Whole community and functional gene changes of biofilms on marine plastic debris in response to ocean acidification.

Plastics are accumulating in the world's oceans, while ocean waters are becoming acidified by increased CO2. We compared metagenome of biofilms on tethered plastic bottles in subtidal waters off Japan naturally enriched in CO2, compared to normal ambient CO2 levels. Extending from an earlier amplicon study of bacteria, we used metagenomics to provide direct insights into changes in the full range of functional genes and the entire taxonomic tree of life in the context of the changing plastisphere. We found changes in the taxonomic community composition of all branches of life. This included a large increase in diatom relative abundance across the treatments but a decrease in diatom diversity. Network complexity among families decreased with acidification, showing overall simplification of biofilm integration. With acidification, there was decreased prevalence of genes associated with cell-cell interactions and antibiotic resistance, decreased detoxification genes, and increased stress tolerance genes. There were few nutrient cycling gene changes, suggesting that the role of plastisphere biofilms in nutrient processes within an acidified ocean may not change greatly. Our results suggest that as ocean CO2 increases, the plastisphere will undergo broad-ranging changes in both functional and taxonomic composition, especially the ecologically important diatom group, with possible wider implications for ocean ecology.

Plant Gender Affects Soil Fungal Microbiota Associated with Welwitschia mirabilis, an Unusual Desert Gymnosperm.

In a recent study, we found a distinct soil bacterial community associated with male and female plants of the desert gymnosperm Welwitschia mirabilis. In this subsequent study, we also found that the soil fungal community associated with Welwitschia differs between male and female plants, and between unvegetated areas and the soil under plants. Site location, pH, and soil moisture also had an important influence on the composition of the fungal community. A number of Ascomycota and Chytrid species were found to be distinct indicators of male and female plants, respectively, but there was no overall difference at the phylum level or in terms of diversity. The unvegetated areas between plants also differed in terms of several Ascomycota OTUs. Network connectivity of the fungal communities was found to be higher under both male and female Welwitschia plants than in unvegetated control areas. As with the bacterial community, it is unclear what processes produce the gender-distinct fungal community, and also the more general plant-associated community, and also what the effects on the biology of the plants are. One possibility behind the gender-related difference in fungal community is that there are differences in the production of pollen or nectar between the two plant genders, affecting the below-ground soil community.

Metal-Tolerant Fungal Communities Are Delineated by High Zinc, Lead, and Copper Concentrations in Metalliferous Gobi Desert Soils.

The soil fungal ecology of the southern Gobi region of Mongolia has been little studied. We utilized the ITS1 region from soil DNA to study possible influences soil metal concentrations on soil fungal community variation. In the sample network, a distinctive fungal community was closely associated with high zinc (Zn), lead (Pb), and copper (Cu) concentrations. The pattern of occurrence suggests that high metal concentrations are natural and not a product of mining activities. The metal-associated fungal community differs little from the "normal" community in its major OTUs, and in terms of major fungal guilds and taxa, and its distinctiveness depends on a combination of many less common OTUs. The fungal community in the sites with high metal concentrations is no less diverse than that in areas with normal background levels. Overall, these findings raise interesting questions of the evolutionary origin and functional characteristics of this apparently "metal-tolerant" community, and of the associated soil biota in general. It is possible that rehabilitation of metal-contaminated mined soils from spoil heaps could benefit from the incorporation of fungi derived from these areas.

From the High Arctic to the Equator: Do Soil Metagenomes Differ According to Our Expectations?

Comparing the functional gene composition of soils at opposite extremes of environmental gradients may allow testing of hypotheses about community and ecosystem function. Here, we were interested in comparing how tropical microbial ecosystems differ from those of polar climates. We sampled several sites in the equatorial rainforest of Malaysia and Brunei, and the high Arctic of Svalbard, Canada, and Greenland, comparing the composition and the functional attributes of soil biota between the two extremes of latitude, using shotgun metagenomic Illumina HiSeq2000 sequencing. Based upon "classical" views of how tropical and higher latitude ecosystems differ, we made a series of predictions as to how various gene function categories would differ in relative abundance between tropical and polar environments. Results showed that in some respects our predictions were correct: the polar samples had higher relative abundance of dormancy related genes, and lower relative abundance of genes associated with respiration, and with metabolism of aromatic compounds. The network complexity of the Arctic was also lower than the tropics. However, in various other respects, the pattern was not as predicted; there were no differences in relative abundance of stress response genes or in genes associated with secondary metabolism. Conversely, CRISPR genes, phage-related genes, and virulence disease and defense genes, were unexpectedly more abundant in the Arctic, suggesting more intense biotic interaction. Also, eukaryote diversity and bacterial diversity were higher in the Arctic of Svalbard compared to tropical Brunei, which is consistent with what may expected from amplicon studies in terms of the higher pH of the Svalbard soil. Our results in some respects confirm expectations of how tropical versus polar nature may differ, and in other respects challenge them.

Extensive Overlap of Tropical Rainforest Bacterial Endophytes between Soil, Plant Parts, and Plant Species.

The extent to which distinct bacterial endophyte communities occur between different plant organs and species is poorly known and has implications for bioprospecting efforts. Using the V3 region of the bacterial 16S ribosomal RNA (rRNA) gene, we investigated the diversity patterns of bacterial endophyte communities of three rainforest plant species, comparing leaf, stem, and root endophytes plus rhizosphere soil community. There was extensive overlap in bacterial communities between plant organs, between replicate plants of the same species, between plant species, and between plant organ and rhizosphere soil, with no consistent clustering by compartment or host plant species. The non-metric multidimensional scaling (NMDS) analysis highlighted an extensively overlapping bacterial community structure, and the ß-nearest taxon index (ßNTI) analysis revealed dominance of stochastic processes in community assembly, suggesting that bacterial endophyte operational taxonomic units (OTUs) were randomly distributed among plant species and organs and rhizosphere soil. Percentage turnover of OTUs within pairs of samples was similar both for plant individuals of the same species and of different species at around 80-90%. Our results suggest that sampling extra individuals, extra plant organs, extra species, or use of rhizosphere soil, might be about equally effective for obtaining new OTUs for culture. These observations suggest that the plant endophyte community may be much more diverse, but less predictable, than would be expected from culturing efforts alone.

Distinctive Soil Archaeal Communities in Different Variants of Tropical Equatorial Forest.

Little is known of how soil archaeal community composition and diversity differ between local variants of tropical rainforests. We hypothesized that (1) as with plants, animals, fungi, and bacteria, the soil archaeal community would differ between different variants of tropical forest; (2) that spatially rarer forest variants would have a less diverse archaeal community than common ones; (3) that a history of forest disturbance would decrease archaeal alpha- and beta-diversity; and (4) that archaeal distributions within the forest would be governed more by deterministic than stochastic factors. We sampled soil across several different forest types within Brunei, Northwest Borneo. Soil DNA was extracted, and the 16S rRNA gene of archaea was sequenced using Illumina MiSeq. We found that (1) as hypothesized, there are distinct archaeal communities for each forest type, and community composition significantly correlates with soil parameters including pH, organic matter, and available phosphorous. (2) As hypothesized, the "rare" white sand forest variants kerangas and inland heath had lower archaeal diversity. A nestedness analysis showed that archaeal community in inland heath and kerangas was mainly a less diverse subset of that in dipterocarp forests. However, primary dipterocarp forest had the lowest beta-diversity among the other tropical forest types. (3) Also, as predicted, forest disturbance resulted in lower archaeal alpha-diversity-but increased beta-diversity in contrast with our predictions. (4) Contrary to our predictions, the BetaNTI of the various primary forest types indicated community assembly was mainly stochastic. The possible effects of these habitat and disturbance-related effects on N cycling should be investigated.

Development of Soil Bacterial Communities in Volcanic Ash Microcosms in a Range of Climates.

There is considerable interest in understanding the processes of microbial development in volcanic ash. We tested the predictions that there would be (1) a distinctive bacterial community associated with soil development on volcanic ash, including groups previously implicated in weathering studies; (2) a slower increase in bacterial abundance and soil C and N accumulation in cooler climates; and (3) a distinct communities developing on the same substrate in different climates. We set up an experiment, taking freshly fallen, sterilized volcanic ash from Sakurajima volcano, Japan. Pots of ash were positioned in multiple locations, with mean annual temperature (MAT) ranging from 18.6 to -3 °C. Within 12 months, bacteria were detectable by qPCR in all pots. By 24 months, bacterial copy numbers had increased by 10-100 times relative to a year before. C and N content approximately doubled between 12 and 24 months. HiSeq and MiSeq sequencing of the 16S rRNA gene revealed a distinctive bacterial community, different from developed vegetated soils in the same areas, for example in containing an abundance of unclassified bacterial groups. Community composition also differed between the ash pots at different sites, while showing no pattern in relation to MAT. Contrary to our predictions, the bacterial abundance did not show any relation to MAT. It also did not correlate to pH or N, and only C was statistically significant. It appears that bacterial community development on volcanic ash can be a rapid process not closely sensitive to temperature, involving distinct communities from developed soils.

Rainforest Conversion to Rubber Plantation May Not Result in Lower Soil Diversity of Bacteria, Fungi, and Nematodes.

Large areas of rainforest in Asia have been converted to plantations, with uncertain effects on soil biodiversity. Using standard metagenetic methods, we compared the soil biota of bacteria, fungi, and nematodes at three rainforest sites in Malaysia with two rubber plantation sites with similar soils and geology. We predicted the following: (1) that the rubber sites would have a lower α- and ß-diversity than the rainforest sites, due to the monospecific canopy cover and intensive management with herbicides, pesticides, and fertilizers, and (2) that due to differences in the physical and biotic environment associated with cultivation, there would be distinct communities of bacteria, fungi, and nematodes. However, regarding (1), the results showed no consistent difference in α- and ß-diversity of bacteria, fungi, or nematodes between rainforest and rubber plantation sites. It appears that conversion of rainforest to rubber plantations does not necessarily result in a decrease in diversity of soil biota. It may be that heterogeneity associated with the cultivation regimen compensates for loss of biotically imposed heterogeneity of the original rainforest. Regarding (2), as predicted there were statistically significant differences in community composition between rainforest and rubber plantation for bacteria, fungi, and nematodes. These differences could be related to a range of factors including light level, litter fall composition, pH, C and N, selecting a distinct set of soil taxa, and it is possible that this in itself would affect long-term soil function.

Shallow water marine sediment bacterial community shifts along a natural CO2 gradient in the Mediterranean Sea off Vulcano, Italy.

The effects of increasing atmospheric CO(2) on ocean ecosystems are a major environmental concern, as rapid shoaling of the carbonate saturation horizon is exposing vast areas of marine sediments to corrosive waters worldwide. Natural CO(2) gradients off Vulcano, Italy, have revealed profound ecosystem changes along rocky shore habitats as carbonate saturation levels decrease, but no investigations have yet been made of the sedimentary habitat. Here, we sampled the upper 2 cm of volcanic sand in three zones, ambient (median pCO(2) 419 µatm, minimum Ω(arag) 3.77), moderately CO(2)-enriched (median pCO(2) 592 µatm, minimum Ω(arag) 2.96), and highly CO(2)-enriched (median pCO(2) 1611 µatm, minimum Ω(arag) 0.35). We tested the hypothesis that increasing levels of seawater pCO(2) would cause significant shifts in sediment bacterial community composition, as shown recently in epilithic biofilms at the study site. In this study, 454 pyrosequencing of the V1 to V3 region of the 16S rRNA gene revealed a shift in community composition with increasing pCO(2). The relative abundances of most of the dominant genera were unaffected by the pCO(2) gradient, although there were significant differences for some 5 % of the genera present (viz. Georgenia, Lutibacter, Photobacterium, Acinetobacter, and Paenibacillus), and Shannon Diversity was greatest in sediments subject to long-term acidification (>100 years). Overall, this supports the view that globally increased ocean pCO(2) will be associated with changes in sediment bacterial community composition but that most of these organisms are resilient. However, further work is required to assess whether these results apply to other types of coastal sediments and whether the changes in relative abundance of bacterial taxa that we observed can significantly alter the biogeochemical functions of marine sediments.

Global comparison shows that soil bacterial communities in extreme pH soils are more structured by deterministic processes.

A major aim of microbial ecology is the search for basic 'rules' that dominate variation in microbial communities. An earlier comparison of several soil successional series showed that pH explained variation in the relative importance of stochastic versus deterministic processes in bacterial communities. In neutral pH soils, bacterial communities were more strongly influenced by stochastic processes than in low or high pH soils. Here, we took a broad level approach to attempt a more definitive answer of whether soil pH dominates bacterial community structuring using the global database of 237 samples. The beta-NTI showed that at both a global and continental scale, samples with low pH were dominated by deterministic processes, while in samples at around neutral pH, stochastic processes dominated. At high pH, stochasticity dominated on the global scale, but on several continents, the beta-NTI showed determinism predominating. Overall, it appears that bacterial community structuring is strongly and predictably affected by pH, with the most consistent difference observed between determinism at low pH and stochasticity at neutral pH. There is a need for hypothesis testing to explain why this trend exists. It is possible that at low pH, there is a greater selection for consortia to exploit resources, which leads to more predictable, deterministic combinations of species co-occurring. Additionally, the high energy demands for homeostasis and the constraints from the lack of available nutrient resources may impose greater niche-based competition, resulting in more deterministic community structuring at low pH.

Elevational Variation in and Environmental Determinants of Fungal Diversity in Forest Ecosystems of Korean Peninsula.

Exploring species diversity along elevational gradients is important for understanding the underlying mechanisms. Our study focused on analyzing the species diversity of fungal communities and their subcommunities at different trophic and taxonomic levels across three high mountains of the Korean Peninsula, each situated in a different climatic zone. Using high-throughput sequencing, we aimed to assess fungal diversity patterns and investigate the primary environmental factors influencing fungal diversity. Our results indicate that soil fungal diversity exhibits different elevational distribution patterns on different mountains, highlighting the combined effects of climate, soil properties, and geographic topology. Notably, the total and available phosphorus contents in the soil emerged as key determinants in explaining the differences in diversity attributed to soil properties. Despite the varied responses of fungal diversity to elevational gradients among different trophic guilds and taxonomic levels, their primary environmental determinants remained remarkably consistent. In particular, total and available phosphorus contents showed significant correlations with the diversity of the majority of the trophic guilds and taxonomic levels. Our study reveals the absence of a uniform diversity pattern along elevational gradients, underscoring the general sensitivity of fungi to soil conditions. By enriching our understanding of fungal diversity dynamics, this research enhances our comprehension of the formation and maintenance of elevational fungal diversity and the response of microbial communities in mountain ecosystems to climate change. This study provides valuable insights for future ecological studies of similar biotic communities.

Soil microbial co-occurrence networks become less connected with soil development in a high Arctic glacier foreland succession.

Classically, ecologists have considered that biota becomes more integrated and interdependent with ecosystem development in primary successional environments. However, recent work on soil microbial communities suggests that there may in fact be no change in network integration over successional time series. Here, we performed a test of this principle by identifying network-level topological features of the soil microbial co-occurrence networks in the primary successional foreland environment of the retreating high-Arctic glacier of Midtre Lovénbreen, Svalbard. Soil was sampled at sites along the foreland of inferred ages 10-90 years since deglaciation. DNA was extracted and amplicon sequenced for 16 s rRNA genes for bacteria and ITS1 region for fungi. Despite the chronologically-related soil pH decline and organic C/N accumulation, analysis on network-level topological features showed network integration did not change with inferred chronological ages, whereas network integration declined with decreasing pH and increasing total organic carbon (TOC) - both factors that can be viewed as an indicator of soil development. We also found that bacteria played a greater role in the network structure than fungi, with all keystone species in the microbial co-occurrence network being bacteria species. Both number and relative abundance of the keystone species were significantly higher when soil pH increased or TOC decreased. It appears that in the more extreme and less productive conditions of early primary succession, integration between members of soil biota into consortia may play a greater role in niche adaptation and survival. Our finding also emphasizes that ecosystem development is not simply a product of time but is influenced by locally heterogeneous factors.

Elevation-related climate trends dominate fungal co-occurrence network structure and the abundance of keystone taxa on Mt. Norikura, Japan.

Soil fungi play an important role in promoting nutrient cycling and maintaining ecosystem stability. Yet, there has been little understanding of how fungal co-occurrence networks differ along elevational climate gradients, a topic of interest to both macroecology and climate change studies. Based on high-throughput sequencing technology, we investigated the trend in co-occurrence network structure of soil fungal communities at 11 elevation levels along a 2300 m elevation gradient on Mt. Norikura, Japan, and identified the keystone taxa in the network, hypothesizing a progressive decline in network connectivity with elevation due to decreased plant diversity and enhanced environmental stress caused by changes in climate and soil characteristics. Our results demonstrated that network-level topological features such as network size, average degree, clustering coefficient, and modularity decreased significantly with increasing elevation, indicating that the fungal OTUs at low elevation were more closely associated and the network structure was more compact at low elevations. This conclusion was verified by the negative correlation between positive cohesion, negative cohesion and elevation. Moreover, the negative/positive cohesion ratio reached its peak value in mid-elevations with moderate environmental stress, indicating that the fungal community structure in mid-elevations was more stable than that at other elevations. We also found that the keystone taxa were more abundant at lower elevations. Furthermore, statistical analysis revealed that against a background of uniform geology, climate may play a dominant role in determining the properties and intensity of soil fungal networks, and significantly affect the abundance distribution of keystone taxa. These findings enhance understanding of the pattern and mechanism of the fungal community co-occurrence network along elevation, as well as the responses of microorganisms to climate change on a vertical scale in montane ecosystems. IMPORTANCE: Exploration of the elevational distribution of microbial networks and their driving factors and mechanisms may provide opportunities for predicting potential impacts of environmental changes, on ecosystem functions and biogeographic patterns at a broad scale. Although many studies have explored patterns of fungal community diversity and composition along various environmental gradients, it is unclear how the topological structure of co-occurrence networks shifts along elevational temperature gradients. In this study, we found that the connectivity of the fungal community decreased with increasing elevation and that climate was the dominant factor regulating co-occurrence patterns, apparently acting indirectly through soil characteristics. Our results also suggest that higher elevations on mountains have fewer keystone taxa than low elevations. These patterns may be related to the decrease of plant diversity and the increase of environmental stress along elevation gradients.

Elevation-related climatic factors dominate soil free-living nematode communities and their co-occurrence patterns on Mt. Halla, South Korea.

Nematodes play vital roles in soil ecosystems. To understand how their communities and coexistence patterns change along the elevation as well as to determine the best explanatory factors underlying these changes, we investigated free-living soil nematodes on Mt. Halla, South Korea, using an amplicon sequencing approach targeting the 18S rRNA gene. Our results showed that there was significant variation in the community diversity and composition of soil nematodes in relation to elevation. The network interactions between soil nematodes were more intensive at the lower elevations. Climatic variables were responsible explaining the elevational variation in community composition and co-occurrence pattern of the nematode community. Our study indicated that climatic factors served as the critical environmental filter that influenced not only the community structure but also the potential associations of soil nematodes in the mountain ecosystem of Mt. Halla. These findings enhance the understanding of the community structure and co-occurrence network patterns and mechanisms of soil nematode along elevation, and the response of soil nematodes to climate change on the vertical scale of mountain ecosystems.

Responses of Intertidal Bacterial Biofilm Communities to Increasing pCO2.

The effects of ocean acidification on ecosystems remain poorly understood, because it is difficult to simulate the effects of elevated CO2 on entire marine communities. Natural systems enriched in CO2 are being used to help understand the long-term effects of ocean acidification in situ. Here, we compared biofilm bacterial communities on intertidal cobbles/boulders and bedrock along a seawater CO2 gradient off Japan. Samples sequenced for 16S rRNA showed differences in bacterial communities with different pCO2 and between habitat types. In both habitats, bacterial diversity increased in the acidified conditions. Differences in pCO2 were associated with differences in the relative abundance of the dominant phyla. However, despite the differences in community composition, there was no indication that these changes would be significant for nutrient cycling and ecosystem function. As well as direct effects of seawater chemistry on the biofilm, increased microalgal growth and decreased grazing may contribute to the shift in bacterial composition at high CO2, as documented by other studies. Thus, the effects of changes in bacterial community composition due to globally increasing pCO2 levels require further investigation to assess the implications for marine ecosystem function. However, the apparent lack of functional shifts in biofilms along the pCO2 gradient is a reassuring indicator of stability of their ecosystem functions in shallow ocean margins.

Ocean acidification alters bacterial communities on marine plastic debris.

The increasing quantity of plastic waste in the ocean is providing a growing and more widespread novel habitat for microbes. Plastics have taxonomically distinct microbial communities (termed the 'Plastisphere') and can raft these unique communities over great distances. In order to understand the Plastisphere properly it will be important to work out how major ocean changes (such as warming, acidification and deoxygenation) are shaping microbial communities on waste plastics in marine environments. Here, we show that common plastic drinking bottles rapidly become colonised by novel biofilm-forming bacterial communities, and that ocean acidification greatly influences the composition of plastic biofilm assemblages. We highlight the potential implications of this community shift in a coastal community exposed to enriched CO2 conditions.

The 'fertile island effect' of Welwitschia plants on soil microbiota is influenced by plant gender.

Desert and semi-desert plants are often associated with a distinct soil biota under the plants and close to the root system. We aimed to understand if similar effects could be found in the taxonomically isolated desert gymnosperm Welwitschia mirabilis in the Namib Desert, and whether this island effect varied with climate and with gender of plants. We took soil cores adjacent to the plants in environments ranging from extreme desert to arid shrubland, and in nearby control sites between the plants. Soil chemistry was analysed, and deoxyribonucleic acid was extracted and sequenced for the bacterial 16s region. Soil under the plants was richer in organic C, N and moisture. Despite the range of climates, the soil around Welwitschia plants was consistently associated with a particular bacterial community composition that was distinct from samples further away. Compared to unvegetated control patches, bacterial diversity close to the plants was reduced. In the plant-associated soil community, there was a clear gender effect across all sites with a distinct community composition and greater diversity under male plants. It is unclear what differences in the soil environment might be producing these gender-associated differences, which provide an additional dimension to the fertile island effect.

Metagenomic analysis reveals rapid development of soil biota on fresh volcanic ash.

Little is known of the earliest stages of soil biota development of volcanic ash, and how rapidly it can proceed. We investigated the potential for soil biota development during the first 3 years, using outdoor mesocosms of sterile, freshly fallen volcanic ash from the Sakurajima volcano, Japan. Mesocosms were positioned in a range of climates across Japan and compared over 3 years, against the developed soils of surrounding natural ecosystems. DNA was extracted from mesocosms and community composition assessed using 16S rRNA gene sequences. Metagenome sequences were obtained using shotgun metagenome sequencing. While at 12 months there was insufficient DNA for sequencing, by 24 months and 36 months, the ash-soil metagenomes already showed a similar diversity of functional genes to the developed soils, with a similar range of functions. In a surprising contrast with our hypotheses, we found that the developing ash-soil community already showed a similar gene function diversity, phylum diversity and overall relative abundances of kingdoms of life when compared to developed forest soils. The ash mesocosms also did not show any increased relative abundance of genes associated with autotrophy (rbc, coxL), nor increased relative abundance of genes that are associated with acquisition of nutrients from abiotic sources (nifH). Although gene identities and taxonomic affinities in the developing ash-soils are to some extent distinct from the natural vegetation soils, it is surprising that so many of the key components of a soil community develop already by the 24-month stage. In this system, however, rapid development may be facilitated by the relatively moderate pH of the Sakurajima ash, proximity of our mesocosms to propagule sources, and the rapid establishment of a productive bryophyte and lichen layer on the surface. Ash from other volcanoes richer in acids or more distant from propagule sources could show a different pattern and slower soil biota development.

Modulation of Gut Microbiota in Korean Navy Trainees following a Healthy Lifestyle Change.

Environmental factors can influence the composition of gut microbiota, but understanding the combined effect of lifestyle factors on adult gut microbiota is limited. Here, we investigated whether changes in the modifiable lifestyle factors, such as cigarette smoking, alcohol consumption, sleep duration, physical exercise, and body mass index affected the gut microbiota of Korean navy trainees. The navy trainees were instructed to stop smoking and alcohol consumption and follow a sleep schedule and physical exercise regime for eight weeks. For comparison, healthy Korean civilians, who had no significant change in lifestyles for eight weeks were included in this study. A total of 208 fecal samples were collected from navy trainees (n = 66) and civilians (n = 38) at baseline and week eight. Gut flora was assessed by sequencing the highly variable region of the 16S rRNA gene. The α-and ß -diversity of gut flora of both the test and control groups were not significantly changed after eight weeks. However, there was a significant difference among individuals. Smoking had a significant impact in altering α-diversity. Our study showed that a healthy lifestyle, particularly cessation of smoking, even in short periods, can affect the gut microbiome by enhancing the abundance of beneficial taxa and reducing that of harmful taxa.

Soil pH rather than elevation determines bacterial phylogenetic community assembly on Mt. Norikura.

There is considerable interest in the factors which may explain variation in microbial community assembly processes. In this study, we investigated bacterial community assembly, phylogenetic diversity and the relative role of deterministic and stochastic processes along environmental gradients on Mt. Norikura, Japan. DNA extracted from soil samples collected at a range of elevations was PCR-amplified targeting the V3 and V4 regions of the bacterial 16S rRNA gene, and sequenced using Illumina MiSeq. We hypothesized that elevation would be a strong predictor of phylogenetic community assembly, with communities being more phylogenetically clustered towards higher elevations, due to more extreme physiological conditions. We also hypothesized a greater role of stochasticity at the highest elevations, due to more frequent soil disturbance. Contrary to our hypotheses, we found that the strength of phylogenetic clustering and the role of stochasticity were strongly related to soil pH, with phylogenetic clustering and deterministic processes being strongest at lower soil pH values. Moreover, there was no trend towards stronger influence of phylogenetic clustering and stochasticity in the upper elevations of Mt. Norikura. These results reveal an overwhelming influence of soil pH on phylogenetic community assembly of soil bacteria, even when a range of other environmental gradients are present.