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Propionate is one of the major intermediates in anaerobic digestion of organic waste to CO2 and CH4. In methanogenic environments, propionate is degraded through a mutualistic interaction between symbiotic propionate oxidizers and methanogens. Although temperature heavily influences the microbial ecology and performance of methanogenic processes, its effect on syntrophic interaction during propionate degradation remains poorly understood. In this study, metagenomics and metatranscriptomics were employed to compare mesophilic and thermophilic propionate degradation communities. Mesophilic propionate degradation involved multiple syntrophic organisms (Syntrophobacter, Smithella, and Syntrophomonas), pathways, interactions, and preference toward formate-based electron transfer to methanogenic partners (i.e., Methanoculleus). In thermophilic propionate degradation, one syntrophic organism predominated (Pelotomaculum), interspecies H2 transfer played a major role, and phylogenetically and metabolically diverse H2-oxidizing methanogens were present (i.e., Methanoculleus, Methanothermobacter, and Methanomassiliicoccus). This study showed that microbial interactions, metabolic pathways, and niche diversity are distinct between mesophilic and thermophilic microbial communities responsible for syntrophic propionate degradation. [ABSTRACT FROM AUTHOR]

Investigating the effects of different kinds of inhibitors on the activity and structure of acetate-degrading microbial community involved in methane fermentation is critically important for developing countermeasures to make the fermentation process stable under different inhibitory conditions. In the present study, a mesophilic chemostat fed with acetate as the sole carbon source was constructed. Microbial community analysis based on high-throughput sequencing of 16S rRNA revealed that Methanothrix was the dominant methanogen and a variety of bacteria including acetate-oxidizing bacteria such as Tepidanaerobacter , Mesotoga , Geovibrio , and Geobacter were found. The activity and dynamic changes of the acetate-degrading microbial community under different inhibitory conditions were investigated. Addition of 600 mg L−1 ammonium and 150 mg L−1 sulfide reduced nearly half of the biogas production. The response of microbial community to sulfide inhibition was quicker than ammonium but the structure could recover in a short time. Addition of 8 mg L−1 chlortetracycline (CTC) and 160 mg L−1 enrofloxacin (EFX) exhibited a similar inhibitory effect on biogas production, with approximately 35% reduction. Compared to ammonium and sulfide, antibiotics showed stronger selective inhibition on some bacterial species. The genera related to acetate-oxidizing and sulfate-reducing bacteria showed stronger tolerance to CTC, which may be due to their low growth rates. Network analysis suggested that some genera which had close phylogenic relationship and similar functions showed constant positive correlation under different inhibitory conditions. [ABSTRACT FROM AUTHOR]

Syntrophic oxidization of acetate and propionate are both critical steps of methanogenesis during thermophilic anaerobic digestion. However, knowledge on syntrophic acetate-oxidizing bacteria (SAOB) and syntrophic propionate-oxidizing bacteria (SPOB) is limited because of the difficulty in pure culture isolation due to symbiotic relationship. In this study, two thermophilic acetate-fed anaerobic chemostats, ATL (dilution rate of 0.025 day−1) and ATH (0.05 day−1) and one thermophilic propionate-fed anaerobic chemostat PTL (0.025 day−1) were constructed, AOB and POB in these chemostats were studied via microbial community analysis and DNA stable-isotope probing (SIP). The results showed that, in addition to Tepidanaerobacter, a known SAOB, species of Thauera, Thermodesulfovibrio, Anaerobaculum, Ruminiclostridium, Comamonas, and uncultured bacteria belonging to Lentimicrobiaceae, o_MBA03, Thermoanaerobacteraceae, Anaerolineaceae, Clostridiales, and Ruminococcaceae were determined to be potential AOB in chemostats. Pelotomaculum was the key SPOB detected in the propionate-fed chemostat. Based on the intense fluorescence of coenzyme F420, majority of Methanosarcina cells in acetate-fed chemostats were involved in hydrogenotrophic methanogenesis, suggesting the existence of highly active SAOB among the detected AOB. In the propionate-fed chemostat, most of the species detected as AOB were similar to those detected in the acetate-fed chemostats, suggesting the contribution of the syntrophic acetate oxidization pathway for methane generation. These results revealed the existence of previously unknown AOB with high diversity in thermophilic chemostats and suggested that methanogenesis from acetate via the syntrophic oxidization pathway is relevant for thermophilic anaerobic digestion. [ABSTRACT FROM AUTHOR]

Propionate is one of the most important intermediates of anaerobic fermentation. Its oxidation performed by syntrophic propionate-oxidizing bacteria coupled with hydrogenotrophic methanogens is considered to be a rate-limiting step for methane production. However, the current understanding of SPOB is limited due to the difficulty of pure culture isolation. In the present study, two anaerobic chemostats fed with propionate as the sole carbon source were operated at different dilution rates (0.05 d−1 and 0.15 d−1). The propionate- and acetate-oxidizing bacteria in the two methanogenic chemostats were investigated combining DNA-stable isotope probing (DNA-SIP) and 16S rRNA gene high-throughput sequencing. The results of DNA-SIP with 13C-propionate/acetate suggested that, Smithella, Syntrophobacter, Cryptanaerobacter, and unclassified Rhodospirillaceae may be putative propionate-oxidizing bacteria; unclassified Spirochaetaceae, unclassified Synergistaceae, unclassified Elusimicrobia, Mesotoga, and Gracilibacter may contribute to acetate oxidation; unclassified Syntrophaceae and Syntrophomonas may be butyrate oxidizers. By DNA-SIP, unclassified OTUs with 16S rRNA gene abundance higher than 62% of total Bacteria in the PL chemostat and 38% in the PH chemostat were revealed to be related to the degradation of propionate. These results suggest that a variety of uncultured bacteria contribute to propionate degradation during anaerobic digestion. The functions and metabolic characteristics of these bacteria require further investigation. [ABSTRACT FROM AUTHOR]