[Taxonomic and Functional Diversity of Soil Microbial Communities in Subalpine Meadow with Different Degradation Degrees in Mount Wutai].

Although soil microbes play a key role in grassland ecosystem functioning, the response of their diversity to grassland degradation has not been fully investigated. Here, we used shotgun metagenomic sequencing to analyze the characteristics and influencing factors of soil microbial taxonomic and functional diversity at four different degradation stages[i.e., non-degraded (ND), lightly degraded (LD), moderately degraded (MD), and heavily degraded (HD)]of subalpine meadow in the Mount Wutai. The results showed that there were significant differences in the relative abundances of Actinobacteria, Bacteroidetes, Nitrospirae, and Parcubacteria among the four subalpine grasslands with different degradation degrees (P<0.05).Compared with that in ND, the degraded meadows increased the proportion of genes related to carbon metabolism, biosynthesis of amino acids, pyruvate metabolism, citric acid cycle, propanoate metabolism, butanoate metabolism, and fatty acid metabolism (P<0.05), indicating that the degradation of subalpine grassland changed the metabolic potential of energy metabolism and the nutrient cycle of the soil microbial community. Grassland degradation changed soil microbial taxonomic and functional α diversity, especially in MD and HD.Grassland degradation resulted in significant changes in the taxonomic and functional compositions of the microbial communities. The total nitrogen, pH, and soil organic carbon significantly affected the taxonomic and functional compositions of the microbial communities.The ß diversity of the plant community was significantly correlated with the taxonomic and functional ß diversity of the microbial community (P<0.05), indicating strong coupling. The results of this study revealed the changes and driving mechanisms of subsurface microbial taxonomic and functional diversity during grassland degradation, which can provide a theoretical basis for subalpine meadow protection and ecological restoration.

[Responses of Soil Fungal Communities to Subalpine Meadow Degradation in Mount Wutai].

Grassland degradation has become a worldwide ecological problem. Although soil microorganisms, as the main participants in the process of grassland degradation, play a key role in maintaining ecosystem function and improving soil productivity, little is known about the changes in microbial communities caused by grassland degradation and their relationship with soil properties and plant communities. In this study, we used Illumina MiSeq sequencing to analyze the soil fungal communities of subalpine meadow soil at four different degradation stages[i.e., non-degraded (ND), lightly degraded (LD), moderately degraded (MD), and heavily degraded (HD)] on Mount Wutai. The results showed that Ascomycota, Basidiomycota, and Zygomycota were the dominant phyla of soil fungi in the subalpine meadow, regardless of degradation stage. LEfSe showed that the subalpine meadows with different degradation degrees were enriched with different biomarkers. Compared with ND, MD and HD were enriched with more pathogenic fungi. Moreover, HD apparently decreased the richness and Shannon indexes of soil fungal communities compared with those of ND. Non-metric multidimensional scaling (NMDS) and similarity analysis (ANOSIM) indicated that the compositions and structures of fungal communities were significantly different among meadows with different degradation degrees (P<0.05). Redundancy analysis (RDA) showed that soil water content, total nitrogen, plant richness, and ammonium nitrogen were significantly correlated with the compositions and structures of fungal communities (P<0.05). There were significant correlations between α diversity and ß diversity between plant and fungal communities (P<0.05), indicating strong coupling. The results of our study provide a theoretical basis for further research on the changes in soil fungal communities and their driving mechanism in different degradation stages of subalpine meadows.

Bacterial Community Spacing Is Mainly Shaped by Unique Species in the Subalpine Natural Lakes of China.

Bacterial communities have been described as early indicators of both regional and global climatic change and play a critical role in the global biogeochemical cycle. Exploring the mechanisms that determine the diversity patterns of bacterial communities and how they share different habitats along environmental gradients are, therefore, a central theme in microbial ecology research. We characterized the diversity patterns of bacterial communities in Pipahai Lake (PPH), Mayinghai Lake (MYH), and Gonghai Lake (GH), three subalpine natural lakes in Ningwu County, Shanxi, China, and analyzed the distribution of their shared and unique taxa (indicator species). Results showed that the species composition and structure of bacterial communities were significantly different among the three lakes. Both the structure of the entire bacterial community and the unique taxa were significantly influenced by the carbon content (TOC and IC) and space distance; however, the structure of the shared taxa was affected by conductivity (EC), pH, and salinity. The structure of the entire bacterial community and unique taxa were mainly affected by the same factors, suggesting that unique taxa may be important in maintaining the spatial distribution diversity of bacterial communities in subalpine natural freshwater lakes. Our results provide new insights into the diversity maintenance patterns of the bacterial communities in subalpine lakes, and suggest dispersal limitation on bacterial communities between adjacent lakes, even in a small local area. We revealed the importance of unique taxa in maintaining bacterial community structure, and our results are important in understanding how bacterial communities in subalpine lakes respond to environmental change in local habitats.

Soil Bacterial Community Response and Nitrogen Cycling Variations Associated with Subalpine Meadow Degradation on the Loess Plateau, China.

Grassland degradation is an ecological problem worldwide. This study aimed to reveal the patterns of the variations in bacterial diversity and community structure and in nitrogen cycling functional genes along a subalpine meadow degradation gradient on the Loess Plateau, China. Meadow degradation had a significant effect on the beta diversity of soil bacterial communities (P < 0.05) but not on the alpha diversity (P > 0.05). Nonmetric multidimensional scaling (NMDS) and analysis of similarity (ANOSIM) indicated that the compositions of bacterial and plant communities changed remarkably with increasing meadow degradation (all P < 0.05). The beta diversities of the plant and soil bacterial communities were significantly correlated (P < 0.05), while their alpha diversities were weakly correlated (P > 0.05) along the meadow degradation gradient. Redundancy analysis (RDA) showed that the structure of the bacterial community was strongly correlated with total nitrogen (TN), nitrate nitrogen (NO3--N), plant Shannon diversity, plant coverage, and soil bulk density (all P < 0.05). Moreover, the abundances of N fixation and denitrification genes of the bacterial community decreased along the degradation gradient, but the abundance of nitrification genes increased along the gradient. The structure of the set of N cycling genes present at each site was more sensitive to subalpine meadow degradation than the structure of the total bacterial community. Our findings revealed compositional shifts in the plant and bacterial communities and in the abundances of key N cycling genes as well as the potential drivers of these shifts under different degrees of subalpine meadow degradation.IMPORTANCE Soil microbes play a crucial role in the biogeochemical cycles of grassland ecosystems, yet information on how their community structure and functional characteristics change with subalpine meadow degradation is scarce. In this study, we evaluated the changes in bacterial community structure and nitrogen functional genes in degraded meadow soils. Meadow degradation had a significant effect on bacterial community composition. Soil total nitrogen was the best predictor of bacterial community structure. The beta diversities of the plant and soil bacterial communities were significantly correlated, while their alpha diversities were only weakly correlated. Meadow degradation decreased the potential for nitrogen fixation and denitrification but increased the potential for nitrification. These results have implications for the restoration and reconstruction of subalpine meadow ecosystem on the Loess Plateau.

[Spatio-temporal Patterns of Microbial Communities and Their Driving Mechanisms in Subalpine Lakes, Ningwu, Shanxi].

Human activities and climate change cause the degradation of subalpine lake ecosystems, which induce the shift of microbial community structure. The spatio-temporal dynamics and the diversity maintenance mechanisms of bacterial communities in Gonghai Lake in Ningwu, Shanxi, were investigated by using Q-PCR and DGGE. The results showed that the temperature), pH, dissolved oxygen, electrical conductivity salinity, and ammonium nitrogen (NH4+) contestation were significantly different among the different sampling depths during different months. Bacterial abundance was the highest in August and the lowest in November, and the abundance was higher in the middle water layer (2 m, 4 m, and 6 m depths), but relatively low in the surface layer and bottom layer (0 m and 8 m depths, respectively). The α diversity index of bacterial communities had significant differences among the different months and depths, and showed an initial decreasing trend and then an increasing trend from April to December. A PERMANOVA test showed that the spatial distribution of bacterial communities was significantly different among depths (P<0.001). The results of redundancy analysis and variation partitioning indicated that environmental selection and diffusion limitation had an effect on the maintenance of the diversity patterns of bacterial communities at the different depths of GH. However, the relative effect of environmental factors was stronger, of which the concentration of NO3-, NO2-, and NH4+ were the main influencing factors. In conclusion, the bacterial communities in GH subalpine lake showed clear spatio-temporal distribution patterns, and environmental variables had a significant effect on shaping the community diversity.

Assembly mechanisms of soil bacterial communities in subalpine coniferous forests on the Loess Plateau, China.

Microbial community assembly is affected by trade-offs between deterministic and stochastic processes. However, the mechanisms underlying the relative influences of the two processes remain elusive. This knowledge gap limits our ability to understand the effects of community assembly processes on microbial community structures and functions. To better understand community assembly mechanisms, the community dynamics of bacterial ecological groups were investigated based on niche breadths in 23 soil plots from subalpine coniferous forests on the Loess Plateau in Shanxi, China. Here, the overall community was divided into the ecological groups that corresponded to habitat generalists, 'other taxa' and specialists. Redundancy analysis based on Bray-Curtis distances (db-RDA) and multiple regression tree (MRT) analysis indicated that soil organic carbon (SOC) was a general descriptor that encompassed the environmental gradients by which the communities responded to, because it can explain more significant variations in community diversity patterns. The three ecological groups exhibited different niche optima and degrees of specialization (i.e., niche breadths) along the SOC gradient, suggesting the presence of a gradient in tolerance for environmental heterogeneity. The inferred community assembly processes varied along the SOC gradient, wherein a transition was observed from homogenizing dispersal to variable selection that reflects increasing deterministic processes. Moreover, the ecological groups were inferred to perform different community functions that varied with community composition, structure. In conclusion, these results contribute to our understanding of the trade-offs between community assembly mechanisms and the responses of community structure and function to environmental gradients.

[Distribution Pattern and Diversity Maintenance Mechanisms of Fungal Community in Subalpine Lakes].

The composition and diversity of fungal communities are essential to maintain the ecosystem balance of subalpine lakes. The aquatic fungal communities at different depths from the subalpine Pipahai (PPH, 0, 2, 4 m), Mayinghai (MYH, 0, 2, 4, 6 m), and Gonghai (GH, 0, 2, 4, 6, 8 m) lakes were studied. In addition to that, the distribution pattern and diversity maintenance mechanism (determination process vs. random process) of fungal communities were explored using high-throughput sequencing. The results showed that the physicochemical parameters of the water were significantly different among the three lakes. The pH, electrical conductivity (EC), ammonia nitrogen (NH4+-N), total carbon (TC), and inorganic carbon (IC) of GH were significantly higher than in the other two lakes. The fungal community was mainly composed of Ascomycota (0.82%-21.05%), Basidiomycota (1.26%-11.79%), Chytridiomycota (0.42%-4.26%), and Rozellomycota (0.11%-0.33%). Cystobasidiomycetes, Dothideomycetes, Chytridiomycetes, and Sordariomycetes were the dominant classes shared by the three lakes. The α-diversity index and the relative abundance of dominant classes were significantly different among the three lakes (P<0.05), but there were no significant differences between the various depths on each lake. The results of the ANOSIM analysis showed that the ß-diversity of the fungal communities were significantly different (r=0.99, P<0.01) among the lakes. There was also expressive differences at various depths on MYH (r=0.98, P<0.01) and GH (r=0.25, P<0.05), but no significant difference in PPH (r=0.23, P>0.05). The analysis results of redundancy and variation partitioning showed that the ß-diversity pattern of fungal communities in small region areas (among the three lakes) and local areas (different depths of MYH) were driven by environmental selection and dispersal limitation. However, the relative role of environmental selection was more significant, with water pH, dissolved oxygen (DO), TC, and EC being the main influencing factors. The results of the null model analysis showed that the interspecific interactions promoted the maintenance of the ß-diversity pattern of the fungal community in GH. In summary, the ß-diversity pattern of fungal communities in the subalpine lakes was mainly driven by a deterministic process.

Biogeographic Patterns and Assembly Mechanisms of Bacterial Communities Differ Between Habitat Generalists and Specialists Across Elevational Gradients.

A core issue in microbial ecology is the need to elucidate the ecological processes and underlying mechanisms involved in microbial community assembly. However, the extent to which these mechanisms differ in importance based on traits of taxa with different niche breadth is poorly understood. Here, we used high-throughput sequencing to examine the relative importance of environmental selection and stochastic processes in shaping soil bacterial sub-communities with different niche breadth (including habitat generalists, specialists and other taxa) across elevational gradients on the subalpine slope of Mount Wutai, Northern China. Our findings suggested that the composition of soil bacterial communities differed significantly different among elevational gradients. According to the niche breadth index, 10.9% of OTUs were defined as habitat generalists (B-value >8.7) and 10.0% of OTUs were defined as habitat specialists (B-value <1.5). Generalists and specialists differed distinctly in diversity and biogeographic patterns across elevational gradients. Environmental selection (deterministic processes) and spatial factors (stochastic processes) seemed to determine the assembly and biogeography of habitat generalists. However, for specialists, deterministic processes strongly influenced the distribution, while stochastic processes were not at play. Environmental drivers for generalists and specialists differed, as did their importance. Elevation, total nitrogen and pH were the main factors determining habitat generalists, and soil water content, nitrate nitrogen and pH had the strongest impacts on specialists. Moreover, variation partitioning analysis revealed that environmental selection had a much greater impact on both generalists (17.7% of pure variance was explained) and specialists (3.6%) than spatial factors. However, generalists had a much stronger response to spatial factors (2.3%) than specialists (0.3%). More importantly, null models of ß-diversity suggested that specialists deviated significantly from non-neutral assembly mechanisms (relative null deviation= 0.64-0.74) relative to generalists (0.16-0.65) (P < 0.05). These results indicate that generalists and specialists are governed by different assembly mechanisms and present distinct biogeographical patterns. The large proportion of unexplained variation in specialists (93.3%) implies that very complex assembly mechanisms exist in the assembly of specialists across elevational gradients on the subalpine slope of Mount Wutai. It is essential to understand the microbial community assembly at a more refined level, and to expand the current understanding of microbial ecological mechanisms.

New Insights into the Electron-Collection Efficiency Improvement of CdS-Sensitized TiO2 Nanorod Photoelectrodes by Interfacial Seed-Layer Mediation.

Titanium dioxide (TiO2) nanorods (NRs) are widely used as photoanodes in photoelectrochemical (PEC) solar fuel production because of their remarkable photoactivity and stability. In addition, TiO2 NR electrode materials can be decorated with active CdS quantum dots (QDs) to expand the sunlight photon capture. The overall photoelectric conversion efficiency for TiO2 NR or QD-sensitized TiO2 NR electrode materials in PEC is typically dominated by their interfacial electron transfer (ET) properties. To understand the key factors affecting the ET, the anatase TiO2 seed layer was added into the interface between the rutile TiO2 NRs and fluorine-doped tin oxide (FTO) substrate. This seed layer enhanced the photocatalytic performance of both the TiO2 NR and CdS QD-sensitized TiO2 NR photoanodes in PEC. Time-resolved photoluminescence spectroscopy and PEC analyses, including Mott-Schottky, electrochemical impedance spectroscopy, and photovoltage ( Vph) measurements, were used to study the charge-carrier dynamics at the interfaces between the FTO, TiO2, and CdS QD. Analysis of the results showed that band alignment at the anatase/rutile junction between the TiO2 and FTO promoted electron-collection efficiency ( eEC) at the FTO/TiO2 interface and ET rate constant ( kET) at the TiO2/CdS QD interface. Furthermore, 34% enhancement of the efficiency in hydrogen (H2) generation demonstrated the potential of the TiO2 seed-layer-mediated TiO2/CdS QD NR photoanode in the application of PEC solar fuel production. The current work represents new insights into the mechanism of ET in TiO2 and TiO2/CdS QD NR, which is very useful for the development of photoelectrode materials in solar energy conversions.

Ecological patterns and adaptability of bacterial communities in alkaline copper mine drainage.

Environmental gradient have strong effects on community assembly processes. In order to reveal the effects of alkaline mine drainage (AlkMD) on bacterial and denitrifying bacterial community compositions and diversity in tailings reservoir, here we conducted an experiment to examine all and core bacterial taxa and denitrifying functional genes's (nirS, nirK, nosZΙ) abundance along a chemical gradient in tailings water in Shibahe copper tailings in Zhongtiaoshan, China. Differences in bacterial and denitrifying bacterial community compositions in different habitats and their relationships with environmental parameters were analyzed. The results showed that the richness and diversity of bacterial community in downstream seeping water (SDSW) were the largest, while that in upstream tailings water (STW1) were the lowest. The diversity and abundance of bacterial communities tended to increase from STW1 to SDSW. The variation of bacterial community diversity was significantly related to electroconductibility (EC), nitrate (NO3-), nitrite (NO2-), total carbon (TC), inorganic carbon (IC) and sulfate (SO42-), but was not correlated with geographic distance in local scale. Core taxa from class to genus were all significantly related to NO3- and NO2-. Core taxa Rhodobacteraceae, Rhodobacter, Acinetobacter and Hydrogenophaga were typical denitrifying bacteria. The variation trends of these groups were consistent with the copy number of nirS, nirK and nosZΙ, demonstrating their importance in the process of nitrogen reduction. The copy number of nirK, nosZΙ and nirS/16S rDNA, nirK/16Sr DNA correlated strongly with NO3-, NO2- and IC, but nirS and nosZI/16SrDNA had no significant correlation with NO3- and NO2-. The copy numbers of denitrifying functional genes (nirS, nirK and nosZΙ) were negatively correlated with heavy metal plumbum (Pb) and zinc (Zn). It showed that heavy metal contamination was an important factor affecting the structure of denitrifying bacterial community in AlkMD. In this study we have identified the distribution pattern of bacterial community along physiochemical gradients in alkaline tailings reservoir and displayed the driving force of shaping the structure of bacterial community. The influence of NO3-, NO2-, IC and heavy metal Pb and Zn on bacterial community might via their influence on the functional groups involving nitrogen, carbon and metal metabolisms.

Application of Monte Carlo simulation to cavity theory based on the virtual electron source concept.

A cavity theory based on the application of the electron transport equation in the cavity and the surrounding medium, referred to as L-L cavity theory in this paper, is discussed. The L-L cavity theory is a very detailed theory that considers electron production in the cavity as well as in the wall, and is applicable to all cavity sizes. The main idea of this cavity theory is that the effect of the cavity on the energy deposition of electrons inside it can be attributed to a virtual electron source uniformly distributed in the cavity. Monte Carlo calculations have been used to determine the effect of the virtual electron source on the relative ionization density in the cavity. In particular, the Monte Carlo method has been used to compute the parameter k(E, a), which is the fraction of the energy deposited in the cavity relative to the initial energy of electrons, given an electron source uniformly distributed in the cavity. The total energy deposition of the virtual electron source in the cavity may also be obtained. The calculated results agree well with the experimental results from Attix et al (Attix F H, Vergne L D and Ritz V H 1958 J. Res. NBS 60 235-43) and show that the L-L cavity theory gives a more accurate prediction of the cavity response at low energies than do other cavity theories.