classification of these ASVs remains a challenge due to inadequate reference databases. To address this, we assemble a comprehensive data set of all publicly available bee-associated 16S rRNA gene sequences, systematically annotate poorly resolved identities via inclusion of 618 placeholder labels for uncultivated microbial dark matter, and correct for phylogenetic inconsistencies using a complementary set of distance-based and maximum likelihood correction strategies. To benchmark the resultant database (BEExact), we compare performance against all existing reference databases in silico using a variety of classifier algorithms to produce probabilistic confidence scores. We also validate realistic classification rates on an independent set of ∼234 million short-read sequences derived from 32 studies encompassing 50 different bee types (36 eusocial and 14 solitary). Species-level classification rates on short-read ASVs range from 80 to 90% using BEExact (with ∼20% due to "bxid" placeholder names), whereas only ∼30% at best can be resolved with current universal databases. A series of data-driven recommendations are developed for future studies. We conclude that BEExact (https://github.com/bdaisley/BEExact) enables accurate and standardized microbiota profiling across a broad range of bee species-two factors of key importance to reproducibility and meaningful knowledge exchange within the scientific community that together, can enhance the overall utility and ecological relevance of routine 16S rRNA gene-based sequencing endeavors.IMPORTANCE The failure of current universal taxonomic databases to support the rapidly expanding field of bee microbiota research has led to many investigators relying on "in-house" reference sets or manual classification of sequence reads (usually based on BLAST searches), often with vague identity thresholds and subjective taxonomy choices. This time-consuming, error- and bias-prone process lacks standardization, cripples the potential for comparative cross-study analysis, and in many cases is likely to incorrectly sway study conclusions. BEExact is structured on and leverages several complementary bioinformatic techniques to enable refined inference of bee host-associated microbial communities without any other methodological modifications necessary. It also bridges the gap between current practical outcomes (i.e., phylotype-to-genus level constraints with 97% operational taxonomic units [OTUs]) and the theoretical resolution (i.e., species-to-strain level classification with 100% ASVs) attainable in future microbiota investigations. Other niche habitats could also likely benefit from customized database curation via implementation of the novel approaches introduced in this study.

Connecting structure to function with the recovery of over 1000 high-quality metagenome-assembled genomes from activated sludge using long-read sequencing

Microorganisms play crucial roles in water recycling, pollution removal and resource recovery in the wastewater industry. The structure of these microbial communities is increasingly understood based on 16S rRNA amplicon sequencing data. However, such data cannot be linked to functional potential in the absence of high-quality metagenome-assembled genomes (MAGs) for nearly all species. Here, we use long-read and short-read sequencing to recover 1083 high-quality MAGs, including 57 closed circular genomes, from 23 Danish full-scale wastewater treatment plants. The MAGs account for ~30% of the community based on relative abundance, and meet the stringent MIMAG high-quality draft requirements including full-length rRNA genes. We use the information provided by these MAGs in combination with >13 years of 16S rRNA amplicon sequencing data, as well as Raman microspectroscopy and fluorescence in situ hybridisation, to uncover abundant undescribed lineages belonging to important functional groups.

Identification of microorganisms responsible for foam formation in mesophilic anaerobic digesters treating surplus activated sludge

Foaming is a common operational problem in anaerobic digestion (AD) systems, where hydrophobic filamentous microorganisms are usually considered to be the major cause. However, little is known about the identity of foam-stabilising microorganisms in AD systems, and control measures are lacking. This study identified putative foam forming microorganisms in 13 full-scale mesophilic digesters located at 11 wastewater treatment plants in Denmark, using 16S rRNA gene amplicon sequencing with species-level resolution and fluorescence in situ hybridization (FISH) for visualization. A foaming potential aeration test was applied to classify the digester sludges according to their foaming propensity. A high foaming potential for sludges was linked to the abundance of species from the genus Candidatus Microthrix, immigrating with the feed stream (surplus activated sludge), but also to several novel phylotypes potentially growing in the digester. These species were classified to the genera Ca. Brevefilum (Ca. B. fermentans) and Tetrasphaera (midas_s_5), the families ST-12K33 (midas_s_22), and Rikenellaceae (midas_s_141), and the archaeal genus Methanospirillum (midas_s_2576). Application of FISH showed that these potential foam-forming organisms all had a filamentous morphology. Additionally, it was shown that concentrations of ammonium and total nitrogen correlated strongly to the presence of foam-formers. This study provided new insight into the identity of putative foam-forming microorganisms in mesophilic AD systems, allowing for the subsequent surveillance of their abundances and studies of their ecology. Such information will importantly inform the development of control measures for these problematic microorganisms.

Integrated air cathode microbial fuel cell-aerobic bioreactor set-up for enhanced bioelectrodegradation of azo dye Acid Blue 29

In this study, an azo dye (Acid Blue 29 or AB29) was efficiently degraded with acetate as co-substrate into less contaminated biodegraded products using an integrated single chamber microbial fuel cell (SMFC)-aerobic bioreactor set-up. The decolorization efficiencies were varied from 91 ± 2% to 94 ± 1.9% and more than 85% of chemical oxygen demand (COD) removal was achieved for all dye concentrations after different operating time. The highest coulombic efficiency (CE) and cell potential were 3.18 ± 0.45% and 287.2 mV, respectively, for SMFC treating 100 mg L^-1 of AB29. Electrochemical impedance spectroscopy (EIS) revealed that the anode resistance was 0.3 Ω representing an entirely grown biofilm on the anode surface resulted in higher electron transfer rate. Gas chromatography coupled mass spectrometry (GC-MS) investigation demonstrated that initially biodegradation of AB29 started with the cleavage of the azo bond (-N=N-), resulted the biotransformation into aromatic amines. In successive aerobic treatment stage, these amines were biodegraded into lower molecular weight compounds. The 16S rRNA microbial community analysis indicated that at phylum level, both inoculum and dye acclimated cultures were mainly consisting of Proteobacteria which was 27.9, 53.6 and 68.9% in inoculum, suspension and anodic biofilm, respectively. At genus level, both suspension and biofilm contained decolorization as well as electrochemically active bacteria. The outcomes exhibited that the AB29 decolorization would contest with electrogenic bacteria for electrons.

Microbial Communities in Flexible Biomethanation of Hydrogen Are Functionally Resilient Upon Starvation

Ex situ biomethanation allows the conversion of hydrogen produced from surplus electricity to methane. The flexibility of the process was recently demonstrated, yet it is unknown how intermittent hydrogen feeding impacts the functionality of the microbial communities. We investigated the effect of starvation events on the hydrogen consumption and methane production rates (MPRs) of two different methanogenic communities that were fed with hydrogen and carbon dioxide. Both communities showed functional resilience in terms of hydrogen consumption and MPRs upon starvation periods of up to 14 days. The origin of the inoculum, community structure and dominant methanogens were decisive for high gas conversion rates. Thus, pre-screening a well performing inoculum is essential to ensure the efficiency of biomethanation systems operating under flexible gas feeding regimes. Our results suggest that the type of the predominant hydrogenotrophic methanogen (here: Methanobacterium) is important for an efficient process. We also show that flexible biomethanation of hydrogen and carbon dioxide with complex microbiota is possible while avoiding the accumulation of acetate, which is relevant for practical implementation. In our study, the inoculum from an upflow anaerobic sludge blanket reactor treating wastewater from paper industry performed better compared to the inoculum from a plug flow reactor treating cow manure and corn silage. Therefore, the implementation of the power-to-gas concept in wastewater treatment plants of the paper industry, where biocatalytic biomass is readily available, may be a viable option to reduce the carbon footprint of the paper industry.

Tropical and temperate wastewater treatment plants assemble different and diverse microbiomes

The diversity and assembly of activated sludge microbiomes play a key role in the performances of municipal wastewater treatment plants (WWTPs), which are the most widely applied biotechnological process systems. In this study, we investigated the microbiomes of municipal WWTPs in Bangkok, Wuhan, and Beijing that respectively represent tropical, subtropical, and temperate climate regions, and also explored how microbiomes assembled in these municipal WWTPs. Our results showed that the microbiomes from these municipal WWTPs were significantly different. The assembly of microbiomes in municipal WWTPs followed deterministic and stochastic processes governed by geographical location, temperature, and nutrients. We found that both taxonomic and phylogenetic α-diversities of tropical Bangkok municipal WWTPs were the highest and were rich in yet-to-be-identified microbial taxa. Nitrospirae and β-Proteobacteria were more abundant in tropical municipal WWTPs, but did not result in better removal efficiencies of ammonium and total nitrogen. Overall, these results suggest that tropical and temperate municipal WWTPs harbored diverse and unique microbial resources, and the municipal WWTP microbiomes were assembled with different processes. Implications of these findings for designing and running tropical municipal WWTPs were discussed. KEY POINTS: • Six WWTPs of tropical Thailand and subtropical and temperate China were investigated. • Tropical Bangkok WWTPs had more diverse and yet-to-be-identified microbial taxa. • Microbiome assembly processes were associated with geographical location.

Methanogenic Microorganisms in Industrial Wastewater Anaerobic Treatment

Over the past decades, anaerobic biotechnology is commonly used for treating high-strength wastewaters from different industries. This biotechnology depends on interactions and co-operation between microorganisms in the anaerobic environment where many pollutants’ transformation to energy-rich biogas occurs. Properties of wastewater vary across industries and significantly affect microbiome composition in the anaerobic reactor. Methanogenic archaea play a crucial role during anaerobic wastewater treatment. The most abundant acetoclastic methanogens in the anaerobic reactors for industrial wastewater treatment are Methanosarcina sp. and Methanotrix sp. Hydrogenotrophic representatives of methanogens presented in the anaerobic reactors are characterized by a wide species diversity. Methanoculleus sp., Methanobacterium sp. and Methanospirillum sp. prevailed in this group. This work summarizes the relation of industrial wastewater composition and methanogen microbial communities present in different reactors treating these wastewaters.

Multistability and Reversibility of Aerobic Granular Sludge Microbial Communities Upon Changes From Simple to Complex Synthetic Wastewater and Back

Aerobic granular sludge (AGS) is a promising alternative wastewater treatment to the conventional activated sludge system allowing space and energy saving. Basic understanding of AGS has mainly been obtained using simple wastewater containing acetate and propionate as carbon source. Yet, the aspect and performances of AGS grown in such model systems are different from those obtained in reactor treating real wastewater. The impact of fermentable and hydrolyzable compounds on already formed AGS was assessed separately by changing the composition of the influent from simple wastewater containing volatile fatty acids to complex monomeric wastewater containing amino acids and glucose, and then to complex polymeric wastewater containing also starch and peptone. The reversibility of the observed changes was assessed by changing the composition of the wastewater from complex monomeric back to simple. The introduction of fermentable compounds in the influent left the settling properties and nutrient removal performance unchanged, but had a significant impact on the bacterial community. The proportion of Gammaproteobacteria diminished to the benefit of Actinobacteria and the Saccharibateria phylum. On the other hand, the introduction of polymeric compounds altered the settling properties and denitrification efficiency, but induced smaller changes in the bacterial community. The changes induced by the wastewater transition were only partly reversed. Seven distinct stables states of the bacterial community were detected during the 921 days of experiment, four of them observed with the complex monomeric wastewater. The transitions between these states were not only caused by wastewater changes but also by operation failures and other incidences. However, the nutrient removal performance and settling properties of the AGS were globally maintained due to the functional redundancy of its bacterial community.

Effects of set cathode potentials on microbial electrosynthesis system performance and biocathode methanogen function at a metatranscriptional level

Microbial electrosynthesis exploits the catalytic activity of microorganisms to utilize a cathode as an electron donor for reducing waste CO2 to valuable fuels and chemicals. Electromethanogenesis is the process of CO2 reduction to CH4 catalyzed by methanogens using the cathode directly as a source of electrons or indirectly via H2. Understanding the effects of different set cathode potentials on the functional dynamics of electromethanogenic communities is crucial for the rational design of cathode materials. Replicate enriched electromethanogenic communities were subjected to different potentials (- 1.0 V and - 0.7 V vs. Ag/AgCl) and the potential-induced changes were analyzed using a metagenomic and metatranscriptomic approach. The most abundant and transcriptionally active organism on the biocathodes was a novel species of Methanobacterium sp. strain 34x. The cathode potential-induced changes limited electron donor availability and negatively affected the overall performance of the reactors in terms of CH4 production. Although high expression of key genes within the methane and carbon metabolism pathways was evident, there was no significant difference in transcriptional response to the different set potentials. The acetyl-CoA decarbonylase/synthase (ACDS) complex were the most highly expressed genes, highlighting the significance of carbon assimilation under limited electron donor conditions and its link to the methanogenesis pathway.

Revealing antimicrobial resistance in stormwater with MinION

Discharge of urban stormwater containing organic matter, heavy metals and sometime human feces, to the natural aquatic reservoirs without any treatment is not only an environmental problem. It can lead to prevalence of antibiotic resistant bacteria in stormwater systems and transmission of antibiotic resistance genes to the environment. We performed antibiotic resistome identification and virus detection in stormwater samples from Stockholm, using publicly available metagenomic sequencing MinION data. A MinION platform offers low-cost, precise environmental metagenomics analysis. 37 groups of antibiotic resistant bacteria (ARB), 11 resistance types with 26 resistance mechanisms - antibiotic resistance genes (ARGs) giving tolerance to the aminoglycoside, beta-lactams, fosmidomycin, MLS, multidrug and vancomycin were identified using ARGpore pipeline. The majority of the identified bacteria species were related to the natural environment such as soil and were not dangerous to human. Alarmingly, human pathogenic bacteria carrying resistance to antibiotics currently used against them (Bordetella resistant to macrolides and multidrug resistant Propionibacterium avidum) were also found in the samples. Most abundant viruses identified belonged to Caudovirales and Herpesvirales and they were not carrying ARGs. Unlike the virome, resistome and ARB were not unique for stormwater sampling points. This results underline the need for extensive monitoring of the microbial community structure in the urban stormwater systems to assess antimicrobial resistance spread.

Generation of Comprehensive Ecosystem-Specific Reference Databases with Species-Level Resolution by High-Throughput Full-Length 16S rRNA Gene Sequencing and Automated Taxonomy Assignment (AutoTax)

High-throughput 16S rRNA gene amplicon sequencing is an essential method for studying the diversity and dynamics of microbial communities. However, this method is presently hampered by the lack of high-identity reference sequences for many environmental microbes in the public 16S rRNA gene reference databases and by the absence of a systematic and comprehensive taxonomy for the uncultured majority. Here, we demonstrate how high-throughput synthetic long-read sequencing can be applied to create ecosystem-specific full-length 16S rRNA gene amplicon sequence variant (FL-ASV) resolved reference databases that include high-identity references (>98.7% identity) for nearly all abundant bacteria (>0.01% relative abundance) using Danish wastewater treatment systems and anaerobic digesters as an example. In addition, we introduce a novel sequence identity-based approach for automated taxonomy assignment (AutoTax) that provides a complete seven-rank taxonomy for all reference sequences, using the SILVA taxonomy as a backbone, with stable placeholder names for unclassified taxa. The FL-ASVs are perfectly suited for the evaluation of taxonomic resolution and bias associated with primers commonly used for amplicon sequencing, allowing researchers to choose those that are ideal for their ecosystem. Reference databases processed with AutoTax greatly improves the classification of short-read 16S rRNA ASVs at the genus- and species-level, compared with the commonly used universal reference databases. Importantly, the placeholder names provide a way to explore the unclassified environmental taxa at different taxonomic ranks, which in combination with in situ analyses can be used to uncover their ecological roles.

Prokaryotic diversity in stream sediments affected by acid mine drainage

The microbial communities in mining impacted areas rely on a variety of mechanisms to survive in such extreme environments. In this work, a meta-taxonomic approach using 16S rRNA gene sequences was used to investigate the prokaryotic diversity of sediment samples from water bodies affected by acid mine drainage at the São Domingos mining area in the south of Portugal. Samples were collected in summer and winter from the most contaminated sites from where the water flows downstream to the freshwater of Chança's river reservoir. The prokaryotic diversity on water bodies' sediments allowed us to distinguish the highly contaminated sites (pH ≈ 2) from sites with intermediate levels of contamination (pH ≈ 3-6.5), and from sites without contamination (pH ≈ 7.5). The abundances of acidophiles of genera Acidiphilium, Acidibacter, Acidobacterium and Acidocella in the sediments were correlated with the level of acid mine drainage contamination. The two first genera were among the 30 most abundant prokaryotes in all contaminated samples, including one (SS2w), where the contamination was very diluted, thereby emphasizing the impact that such type of pollution can have in the microbial communities of sediments. In addition, the high abundances of archaeal taxa from class Thermoplasmata and of bacteria from family RCP1-48 in the sediments from the most contaminated site corroborate their importance in such ecosystems and a putative role in the generation of acid mine drainage.

Pilot-scale aerobic granular sludge in the treatment of municipal wastewater: Optimizations in the start-up, methodology of sludge discharge, and evaluation of resource recovery

This work evaluated the formation, maintenance, performance, and microbiology of a pilot-scale aerobic granular sludge reactor treating low-strength municipal wastewater under tropical climate conditions. Additionally, different resource recovery possibilities (phosphorous, tryptophan, and alginate-like exopolysaccharides) were investigated from the produced sludge. Granulation occurred after 35 days without external carbon source supplementation (COD_inf ≈ 461 mg/L; COD/DBO₅ ≈ 3.2). Some protocols were implemented: (i) fat separation to decrease granule flotation; (ii) high exchange rates (60%) during rainy periods to increase the organic load; (iii) selective sludge discharge methodology. After granules formation, optimizations were done to improve reactor performance (COD, BOD, NH₄⁺, and PO₄^3- removals close to 90%), and energy demand reduced from 0.43 (start-up) to 0.25 kWh/m³ (after optimizations). The produced sludge had a high concentration of phosphorus (0.020 g P/g VSS), tryptophan (0.048 g tryptophan/g VSS), and alginate-like exopolysaccharides (0.219 g ALE/g VSS), indicating a good resource recovery possibility.

Fat, oil, and grease (FOG) deposits yield higher methane than FOG in anaerobic co-digestion with waste activated sludge

The formation of fat, oil, and grease (FOG) deposits in sewers is a global challenge for the maintenance of sewer collection systems. Tons of FOG deposits (FDs) are removed from sewer systems every year and present an opportunity for increased methane production via anaerobic co-digestion with waste activated sludge (WAS) at water resource recovery facilities with existing anaerobic digesters. We hypothesized that FDs have higher biomethane potential than that of FOG (e.g., FOG collected in grease interceptors), because of the reduction of inhibition of long chain fatty acids due to saponification. In this study, substantially enhanced methane production was found in anaerobic co-digestion of WAS with FDs within the substrate to inoculum (S/I) ratio range of 0.25-1.2, and the maximum ultimate methane production (685.7 ± 24.1 mL/gVS_added, at S/I = 0.5) was 4.0 times higher than in the control (with WAS only) after 42 days of incubation. Although the lag phase period was longer in FD co-digestion (S/I = 0.5) than in FOG co-digestion (S/I = 0.5) under the same organic loading (gVS) and two times the COD loading, the daily methane production rate became higher after Day 15 in FD co-digestion. Significantly higher cumulative methane production (10.2%, p < 0.05) was obtained in FD co-digestion than in FOG co-digestion after 42-days. Microbial community analysis revealed higher levels of Geobacter in FD co-digestion, possibly suggesting a role for direct interspecies electron transfer (DIET) between Methanosaeta and Geobacter. This work provides fundamental insights supporting anaerobic co-digestion of FDs with WAS, demonstrating the advantages of FDs compared to FOG as co-substrate for enhanced biomethane recovery.

The next frontier of the anaerobic digestion microbiome: From ecology to process control

The anaerobic digestion process has been one of the key processes for renewable energy recovery from organic waste streams for over a century. The anaerobic digestion microbiome is, through the continuous development of novel techniques, evolving from a black box to a well-defined consortium, but we are not there yet. In this perspective, I provide my view on the current status and challenges of the anaerobic digestion microbiome, as well as the opportunities and solutions to exploit it. I consider identification and fingerprinting of the anaerobic digestion microbiome as complementary tools to monitor the anaerobic digestion microbiome. However, data availability, method-inherent biases and correct taxa identification hamper the accuracy and reproducibility of anaerobic digestion microbiome characterization. Standardisation of microbiome research in anaerobic digestion and other engineered systems will be essential in the coming decades, for which I proposed some targeted solutions. These will bring anaerobic digestion from a single-purpose energy-recovery technology to a versatile process for integrated resource recovery. It is my opinion that the exploitation of the microbiome will be a driver of innovation, and that it has a key role to play in the bio-based economy of the decades to come.

Candidatus Amarolinea and Candidatus Microthrix Are Mainly Responsible for Filamentous Bulking in Danish Municipal Wastewater Treatment Plants

Filamentous bulking is a common serious operational problem leading to deteriorated sludge settling that has long been observed in activated sludge biological wastewater treatment systems. A number of bacterial genera found therein possess filamentous morphology, where some have been shown to be implicated in bulking episodes (e.g., Ca. Microthrix), the impact of many others is still not clear. In this study we performed a survey of 17 Danish municipal wastewater treatment plants (WWTPs) with nutrient removal using 16S rRNA amplicon sequencing over a period of 13 years, where all known filamentous bacteria from 30 genera were analyzed. The filamentous community constituted on average 13 ± 6%, and up to 43% of total read abundance with the same genera common to all plants. Ca. Microthrix and several genera belonging to phylum Chloroflexi were among the most abundant filamentous bacteria. The effect of filamentous bacteria on sludge settling properties was analyzed using measurements of the diluted sludge volume index (DSVI). Strong positive correlations with DSVI were observed only for Ca. Microthrix and Ca. Amarolinea, the latter being a novel, recently characterized genus belonging to the phylum Chloroflexi. The bulking potential of other filamentous bacteria was not significant despite their presence in many plants. Low phylogenetic diversity was observed for both Ca. Microthrix and Ca. Amarolinea, making physiological characterization of individual species and potential development of control strategies more feasible. In this study we show that, despite the high diversity of filamentous phylotypes in Danish WWTPs, only few of them were responsible for severe bulking episodes.

Construction of habitat-specific training sets to achieve species-level assignment in 16S rRNA gene datasets

BACKGROUND

The low cost of 16S rRNA gene sequencing facilitates population-scale molecular epidemiological studies. Existing computational algorithms can resolve 16S rRNA gene sequences into high-resolution amplicon sequence variants (ASVs), which represent consistent labels comparable across studies. Assigning these ASVs to species-level taxonomy strengthens the ecological and/or clinical relevance of 16S rRNA gene-based microbiota studies and further facilitates data comparison across studies.

RESULTS

To achieve this, we developed a broadly applicable method for constructing high-resolution training sets based on the phylogenic relationships among microbes found in a habitat of interest. When used with the naïve Bayesian Ribosomal Database Project (RDP) Classifier, this training set achieved species/supraspecies-level taxonomic assignment of 16S rRNA gene-derived ASVs. The key steps for generating such a training set are (1) constructing an accurate and comprehensive phylogenetic-based, habitat-specific database; (2) compiling multiple 16S rRNA gene sequences to represent the natural sequence variability of each taxon in the database; (3) trimming the training set to match the sequenced regions, if necessary; and (4) placing species sharing closely related sequences into a training-set-specific supraspecies taxonomic level to preserve subgenus-level resolution. As proof of principle, we developed a V1-V3 region training set for the bacterial microbiota of the human aerodigestive tract using the full-length 16S rRNA gene reference sequences compiled in our expanded Human Oral Microbiome Database (eHOMD). We also overcame technical limitations to successfully use Illumina sequences for the 16S rRNA gene V1-V3 region, the most informative segment for classifying bacteria native to the human aerodigestive tract. Finally, we generated a full-length eHOMD 16S rRNA gene training set, which we used in conjunction with an independent PacBio single molecule, real-time (SMRT)-sequenced sinonasal dataset to validate the representation of species in our training set. This also established the effectiveness of a full-length training set for assigning taxonomy of long-read 16S rRNA gene datasets.

CONCLUSION

Here, we present a systematic approach for constructing a phylogeny-based, high-resolution, habitat-specific training set that permits species/supraspecies-level taxonomic assignment to short- and long-read 16S rRNA gene-derived ASVs. This advancement enhances the ecological and/or clinical relevance of 16S rRNA gene-based microbiota studies. Video Abstract.