Multiple‐trophic patterns of primary succession following retreat of a high‐elevation glacier

Principal role of fungi in soil carbon stabilization during early pedogenesis in the high Arctic

Climate warming is causing widespread deglaciation and pioneer soil formation over glacial deposits. Melting glaciers expose rocky terrain and glacial till sediment that is relatively low in biomass, oligotrophic, and depleted in nutrients. Following initial colonization by microorganisms, glacial till sediments accumulate organic carbon and nutrients over time. However, the mechanisms driving soil nutrient stabilization during early pedogenesis after glacial retreat remain unclear. Here, we traced amino acid uptake by microorganisms in recently deglaciated high-Arctic soils and show that fungi play a critical role in the initial stabilization of the assimilated carbon. Pioneer basidiomycete yeasts were among the predominant taxa responsible for carbon assimilation, which were associated with overall high amino acid use efficiency and reduced respiration. In intermediate- and late-stage soils, lichenized ascomycete fungi were prevalent, but bacteria increasingly dominated amino acid assimilation, with substantially decreased fungal:bacterial amino acid assimilation ratios and increased respiration. Together, these findings demonstrate that fungi are important drivers of pedogenesis in high-Arctic ecosystems that are currently subject to widespread deglaciation from global warming.

Rapid growth rate responses of terrestrial bacteria to field warming on the Antarctic Peninsula

Ice-free terrestrial environments of the western Antarctic Peninsula are expanding and subject to colonization by new microorganisms and plants, which control biogeochemical cycling. Measuring growth rates of microbial populations and ecosystem carbon flux is critical for understanding how terrestrial ecosystems in Antarctica will respond to future warming. We implemented a field warming experiment in early (bare soil; +2 °C) and late (peat moss-dominated; +1.2 °C) successional glacier forefield sites on the western Antarctica Peninsula. We used quantitative stable isotope probing with H218O using intact cores in situ to determine growth rate responses of bacterial taxa to short-term (1 month) warming. Warming increased the growth rates of bacterial communities at both sites, even doubling the number of taxa exhibiting significant growth at the early site. Growth responses varied among taxa. Despite that warming induced a similar response for bacterial relative growth rates overall, the warming effect on ecosystem carbon fluxes was stronger at the early successional site-likely driven by increased activity of autotrophs which switched the ecosystem from a carbon source to a carbon sink. At the late-successional site, warming caused a significant increase in growth rate of many Alphaproteobacteria, but a weaker and opposite gross ecosystem productivity response that decreased the carbon sink-indicating that the carbon flux rates were driven more strongly by the plant communities. Such changes to bacterial growth and ecosystem carbon cycling suggest that the terrestrial Antarctic Peninsula can respond fast to increases in temperature, which can have repercussions for long-term elemental cycling and carbon storage.

The microbiome of the ice-capped Cayambe Volcanic Complex in Ecuador

A major challenge in microbial ecology is to understand the principles and processes by which microbes associate and interact in community assemblages. Microbial communities in mountain glaciers are unique as first colonizers and nutrient enrichment drivers for downstream ecosystems. However, mountain glaciers have been distinctively sensitive to climate perturbations and have suffered a severe retreat over the past 40  years, compelling us to understand glacier ecosystems before their disappearance. This is the first study in an Andean glacier in Ecuador offering insights into the relationship of physicochemical variables and altitude on the diversity and structure of bacterial communities. Our study covered extreme Andean altitudes at the Cayambe Volcanic Complex, from 4,783 to 5,583 masl. Glacier soil and ice samples were used as the source for 16S rRNA gene amplicon libraries. We found (1) effects of altitude on diversity and community structure, (2) the presence of few significantly correlated nutrients to community structure, (3) sharp differences between glacier soil and glacier ice in diversity and community structure, where, as quantified by the Shannon γ-diversity distribution, the meta-community in glacier soil showed more diversity than in glacier ice; this pattern was related to the higher variability of the physicochemical distribution of variables in the former substrate, and (4) significantly abundant genera associated with either high or low altitudes that could serve as biomarkers for studies on climate change. Our results provide the first assessment of these unexplored communities, before their potential disappearance due to glacier retreat and climate change.

Community assembly in the wake of glacial retreat: A meta-analysis

Ecosystems shaped by retreating glaciers provide a unique opportunity to study the order and timing of biotic colonization, and how this influences the structure of successive ecological communities. In the last century glaciers across most of the cryosphere have receded at an unprecedented pace. Many studies have been published from different parts of the world testing hypotheses about how soil ecosystems are responding to rapid, contemporary deglaciation events. To better understand and draw general conclusions about how soil ecosystems respond to deglaciation, we conducted a global meta-analysis of 95 published articles focused on the succession of various organisms and soil physicochemical properties in glacier forefields along the chronosequence. Our global synthesis reveals that key soil properties and the abundance and richness of biota follow two conspicuous patterns: (1) some taxa demonstrate a persistent increase in abundance and richness over the entire chronosequence, (2) other taxa increase in abundance and richness during the first 50 years of succession, then gradually decline 50 years onward. The soil properties and soil organisms that are intimately tied to vegetation follow the first pattern, consistent with the idea that aboveground patterns of vegetation can drive patterns of belowground biodiversity. The second pattern may be due to an initial increase and subsequent decline in available nutrients and habitat suitability caused by increased biotic interactions, including resource competition among soil biota. A consensus view of the patterns of historical and contemporary soil ecosystem responses to deglaciation provides a better understanding of the processes that generate these patterns and informs predictions of ongoing and future responses to environmental changes.

Dynamic trophic shifts in bacterial and eukaryotic communities during the first 30 years of microbial succession following retreat of an Antarctic glacier

We examined microbial succession along a glacier forefront in the Antarctic Peninsula representing ∼30 years of deglaciation to contrast bacterial and eukaryotic successional dynamics and abiotic drivers of community assembly using sequencing and soil properties. Microbial communities changed most rapidly early along the chronosequence, and co-occurrence network analysis showed the most complex topology at the earliest stage. Initial microbial communities were dominated by microorganisms derived from the glacial environment, whereas later stages hosted a mixed community of taxa associated with soils. Eukaryotes became increasingly dominated by Cercozoa, particularly Vampyrellidae, indicating a previously unappreciated role for cercozoan predators during early stages of primary succession. Chlorophytes and Charophytes (rather than cyanobacteria) were the dominant primary producers and there was a spatio-temporal sequence in which major groups became abundant succeeding from simple ice Chlorophytes to Ochrophytes and Bryophytes. Time since deglaciation and pH were the main abiotic drivers structuring both bacterial and eukaryotic communities. Determinism was the dominant assembly mechanism for Bacteria, while the balance between stochastic/deterministic processes in eukaryotes varied along the distance from the glacier front. This study provides new insights into the unexpected dynamic changes and interactions across multiple trophic groups during primary succession in a rapidly changing polar ecosystem.

Unexpected high carbon losses in a continental glacier foreland on the Tibetan Plateau

Closely related with microbial activities, soil developments along the glacier forelands are generally considered a carbon sink; however, those of continental glacier forelands remain unclear. Continental glaciers are characterized by dry conditions and low temperature that limit microbial growth. We investigated the carbon characteristics along a chronosequence of the Laohugou Glacier No. 12 foreland, a typical continental glacier on the Tibetan Plateau, by analyzing soil bacterial community structure and microbial carbon-related functional potentials. We found an unexpected carbon loss in which soil organic carbon decreased from 22.21 g kg-1 to 10.77 g kg-1 after receding 50 years. Structural equation modeling verified the important positive impacts from bacterial community. Lower carbon fixation efficiency along the chronosequence was supported by less autotrophic bacteria and carbon fixation genes relating to the reductive tricarboxylic acid cycle. Lower carbon availability and higher carbon requirements were identified by an increasing bacterial copy number and a shift of the dominant bacterial community from Proteobacteria and Bacteroidetes (r-strategists) to Actinobacteria and Acidobacteria (K-strategists). Our findings show that the carbon loss of continental glacier foreland was significantly affected by the changes of bacterial community, and can help to avoid overestimating the carbon sink characteristics of glacier forelands in climate models.

Estimating biodiversity across the tree of life on Mount Everest’s southern flank with environmental DNA

Species composition in high-alpine ecosystems is a useful indicator for monitoring climatic and environmental changes at the upper limits of habitable environments. We used environmental DNA (eDNA) analysis to document the breadth of high-alpine biodiversity present on Earth’s highest mountain, Mt. Everest (8,849 m a.s.l.) in Nepal’s Khumbu region. In April-May 2019, we collected eDNA from ten ponds and streams between 4,500 m and 5,500 m. Using multiple sequencing and bioinformatic approaches, we identified taxa from 36 phyla and 187 potential orders across the Tree of Life in Mt. Everest’s high-alpine and aeolian ecosystem. These organisms, all recorded above 4,500 m—an elevational belt comprising <3% of Earth’s land surface—represents ∼16% of global taxonomic order estimates. Our eDNA inventory will aid future high-Himalayan biomonitoring and retrospective molecular studies to assess changes over time as climate-driven warming, glacial melt, and anthropogenic influences reshape this rapidly transforming world-renowned ecosystem.

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First comprehensive eDNA biodiversity survey conducted on Earth’s highest mountain

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One-sixth (16%) of global orders detected are >4,500m on the south flank of Everest

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Identified 187 unique orders from 36 phyla across the six kingdoms

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Metabarcoding and WGS approaches provide distinct yet complementary information

Landscape scale ecology of Tetracladium spp. fungal root endophytes

BACKGROUND

The genus Tetracladium De Wild. (Ascomycota) has been traditionally regarded as a group of Ingoldian fungi or aquatic hyphomycetes-a polyphyletic group of phylogenetically diverse fungi which grow on decaying leaves and plant litter in streams. Recent sequencing evidence has shown that Tetracladium spp. may also exist as root endophytes in terrestrial environments, and furthermore may have beneficial effects on the health and growth of their host. However, the diversity of Tetracladium spp. communities in terrestrial systems and the factors which shape their distribution are largely unknown.

RESULTS

Using a fungal community internal transcribed spacer amplicon dataset from 37 UK Brassica napus fields we found that soils contained diverse Tetracladium spp., most of which represent previously uncharacterised clades. The two most abundant operational taxonomic units (OTUs), related to previously described aquatic T. furcatum and T. maxilliforme, were enriched in roots relative to bulk and rhizosphere soil. For both taxa, relative abundance in roots, but not rhizosphere or bulk soil was correlated with B. napus yield. The relative abundance of T. furcatum and T. maxilliforme OTUs across compartments showed very similar responses with respect to agricultural management practices and soil characteristics. The factors shaping the relative abundance of OTUs homologous to T. furcatum and T. maxilliforme OTUs in roots were assessed using linear regression and structural equation modelling. Relative abundance of T. maxilliforme and T. furcatum in roots increased with pH, concentrations of phosphorus, and increased rotation frequency of oilseed rape. It decreased with increased soil water content, concentrations of extractable phosphorus, chromium, and iron.

CONCLUSIONS

The genus Tetracladium as a root colonising endophyte is a diverse and widely distributed part of the oilseed rape microbiome that positively correlates to crop yield. The main drivers of its community composition are crop management practices and soil nutrients.

Abundant and rare soil fungi exhibit distinct succession patterns in the forefield of Dongkemadi glacier on the central Qinghai-Tibet Plateau

Glaciers are retreating rapidly, exposing extensive new soil habitats in glacier forefields and providing unique areas for studying primary succession. However, understanding the variation patterns and assembly mechanisms of abundant and rare fungi subcommunities along the glacier-retreating chronosequence remains a knowledge gap, especially true for the vast Qinghai-Tibet Plateau (QTP). Here, we investigated fungal communities in the glacier forefield in Dongkemadi Glaicer on the central QTP. The results showed that fungal alpha diversity exhibited a clear increasing pattern in response to increasing of distance to glacier. The percentage of abundant OTUs decreased while the percentage of rare OTUs increased, suggesting that soil development is more beneficial to the rare taxa. The distributions of both abundant and rare subcommunities exhibited a clear spatial pattern along the distance to glacier, and might be strongly controlled by multiple environmental variables, including pH, soil moisture, vegetation status, soil organic carbon, total nitrogen, and soluble reactive phosphorus. Abundant and rare fungal subcommunities were structured in different assembly regimes. Dispersal limitation processes were dominant for both abundant and rare subcommunities but with a stronger contribution to abundant subcommunity assembly. Heterogeneous selection processes contributed higher and non-dominant processes contributed lower to abundant subcommunities than to rare subcommunities. The modular structure of the fungal co-occurrence network was highly localized along the soil chronosequence. By revealing distinct diversity patterns and community assembly mechanisms of abundant and rare fungal subcommunities, our study improved our understanding of ecological succession along the glacier-retreating chronosequence.