Wind dispersal of soil invertebrates in the McMurdo Dry Valleys, Antarctica

Tardigrade distribution in soils of high Arctic habitats

Tardigrades are omnipresent microfauna with scarce record on their ecology in soils. Here, we investigated soil inhabiting tardigrade communities in five contrasting polar habitats, evaluating their abundance, diversity, species richness, and species composition. Moreover, we measured selected soil physico-chemical properties to find the drivers of tardigrade distribution among these habitats. In spite of reported tardigrade viability in extreme conditions, glacier forelands represented a habitat almost devoid of tardigrades. Even dry and wet tundra with soil developing for over more than 10 000 years held low abundances compared to usual numbers of tardigrades in temperate habitats. Polar habitats also differ in species composition, with Diaforobiotus islandicus being typical species for dry and Hypsibius exemplaris for wet tundra. Overall, tardigrade abundance was affected by the content of nutrients as well as physical properties of soil, i.e. content of total nitrogen (TN), total organic carbon (TOC), stoniness, soil texture and the water holding capacity (WHC). While diversity and species composition were significantly related to soil physical properties such as the bulk density (BD), soil texture, stoniness, and WHC. Physical structure of environment was, therefore, an important predictor of tardigrade distribution in polar habitats. Since many studies failed to identify significant determinants of tardigrade distribution, we encourage scientists to include physical properties of tardigrade habitats as explanatory variables in their studies.

Clearing the air: unraveling past and guiding future research in atmospheric chemosynthesis

SUMMARY

Atmospheric chemosynthesis is a recently proposed form of chemoautotrophic microbial primary production. The proposed process relies on the oxidation of trace concentrations of hydrogen (≤530 ppbv), carbon monoxide (≤90 ppbv), and methane (≤1,870 ppbv) gases using high-affinity enzymes. Atmospheric hydrogen and carbon monoxide oxidation have been primarily linked to microbial growth in desert surface soils scarce in liquid water and organic nutrients, and low in photosynthetic communities. It is well established that the oxidation of trace hydrogen and carbon monoxide gases widely supports the persistence of microbial communities in a diminished metabolic state, with the former potentially providing a reliable source of metabolic water. Microbial atmospheric methane oxidation also occurs in oligotrophic desert soils and is widespread throughout copiotrophic environments, with established links to microbial growth. Despite these findings, the direct link between trace gas oxidation and carbon fixation remains disputable. Here, we review the supporting evidence, outlining major gaps in our understanding of this phenomenon, and propose approaches to validate atmospheric chemosynthesis as a primary production process. We also explore the implications of this minimalistic survival strategy in terms of nutrient cycling, climate change, aerobiology, and astrobiology.

External and Internal Microbiomes of Antarctic Nematodes are Distinct, but More Similar to each other than the Surrounding Environment

Host-associated microbiomes have primarily been examined in the context of their internal microbial communities, but many animal species also contain microorganisms on external host surfaces that are important to host physiology. For nematodes, single strains of bacteria are known to adhere to the cuticle (e.g., Pasteuria penetrans), but the structure of a full external microbial community is uncertain. In prior research, we showed that internal gut microbiomes of nematodes (Plectus murrayi, Eudorylaimus antarcticus) and tardigrades from Antarctica's McMurdo Dry Valleys were distinct from the surrounding environment and primarily driven by host identity. Building on this work, we extracted an additional set of individuals containing intact external microbiomes and amplified them for 16S and 18S rRNA metabarcoding. Our results showed that external bacterial microbiomes were more diverse than internal microbiomes, but less diverse than the surrounding environment. Host-specific bacterial compositional patterns were observed, and external microbiomes were most similar to their respective internal microbiomes. However, external microbiomes were more influenced by the environment than the internal microbiomes were. Non-host eukaryotic communities were similar in diversity to internal eukaryotic communities, but exhibited more stochastic patterns of assembly compared to bacterial communities, suggesting the lack of a structured external eukaryotic microbiome. Altogether, we provide evidence that nematode and tardigrade cuticles are inhabited by robust bacterial communities that are substantially influenced by the host, albeit less so than internal microbiomes are.

Islands in the ice: Potential impacts of habitat transformation on Antarctic biodiversity

Antarctic biodiversity faces an unknown future with a changing climate. Most terrestrial biota is restricted to limited patches of ice-free land in a sea of ice, where they are adapted to the continent's extreme cold and wind and exploit microhabitats of suitable conditions. As temperatures rise, ice-free areas are predicted to expand, more rapidly in some areas than others. There is high uncertainty as to how species' distributions, physiology, abundance, and survivorship will be affected as their habitats transform. Here we use current knowledge to propose hypotheses that ice-free area expansion (i) will increase habitat availability, though the quality of habitat will vary; (ii) will increase structural connectivity, although not necessarily increase opportunities for species establishment; (iii) combined with milder climates will increase likelihood of non-native species establishment, but may also lengthen activity windows for all species; and (iv) will benefit some species and not others, possibly resulting in increased homogeneity of biodiversity. We anticipate considerable spatial, temporal, and taxonomic variation in species responses, and a heightened need for interdisciplinary research to understand the factors associated with ecosystem resilience under future scenarios. Such research will help identify at-risk species or vulnerable localities and is crucial for informing environmental management and policymaking into the future.

Glacial Legacies: Microbial Communities of Antarctic Refugia

In the cold deserts of the McMurdo Dry Valleys (MDV) the suitability of soil for microbial life is determined by both contemporary processes and legacy effects. Climatic changes and accompanying glacial activity have caused local extinctions and lasting geochemical changes to parts of these soil ecosystems over several million years, while areas of refugia may have escaped these disturbances and existed under relatively stable conditions. This study describes the impact of historical glacial and lacustrine disturbance events on microbial communities across the MDV to investigate how this divergent disturbance history influenced the structuring of microbial communities across this otherwise very stable ecosystem. Soil bacterial communities from 17 sites representing either putative refugia or sites disturbed during the Last Glacial Maximum (LGM) (22-17 kya) were characterized using 16 S metabarcoding. Regardless of geographic distance, several putative refugia sites at elevations above 600 m displayed highly similar microbial communities. At a regional scale, community composition was found to be influenced by elevation and geographic proximity more so than soil geochemical properties. These results suggest that despite the extreme conditions, diverse microbial communities exist in these putative refugia that have presumably remained undisturbed at least through the LGM. We suggest that similarities in microbial communities can be interpreted as evidence for historical climate legacies on an ecosystem-wide scale.

Tardigrades and their emergence as model organisms

Experimentally tractable organisms like C. elegans, Drosophila, zebrafish, and mouse are popular models for addressing diverse questions in biology. In 1997, two of the most valuable invertebrate model organisms to date-C. elegans and Drosophila-were found to be much more closely related to each other than expected. C. elegans and Drosophila belong to the nematodes and arthropods, respectively, and these two phyla and six other phyla make up a clade of molting animals referred to as the Ecdysozoa. The other ecdysozoan phyla could be valuable models for comparative biology, taking advantage of the rich and continual sources of research findings as well as tools from both C. elegans and Drosophila. But when the Ecdysozoa was first recognized, few tools were available for laboratory studies in any of these six other ecdysozoan phyla. In 1999 I began an effort to develop tools for studying one such phylum, the tardigrades. Here, I describe how the tardigrade species Hypsibius exemplaris and tardigrades more generally have emerged over the past two decades as valuable new models for answering diverse questions. To date, these questions have included how animal body plans evolve and how biological materials can survive some remarkably extreme conditions.

Blowin' in the wind: Dispersal, structure, and metacommunity dynamics of aeolian diatoms in the McMurdo Sound region, Antarctica

Diatom metacommunities are structured by environmental, historical, and spatial factors that are often attributed to organism dispersal. In the McMurdo Sound region (MSR) of Antarctica, wind connects aquatic habitats through delivery of inorganic and organic matter. We evaluated the dispersal of diatoms in aeolian material and its relation to the regional diatom metacommunity using light microscopy and 18S rRNA high-throughput sequencing. The concentration of diatoms ranged from 0 to 8.76 * 10⁶  valves · g^-1 dry aeolian material. Up to 15% of whole cells contained visible protoplasm, indicating that up to 3.43 * 10⁴ potentially viable individuals could be dispersed in a year to a single 2 -cm² site. Diatom DNA and RNA was detected at each site, reinforcing the likelihood that we observed dispersal of viable diatoms. Of the 50 known morphospecies in the MSR, 72% were identified from aeolian material using microscopy. Aeolian community composition varied primarily by site. Meanwhile, each aeolian community was comprised of morphospecies found in aquatic communities from the same lake basin. These results suggest that aeolian diatom dispersal in the MSR is spatially structured, is predominantly local, and connects local aquatic habitats via a shared species pool. Nonetheless, aeolian community structure was distinct from that of aquatic communities, indicating that intrahabitat dispersal and environmental filtering also underlie diatom metacommunity dynamics. The present study confirms that a large number of diatoms are passively dispersed by wind across a landscape characterized by aeolian processes, integrating the regional flora and contributing to metacommunity structure and landscape connectivity.

Colonization of macroalgal deposits by estuarine nematodes through air and potential for rafting inside algal structures

Dispersal is an important life-history trait. In marine meiofauna, and particularly in nematodes, dispersal is generally considered to be mainly passive, i.e. through transport with water currents and bedload transport. Because nematodes have no larval dispersal stage and have a poor swimming ability, their per capita dispersal capacity is expected to be limited. Nevertheless, many marine nematode genera and even species have near-cosmopolitan distributions, and at much smaller spatial scales, can rapidly colonise new habitat patches. Here we demonstrate that certain marine nematodes, like the morphospecies Litoditis marina, can live inside macroalgal structures such as receptacula and-to a lesser extent-floating bladders, which may allow them to raft over large distances with drifting macroalgae. We also demonstrate for the first time that these nematodes can colonize new habitat patches, such as newly deposited macroalgal wrack in the intertidal, not only through seawater but also through air. Our experimental set-up demonstrates that this aerial transport is probably the result of hitchhiking on vectors such as insects, which visit, and move between, the patches of deposited algae. Transport by wind, which has been observed for terrestrial nematodes and freshwater zooplankton, could not be demonstrated. These results can be important for our understanding of both large-scale geographic distribution patterns and of the small-scale colonization dynamics of habitat patches by marine nematodes.

The limited spatial scale of dispersal in soil arthropods revealed with whole-community haplotype-level metabarcoding

Soil arthropod communities are highly diverse and critical for ecosystem functioning. However, our knowledge of spatial structure and the underlying processes of community assembly are scarce, hampered by limited empirical data on species diversity and turnover. We implement a high-throughput sequencing approach to generate comparative data for thousands of arthropods at three hierarchical levels: genetic, species and supra-specific lineages. A joint analysis of the spatial arrangement across these levels can reveal the predominant processes driving the variation in biological assemblages at the local scale. This multihierarchical approach was performed using haplotype-level COI metabarcoding of entire communities of mites, springtails and beetles from three Iberian mountain regions. Tens of thousands of specimens were extracted from deep and superficial soil layers and produced comparative phylogeographic data for >1,000 codistributed species and nearly 3,000 haplotypes. Local assemblage composition differed greatly between grasslands and forests and, within each habitat, showed strong spatial structure and high endemicity. Distance decay was high at all levels, even at the scale of a few kilometres or less. The local distance decay patterns were self-similar for the haplotypes and higher hierarchical entities, and this fractal structure was similar in all regions, suggesting that uniform processes of limited dispersal determine local-scale community assembly. Our results from whole-community metabarcoding provide insight into how dispersal limitations constrain mesofauna community structure within local spatial settings over evolutionary timescales. If generalized across wider areas, the high turnover and endemicity in the soil locally may indicate extremely high richness globally, challenging our current estimations of total arthropod diversity on Earth.

Eukaryotic Colonization of Micrometer-Scale Cracks in Rocks: A "Microfluidics" Experiment Using Naturally Weathered Meteorites from the Nullarbor Plain, Australia

The advent of microfluidics has revolutionized the way we understand how microorganisms propagate through microporous spaces. Here, we apply this understanding to the study of how endolithic environmental microorganisms colonize the interiors of sterile rock. The substrates used for our study are stony meteorites from the Nullarbor Plain, Australia; a semiarid limestone karst that provides an ideal setting for preserving meteorites. Periodic flooding of the Nullarbor provides a mechanism by which microorganisms and exogenous nutrients may infiltrate meteorites. Our laboratory experiments show that environmental microorganisms reach depths greater than 400 μm by propagating through existing brecciation, passing through cracks no wider than the diameter of a resident cell (i.e., ∼5 μm). Our observations are consistent with the propagation of these eukaryotic cells via growth and cell division rather than motility. The morphology of the microorganisms changed as a result of propagation through micrometer-scale cracks, as has been observed previously for bacteria on microfluidic chips. It has been suggested that meteorites could have served as preferred habitats for microorganisms on ancient Mars. Based on our results, the depths reached by terrestrial microorganisms within meteorites would be sufficient to mitigate against the harmful effects of ionizing radiation, such as UV light, in Earth's deserts and potentially on Mars, if similar processes of microbial colonization had once been active there. Thus, meteorites landing in ancient lakes on Mars, that later dried out, could have been some of the last inhabited locations on the surface, serving as refugia before the planet's surface became inhospitable. Finally, our observations suggest that terrestrial microorganisms can colonize very fine cracks within meteorites (and potentially spaceships and rovers) on unexpectedly short timescales, with important implications for both recognition of extraterrestrial life in returned geological samples and planetary protection.

First 12 years of tardigrade succession in the young soils of a quickly evolving ecosystem

Open-cast coal-mining creates a severe disturbance of ecological systems, resulting in post-mining areas that have lost their soils entirely and are exposed to extreme conditions. Nonetheless, these areas allow us to investigate the process of primary succession. We studied the first 12 years of soil tardigrade succession at ‘Chicken Creek’, an experimental site within the open-cast coal-mining area in Welzow Süd, Germany. A total of ten tardigrade genera and 13 species or species groups were identified at the site. Over time, the organic carbon content, proportion of silt and nutrient availability in the soil increased, and the soil particle size distribution and pH value changed significantly. The vegetation cover and nematode abundance increased, whereas tardigrade densities peaked in 2008 and decreased thereafter, as did their mean body size. Species replacement was obvious within this 12 year study period, and tardigrade species diversity increased, indicating that ecological niche diversity also increased throughout this time. Vegetation cover, soil particle size distribution and time were the most important factors influencing the tardigrade community. We also discuss further potential factors and the ability of terrestrial tardigrades to persist despite many environmental constraints, such as fluctuations in pH and moisture, and to colonize new habitats faster than most other animal groups.

Anemochory of diapausing stages of microinvertebrates in North American drylands

1. Dry, ephemeral, desert wetlands are major sources of windblown sediment, as well as repositories for diapausing stages (propagules) of aquatic invertebrates. Zooplankton propagules are of the same size range as sand and dust grains. They can be deflated and transported in windstorm events. This study provides the evidence that dust storms aid in dispersal of microinvertebrate propagules via anemochory (aeolian transport). 2. We monitored 91 windstorms at six sites in the southwestern U.S. over a 17-year period. The primary study site was located in El Paso, Texas in the northern Chihuahuan Desert. Additional samples were collected from the Southern High Plains region. Dust carried by these events was collected and rehydrated to hatch viable propagules transported with it. 3. Using samples collected over a six-year period, 21 m above the ground which included 59 storm events, we tested the hypothesis that transport of propagules is correlated with storm intensity by monitoring meteorological conditions such as storm duration, wind direction, wind speed, and PM₁₀ (fine dust concentration). An air quality monitoring site located adjacent to the dust samplers provided quantitative hourly measurements. 4. Rehydration results from all events showed that ciliates were found in 92% of the samples, rotifers in 81%, branchiopods in 29%, ostracods in 4%, nematodes in 13%, gastrotrichs in 16%, and tardigrades in 3%. Overall, four bdelloid and 11 monogonont rotifer species were identified from rehydrated windblown dust samples. 5. PCA results indicated gastrotrichs, branchiopods, nematodes, tardigrades, and monogonont rotifer occurrence positively correlated with PM₁₀ and dust event duration. Bdelloid rotifers were correlated with amount of sediment deposited. NMDS showed a significant relationship between PM₁₀ and occurrence of some taxa. Zero-inflated, general linear models with mixed-effects indicated significant relationships with bdelloid and nematode transport and PM₁₀. 6. Thus, windstorms with high particulate matter concentration and long duration are more likely to transport microinvertebrate diapausing stages in drylands.

Diversity patterns of microbial eukaryotes mirror those of bacteria in Antarctic cryoconite holes

Ice-lidded cryoconite holes on glaciers in the Taylor Valley, Antarctica, provide a unique system of natural mesocosms for studying community structure and assembly. We used high-throughput DNA sequencing to characterize both microbial eukaryotic communities and bacterial communities within cryoconite holes across three glaciers to study similarities in their spatial patterns. We expected that the alpha (phylogenetic diversity) and beta (pairwise community dissimilarity) diversity patterns of eukaryotes in cryoconite holes would be related to those of bacteria, and that they would be related to the biogeochemical gradient within the Taylor Valley. We found that eukaryotic alpha and beta diversity were strongly related to those of bacteria across scales ranging from 140 m to 41 km apart. Alpha diversity of both was significantly related to position in the valley and surface area of the cryoconite hole, with pH also significantly correlated with the eukaryotic diversity. Beta diversity for both bacteria and eukaryotes was significantly related to position in the valley, with bacterial beta diversity also related to nitrate. These results are consistent with transport of sediments onto glaciers occurring primarily at local scales relative to the size of the valley, thus creating feedbacks in local chemistry and diversity.

Long-distance passive dispersal in microscopic aquatic animals

Given their dormancy capability (long-term resistant stages) and their ability to colonise and reproduce, microscopic aquatic animals have been suggested having cosmopolitan distribution. Their dormant stages may be continuously moved by mobile elements through the entire planet to any suitable habitat, preventing the formation of biogeographical patterns. In this review, I will go through the evidence we have on the most common microscopic aquatic animals, namely nematodes, rotifers, and tardigrades, for each of the assumptions allowing long-distance dispersal (dormancy, viability, and reproduction) and all the evidence we have for transportation, directly from surveys of dispersing stages, and indirectly from the outcome of successful dispersal in biogeographical and phylogeographical studies. The current knowledge reveals biogeographical patterns also for microscopic organisms, with species-specific differences in ecological features that make some taxa indeed cosmopolitan with the potential for long-distance dispersal, but others with restricted geographic distributions.

Evidence for regional aeolian transport of freshwater micrometazoans in arid regions

While separated by large expanses of dry terrain unsuitable for aquatic biota, aridland waters possess high biodiversity. How aquatic micrometazoans disperse to, and colonize, these isolated ephemeral habitats are not well understood. We used a multi-faceted approach including wind tunnel and rehydration experiments, and next-generation sequencing to assess potential movement of diapausing propagules of aquatic invertebrates by anemochory across regional scales (102-105 km). Wind tunnel experiments using dry playa sediments with added micrometazoan propagules demonstrated that after entrainment by saltation and downwind transport were subsequently recoverable as viable animals when rehydrated. Further, rehydration of fallen natural dust yielded micrometazoans, including rotifers, gastrotrichs, microcrustaceans, and nematodes. Using conserved DNA primers, we identified >3,300 eukaryotic Operational Taxonomic Units (excluding fungi) in the dust including some taxa found in rehydration experiments. Thus, we provide strong evidence that anemochory can disperse micrometazoans among isolated, ephemeral ecosystems in North American deserts and likely elsewhere.

Evidence of avian-mediated long distance dispersal in American tardigrades

Terrestrial tardigrades, commonly known as “water bears”, are part of a phylum of microscopic, aquatic invertebrates famous for cryptobiosis and space travel, but little is known about their modes of dispersal on Earth. Wind is assumed, but not truly demonstrated, to be the major method of global dispersal. Yet, some water bear distribution patterns cannot be explained by patterns of prevailing winds. Mammals and birds have been proposed as potential animal vectors. Importantly, most nearctic-neotropical migrant birds move north and south, with many crossing the equator, whereas prevailing winds move west to east or east to west but do not cross the equator. When multiplied by billions of birds over tens of millions of years, if the ectozoochory of tardigrades by birds is true then both regional and intercontinental patterns can be better explained. To test for the potential role of birds in tardigrade dispersal, the nests of 10 species for birds were examined. Seventy percent of nests were positive for tardigrades, demonstrating that some birds are in a position for transference. The carcasses of eight birds (six species) found dead from window strikes and a Sandhill Crane (Grus canadensis) found dead during routine surveys were also examined. Of the birds examined, 66% yielded tardigrades from two classes, three orders, and five species, including juveniles, adults, and eggs, suggesting that many bird species are potential vectors for many species of tardigrades. Our data support the hypothesis of avian-mediated long distance dispersal of tardigrades and provide evidence that further investigation is warranted.

The extent of wind-mediated dispersal of small metazoans, focusing nematodes

Wind-mediated transport is an important mechanism in the dispersal of small metazoans. Yet, concrete dispersal rates have hardly been examined. Here we present the results of an one-year field experiment investigating the composition and dispersal rates of aeroplankton. To gain insights into the dynamics of dispersal at the species level, we focused on nematodes, worldwide the most common metazoan taxon. Among the six taxa collected in this study (nematodes, rotifers, collembolans, tardigrades, mites, and thrips), nematodes had the highest dispersal rates (up to >3000 individuals m^-2 in 4 weeks, 27 species identified) and represented >44% of aeroplankton. Only living nematodes, and no propagules, were dispersed. All taxa had a higher dispersal potential in environments linked to the source habitat, evidenced by the much higher deposition of organisms in funnels placed on the ground than on the rooftop of a ten-story building. Nematodes under conditions of high humidity and wind speed had the highest dispersal rates, while increasing temperatures and dryness had a significantly positive impact on the wind drift of mites and thrips. The results indicated that wind dispersal over long distances is possible. The notable organismal input by wind dispersal may contribute to biodiversity and ecosystem functions.

Diversity and feeding strategies of soil microfauna along elevation gradients in Himalayan cold deserts

High-elevation cold deserts in Tibet and Himalaya are one of the most extreme environments. One consequence is that the diversity of macrofauna in this environment is often limited, and soil microorganisms have a more influential role in governing key surface and subsurface bioprocesses. High-elevation soil microfauna represent important components of cold ecosystems and dominant consumers of microbial communities. Still little is known about their diversity and distribution on the edge of their reproductive and metabolic abilities. In this study, we disentangle the impact of elevation and soil chemistry on diversity and distribution of rotifers, nematodes and tardigrades and their most frequent feeding strategies (microbial filter-feeders, bacterivores, fungivores, root-fungal feeders, omnivores) along two contrasting altitudinal gradients in Indian NW Himalaya (Zanskar transect from 3805 to 4714 m a.s.l.) and southwestern Tibet (Tso Moriri transect from 4477 to 6176 m a.s.l.), using a combination of multivariate analysis, variation partitioning and generalized additive models. Zanskar transect had higher precipitation, soil moisture, organic matter and available nutrients than dry Tso Moriri transect. In total, 40 species of nematodes, 19 rotifers and 1 tardigrade were discovered. Species richness and total abundance of rotifers and nematodes showed mid-elevation peaks in both investigated transects. The optimum for rotifers was found at higher elevation than for nematodes. Diversity and distribution of soil microfauna was best explained by soil nitrogen, phosphorus and organic matter. More fertile soils hosted more diverse and abundant faunal communities. In Tso Moriri, bacterivores represented 60% of all nematodes, fungivores 35%, root-fungal feeders 1% and omnivores 3%. For Zanskar the respective proportions were 21%, 13%, 56% and 9%. Elevational optima of different feeding strategies occurred in Zanskar in one elevation zone (4400–4500 m), while in Tso Moriri each feeding strategy had their unique optima with fungivores at 5300 m (steppes), bacterivores at 5500 m (alpine grassland), filter-feeders at 5600 m and predators and omnivores above 5700 m (subnival zone). Our results shed light on the diversity of microfauna in the high-elevation cold deserts and disentangle the role of different ecological filters in structuring microfaunal communities in the rapidly-warming Himalayas.

Microbial ecology of mountain glacier ecosystems: biodiversity, ecological connections and implications of a warming climate

Glacier ecosystems are teeming with life on, beneath, and to a lesser degree, within their icy masses. This conclusion largely stems from polar research, with less attention paid to mountain glaciers that overlap environmentally and ecologically with their polar counterparts in some ways, but diverge in others. One difference lies in the susceptibility of mountain glaciers to the near-term threat of climate change, as they tend to be much smaller in both area and volume. Moreover, mountain glaciers are typically steeper, more dependent upon basal sliding for movement, and experience higher seasonal precipitation. Here, we provide a modern synthesis of the microbial ecology of mountain glacier ecosystems, and particularly those at low- to mid-latitudes. We focus on five ecological zones: the supraglacial surface, englacial interior, subglacial bedrock-ice interface, proglacial streams and glacier forefields. For each, we discuss the role of microbiota in biogeochemical cycling and outline ecological and hydrological connections among zones, underscoring the interconnected nature of these ecosystems. Collectively, we highlight the need to: better document the biodiversity and functional roles of mountain glacier microbiota; describe the ecological implications of rapid glacial retreat under climate change and resolve the relative contributions of ecological zones to broader ecosystem function.

The D3-D5 region of large subunit ribosomal DNA provides good resolution of German limnic and terrestrial nematode communities

Reliable and well-developed DNA barcode databases are indispensable for the identification of microscopic life. However, effectiveness of molecular barcoding in identifying terrestrial specimens, and nematodes in particular, has received little attention. In this study, ca 600 ribosomal large subunit DNA fragments (D3-D5 region) were successfully amplified for 79 limnic and terrestrial nematode species sampled at 147 locations across Germany. Distinctive DNA motifs in the LSU region were identified in 80% of all species examined. For 13 supposedly single morphospecies, 2-7 LSU barcode groups were detected with a wide range of intraspecific variations (0.09-7.9%). This region seems to be more suitable for the assessment of limno-terrestrial nematode diversity than the frequently used mitochondrial gene COI, as amplification success of the latter fragment is low for several nematode species. Our reference database for nematodes may serve as a starting point for applied and fundamental studies for these ubiquitous, ecologically highly relevant, organisms.

Estimation of phylogenetic divergence times in Panagrolaimidae and other nematodes using relaxed molecular clocks calibrated with insect and crustacean fossils

This study presents the use of relaxed molecular clock methods to infer the dates of divergence between Panagrolaimus species. Autocorrelated relaxed tree methods, combined with well characterised fossil calibration dates, yield estimates of nematode divergence dates in accordance with the palaeontological age of fossil ascarid eggs and with the previously estimated date of 18 Ma (range 11.6 to 29.9 Ma) for the divergence of the Caenorhabditis lineage. Our data indicate that Panagrolaimus davidi from Antarctica separated ca 21.98 Ma from its currently known, most closely related strain. Thus, P. davidi may have existed in Antarctica prior to the Last Glacial Maximum, although this seems unlikely as it shares physiological and life history traits with closely related nematodes from temperate climates. These traits may have facilitated colonisation of Antarctica by P. davidi after the quaternary glaciation, analogous to the colonisation of Surtsey Island, Iceland, by P. superbus after its volcanic formation. This study demonstrates that autocorrelated relaxed tree methods combined with well characterised fossil calibration dates may be used as a method to estimate the divergence dates within nematodes in order to gain insight into their evolutionary history.

Changes of the Bacterial Abundance and Communities in Shallow Ice Cores from Dunde and Muztagata Glaciers, Western China

In this study, six bacterial community structures were analyzed from the Dunde ice core (9.5-m-long) using 16S rRNA gene cloning library technology. Compared to the Muztagata mountain ice core (37-m-long), the Dunde ice core has different dominant community structures, with five genus-related groups Blastococcus sp./Propionibacterium, Cryobacterium-related., Flavobacterium sp., Pedobacter sp., and Polaromas sp. that are frequently found in the six tested ice layers from 1990 to 2000. Live and total microbial density patterns were examined and related to the dynamics of physical-chemical parameters, mineral particle concentrations, and stable isotopic ratios in the precipitations collected from both Muztagata and Dunde ice cores. The Muztagata ice core revealed seasonal response patterns for both live and total cell density, with high cell density occurring in the warming spring and summer months indicated by the proxy value of the stable isotopic ratios. Seasonal analysis of live cell density for the Dunde ice core was not successful due to the limitations of sampling resolution. Both ice cores showed that the cell density peaks were frequently associated with high concentrations of particles. A comparison of microbial communities in the Dunde and Muztagata glaciers showed that similar taxonomic members exist in the related ice cores, but the composition of the prevalent genus-related groups is largely different between the two geographically different glaciers. This indicates that the micro-biogeography associated with geographic differences was mainly influenced by a few dominant taxonomic groups.

Response of Antarctic cryoconite microbial communities to light

Microbial communities on polar glacier surfaces are found dispersed on the ice surface, or concentrated in cryoconite holes and cryolakes, which are accumulations of debris covered by a layer of ice for some or all of the year. The ice lid limits the penetration of photosynthetically available radiation (PAR) to the sediment layer, since the ice attenuates up to 99% of incoming radiation. This suite of field and laboratory experiments demonstrates that PAR is an important control on primary production in cryoconite and cryolake ecosystems. Increased light intensity increased efficiency of primary production in controlled laboratory incubations of debris from the surface of Joyce Glacier, McMurdo Dry Valleys, Antarctica. However, when light intensity was increased to levels near that received on the ice surface, without the protection of an ice lid, efficiency decreased and measurements of photophysiology showed that the communities suffered light stress. The communities are therefore well adapted to low light levels. Comparison with Arctic cryoconite communities, which are typically not covered by an ice lid for the majority of the ablation season, showed that these organisms were also stressed by high light, so they must employ strategies to protect against photodamage.

Microbial communities in cryoconite holes and cryolakes on glaciers in Antarctica are adapted to low light levels and exhibit stress when exposed to high light.

Nearing the cold-arid limits of microbial life in permafrost of an upper dry valley, Antarctica

Some of the coldest and driest permafrost soils on Earth are located in the high-elevation McMurdo Dry Valleys (MDVs) of Antarctica, but little is known about the permafrost microbial communities other than that microorganisms are present in these valleys. Here, we describe the microbiology and habitable conditions of highly unique dry and ice-cemented permafrost in University Valley, one of the coldest and driest regions in the MDVs (1700 m above sea level; mean temperature −23 °C; no degree days above freezing), where the ice in permafrost originates from vapour deposition rather than liquid water. We found that culturable and total microbial biomass in University Valley was extremely low, and microbial activity under ambient conditions was undetectable. Our results contrast with reports from the lower-elevation Dry Valleys and Arctic permafrost soils where active microbial populations are found, suggesting that the combination of severe cold, aridity, oligotrophy of University Valley permafrost soils severely limit microbial activity and survival.

Genetic diversity and biogeography of the south polar water bear Acutuncus antarcticus (Eutardigrada : Hypsibiidae) – evidence that it is a truly pan-Antarctic species

Antarctica is an ice-dominated continent and all its terrestrial and freshwater habitats are fragmented, which leads to genetic divergence and, eventually, speciation. Acutuncus antarcticus is the most common Antarctic tardigrade and its cryptobiotic capabilities, small size and parthenogenetic reproduction present a high potential for dispersal and colonisation. Morphological (light and electron microscopy, karyology) and molecular (18S rRNA and cytochrome c oxidase subunit I (COI) genes) analyses on seven populations of A. antarcticus elucidated the genetic diversity and distribution of this species. All analysed populations were morphologically indistinguishable and made up of diploid females. All specimens presented the same 18S rRNA sequence. In contrast, COI analysis showed higher variability, with most Victoria Land populations presenting up to five different haplotypes. Genetic distances between Victoria Land specimens and those found elsewhere in Antarctica were low, while distances between Dronning Maud Land and specimens from elsewhere were high. Our analyses show that A. antarcticus can still be considered a pan-Antarctic species, although the moderately high genetic diversity within Victoria Land indicates the potential for speciation events. Regions of Victoria Land are considered to have been possible refugia during the last glacial maximum and a current biodiversity hotspot, which the populations of A. antarcticus mirror with a higher diversity than in other regions of Antarctica.

Soil biodiversity and human health

Soil biodiversity is increasingly recognized as providing benefits to human health because it can suppress disease-causing soil organisms and provide clean air, water and food. Poor land-management practices and environmental change are, however, affecting belowground communities globally, and the resulting declines in soil biodiversity reduce and impair these benefits. Importantly, current research indicates that soil biodiversity can be maintained and partially restored if managed sustainably. Promoting the ecological complexity and robustness of soil biodiversity through improved management practices represents an underutilized resource with the ability to improve human health.

Benthic microbial communities of coastal terrestrial and ice shelf Antarctic meltwater ponds

The numerous perennial meltwater ponds distributed throughout Antarctica represent diverse and productive ecosystems central to the ecological functioning of the surrounding ultra oligotrophic environment. The dominant taxa in the pond benthic communities have been well described however, little is known regarding their regional dispersal and local drivers to community structure. The benthic microbial communities of 12 meltwater ponds in the McMurdo Sound of Antarctica were investigated to examine variation between pond microbial communities and their biogeography. Geochemically comparable but geomorphologically distinct ponds were selected from Bratina Island (ice shelf) and Miers Valley (terrestrial) (<40 km between study sites), and community structure within ponds was compared using DNA fingerprinting and pyrosequencing of 16S rRNA gene amplicons. More than 85% of total sequence reads were shared between pooled benthic communities at different locations (OTU_0.05), which in combination with favorable prevailing winds suggests aeolian regional distribution. Consistent with previous findings Proteobacteria and Bacteroidetes were the dominant phyla representing over 50% of total sequences; however, a large number of other phyla (21) were also detected in this ecosystem. Although dominant Bacteria were ubiquitous between ponds, site and local selection resulted in heterogeneous community structures and with more than 45% of diversity being pond specific. Potassium was identified as the most significant contributing factor to the cosmopolitan community structure and aluminum to the location unique community based on a BEST analysis (Spearman's correlation coefficient of 0.632 and 0.806, respectively). These results indicate that the microbial communities in meltwater ponds are easily dispersed regionally and that the local geochemical environment drives the ponds community structure.

Anhydrobiosis and freezing-tolerance: adaptations that facilitate the establishment of Panagrolaimus nematodes in polar habitats

Anhydrobiotic animals can survive the loss of both free and bound water from their cells. While in this state they are also resistant to freezing. This physiology adapts anhydrobiotes to harsh environments and it aids their dispersal. Panagrolaimus davidi, a bacterial feeding anhydrobiotic nematode isolated from Ross Island Antarctica, can survive intracellular ice formation when fully hydrated. A capacity to survive freezing while fully hydrated has also been observed in some other Antarctic nematodes. We experimentally determined the anhydrobiotic and freezing-tolerance phenotypes of 24 Panagrolaimus strains from tropical, temperate, continental and polar habitats and we analysed their phylogenetic relationships. We found that several other Panagrolaimus isolates can also survive freezing when fully hydrated and that tissue extracts from these freezing-tolerant nematodes can inhibit the growth of ice crystals. We show that P. davidi belongs to a clade of anhydrobiotic and freezing-tolerant panagrolaimids containing strains from temperate and continental regions and that P. superbus, an early colonizer at Surtsey island, Iceland after its volcanic formation, is closely related to a species from Pennsylvania, USA. Ancestral state reconstructions show that anhydrobiosis evolved deep in the phylogeny of Panagrolaimus. The early-diverging Panagrolaimus lineages are strongly anhydrobiotic but weakly freezing-tolerant, suggesting that freezing tolerance is most likely a derived trait. The common ancestors of the davidi and the superbus clades were anhydrobiotic and also possessed robust freezing tolerance, along with a capacity to inhibit the growth and recrystallization of ice crystals. Unlike other endemic Antarctic nematodes, the life history traits of P. davidi do not show evidence of an evolved response to polar conditions. Thus we suggest that the colonization of Antarctica by P. davidi and of Surtsey by P. superbus may be examples of recent “ecological fitting” of freezing-tolerant anhydrobiotic propagules to the respective abiotic conditions in Ross Island and Surtsey.

Mitochondrial DNA analyses reveal widespread tardigrade diversity in Antarctica

Antarctica contains some of the most challenging environmental conditions on the planet due to freezing temperatures, prolonged winters and lack of liquid water. Whereas 99.7% of Antarctica is permanently covered by ice and snow, some coastal areas and mountain ridges have remained ice-free and are able to sustain populations of microinvertebrates. Tardigrades are one of the more dominant groups of microfauna in soil and limno-terrestrial habitats, but little is known of their diversity and distribution across Antarctica. Here, we examine tardigrades sampled from across an extensive region of continental Antarctica, and analyse and compare their partial mitochondrial cytochrome c oxidase subunit 1 (COI) gene sequences with those from the Antarctic Peninsula, maritime and sub-Antarctica, Tierra del Fuego and other worldwide locations in order to recognise operational taxonomic units (OTUs). From 439 new tardigrade COI sequences, we identified 98 unique haplotypes (85 from Antarctica) belonging to Acutuncus, Diphascon, Echiniscus, Macrobiotus, Milnesium and unidentified Parachela. Operational taxonomic units were delimited by Poisson tree processes and general mixed Yule coalescent methods, resulting in 58 and 55 putative species, respectively. Most tardigrades appear to be locally endemic (i.e. restricted to a single geographic region), but some (e.g. Acutuncus antarcticus (Richters, 1904)) are widespread across continental Antarctica. Our molecular results reveal: (i) greater diversity than has previously been appreciated with distinct OTUs that potentially represent undescribed species, and (ii) a lack of connectivity between most OTUs from continental Antarctica and those from other Antarctic geographical zones.

Ecological biogeography of the terrestrial nematodes of victoria land, antarctica

The terrestrial ecosystems of Victoria Land, Antarctica are characteristically simple in terms of biological diversity and ecological functioning. Nematodes are the most commonly encountered and abundant metazoans of Victoria Land soils, yet little is known of their diversity and distribution. Herein we present a summary of the geographic distribution, habitats and ecology of the terrestrial nematodes of Victoria Land from published and unpublished sources. All Victoria Land nematodes are endemic to Antarctica, and many are common and widely distributed at landscape scales. However, at smaller spatial scales, populations can have patchy distributions, with the presence or absence of each species strongly influenced by specific habitat requirements. As the frequency of nematode introductions to Antarctica increases, and soil habitats are altered in response to climate change, our current understanding of the environmental parameters associated with the biogeography of Antarctic nematofauna will be crucial to monitoring and possibly mitigating changes to these unique soil ecosystems.

Airborne bacterial populations above desert soils of the McMurdo Dry Valleys, Antarctica

Bacteria are assumed to disperse widely via aerosolized transport due to their small size and resilience. The question of microbial endemicity in isolated populations is directly related to the level of airborne exogenous inputs, yet this has proven hard to identify. The ice-free terrestrial ecosystem of Antarctica, a geographically and climatically isolated continent, was used to interrogate microbial bio-aerosols in relation to the surrounding ecology and climate. High-throughput sequencing of bacterial ribosomal RNA (rRNA) genes was combined with analyses of climate patterns during an austral summer. In general terms, the aerosols were dominated by Firmicutes, whereas surrounding soils supported Actinobacteria-dominated communities. The most abundant taxa were also common to aerosols from other continents, suggesting that a distinct bio-aerosol community is widely dispersed. No evidence for significant marine input to bioaerosols was found at this maritime valley site, instead local influence was largely from nearby volcanic sources. Back trajectory analysis revealed transport of incoming regional air masses across the Antarctic Plateau, and this is envisaged as a strong selective force. It is postulated that local soil microbial dispersal occurs largely via stochastic mobilization of mineral soil particulates.

First evidence of cryptic species diversity and significant population structure in a widespread freshwater nematode morphospecies (Tobrilus gracilis)

Free-living nematodes are ubiquitous and highly abundant in terrestrial and aquatic environments, where they sustain ecosystem functioning by mineralization processes and nutrient cycling. Nevertheless, very little is known about their true diversity and intraspecific population structure. Recent molecular studies on marine nematodes indicated cryptic diversity and strong genetic differentiation of distinct populations, but for freshwater nematode species, analogous studies are lacking. Here, we present the first extensive molecular study exploring cryptic species diversity and genetic population structure of a widespread freshwater nematode morphospecies, Tobrilus gracilis, from nine postglacially formed European lakes. Taxonomic species status of individuals, analysed for fragments of the mitochondrial COI gene and for the large (LSU) and small (SSU) ribosomal subunits, were determined by morphological characteristics. Mitochondrial and nuclear markers strongly supported the existence of three distinct genetic lineages (Tg I-III) within Tobrilus gracilis, suggesting that this morphospecies indeed represents a complex of highly differentiated biological species. High genetic diversity was also observed at the population level. Across the nine lakes, 19 mitochondrial, and seven (LSU) and four (SSU) nuclear haplotypes were determined. A phylogeographical analysis revealed remarkable genetic differentiation even among neighbouring lake populations for one cryptic lineage. Priority and persistent founder effects are possible explanations for the observed population structure in the postglacially colonized lakes, but ask for future studies providing direct estimates of freshwater nematode dispersal rates. Our study suggests therefore that overall diversity of limnetic nematodes has been so far drastically underestimated and challenges the assumed ubiquitous distribution of other, single freshwater nematode morphospecies.