Background: Sequencing prokaryotic genomes has revolutionized our understanding of the many roles played by microorganisms. However, the cell and taxon proportions of genome-sequenced bacteria or archaea on earth remain unknown. This study aimed to explore this basic question using large-scale alignment between the sequences released by the Earth Microbiome Project and 155,810 prokaryotic genomes from public databases. Results: Our results showed that the median proportions of the genome-sequenced cells and taxa (at 100% identities in the 16S-V4 region) in different biomes reached 38.1% (16.4–86.3%) and 18.8% (9.1–52.6%), respectively. The sequenced proportions of the prokaryotic genomes in biomes were significantly negatively correlated with the alpha diversity indices, and the proportions sequenced in host-associated biomes were significantly higher than those in free-living biomes. Due to a set of cosmopolitan OTUs that are found in multiple samples and preferentially sequenced, only 2.1% of the global prokaryotic taxa are represented by sequenced genomes. Most of the biomes were occupied by a few predominant taxa with a high relative abundance and much higher genome-sequenced proportions than numerous rare taxa. Conclusions: These results reveal the current situation of prokaryotic genome sequencing for earth biomes, provide a more reasonable and efficient exploration of prokaryotic genomes, and promote our understanding of microbial ecological functions.

Different habitats harbor different microbial communities with elusive assembly mechanisms. This study comprehensively investigated the global assembly mechanisms of microbial communities and effects of community-internal influencing factors using the Earth Microbiome Project (EMP) data set. We found that deterministic and stochastic processes contribute approximately equally to global microbial community assembly, and, specifically, deterministic processes generally play a major role in free-living and plant-associated (but not plant corpus) environments, while stochastic processes are the major contributor in animal-associated environments. In contrast with the assembly of microorganisms, the assembly of functional genes, predicted from PICRUSt, is mainly attributed to deterministic processes in all microbial communities. The sink and source microbial communities are normally assembled using similar mechanisms, and the core microorganisms are specific to different environment types. On a global scale, deterministic processes are positively related to the community alpha diversity, microbial interaction degree and bacterial predatory-specific gene abundance. Our analysis provides a panoramic picture and regularities of global and environment-typical microbial community assemblies. IMPORTANCE: With the development of sequencing technologies, the research topic of microbial ecology has evolved from the analysis of community composition to community assembly, including the relative contribution of deterministic and stochastic processes for the formation and maintenance of community diversity. Many studies have reported the microbial assembly mechanisms in various habitats, but the assembly regularities of global microbial communities remain unknown. In this study, we analyzed the EMP data set using a combined pipeline to explore the assembly mechanisms of global microbial communities, microbial sources to construct communities, core microbes in different environment types, and community-internal factors influencing assembly. The results provide a panoramic picture and rules of global and environment-typical microbial community assemblies, which enhances our understandings of the mechanisms globally controlling community diversity and species coexistence.

The oral cavity is an important window for microbial communication between the environment and the human body. The oral microbiome plays an important role in human health. However, compared to the gut microbiome, the oral microbiome has been poorly explored. Here, we analyzed 404 datasets from human oral saliva samples published by the Earth Microbiome Project (EMP) and compared them with 815 samples from the human gut, nose/pharynx, and skin. The diversity of the human saliva microbiome varied significantly among individuals, and the community compositions were complex and diverse. The saliva microbiome showed the lowest species diversity among the four environment types. Human oral habitats shared a small core bacterial community containing only 14 operational taxonomic units (OTUs) under 5 phyla, which occupied over 75% of the sequence abundance. For the four habitats, the core taxa of the saliva microbiome had the greatest impact on saliva habitats than other habitats and were mostly unique. In addition, the saliva microbiome showed significant differences in the populations of different regions, which may be determined by the living environment and lifestyle/dietary habits. Finally, the correlation analysis showed high similarity between the saliva microbiome and the microbiomes of Aerosol (non-saline) and Surface (non-saline), i.e., two environment types closely related to human, suggesting that contact and shared environment being the driving factors of microbial transmission. Together, these findings expand our understanding of human oral diversity and biogeography.

The tailings of the Shimen realgar mine have unique geochemical features. Arsenite oxidation is one of the major biogeochemical processes that occurs in the tailings. However, little is known about the functional and molecular aspects of the microbial community involved in arsenite oxidation. Here, we fully explored the functional and molecular features of the microbial communities from the tailings of the Shimen realgar mine. We collected six samples of tailings from sites A, B, C, D, E, and F. Microcosm assays indicated that all of the six sites contain both chemoautotrophic and heterotrophic arsenite-oxidizing microorganisms; their activities differed considerably from each other. The microbial arsenite-oxidizing activities show a positive correlation with soluble arsenic concentrations. The microbial communities of the six sites contain 40 phyla of bacteria and 2 phyla of archaea that show extremely high diversity. Soluble arsenic, sulfate, pH, and total organic carbon (TOC) are the key environmental factors that shape the microbial communities. We further identified 114 unique arsenite oxidase genes from the samples; all of them code for new or new-type arsenite oxidases. We also isolated 10 novel arsenite oxidizers from the samples, of which 4 are chemoautotrophic and 6 are heterotrophic. These data highlight the unique diversities of the arsenite-oxidizing microorganisms and their oxidase genes from the tailings of the Shimen realgar mine. To the best of our knowledge, this is the first report describing the functional and molecular features of microbial communities from the tailings of a realgar mine. IMPORTANCE This study focused on the functional and molecular characterizations of microbial communities from the tailings of the Shimen realgar mine. We fully explored, for the first time, the arsenite-oxidizing activities and the functional gene diversities of microorganisms from the tailings, as well as the correlation of the microbial activities/diversities with environmental factors. The findings of this study help us to better understand the diversities of the arsenite-oxidizing bacteria and the geochemical cycle of arsenic in the tailings of the Shimen realgar mine and gain insights into the microbial mechanisms by which the secondary minerals of the tailings were formed. This work also offers a set of unique arsenite-oxidizing bacteria for basic research of the molecular regulation of arsenite oxidation in bacterial cells and for the environmentally friendly bioremediation of arsenic-contaminated groundwater.