Comparison of functional microbial prole between mesophilic and thermophilic anaerobic digestion of vegetable waste

With increasing accumulation of vegetable waste in China, the valorization of vegetable waste becomes an urgent concern. Based on the characteristics of high moisture content and biodegradable organic matter of vegetable waste, anaerobic digestion as an effective technique was selected to reuse this kind of agriculture waste. The anaerobic digestion performance is highly correlated with the functional microbial community. In this study, mesophilic and thermophilic digestions of vegetable waste were conducted, and dynamics of the microbial community were investigated. The mesophilic and thermophilic collapsed stages occurred at organic loading rates of 1.5 and 2.0 g volatile solid (VS)/(L·d), respectively, due to severe accumulation of volatile fatty acids. The mesophilic digestion exhibited a higher microbial diversity and richness than the thermophilic digestion. Syntrophic acetate-oxidizing coupled with hydrogenotrophic methanogenesis was the dominant pathway in the thermophilic stable system, and acetoclastic methanogenesis in the mesophilic stable system. The dominant acidogens, syntrophus, and methanogens were Candidatus_Cloacamonas, norank_f__Synergistaceae, Methanosaeta, and Methanosarcina in the mesophilic stable stage, and Anaerobaculum, Syntrophaceticus, Methanosarcina, and Methanothermobacter in thermophilic stable stage. Spirochaetae and Thermotogae phyla were the characteristic microorganisms in the mesophilic and thermophilic collapsed stages, respectively. This study unveiled the distribution of the functional microbial consortia at the stable and collapsed periods of the anaerobic digestion of vegetable waste under mesophilic and thermophilic conditions to providing guidelines for the further research of anaerobic digestion of vegetable waste. in the mesophilic stable stage, and Anaerobaculum, Syntrophaceticus, Methanosarcina, and Methanothermobacter in the thermophilic stable stage.


Introduction
The accumulation of vegetable waste (VW), originally produced from vegetable planting eld and secondly collected from wholesale and farmer's markets, has caused severe rural and urban environmental concern. According to China Statistical Abstract 2015, approximately 760 × 10 6 t of VW was generated in 2014 in China [1]. With high moisture content and biodegradable characteristics, the land ll of VW generates a harmful leachate, which leads to adverse environmental effects. In order to achieve the green-sustainable agriculture development, exploring an effective method to realize the valorization of VW is necessary. VW comprises 75% easily biodegradable organic matter (i.e., sugar, hemicellulose) and 15% recalcitrant organic matter (i.e., cellulose, lignin) [2]. Such types of organic waste are suitable for anaerobic digestion (AD) to produce biogas for energy generation and organic fertilizer for agricultural application [3]. Li et al. studied the performance and stability of the AD of VW under thermophilic condition, and showed that VW was easily acidi ed and could only function stably at an organic loading rate (OLR) below 1.5 g volatile solid (VS)/(L • d) [1]. To improve the stability of the AD of VW, two-phase AD for VW was conducted by Zuo et al., which contributed to the improvement of stable OLR threshold from 2.6 to 3 g VS/(L·d) [4]. However, the composition and distribution of the microbial community in the AD of VW, which is the key factor for its stable operation, are not yet understood.
VW undergoes a fast degradation by fermentative and acidogenic bacterial consortium, which generates a large amount of volatile fatty acids (VFAs) that lead to rapid acidi cation during the AD process [2]. The acidic environment has been found to seriously inhibit the activities of methanogens. Li et al. quoted that the methanogens in an AD reactor would face acidic inhibition when the pH value is below 6.5 [5]. Finally, the collapse of the AD system is occurred. Three types of microbial consortia are involved in an AD system, including fermentative and acidogenic bacterial consortium, syntrophic bacterial consortium, and methanogenic consortium [6]. The stability of the AD process is highly relevant to the balance between the different groups of functional microbial consortia. Therefore, it is signi cant to unveil the speci c functional microbial consortium for a better understanding of the AD system of VW.
Temperature is a key environmental factor that affects the AD performance signi cantly. Generally, AD conducts at mesophilic (30-40 °C) and thermophilic (50-60 °C) conditions. In fact, temperature was found to be the main cause that causes the apparent shifts of different types of functional microorganisms, which in turn in uence the performance of AD. The mesophilic and thermophilic AD of soybean curd residue was conducted by Zhang et al., who systemically analyzed the microbial compositions and dynamics under different temperature conditions [7]. Ryue et al. also compared the differences of the microbial community structures between mesophilic and thermophilic AD for food waste [8]. Nonetheless, little is known about the differences of the functional microbial consortia between the mesophilic and thermophilic AD of VW.
The present study systematically unveiled the distribution of the functional microbial consortia at the stable and collapsed periods of the AD of VW under mesophilic and thermophilic conditions. Two continuous stirred-tank reactors (CSTRs) fed with VW were conducted under anaerobic mesophilic and thermophilic conditions. The parameters of the gaseous and liquid phases were monitored daily to evaluate the performance of the AD for VW. The microbial community shifts at different periods were analyzed via high-throughput sequencing technology.

Substrate and inoculum
VW was acquired from Chengdu HIGREEN wholesale vegetable market, and contained cabbage and lettuce leaves among others. Before being fed into the mesophilic and thermophilic reactors, the VW was chopped into small pieces (4-5 mm) and mixed homogeneously, after which they were stored in a refrigerator at 4 °C. The total solid (TS) and volatile solid (VS) contents of the VW were 105.7 g/kg and 92.1% of TS, respectively. The crude protein, lipid, and ber of VW were 13.2% of TS, 3.4% of TS, and 11.9% of TS. The C/N ratio of VW were 17.1. The original inoculum was gained from a mesophilic (35 ± 2 °C) anaerobic digester fed with swine manure. The inoculum was introduced into a mesophilic reactor under anaerobic conditions, and acclimated for about 30 days via feeding the VW with an OLR of 0.5 g VS/(L·d) into the reactor daily until the methane content was above 60%. The inoculum in the thermophilic anaerobic reactor was also acclimated for about 30 days by stepwise increasing the temperature (i.e. increase 1 °C per day) from ambient temperature to 55 ± 2 °C while simultaneously feeding the VW with an OLR of 0.5 g VS/(L·d) into the reactor daily until the methane content was above 60%. The TS and VS contents and pH value of the original inoculum were 11.2 g/kg, 72.5% TS, and 7.6, respectively.

Experimental setup and design
The experimental setup comprised two CSTRs. Each reactor has a total and working volume of 70 L and 55 L, respectively. Furthermore, a set of self-designed online monitor systems was also equipped on the reactors to record the daily changes of biogas production, temperature, and pH. The operational temperatures for the mesophilic and thermophilic reactors were maintained at 35 ± 2 °C and 55 ± 2 °C using heating jackets, respectively. The contents in the reactors were stirred 8 times per day at 40 rpm for 30 min via the impellers, which were equipped with two 200 mm × 40 mm at blades. The mesophilic AD of the VW was conducted at OLRs of 0.5, 1.0, and 1.5 g VS/(L·d) and a xed hydrolytic retention time (HRT) of 20 days. The thermophilic AD of the VW was carried out at OLRs of 0.5, 1.0, 1.5, 2.0, and 2.5 g VS/(L·d) and a xed HTR of 20 days. According to the OLR values, a certain amount of mixture (i.e., VW mixed with water, 2,750 g) was fed into the reactors daily after the same amount of digestate (2,750 g) was discharged from the outlet. The overall running times for the mesophilic and thermophilic experiments were 90 and 129 days, respectively.

Analytical methods
The TS and VS contents were measured using the standard techniques provided by [9]. The compositions of crude ber, crude lipid, and crude protein in the VW were measured referring to Chinese standard methods (GB/T 5009 − 2003). A gas owmeter (Beijing Sevenstar Electronics Co., Ltd, China) was applied to measure the biogas production. The biogas contents (i.e., methane and carbon dioxide) were analyzed via an Agilent 7890A gas chromatograph (Agilent Technologies, Santa Clara, CA, USA). The daily pH value of the liquid digestate in the reactor was determined online. Liquid samples were obtained daily by centrifuging the liquid digestate at 12,000 rpm for 10 min to remove the large insoluble particles, after which the centrifuged liquid samples were ltered with a 0.45-µm membrane lter to analyze the total ammonium nitrogen (TAN) and VFA concentrations. The concentrations of the TAN and VFAs were measured using the methods described by Li, Ran [10].

Sequence processing and analysis
The digestate samples were extracted from the mesophilic and thermophilic reactors at the stable stages (days 46, 50, and 54; and days 46, 50, and 55, respectively) and the collapsed stages (days 62, 72, 81, and 89; and days 100, 117, and 126, respectively). These stages are de ned in Sect. 3. Each of the collected samples (50 ml) were centrifuged at 5,000 rpm for 10 min. The supernatant was then removed and the remaining solid digestate was stored in a freezer at -80 °C for further analysis. Microbial DNA was extracted using E.Z.N.A DNA Extraction Kit for soil (Omega Biotek, Norcross, GA, USA) following the manufacturer's protocols. Furthermore, universal 16S rRNA gene primers of 515F and 806R were used to perform polymerase chain reaction with TransStart FastPfu DNA Polymerase (TransGen Biotech, Beijing, China) [11]. The ampli ed products were further puri ed, and the puri ed amplicons were pooled in equimolar concentrations and paired-end sequenced (2 × 300 bp) on an Illumina MiSeq platform The microbial community structure and dynamics at different reactors and operational stages were analyzed using the Majorbio I-Sanger Cloud Platform (www.i-sanger.com) online tool. The mothur software (version v.1.30.1) was adopted to measure the alpha diversity, while the Bray-Curtis matrix was used to calculate the beta diversity [12]. R Stat was chosen in this study to conduct the statistical analyses to exhibit the signi cant differences in the relative abundances of microorganisms at difference temperature conditions. All the data from the aforementioned microbial analyses are provided below as the average ± standard deviation of triplicate measurements.

Results And Discussion
3.1 Operation and performance of mesophilic and thermophilic anaerobic reactors As in Fig. 1, the mesophilic experiment was conducted for 90 days in total. Days 31-60 were de ned as the mesophilic stable stage (M Stable ) and days 61-90 as the mesophilic collapsed stage (M Collapsed ). The overall thermophilic experiment was to last for 129 days. The thermophilic stable stage (T Stable ) occurred on days 39-64, and the thermophilic collapsed stage (T Collapsed ) on days 100-129.
In this study, the performance of the mesophilic and thermophilic reactors was re ected by the uctuation of gaseous parameters (i.e., the methane content and volumetric methane production rate (VMPR)) and liquid parameters (i.e., VFAs, TAN, and pH value). The VMPR and Total VFA (TVFA) are presented in Fig. 1 while the other parameters are summarized in Table 1. The TVFA sharply increased to 2276.84 and 6476.56 mg/L at M Collapsed and T Collapsed stages, respectively, while the VMPR at M Collapsed and T Collapsed stages were both decreased to zero. Both the mesophilic and thermophilic anaerobic reactors experienced process failure caused by the severe accumulation of VFAs. The maximum OLRs for the stable mesophilic and thermophilic reactors were both 1.0 g VS/(L·d). The balance between acidogens and methanogens is indispensable for a stable AD system. However, methanogens are more vulnerable to the acid stress than acidogens, which nally led to the disturbance between the acidogens and methanogens and resulted in severe acidi cation [13]. In addition, VW is a type of carbohydrate-rich substrate with a weak buffering capacity in the AD process [14]. High temperatures accelerated the degradation of VW; thus, improved the average VMPR value from 0.16 ± 0.03 L/(L·d) at the mesophilic condition to 0.29 ± 0.02 L/(L·d) at the thermophilic condition when the two reactors operated at a stable OLR of 1.0 g VS/(L·d).

Overall microbial diversity
The microbial composition and dynamics of the mesophilic and thermophilic reactors at the stable and collapsed stages were obtained through high-throughput sequencing technology based on the Illumina MiSeq platform. Moreover, the alpha diversity indexes, including the Simpson, Chao, and Simpsoneven indexes, were analyzed in this study to exhibit the differences in the microbial diversity at different temperatures and stages ( Table 2). The Simpson index decreased as the microbial diversity increased, while the Chao and Simpsoneven indexes increased as microbial richness and evenness increased, respectively [15]. The Simpson index increased slightly from M Stable to M Collapsed , and the Chao index decreased slightly from M Stable to M Collapsed . This result re ects that the microbial diversity and richness decreased at M Collapsed . The accumulation of VFAs at M Collapsed resulted in the inhibition of microorganisms, which might lead to the reduction of most types of bacteria and methanogens. The decrease of the Simpsoneven index at M Collapsed also con rmed the aforementioned speculation that the microbial evenness was less in M Collapsed compared with that in M Stable . A similar tendency was found for the thermophilic anaerobic system. The accumulation of the total VFAs at T Collapsed was far more than that at M Collapsed (Table 1), indicating that the severe acidi cation resulted in a clear reduction of most types of microorganisms at the thermophilic collapsed condition. The microbial diversity, richness, and evenness at M Stable were slightly higher than those at T Stable . Greses et al. found that the microbial diversity was higher at the mesophilic condition in comparison with that at the thermophilic condition using Scenedesmus spp as the substrate [16]. Vanwonterghem et al. also clari ed that the increase of temperature from the mesophilic to thermophilic anaerobic conditions resulted in a limited population of microorganisms [17]. Therefore, temperature is one of the most important factors that in uenced the microbial diversity. Notes: a Diversity of the microbial community, b Richness of microbial species, c Distribution evenness of the microbial community.
The principal coordinate analysis results of the microorganisms in both the mesophilic and thermophilic anaerobic reactors at different stages are exhibited in Fig. 2. Clusters 1, 2, 3, and 4 represent the samples from M Stable , M Collapsed , T Stable , and T Collapsed , respectively. The total microbial community showed a 43.04% variation in principal components 1. Under different temperature conditions, clusters 1 and 2 from the mesophilic condition located in the left side of the plot, and clusters 3 and 4 from the thermophilic condition located in the right side of the plot. The clear separation of the microbial communities under different temperature conditions showed that temperature changes signi cantly affect the microbial community shifting under the AD of VW. The microbial community showed a 23.83% total variation in principal components 2. Under the mesophilic condition, the samples from the stable and collapsed stages were separated into two clusters (clusters 1 and 2). However, the distance between clusters 1 and 2 was quite close, indicating that the microbial community structure was only slightly shifted under the mesophilic condition. However, all the samples from the stable and collapsed stages were distinctly divided into two clusters (cluster 3 and 4) under the thermophilic condition, demonstrating that the severe acidi cation caused dramatic shifts of the microbial community structure. The nal collapse of the mesophilic and thermophilic AD of VW occurred at an OLR of 1.5 g VS/(L·d) and 2.5 g VS/(L·d), respectively. A high OLR led to a high VFA concentration; thus, inhibits the growth of the bacterial community more severe compared with a low OLR.

Dynamics of microbial community composition on phylum level
The results of the microbial community analysis that revealed the phylum level are shown in Fig. 3(a). followed by the phyla Euryarchaeota (7.84%), Chloro exi (6.54%), Firmicutes (6.33%), Cloacimonetes (6.01%), Proteobacteria (4.89%), Actinobacteria (4.00%), Thermotogae (2.34%), and Synergistetes (1.44%). Among these phyla, Euryarchaeota was the only phylum that functioned as methanogens in the mesophilic reactor. Most genus within the phyla of Spirochaetae, Synergistetes, and Atribacteria functioned as syntrophic organic acid oxidation bacteria in the AD system [18][19][20]. While the remaining phyla were mainly attributed to the fermentation and degradation processes of the AD system. When the mesophilic anaerobic system collapsed, the abundance of Bacteroidetes notably increased, followed by a slight increase in the Thermotogae, indicating that these two phyla were more tolerant to the acidi ed condition. The Bacteroidetes and Thermotogae phyla play a signi cant role in the degradation of cellulose and protein under anaerobic conditions [18,21]. The phyla of Spirochaetae, which existed uniquely in the collapsed stage, could be used as a microbial early warning indicator for the AD instability of VW. The phyla Firmicutes was dominant in both the thermophilic stable and collapsed stages; however, its abundance in the collapsed stage was lower than it in the stable stage. Thermotogae was the most abundant phyla that existed uniquely in the thermophilic collapsed stage, demonstrating the higher adaptability of Thermotogae to the acidic condition. Thus, Thermotogae might play a key role in relieving the further acidi cation, which has been reported to degrade various organic substrates under acidic anaerobic conditions [22]. The abundance of the phyla Euryarchaeota and Synergistetes sharply decreased at the collapsed stage, which was consistent with the nal failure of AD of VW.

Dynamics of microbial community composition on genus level
Details regarding the major microbial composition and abundance on the genus level were analyzed to further explore the dynamics of the microbial community ( Fig. 3(b)). The most abundant genera from M Stable were Candidatus_Cloacamonas (19.7%) and vadinBC27_wastewater-sludge_group (11.05%), followed by unclassi ed_f__Anaerolineaceae (8.68%), Proteiniphilum (7.78%), Petrimonas (6.55%), Propionimicrobium (6.39%), Methanosaeta (6.10%), norank_f__Anaerolineaceae (4.14%), Methanosarcina (4.08%), and Microbacter (3.43%). In contrast, the most abundant genera from M Collapsed were Bacteroides (15.11%), vadinBC27_wastewater-sludge_group (14.01%), and Sphaerochaeta (10.15%), followed by Candidatus_Cloacamonas (6.01%), Methanosaeta (5.65%), norank_f__V2072-189E03 (5.61%), norank_f__Anaerolineaceae (4.92%), Aeromonas (4.09%), norank_f__Porphyromonadaceae (4.02%), and Propionimicrobium (4.00%). According to previous studies, the speci c functions of each major genus from the mesophilic and thermophilic anaerobic reactors are listed in Table 3. At the mesophilic stable stage, the genera Candidatus_Cloacamonas and vadinBC27_wastewater-sludge_group were classi ed as syntrophic bacteria. Candidatus_Cloacamonas was able to digest cellulose and use hydrolyzing products to produce H 2 and CO 2 , and VadinBC27_wastewater-sludge_group was reported to degrade amino acids in syntrophic association with hydrogenotrophic methanogens. Methanosaeta and Methanosarcina were the only two dominant methanogens in the mesophilic stable stage, which functioned as acetoclastic and mixotrophic (i.e., acetoclastic, hydrogenotrophic, and methylotrophic) methanogens, respectively [18,23]. Thus, only Methanosarcina has the ability of syntrophic association with Candidatus_Cloacamonas and vadinBC27_wastewater-sludge_group, which mainly uses H 2 and CO 2 to produce methane. The genera unclassi ed_f__Anaerolineaceae, Proteiniphilum, Petrimonas, Propionimicrobium, norank_f__Anaerolineaceae, and Microbacter functioned as fermentative and acidogenic bacteria that degraded glucose to produce acetic and propionic acids. The dominant microbes changed clearly from the mesophilic stable to the collapsed stages. The most abundant bacteria shifted from syntrophic bacteria in the mesophilic stable stage to fermentative, acidogenic, and syntrophic bacteria in the mesophilic collapsed stage. Among the three most dominant genera of Bacteroides, vadinBC27_wastewater-sludge_group, and Sphaerochaeta in the mesophilic collapsed stage, two are fermentative and acidogenic bacteria (i.e., Bacteroides and Sphaerochaeta). Bacteroides are reported to have the ability to hydrolyze a complex insoluble substrate, such as polysaccharides, while Sphaerochaeta can ferment carbohydrates to produce acetate, formate, and ethanol (Table 3). Candidatus_Cloacamonas, which was the most abundant syntrophic bacteria, decreased dramatically form the mesophilic stable (19.7%) to the collapsed (6.01%) stage. The only methanogen that existed in the top ten genera at the mesophilic collapsed stage was Methanosaeta, compared with the two dominant methanogens (i.e., Methanosaeta and Methanosarcina) at the stable stages. The decrease of methanogens corresponded with the reduction of syntrophic bacteria at the mesophilic collapsed stage. These results reveal that fermentative and acidogenic bacteria were dominant occupants in the mesophilic collapsed stage, compared with syntrophic bacteria in the stable stage. When the performance of the AD of VW changed from the stable to collapsed state, caused by the accumulation of VFAs, the abundance of fermentative and acidogenic bacteria considerably shifted. All the fermentative and acidogenic bacteria of unclassi ed_f__Anaerolineaceae, Proteiniphilum, Petrimonas, and Microbacter were eliminated from the top ten genera at the mesophilic collapsed stage, and instead comprised the genera of Bacteroides, Sphaerochaeta, norank_f__V2072-189E03, Aeromonas, and norank_f__Porphyromonadaceae. This result indicates that the genera of Bacteroides, Sphaerochaeta, norank_f__V2072-189E03, Aeromonas, and norank_f__Porphyromonadaceae prefer to thrive under acidic environmental conditions, while the genera of unclassi ed_f__Anaerolineaceae, Proteiniphilum, Petrimonas, and Microbacter are vulnerable to high concentration of VFAs. Bacteroides Secrete different hydrolyzing enzymes, such as cellulase, to hydrolyze a complex insoluble substrate, such as polysaccharides. [27] Sphaerochaeta Ferment carbohydrates to produce acetate, formate, and ethanol. [28] Unclassi ed_f__Anaerolineaceae Anaerobic fermentative and acetogenic bacteria. [29] Propionimicrobium Growth under anaerobic conditions and produce propionic acid. [30] Norank_f__Anaerolineaceae Convert glycerol into VFAs. [31] Proteiniphilum Ferment organic substrates to produce acetic and propionic acids. [32] Petrimonas Anaerobic fermentative and acetogenic bacteria. [29] Norank_f__V2072-189E03 Ferment polysaccharides to produce acetate, formate, and ethanol. [33] Microbacter Fermentative bacteria in the anaerobic digestion system. [34] Aeromonas Ferment glucose to produce acetate in the anaerobic digestion system. [35] Norank_f__Porphyromonadaceae Ferment glucose to produce lactate, acetate, butyrate, and isobutyrate. [23] De uviitoga Degrade biomass to produce acetate. [36] Thermoanaerobacterium Degrade lignocellulose to produce acetic acid, butyric acid, and lactic acid. [37] Anaerobaculum Ferment a range of amino acids to acetate, propionate, and hydrogen. [38]

Ruminiclostridium_1
Hydrolyze cellulosic substrates and ferment the resulting sugars to ethanol. [40] Mobilitalea Hydrolyze celluloses, peptides and fermenting amino acids, and carbohydrates to produce acetate, butyrate, lactate, CO 2 , and H 2 . [41] Bacillus Fermentative bacteria; functioned as lignin degraders as they can decompose lignin with little consumption of cellulose. [42] Anaerolinea Use carbohydrates to produce hydrogen and acetic acid. [43] Candidatus_Caldatribacterium Thermophilic bacteria; produce acetate from sugar fermentation. [44] Norank_o__D8A-2 Syntrophic acetate-oxidizing bacteria (SAOB). [45] Norank_o__MBA03 SAOB. [46] Gelria SAOB. [12] Syntrophaceticus SAOB [47] De uviitalea Acid-producing bacteria. [28] Norank_f__Syntrophomonadaceae Syntrophic butyrate oxidizers in anoxic environments. [48] Norank_f__Synergistaceae Can degrade amino acids into volatile fatty acids and contribute to acidogenesis and acetogenesis via syntrophic relationships with methanogens. [49] Norank_f__Lentimicrobiaceae Denitri cation under anaerobic condition. [50] Coprothermobacter Protein-degrading bacteria that established a syntrophy with hydrogenotrophic methanogens. [51] Norank_p__Atribacteria Specialize in either primary fermentation of carbohydrates or secondary fermentation of organic acids, such as propionate. [52] Mesotoga Anaerobic mesophilic acetogens. [53] Hydrogenispora Ferment glucose to produce acetate, ethanol, and hydrogen. [54] Genus Functional description Reference Norank_f__Ruminococcaceae Fermentative bacteria with the ability of ber degradation. [55] Methanosaeta Acetoclastic methanogens that use acetate to produce methane. [23] Methanosarcina Mixotrophic methanogens that can perform acetoclastic, hydrogenotrophic, and methylotrophic methanogenesis. [23] Methanothermobacter Hydrogenotrophic methanogens that can use H 2 as a precursor to produce methane. [56] The most abundant genera from T Stable were Methanosarcina (30.58%) and Anaerobaculum (12.  Table 3, the genera of Anaerobaculum, Bacillus, De uviitalea, Mobilitalea, and Ruminiclostridium functioned as fermentative bacteria, and Syntrophaceticus, norank_o__D8A-2, and norank_f__Syntrophomonadaceae were classi ed as syntrophic bacteria. Methanothermobacter acted as hydrogenotrophic methanogen that can use H 2 as a precursor to produce methane. Syntrophaceticus and norank_o__D8A-2 have been reported to be syntrophic acetate-oxidizing bacteria (SAOB), which possess the syntrophic acetate-oxidizing ability in cocultivation with a hydrogen-utilizing methanogen. The coexistence of Methanothermobacter, Syntrophaceticus, and norank_o__D8A-2 showed that a syntrophic acetate-oxidizing association was developed in the thermophilic stable stage. When the process failure of the thermophilic AD of VW occurred at the T Collapsed , the genus De uviitoga were dominant, and the abundance increased from 4.66% at T Stable to 48.93% at T Collapsed . De uviitoga was revealed to act as acetogens that degraded various biomass to produce acetate under the anaerobic condition. Another dominant genus was Thermoanaerobacterium, which uniquely existed in T Collapsed . Meanwhile, Thermoanaerobacterium exhibited the ability to degrade lignocellulose to produce acetic, butyric, and lactic acids, and was classi ed as fermentative and acidogenic bacteria. These results indicate that both De uviitoga and Thermoanaerobacterium were more tolerant to a highly acidic environment. Furthermore, the dominant functional microbial consortium changed from methanogens and fermentative bacteria at the stable stage to fermentative bacteria at the collapsed stage. All the methanogens decreased at the collapsed stage, particularly for the abundance of Methanosarcina that decreased from 30.58% at the stable stage to 2.77% at the collapsed stage. The sharp decrease of methanogens and the sharp increases of fermentative and acidogenic bacteria both explain the occurrence of the severe accumulation of organic acids in the thermophilic collapsed stage. These results also verify that methanogens are vulnerable to the accumulation of VFAs.

Dynamics of methanogen communities
The dynamics of methanogens among the archaea at different temperatures are presented in Fig. 4. The acetoclastic methanogen of Methanosaeta (59.16%) and the mixotrophic methanogen of Methanosarcina (34.73%) were the dominant methanogens at the mesophilic stable condition, re ecting that the most dominant methanogen in the mesophilic reactor was acetoclastic methanogen, which uses acetate to produce methane. Meanwhile, the most dominant methanogens that existed in the thermophilic reactor were Methanosarcina and Methanothermobacter. Methanothermobacter was reported to function as a hydrogenotrophic methanogen, which can use H 2 as a precursor to produce methane ( Table 3). The relative abundance of Methanosarcina accounted for 90.71% at the thermophilic stable condition, but sharply decreased to 63.61% when the severe acidi cation occurred at T Collapsed .
Nonetheless, the relative abundance of Methanothermobacter dramatically increased from 9.16% at T Stable to 36.01% at T Collapsed , indicating that Methanothermobacter is more favorable to survive under a severe acidic environment. These results demonstrate that temperature can change the methanogenesis pathway in a large extent. Furthermore, methanogenesis might shift from acetoclastic in the mesophilic stable stage to a hydrogenotrophic pathway in the thermophilic stage. However, this result should be further con rmed via genomic and metabonomics analyses.

Differences in microorganisms at mesophilic and thermophilic stable stages
The differences in the microorganisms at the mesophilic and thermophilic stable stages were compared to determine the effect of temperature on the distribution of dominant genera at the stable operational condition of the AD of VW (Fig. 5). The abundances of Candidatus_Cloacamonas (P < 0.01), Propionimicrobium (P < 0.01), Proteiniphilum (P < 0.05), unclassi ed_f__Anaerolineaceae (P < 0.05), and Petrimonas (P < 0.05) were signi cantly higher in the mesophilic stable stage. In contrast, the abundances of Anaerobaculum (P < 0.01) and Mobilitalea (P < 0.05) were signi cantly higher in the thermophilic stable stage. All of these genera were classi ed as fermentative and acidogenic bacteria (Table 3). Candidatus_Cloacamonas functioned as both syntrophic and fermentative bacteria. Propionimicrobium, Proteiniphilum, unclassi ed_f__Anaerolineaceae, and Petrimonas were all reported to ferment biomass to produce acetate. Anaerobaculum and Mobilitalea were both found to hydrolyze celluloses and other polysaccharides, peptides and fermenting amino acids, and carbohydrate to produce acetate, butyrate, lactate, CO 2 , and H 2 . The abundances of syntrophic bacteria of Syntrophaceticus (P < 0.01) and norank_f__Syntrophomonadaceae (P < 0.05) were both signi cantly higher in the thermophilic stable condition. Norank_f__Syntrophomonadaceae functioned as syntrophic butyrate oxidizers, and Syntrophaceticus belonged to SAOB, which are highly associated with hydrogen-utilizing methanogens via syntrophic acetate-oxidizing to produce methane [24]. The abundance of Methanosaeta (P < 0.05) was signi cantly higher in the mesophilic stable condition, while the abundance of Methanosarcina (P < 0.05) was signi cantly higher in the thermophilic stable condition.
An overall comparison of the functional microbial consortium is exhibited in Fig. 6 [13]. Therefore, the dynamic shifts of the functional microbial community from acetoclastic methanogenesis to syntrophic acetate-oxidizing coupled with hydrogenotrophic methanogenesis resulted in the accumulation of VFAs under the thermophilic condition.

Conclusion
With the severe accumulation of VFAs, system failure occurred due to the imbalance between fermentative bacteria and methanogens. The diversity and richness of the microbial community under the mesophilic condition were higher than those under the thermophilic condition. Temperature changes signi cantly affected microbial shifting. Furthermore, syntrophic acetate-oxidizing coupled with hydrogenotrophic methanogenesis was the dominant anaerobic reaction in the thermophilic reactor, while it was mainly acetoclastic methanogenesis in the mesophilic reactor. The dominant acidogens, syntrophic bacteria, and methanogens were Candidatus_Cloacamonas, norank_f__Synergistaceae, Methanosaeta, and Methanosarcina in the mesophilic stable stage, and Anaerobaculum, Syntrophaceticus, Methanosarcina, and Methanothermobacter in the thermophilic stable stage.     Microbial differences between the mesophilic stable (MStable) and thermophilic stable (TStable) stages.