Household biogas digesters are widely used to harvest energy in rural areas of developing countries. Understanding core prokaryotic communities, their co-occurrence patterns, and their relationships to environmental factors is important to manage these small-scale anaerobic digestion systems effectively. In this study, 43 household biogas digesters were collected across eight provinces in China. Prokaryotic communities were investigated using 454 pyrosequencing of 16S rRNA genes. Fourteen core genera and ten core OTUs were identified in household biogas digesters. They were mainly affiliated with the phylum Firmicutes, Synergistetes, Actinobacteria, Chloroflexi, and Spirochaetes. Core prokaryotic genera were mainly composed of Clostridium, Clostridium XI, Syntrophomonas, Cloacibacillus, Sedimentibacter, and Turicibacter. Prokaryotic communities in the 43 samples were clearly divided into two clusters. Cluster I was dominated by Clostridium, while Cluster II was dominated by members of Spirochaetes, Bacteroidales, Clostridia, and abundant syntrophs and methanogens. NH4 +-N and COD contributed significantly to the assembly of the prokaryotic community in Cluster I, while NH4 +-N, pH, and phosphate contributed significantly to Cluster II. Correlation-based network analysis showed that the prokaryotic communities in the biogas digesters were dominated by some functional modules. Cluster I was dominated by acetotrophic methanogenic modules and the Clostridium-driven primary fermentation module, while the network of Cluster II was dominated by hydrogenotrophic and acetogenic methanogenesis modules and multi-group-driven (Spirochaetes, Bacteroidales, and Clostridia) primary fermentation modules. The network of Cluster II was more complex and functionally redundant. Prokaryotic communities identified in the household biogas digesters varied significantly and were affected by environmental factors, such as NH4 +-N, pH, and COD. However, core prokaryotic communities existed, and most of them were also dominant populations. Cosmopolitan OTUs tended to co-occur. Prokaryotic communities in biogas digesters were well organized by some functional modules. The modular structure of the prokaryotic community, which has functional redundancy, enhances the resistance against environmental stress and maintains digestion efficiency in the anaerobic digestion process.

Host population density is positively associated with the strength of social interactions or the frequency of physical contacts, and thus potentially influences microbial transmission among individuals. However, the relationship between host density and gut microbiota remains unknown. Here, we characterized the gut microbiota of plateau pikas (Ochotona curzoniae) in wild experimental fields with different host population densities. The abundance of some gut microbes significantly correlated with host density, such as Ruminococcaceae, Lachnospiraceae and Staphylococcaceae. Intriguingly, host density was positively correlated with alpha diversity (Shannon diversity and observed species) of gut microbial communities. The inter-individual gut microbiota within high-density groups were more similar to each other than those of low-density groups. Host density significantly explained the variations in the gut microbiota, even after controlling sex, weight, diet and geographical locations. Based on the PICRUSt metagenome prediction, positive correlations were observed between host density and the relative abundances of 12 gene functions involved in cellular processes, environmental information processing and metabolism. These results indicate the importance of host density as a factor in shaping gut microbial composition and diversity in plateau pikas, and may further help us understand the social transmission of gut microbiota.

Temperature is an important factor determining the performance and stability of the anaerobic digestion process. However, the microorganism-regulated mechanisms of temperature effects on the performance of anaerobic digestion systems remain further elusive. To address this issue, we investigated the changes in composition, diversity and activities of microbial communities under temperature gradient from 25 to 55 °C using 16S rRNA gene amplicon sequencing approach based on genomic DNA (refer to as “16S rDNA”) and total RNA (refer to as “16S rRNA”). Microbial community structure and activities changed dramatically along the temperature gradient, which corresponded to the variations in digestion performance (e.g., daily CH4 production, total biogas production and volatile fatty acids concentration). The ratios of 16S rRNA to 16S rDNA of microbial taxa, as an indicator of the potentially relative activities in situ, and whole activities of microbial community assessed by the similarity between microbial community based on 16S rDNA and rRNA, varied strongly along the temperature gradient, reflecting different metabolic activities. The daily CH4 production increased with temperature from 25 to 50 °C and declined at 55 °C. Among all the examined microbial properties, the whole activities of microbial community and alpha-diversity indices of both microbial communities and potentially relative activities showed highest correlations to the performance. The whole activities of microbial community and alpha-diversity indices of both microbial communities and potentially relative activities were sensitive indicators for the performance of anaerobic digestion systems under temperature gradient, while beta-diversity could predict functional differences. Microorganism-regulated mechanisms of temperature effects on anaerobic digestion performance were likely realized through increasing alpha-diversity of both microbial communities and potentially relative activities to supply more functional pathways and activities for metabolic network, and increasing the whole activities of microbial community, especially methanogenesis, to improve the strength and efficiency in anaerobic digestion process.