Potential of acetic acid to restore methane production in anaerobic reactors critically intoxicated by ammonia as evidenced by metabolic and microbial monitoring.

BACKGROUND: Biogas and biomethane production from the on-farm anaerobic digestion (AD) of animal manure and agri-food wastes could play a key role in transforming Europe's energy system by mitigating its dependence on fossil fuels and tackling the climate crisis. Although ammonia is essential for microbial growth, it inhibits the AD process if present in high concentrations, especially under its free form, thus leading to economic losses. In this study, which includes both metabolic and microbial monitoring, we tested a strategy to restore substrate conversion to methane in AD reactors facing critical free ammonia intoxication. RESULTS: The AD process of three mesophilic semi-continuous 100L reactors critically intoxicated by free ammonia (> 3.5 g_N L-1; inhibited hydrolysis and heterotrophic acetogenesis; interrupted methanogenesis) was restored by applying a strategy that included reducing pH using acetic acid, washing out total ammonia with water, re-inoculation with active microbial flora and progressively re-introducing sugar beet pulp as a feed substrate. After 5 weeks, two reactors restarted to hydrolyse the pulp and produced CH4 from the methylotrophic methanogenesis pathway. The acetoclastic pathway remained inhibited due to the transient dominance of a strictly methylotrophic methanogen (Candidatus Methanoplasma genus) to the detriment of Methanosarcina. Concomitantly, the third reactor, in which Methanosarcina remained dominant, produced CH4 from the acetoclastic pathway but faced hydrolysis inhibition. After 11 weeks, the hydrolysis, the acetoclastic pathway and possibly the hydrogenotrophic pathway were functional in all reactors. The methylotrophic pathway was no longer favoured. Although syntrophic propionate oxidation remained suboptimal, the final pulp to CH4 conversion ratio (0.41 ± 0.10 LN_CH4 g_VS-1) was analogous to the pulp biochemical methane potential (0.38 ± 0.03 LN_CH4 g_VS-1). CONCLUSIONS: Despite an extreme free ammonia intoxication, the proposed process recovery strategy allowed CH4 production to be restored in three intoxicated reactors within 8 weeks, a period during which re-inoculation appeared to be crucial to sustain the process. Introducing acetic acid allowed substantial CH4 production during the recovery period. Furthermore, the initial pH reduction promoted ammonium capture in the slurry, which could allow the field application of the effluents produced by full-scale digesters recovering from ammonia intoxication.

A holobiont approach towards polysaccharide degradation by the highly compartmentalised gut system of the soil-feeding higher termite Labiotermes labralis.

BACKGROUND: Termites are among the most successful insects on Earth and can feed on a broad range of organic matter at various stages of decomposition. The termite gut system is often referred to as a micro-reactor and is a complex structure consisting of several components. It includes the host, its gut microbiome and fungal gardens, in the case of fungi-growing higher termites. The digestive tract of soil-feeding higher termites is characterised by radial and axial gradients of physicochemical parameters (e.g. pH, O2 and H2 partial pressure), and also differs in the density and structure of residing microbial communities. Although soil-feeding termites account for 60% of the known termite species, their biomass degradation strategies are far less known compared to their wood-feeding counterparts. RESULTS: In this work, we applied an integrative multi-omics approach for the first time at the holobiont level to study the highly compartmentalised gut system of the soil-feeding higher termite Labiotermes labralis. We relied on 16S rRNA gene community profiling, metagenomics and (meta)transcriptomics to uncover the distribution of functional roles, in particular those related to carbohydrate hydrolysis, across different gut compartments and among the members of the bacterial community and the host itself. We showed that the Labiotermes gut was dominated by members of the Firmicutes phylum, whose abundance gradually decreased towards the posterior segments of the hindgut, in favour of Bacteroidetes, Proteobacteria and Verrucomicrobia. Contrary to expectations, we observed that L. labralis gut microbes expressed a high diversity of carbohydrate active enzymes involved in cellulose and hemicelluloses degradation, making the soil-feeding termite gut a unique reservoir of lignocellulolytic enzymes with considerable biotechnological potential. We also evidenced that the host cellulases have different phylogenetic origins and structures, which is possibly translated into their different specificities towards cellulose. From an ecological perspective, we could speculate that the capacity to feed on distinct polymorphs of cellulose retained in soil might have enabled this termite species to widely colonise the different habitats of the Amazon basin. CONCLUSIONS: Our study provides interesting insights into the distribution of the hydrolytic potential of the highly compartmentalised higher termite gut. The large number of expressed enzymes targeting the different lignocellulose components make the Labiotermes worker gut a relevant lignocellulose-valorising model to mimic by biomass conversion industries.

Effluent solids recirculation to municipal sludge digesters enhances long-chain fatty acids degradation capacity.

BACKGROUND: Slow degradation kinetics of long-chain fatty acids (LCFA) and their accumulation in anaerobic digesters disrupt methanogenic activity and biogas production at high loads of waste lipids. In this study, we evaluated the effect of effluent solids recirculation on microbial LCFA (oleate) degradation capacity in continuous stirred-tank sludge digesters, with the overall aim of providing operating conditions for efficient co-digestion of waste lipids. Furthermore, the impacts of LCFA feeding frequency and sulfide on process performance and microbial community dynamics were investigated, as parameters that were previously shown to be influential on LCFA conversion to biogas. RESULTS: Effluent solids recirculation to municipal sludge digesters enabled biogas production of up to 78% of the theoretical potential from 1.0 g oleate l-1 day-1. In digesters without effluent recirculation, comparable conversion efficiency could only be reached at oleate loading rates up to 0.5 g l-1 day-1. Pulse feeding of oleate (supplementation of 2.0 g oleate l-1 every second day instead of 1.0 g oleate l-1 every day) did not have a substantial impact on the degree of oleate conversion to biogas in the digesters that operated with effluent recirculation, while it marginally enhanced oleate conversion to biogas in the digesters without effluent recirculation. Next-generation sequencing of 16S rRNA gene amplicons of bacteria and archaea revealed that pulse feeding resulted in prevalence of fatty acid-degrading Smithella when effluent recirculation was applied, whereas Candidatus Cloacimonas prevailed after pulse feeding of oleate in the digesters without effluent recirculation. Combined oleate pulse feeding and elevated sulfide level contributed to increased relative abundance of LCFA-degrading Syntrophomonas and enhanced conversion efficiency of oleate, but only in the digesters without effluent recirculation. CONCLUSIONS: Effluent solids recirculation improves microbial LCFA degradation capacity, providing possibilities for co-digestion of larger amounts of waste lipids with municipal sludge.

Integrative omics analysis of the termite gut system adaptation to Miscanthus diet identifies lignocellulose degradation enzymes.

Miscanthus sp. biomass could satisfy future biorefinery value chains. However, its use is largely untapped due to high recalcitrance. The termite and its gut microbiome are considered the most efficient lignocellulose degrading system in nature. Here, we investigate at holobiont level the dynamic adaptation of Cortaritermes sp. to imposed Miscanthus diet, with a long-term objective of overcoming lignocellulose recalcitrance. We use an integrative omics approach combined with enzymatic characterisation of carbohydrate active enzymes from termite gut Fibrobacteres and Spirochaetae. Modified gene expression profiles of gut bacteria suggest a shift towards utilisation of cellulose and arabinoxylan, two main components of Miscanthus lignocellulose. Low identity of reconstructed microbial genomes to closely related species supports the hypothesis of a strong phylogenetic relationship between host and its gut microbiome. This study provides a framework for better understanding the complex lignocellulose degradation by the higher termite gut system and paves a road towards its future bioprospecting.

Compositional and functional characterisation of biomass-degrading microbial communities in guts of plant fibre- and soil-feeding higher termites.

BACKGROUND: Termites are among the most successful insect lineages on the globe and are responsible for providing numerous ecosystem services. They mainly feed on wood and other plant material at different stages of humification. Lignocellulose is often a principal component of such plant diet, and termites largely rely on their symbiotic microbiota and associated enzymes to decompose their food efficiently. While lower termites and their gut flagellates were given larger scientific attention in the past, the gut lignocellulolytic bacteria of higher termites remain less explored. Therefore, in this study, we investigated the structure and function of gut prokaryotic microbiomes from 11 higher termite genera representative of Syntermitinae, Apicotermitinae, Termitidae and Nasutitermitinae subfamilies, broadly grouped into plant fibre- and soil-feeding termite categories. RESULTS: Despite the different compositional structures of the studied termite gut microbiomes, reflecting well the diet and host lineage, we observed a surprisingly high functional congruency between gut metatranscriptomes from both feeding groups. The abundance of transcripts encoding for carbohydrate active enzymes as well as expression and diversity profiles of assigned glycoside hydrolase families were also similar between plant fibre- and soil-feeding termites. Yet, dietary imprints highlighted subtle metabolic differences specific to each feeding category. Roughly, 0.18% of de novo re-constructed gene transcripts were shared between the different termite gut microbiomes, making each termite gut a unique reservoir of genes encoding for potentially industrially applicable enzymes, e.g. relevant to biomass degradation. Taken together, we demonstrated the functional equivalence in microbial populations across different termite hosts. CONCLUSIONS: Our results provide valuable insight into the bacterial component of the termite gut system and significantly expand the inventory of termite prokaryotic genes participating in the deconstruction of plant biomass. Video Abstract.

Carbohydrate Hydrolytic Potential and Redundancy of an Anaerobic Digestion Microbiome Exposed to Acidosis, as Uncovered by Metagenomics.

Increased hydrolysis of easily digestible biomass may lead to acidosis of anaerobic reactors and decreased methane production. Previously, it was shown that the structure of microbial communities changed during acidosis; however, once the conditions are back to optimal, biogas (initially CO2) production quickly restarts. This suggests the retention of the community functional redundancy during the process failure. In this study, with the use of metagenomics and downstream bioinformatics analyses, we characterize the carbohydrate hydrolytic potential of the microbial community, with a special focus on acidosis. To that purpose, carbohydrate-active enzymes were identified, and to further link the community hydrolytic potential with key microbes, bacterial genomes were reconstructed. In addition, we characterized biochemically the specificity and activity of selected enzymes, thus verifying the accuracy of the in silico predictions. The results confirm the retention of the community hydrolytic potential during acidosis and indicate Bacteroidetes to be largely involved in biomass degradation. Bacteroidetes showed higher diversity and genomic content of carbohydrate hydrolytic enzymes that might favor the dominance of this phylum over other bacteria in some anaerobic reactors. The combination of bioinformatic analyses and activity tests enabled us to propose a model of acetylated glucomannan degradation by BacteroidetesIMPORTANCE The enzymatic hydrolysis of lignocellulosic biomass is mainly driven by the action of carbohydrate-active enzymes. By characterizing the gene profiles at the different stages of the anaerobic digestion experiment, we showed that the microbiome retains its hydrolytic functional redundancy even during severe acidosis, despite significant changes in taxonomic composition. By analyzing reconstructed bacterial genomes, we demonstrate that Bacteroidetes hydrolytic gene diversity likely favors the abundance of this phylum in some anaerobic digestion systems. Further, we observe genetic redundancy within the Bacteroidetes group, which accounts for the preserved hydrolytic potential during acidosis. This work also uncovers new polysaccharide utilization loci involved in the deconstruction of various biomasses and proposes the model of acetylated glucomannan degradation by Bacteroidetes Acetylated glucomannan-enriched biomass is a common substrate for many industries, including pulp and paper production. Using naturally evolved cocktails of enzymes for biomass pretreatment could be an interesting alternative to the commonly used chemical pretreatments.

Community diversity and potential functions of rhizosphere-associated bacteria of nickel hyperaccumulators found in Albania.

Ultramafic (i.e. serpentine) soils are widespread in the Balkans and particularly in Albania. They account for a large part of plant endemism in that region and host several hyperaccumulator species, which are characterized by leaf nickel concentrations frequently above 1%. This rich nickel hyperaccumulating flora could serve as candidate to be used in phytoextraction and agromining. Despite recent interest in metal hyperaccumulating plants and agromining, very few studies have investigated the bacterial diversity and the influence of environmental factors on microbial gene profiles in the rhizosphere of hyperaccumulator plants growing on ultramafic soils. Because rhizospheric bacteria could be crucial to the success of phytoremediation, we studied a total of 48 nickel-hyperaccumulating plants which were sampled from four species that are widespread in Albania: Noccaea ochroleuca, Odontarrhena smolikana, O. rigida and O. chalcidica. All samples were taken from the ultramafic regions of Librazhd and Pogradec in eastern Albania in October 2015. Our study shows that Proteobacteria, Actinobacteria and Acidobacteria dominated the soil bacterial communities. Of these three phyla, only Proteobacteria was relatively abundant. This study underlines the influence of soil Cation Exchange Capacity on the bacterial community's diversity and structure. Based on the predicted metagenomes, the genes belonging to amino acid, lipid and carbohydrate metabolisms were identified as major gene families. Our study sheds some light on our understanding of how bacterial communities are structured within and affect the rhizosphere of hyperaccumulator plants from ultramafic soils in Albania.

A year of monitoring 20 mesophilic full-scale bioreactors reveals the existence of stable but different core microbiomes in bio-waste and wastewater anaerobic digestion systems.

BACKGROUND: Anaerobic digestion (AD) is a microbe-driven process of biomass decomposition to CH4 and CO2. In addition to renewable and cost-effective energy production, AD has emerged in the European Union as an environmentally friendly model of bio-waste valorisation and nutrient recycling. Nevertheless, due to the high diversity of uncharacterised microbes, a typical AD microbiome is still considered as "dark matter". RESULTS: Using the high-throughput sequencing of small rRNA gene, and a monthly monitoring of the physicochemical parameters for 20 different mesophilic full-scale bioreactors over 1 year, we generated a detailed view of AD microbial ecology towards a better understanding of factors that influence and shape these communities. By studying the broadly distributed OTUs present in over 80% of analysed samples, we identified putatively important core bacteria and archaea to the AD process that accounted for over 70% of the whole microbial community relative abundances. AD reactors localised at the wastewater treatment plants were shown to operate with distinct core microbiomes than the agricultural and bio-waste treating biogas units. We also showed that both the core microbiomes were composed of low (with average community abundance ≤ 1%) and highly abundant microbial populations; the vast majority of which remains yet uncharacterised, e.g. abundant candidate Cloacimonetes. Using non-metric multidimensional scaling, we observed microorganisms grouping into clusters that well reflected the origin of the samples, e.g. wastewater versus agricultural and bio-waste treating biogas units. The calculated diversity patterns differed markedly between the different community clusters, mainly due to the presence of highly diverse and dynamic transient species. Core microbial communities appeared relatively stable over the monitoring period. CONCLUSIONS: In this study, we characterised microbial communities in different AD systems that were monitored over a 1-year period. Evidences were shown to support the concept of a core community driving the AD process, whereas the vast majority of dominant microorganisms remain yet to be characterised.

Influence of new agromining cropping systems on soil bacterial diversity and the physico-chemical characteristics of an ultramafic soil.

Most of the research dedicated to agromining has focused on cultivating a single hyperaccumulator plant, although plant diversity has been shown to positively modify soil characteristics. Hence, we compared the effect of cropping a nickel-hyperaccumulator Alyssum murale with a legume (Vicia sativa) to A. murale's mono-culture, on the bacterial diversity and physico-chemical characteristics of an ultramafic soil. A pot experiment with 5 replicates was conducted in controlled conditions for 11 months. The treatments studied were: co-cropping and rotation vs. mineral fertilization controls and bare soil. The introduction of legumes induced a clearly positive effect on the soil's microbial biomass carbon and nitrogen. Arylsulfatase and urease activities tended to be enhanced in the co-cropping and rotation treatments and to be lessened in the mineral fertilization treatments. However, ß-glucosidase and phosphatase activities were seen to decrease when legumes were used. Our results showed that the rotation treatment induced a higher organic matter content than the fertilized control did. Actinobacteria was the most-represented bacterial phyla and had lower relative abundance in treatments associating legumes. Conversely, the relative abundance of Acidobacteria and Gemmatimonadetes phyla increased but not significantly in treatments with legumes. The relative abundance of Chloroflexi phylum was shown to be significantly higher for the fertilized rotation control. The relative abundance of ß-Proteobacteria subphylum increased but not significantly in treatments with legumes. NMDS analysis showed a clear separation between planted treatments and bare soil and between co-cropping and rotation and fertilized controls. Shannon index showed reduction in microbial diversity that was mainly due to chemical inputs in the soil. This study showed that these new cropping systems influenced both the bacterial diversity and the physico-chemical characteristics of an ultramafic soil. In addition, this study provides evidence that mineral fertilization can negatively impact bacterial communities and some of their functions linked to biogeochemical cycles.

Optimization of a metatranscriptomic approach to study the lignocellulolytic potential of the higher termite gut microbiome.

BACKGROUND: Thanks to specific adaptations developed over millions of years, the efficiency of lignin, cellulose and hemicellulose decomposition of higher termite symbiotic system exceeds that of many other lignocellulose utilizing environments. Especially, the examination of its symbiotic microbes should reveal interesting carbohydrate-active enzymes, which are of primary interest for the industry. Previous metatranscriptomic reports (high-throughput mRNA sequencing) highlight the high representation and overexpression of cellulose and hemicelluloses degrading genes in the termite hindgut digestomes, indicating the potential of this technology in search for new enzymes. Nevertheless, several factors associated with the material sampling and library preparation steps make the metatranscriptomic studies of termite gut prokaryotic symbionts challenging. METHODS: In this study, we first examined the influence of the sampling strategy, including the whole termite gut and luminal fluid, on the diversity and the metatranscriptomic profiles of the higher termite gut symbiotic bacteria. Secondly, we evaluated different commercially available kits combined in two library preparative pipelines for the best bacterial mRNA enrichment strategy. RESULTS: We showed that the sampling strategy did not significantly impact the generated results, both in terms of the representation of the microbes and their transcriptomic profiles. Nevertheless collecting luminal fluid reduces the co-amplification of unwanted RNA species of host origin. Furthermore, for the four studied higher termite species, the library preparative pipeline employing Ribo-Zero Gold rRNA Removal Kit "Epidemiology" in combination with Poly(A) Purist MAG kit resulted in a more efficient rRNA and poly-A-mRNAdepletion (up to 98.44% rRNA removed) than the pipeline utilizing MICROBExpress and MICROBEnrich kits. High correlation of both Ribo-Zero and MICROBExpresse depleted gene expression profiles with total non-depleted RNA-seq data has been shown for all studied samples, indicating no systematic skewing of the studied pipelines. CONCLUSIONS: We have extensively evaluated the impact of the sampling strategy and library preparation steps on the metatranscriptomic profiles of the higher termite gut symbiotic bacteria. The presented methodological approach has great potential to enhance metatranscriptomic studies of the higher termite intestinal flora and to unravel novel carbohydrate-active enzymes.

ICoVeR - an interactive visualization tool for verification and refinement of metagenomic bins.

BACKGROUND: Recent advances in high-throughput sequencing allow for much deeper exploitation of natural and engineered microbial communities, and to unravel so-called "microbial dark matter" (microbes that until now have evaded cultivation). Metagenomic analyses result in a large number of genomic fragments (contigs) that need to be grouped (binned) in order to reconstruct draft microbial genomes. While several contig binning algorithms have been developed in the past 2 years, they often lack consensus. Furthermore, these software tools typically lack a provision for the visualization of data and bin characteristics. RESULTS: We present ICoVeR, the Interactive Contig-bin Verification and Refinement tool, which allows the visualization of genome bins. More specifically, ICoVeR allows curation of bin assignments based on multiple binning algorithms. Its visualization window is composed of two connected and interactive main views, including a parallel coordinates view and a dimensionality reduction plot. To demonstrate ICoVeR's utility, we used it to refine disparate genome bins automatically generated using MetaBAT, CONCOCT and MyCC for an anaerobic digestion metagenomic (AD microbiome) dataset. Out of 31 refined genome bins, 23 were characterized with higher completeness and lower contamination in comparison to their respective, automatically generated, genome bins. Additionally, to benchmark ICoVeR against a previously validated dataset, we used Sharon's dataset representing an infant gut metagenome. CONCLUSIONS: ICoVeR is an open source software package that allows curation of disparate genome bins generated with automatic binning algorithms. It is freely available under the GPLv3 license at https://git.list.lu/eScience/ICoVeR . The data management and analytical functions of ICoVeR are implemented in R, therefore the software can be easily installed on any system for which R is available. Installation and usage guide together with the example files ready to be visualized are also provided via the project wiki. ICoVeR running instance preloaded with AD microbiome and Sharon's datasets can be accessed via the website.

Start-up phase of an anaerobic full-scale farm reactor - Appearance of mesophilic anaerobic conditions and establishment of the methanogenic microbial community.

The goal of this study was to investigate how the microbial community structure establishes during the start-up phase of a full-scale farm anaerobic reactor inoculated with stale and cold cattle slurry. The 16S/18S high-throughput amplicon sequencing results showed an increase of the bacterial, archaeal and eukaryotic diversity, evenness and richness during the settlement of the mesophilic anaerobic conditions. When a steady performing digestion process was reached, the microbial diversity, evenness and richness decreased, indicating the establishment of a few dominant microbial populations, best adapted to biogas production. Interestingly, among the environmental parameters, the temperature, alkalinity, free-NH3, total solids and O2 content were found to be the main drivers of microbial dynamics. Interactions between eukaryotes, characterized by a high number of unknown organisms, and the bacterial and archaeal communities were also evidenced, suggesting that eukaryotes might play important roles in the anaerobic digestion process.

Analysis of dsDNA and RNA viromes in methanogenic digesters reveals novel viral genetic diversity.

Although viruses are not the key players of the anaerobic digestion process, they may affect the dynamics of bacterial and archaeal populations involved in biogas production. Until now viruses have received very little attention in this specific habitat; therefore, as a first step towards their characterization, we optimized a virus filtration protocol from anaerobic sludge. Afterwards, to assess dsDNA and RNA viral diversity in sludge samples from nine different reactors fed either with waste water, agricultural residues or solid municipal waste plus agro-food residues, we performed metagenomic analyses. As a result we showed that, while the dsDNA viromes (21 assigned families in total) were dominated by dsDNA phages of the order Caudovirales, RNA viruses (14 assigned families in total) were less diverse and were for the main part plant-infecting viruses. Interestingly, less than 2% of annotated contigs were assigned as putative human and animal pathogens. Our study greatly extends the existing view of viral genetic diversity in methanogenic reactors and shows that these viral assemblages are distinct not only among the reactor types but also from nearly 30 other environments already studied, including the human gut, fermented food, deep sea sediments and other aquatic habitats.

Microbial community dynamics in replicate anaerobic digesters exposed sequentially to increasing organic loading rate, acidosis, and process recovery.

BACKGROUND: Volatile fatty acid intoxication (acidosis), a common process failure recorded in anaerobic reactors, leads to drastic losses in methane production. Unfortunately, little is known about the microbial mechanisms underlining acidosis and the potential to recover the process. In this study, triplicate mesophilic anaerobic reactors of 100 L were exposed to acidosis resulting from an excessive feeding with sugar beet pulp and were compared to a steady-state reactor. RESULTS: Stable operational conditions at the beginning of the experiment initially led to similar microbial populations in the four reactors, as revealed by 16S rRNA gene T-RFLP and high-throughput amplicon sequencing. Bacteroidetes and Firmicutes were the two dominant phyla, and although they were represented by a high number of operational taxonomic units, only a few were dominant. Once the environment became deterministic (selective pressure from an increased substrate feeding), microbial populations started to diverge between the overfed reactors. Interestingly, most of bacteria and archaea showed redundant functional adaptation to the changing environmental conditions. However, the dominant Bacteroidales were resistant to high volatile fatty acids content and low pH. The severe acidosis did not eradicate archaea and a clear shift in archaeal populations from acetotrophic to hydrogenotrophic methanogenesis occurred in the overfed reactors. After 11 days of severe acidosis (pH 5.2 ± 0.4), the process was quickly recovered (restoration of the biogas production with methane content above 50 %) in the overfed reactors, by adjusting the pH to around 7 using NaOH and NaHCO3. CONCLUSIONS: In this study we show that once the replicate reactors are confronted with sub-optimal conditions, their microbial populations start to evolve differentially. Furthermore the alterations of commonly used microbial parameters to monitor the process, such as richness, evenness and diversity indices were unsuccessful to predict the process failure. At the same time, we tentatively propose the replacement of the dominant Methanosaeta sp. in this case by Methanoculleus sp., to be a potential warning indicator of acidosis.

Upscaling of an electronic nose for completely stirred tank reactor stability monitoring from pilot-scale to real-scale agricultural co-digestion biogas plant.

This study investigated the use of an electronic nose for on-line anaerobic reactor state monitoring at the pilot-scale level and then upscaling to the full-scale level. E-nose indicator was compared to classical state indicators such as pH, alkalinity, volatile fatty acids concentration and to other gas phase compounds. Multivariate statistical process control method, based on principal component analysis and the Hotelling's T(2) statistics was used to derive an indicator representative of the reactor state. At the pilot-scale level, the e-nose indicator was relevant and could distinguish 3 process states: steady-state, transient and collapsing process. At the full-scale level, the e-nose indicator could provide the warning of the major disturbance whereas two slight disturbances were not detected and it gave one major false alarm. This work showed that gas phase relation with anaerobic process should be deeper investigated, as an e-nose could indicate the reactor state, focusing on the gas phase.