Microbial Diversity And Community Assembly In Permafrost Soils From The Kunlun Mountain Pass

Permafrost constitutes a significant proportion of the terrestrial cryosphere of the Earth and represents a key reservoir of belowground organic carbon pool. Microbes, including bacteria, archaea, phototrophic cyanobacteria and green algae, fungi and protozoa, are the major component of living biomass and species diversity within permafrost, and mediate important biogeochemical processes. Recently there has been increasing attention about the impacts of climate warming on the permafrost environments, and the resulting permafrost thaw and emissions of greenhouse-gas(mainly CO2, CH4 and N2O) by microbial decomposition of trapped organic matter is regarded as one of the most important feedback mechanisms from terrestrial ecosystems to the global atmospheric system. Currently many ecologists have focused on the composition of the permafrost microbial community and understanding their potential functional roles and how they could respond to climate change and subsequent permafrost thaw. The Qinghai-Tibet Plateau, where low-latitude, high-altitude permafrost are widely distributed, represents a unique and fragile ecosystem and is sensitive to the influence of climate warming. However, there are far fewer data on permafrost microorganisms of the Qinghai-Tibet Plateau compared with the Arctic and Antarctic, so that it is difficult to provide scientific basis and data reference for the microbial ecology of permafrost in the world and researches in other disciplines in this region. In this dissertation, I collected frozen soil samples from the Kunlun Mountain Pass, Qinghai-Tibet Plateau, to investigate the physiological and biochemical characteristics, diversity and community structure of permafrost microorganisms(Bacteria, Archaea and Fungi), and to explore the spatial patterns of microbial communities as well as their ecological drivers. The main results were summarized as follows,(1) DAPI staining of the permafrost samples collected from the Kunlun Mountain Pass revealed total cell counts of 108 ~ 109 g-1. The numbers of culturable bacteria ranged from 103 ~ 106 CFU/g, and showed that there was no observable correlation with soil depth.Phylogenetic analyses suggested a high diversity of culturable bacteria, and these isolates belonged to 44 bacterial genera affiliated with the phyla Actinobacteria, Firmicutes, Proteobacteria, Bacteroides and Deinococcus-Thermus, among which the genera Arthrobacter, Psychrobacter, Planococcus, Bacillus and Microbacterium were the most abundant. A large number of bacterial strains were psychrotolerant, while very few were psychrophilic or mesophilic. Moreover, 93.6% and 49.0 of these strains were alkalotolerant and halotolerant, respectively. However, enumeration of culturable fungi revealed a low number of 0 ~ 103 CFU/g, and fungal isolates belonged to the phyla Ascomycota and Basidiomycota, with five genera represented: Geomyces, Cladosporium, Alternaria, Rhodotorula and Cryptococcus.(2) A total of 62 fungal phylotypes related to 10 distinct classes representing three phyla(Ascomycota, Basidiomycota and Mucoromycotina) were detected in ITS r RNA gene clone libraries constructed from 13 permafrost samples. Dothideomycetes affiliated with the phylum Ascomycota was the predominant group of fungi, with respect to both clone and phylotype number. A large proportion of the fungal phylotypes(25/62) that were distantly related to known fungal species appeared to be novel diversity. Our results suggested the existence of cosmopolitan psychrophilic or psychrotolerant fungi in permafrost soils. Fungal community composition varied with increasing depth, while these communities largely distributed according to core layers(active layer and permafrost layer). The soil conductivity had significant effect on the composition of fungal community.(3) Bacterial clone libraries constructed from 16 community DNA samples consisted of 191 phylotypes, which could be divided into 14 phyla. Proteobacteria was the most abundant and heterogeneous group in the libraries. A total of 52 orders were identified within the 14 bacterial divisions, among which 19 of the identified orders were novel. The vast majority of the bacterial phylotypes(108/191) had less than 97% sequence similarity to their closest cultivated representatives in the public database, suggesting the possibility of novel species. The alpha diversity of bacterial community varied negatively with sampling depth, and the clone library constructed from active layer samples showed higher phylotype richness. Compositional differences were also observed in different sediment horizons of the permafrost core, and the samples collected from active layer showed greater variability in their community composition. Changes in taxonomic composition of bacterial communities were significantly correlated with the majority of our measured soil variables, among which soil C/N ratio was the most important explanatory variable, followed by soil organic carbon.(4) Archaeal communities were less diverse than the bacterial ones, and consisted of 17 phylotypes, which were assigned to the classes Group I.1a, Group I.1b and Thermoplasmata representing the phyla Thaumarchaeota and Euryarchaeota. One previously unrecognized order(defined at 90% sequence similarity) and 13 novel phylotypes were determined by constructing phylogenetic trees with the bootstrap analysis for each archaeal division. An overwhelming majority(> 99%) of the total archaeal clones was closely related to the known ammonia-oxidizing archaea Candidatus Nitrosoarchaeum limnia, Candidatus Nitrososphaera gargensis and Nitrososphaera viennensis. These findings suggest that ammonia-oxidizing archaea could play an important role in the carbon and nitrogen cycles of permafrost ecosystems in the Kunlun Mountain Pass.(5) Further, we investigated the vertical distributions patterns of bacterial communities in the sediment core taken within permafrost of the Kunlun Mountain Pass. To get a better understanding of the forces that govern these patterns, we examined the diversity, structure, and spatial turnover of bacterial communities from both taxonomic and phylogenetic perspectives. Measures of taxonomic and phylogenetic beta diversity revealed that bacterial communities were not randomly distributed along the soil core, and showed a distance-decay relationship. Bacterial communities in five soil horizons of the permafrost core were phylogenetically random; however, all communities had a tendency to be phylogenetically clustered, containing closely related taxa. We therefore conclude that although the vertical distribution of bacterial communities was governed primarily by deterministic ecological selection, stochastic processes were also at work.