Climate change and alpine stream biology: progress, challenges, and opportunities for the future

Combined effects of river hydromorphological disturbances on macroinvertebrate communities: Multispatial scales analysis of central European rivers

Hydro-morphological threats impact the natural physical characteristics of river ecosystems, such as flow regimes, sediment transport, and channel morphology. These negative effects can occur at multiple scales, ranging from local microhabitats to geographic regions. Understanding these interactions can be useful for an integrated conservation approach and is needed for effective freshwater management. The aim of the study was to elucidate the combined effects of hydro-morphological threats on macroinvertebrates at three spatial scales: macroscale, including whole catchments, mesoscale (hydro-morphological changes in individual river sections) and the microscale, describing the microhabitat conditions of European rivers. The diversity and trophic structure of 1120 local macroinvertebrate communities in 28 catchments of various hydro-morphological disturbance levels, ranging from 0 to 2400 m asl, were analyzed. The response of macroinvertebrates to the main disturbance gradient differed between mountain and lowland communities. Random forest analysis indicated that the most important predictor of the ecological, diversity, and trophic indices was described by flow rate reduction. Generalized additive mixed models showed that decreased flow combined with river incision explained most of the variation in macroinvertebrate indices. Our results emphasize that based on multi-spatial scale analysis, hydro-morphological threats are very important factors in invertebrates biodiversity loss. Thus, to implement effective river management, we should pay more attention to the combined effects of geomorphological threats.

Dataset of a flow intermittency study: Benthic communities of 13 alpine intermittent rivers

In the last few decades, perennial mountain streams are becoming increasingly intermittent, due to global climate change and anthropogenic pressures. This phenomenon leads to negative effects on benthic communities' biodiversity and river ecosystems functionality. However, the impact of flow intermittency in previously perennial Alpine streams is still poorly investigated. This dataset consists of all the data collected during a spring sampling campaign performed in April-May 2017 along 13 mountain streams located in the SW Italian Alps. These watercourses have been selected because it was possible to identify two different sampling sites: one perennial, where water has always been flowing throughout the years, and one intermittent, which showed flowing water during the sampling campaign but, in the last decade, has experienced summer dry phases. All the sites have been characterized defining the microhabitats in which samples were retrieved, and physico-chemical data were collected at each site. Biological sampling included benthic macroinvertebrates and diatoms. Therefore, the present dataset offers various biological, ecological and physico-chemical information regarding Alpine streams which have recently become intermittent. Potentially, it could be used for comparisons with different benthic communities present in mountain rivers worldwide which are facing drying events too. The broad range of information present in this dataset offers the possibility to examine only the perennial sites themselves, as an example of good river functionality due to continuous flowing water, or only the intermittent ones, to better understand the effects of drying events on these peculiar ecosystems.

Comparative analysis of prokaryotic microbiomes in high-altitude active layer soils: insights from Ladakh and global analogues using In-Silico approaches

The active layer is the portion of soil overlaying the permafrost that freezes and thaws seasonally. It is a harsh habitat in which a varied and vigorous microbial population thrives. The high-altitude active layer soil in northern India is a unique and important cryo-ecosystem. However, its microbiology remains largely unexplored. It represents a unique reservoir for microbial communities with adaptability to harsh environmental conditions. In the Changthang region of Ladakh, the Tsokar area is a high-altitude permafrost-affected area situated in the southern part of Ladakh, at a height of 4530 m above sea level. Results of the comparison study with the QTP, Himalayan, Alaskan, Russian, Canadian and Polar active layers showed that the alpha diversity was significantly higher in the Ladakh and QTP active layers as the environmental condition of both the sites were similar. Moreover, the sampling site in the Ladakh region was in a thawing condition at the time of sampling which possibly provided nutrients and access to alternative nitrogen and carbon sources to the microorganisms thriving in it. Analysis of the samples suggested that the geochemical parameters and environmental conditions shape the microbial alpha diversity and community composition. Further analysis revealed that the cold-adapted methanogens were present in the Ladakh, Himalayan, Polar and Alaskan samples and absent in QTP, Russian and Canadian active layer samples. These methanogens could produce methane at slow rates in the active layer soils that could increase the atmospheric temperature owing to climate change.

From Primary Data to Ethnopharmacological Investigations on Achillea erba-rotta subsp. moschata (Wulfen) I.Richardson as a Remedy against Gastric Ailments in Valmalenco (Italy)

(1) Background: Within the framework of the European Interreg Italy-Switzerland B-ICE & Heritage project (2018-2022), this study originated from a three-year ethnobotanical survey in Valmalenco (Sondrio, Italy). Following a preliminary work published by our group, this research further explored the folk therapeutic use of Achillea erba-rotta subsp. moschata (Wulfen) I.Richardson (Asteraceae) for dyspepsia disorders, specifically its anti-inflammatory potential at a gastrointestinal level. (2) Methods: Semi-structured interviews were performed. The bitter taste was investigated through molecular docking software (PLANTS, GOLD), while the anti-inflammatory activity of the hydroethanolic extract, infusion, and decoction was evaluated based on the release of IL-8 and IL-6 after treatment with TNFα or Helicobacter pylori. The minimum inhibitory concentration and bacterial adhesion on the gastric epithelium were evaluated. (3) Results: In total, 401 respondents were interviewed. Molecular docking highlighted di-caffeoylquinic acids as the main compounds responsible for the interaction with bitter taste receptors. The moderate inhibition of IL-6 and IL-8 release was recorded, while, in the co-culture with H. pylori, stronger anti-inflammatory potential was expressed (29-45 μg/mL). The concentration-dependent inhibition of H. pylori growth was recorded (MIC = 100 μg/mL), with a significant anti-adhesive effect. (4) Conclusions: Confirming the folk tradition, the study emphasizes the species' potentiality for dyspepsia disorders. Future studies are needed to identify the components mostly responsible for the biological effects.

Evolutionary history of a cold-adapted limnephilid caddisfly: Effects of climate change and topography on genetic structure

The distribution of organisms is influenced by complex factors such as the phylogenetic evolutionary histories of species, the physiological and ecological characteristics of organisms, climate, and geographical and geohistorical features. In this study, we focused on a caddisfly, Asynarchus sachalinensis (Trichoptera: Limnephilidae), which has adapted to cold habitats. From phylogeographic analyses based on the mitochondrial DNA (mtDNA) cytochrome c oxidase subunit I (COI) and 16S rRNA regions and the nuclear DNA (nDNA) 18S rRNA, 28S rRNA, carbamoyl-phosphate synthetase (CAD), elongation factor-1 alpha (EF1-α), and RNA polymerase II (POLII) regions, two distinct genetic clades were detected. Clade I was shown to be widely distributed from Sakhalin to Honshu, whereas Clade II was only distributed within Honshu. The distributions of these clades overlapped in Honshu. The habitats were located at relatively lower altitudes for Clade I and higher altitudes for Clade II. The divergence time of these clades was estimated to be during the Pleistocene, indicating that repeated climatic changes facilitated distributional shifts. Haplotype network and demographic analyses based on the mtDNA COI region showed contrasting genetic structures in the two clades. It was indicated that the population sizes of Clade I had expanded rapidly in a recent period, whereas Clade II had maintained stable population sizes. The habitats of Clade II were typically isolated and scattered at high altitudes, resulting in restricted migration and dispersal because of their discontinuous "Sky Island" habitats. The habitats of Clade I were located at relatively low altitudes, and it was assumed that the populations were continuous, which resulted in a higher frequency of migration and dispersal between populations. Thus, differences in the spatial scale of the adapted habitats of each clade may have resulted in different patterns of population connectivity and fragmentation associated with repeated climatic changes during the Pleistocene. Our study provided new insight into the distributional patterns of cold-adapted aquatic insects in the Japanese Archipelago. Furthermore, the distributional shifts predicted by ecological niche modeling under future climatic change conditions were different for each clade. Therefore, different principles are required in the assessment of each clade to predict temporal changes in their distributions.

Adaptive traits of cysts of the snow alga Sanguina nivaloides unveiled by 3D subcellular imaging

Sanguina nivaloides is the main alga forming red snowfields in high mountains and Polar Regions. It is non-cultivable. Analysis of environmental samples by X-ray tomography, focused-ion-beam scanning-electron-microscopy, physicochemical and physiological characterization reveal adaptive traits accounting for algal capacity to reside in snow. Cysts populate liquid water at the periphery of ice, are photosynthetically active, can survive for months, and are sensitive to freezing. They harbor a wrinkled plasma membrane expanding the interface with environment. Ionomic analysis supports a cell efflux of K+, and assimilation of phosphorus. Glycerolipidomic analysis confirms a phosphate limitation. The chloroplast contains thylakoids oriented in all directions, fixes carbon in a central pyrenoid and produces starch in peripheral protuberances. Analysis of cells kept in the dark shows that starch is a short-term carbon storage. The biogenesis of cytosolic droplets shows that they are loaded with triacylglycerol and carotenoids for long-term carbon storage and protection against oxidative stress.

Metal enrichment in ice-melt water and uptake by chironomids as possible legacy of World War One in the Italian Alps

Relics of World War One (WW1) were buried in alpine glaciers around 100 years ago. Today, these are emerging from the ice due to widespread glacier retreat, and are in direct contact with glacial meltwater-fed streams. To address a possible emergent contamination, we quantified major and trace elements (M-TEs) by mass spectrometry in water and larvae of Diamesa zernyi from three glacial streams fed by glaciers differently impacted by the Italian Austro-Hungarian war, in the Adamello-Presanella mountain range (Italian Alps): Lares and Presena, the two main battlefields, and Amola, 8 km from the front. M-TEs in stream water were interpreted using the crustal enrichment factor (EFc) while larval uptake was quantified by adopting the bioaccumulation factor (BAF). Despite low M-TEs concentrations in the water, in a range between 1 ng L-1 (Ag, Ta) and 1-2 mg L-1 (Al, Fe, Mg), low to moderate enrichments (10 ≥ EFc≥ 6) were observed for Sb and U in Presena and for Ag, As, Bi, Cd, Li, Mo, Pb, Sb and U in Lares. In addition, M-TE mass concentrations in larvae were up to ninety thousand times higher than in water, from 20 to 50 ng g-1 dry weight (d.w.; for Bi, Sb, Ta, Tl) to 1-4 mg g-1 d.w. (for Al, Fe, Na, and Mg). Larvae from Lares accumulated the largest amount of metals and metalloids, including those mostly used in the manufacture of artillery shells (As, Cu, Ni, Pb, Sb; BAFs from 375 to about 11,500). This was expected as most of the WW1 battles in this mountain range were fought on the Lares glacier, where the greatest number of war relics are emerging. These results provide preliminary evidence of water contamination and bioaccumulation of metals and metalloids by glacial fauna as a possible legacy of WW1 in the Alps.

Glacier foreland insect uptake synthetic compounds: an emerging environmental concern

Pesticides, synthetic fragrances and polycyclic aromatic hydrocarbons contaminated two glacier-fed streams (Amola, Mandrone) and one spring (Grostè) in the Italian Alps. Ten compounds (chlorpyrifos (CPY), chlorpyrifos-methyl (CPY-m), galaxolide (HHCB), tonalide (AHTN), fluorene (Flu), phenanthrene (Phen), anthracene (Ant), fluoranthene (Fl), pyrene (Pyr), benzo[a]anthracene (BaA)) accumulated in aquatic larvae of chironomids (Diamesa steinboecki, D. latitarsis, D. bertrami, D. tonsa, D. zernyi, Pseudokiefferiella parva, Orthocladiinae) and tipulids. Their tissue concentrations (detected by gas chromatography coupled with mass spectrometry) ranged from 1.1 ± 0.1 ng/g d.w. (= dry weight) (CPY-m in D. tonsa from Amola) to 68.0 ± 9.1 ng/g d.w. (Pyr in D. steinboecki from Mandrone). HHCB, AHTN, and CPY, with one exception, were accumulated by all aquatic insects. Six compounds (CPY, CPY-m, HHCB, AHTN, Fl, Pyr) also contaminated carabids (Nebria germarii, N. castanea, N. jockischii) predating adults of merolimnic insects. Their tissue concentrations ranged from 1.1 ± 0.3 ng/g d.w. (CPY-m in N. germarii from Mandrone) to 84.6 ± 0.3 ng/g d.w. (HHCB in N. castanea from Grostè). HHCB and AHTN were accumulated by all Nebria species. Intersite and interspecies differences were observed, which might be attributed to different environmental contamination levels. There was a stronger similarity between species from the same site than among the same species from different sites, suggesting that uptake is not species specific. At all sites, the concentration of xenobiotics was higher in larvae than in water and comparable or higher in carabids than in larvae from the same site, suggesting trophic transfer by emerging aquatic insects to their riparian predators.

Contrasting adaptive genetic consequences of stream insects under changing climate

Freshwater biodiversity undergoes degradation due to climate change. Researchers have inferred the effects of climate change on neutral genetic diversity, assuming the fixed spatial distributions of alleles. However, the adaptive genetic evolution of populations that may change the spatial distribution of allele frequencies along environmental gradients (i.e., evolutionary rescue) have largely been overlooked. We developed a modeling approach that projects the comparatively adaptive and neutral genetic diversities of four stream insects, using empirical neutral/ putative adaptive loci, ecological niche models (ENMs), and a distributed hydrological-thermal simulation at a temperate catchment under climate change. The hydrothermal model was used to generate hydraulic and thermal variables (e.g., annual current velocity and water temperature) at the present and the climatic change conditions, projected based on the eight general circulation models and the three representative concentration pathways scenarios for the two future periods (2031-2050, near future; 2081-2100, far future). The hydraulic and thermal variables were used for predictor variables of the ENMs and adaptive genetic modeling based on machine learning approaches. The increases in annual water temperature in the near- (+0.3-0.7 °C) and far-future (+0.4-3.2 °C) were projected. Of the studied species, with different ecologies and habitat ranges, Ephemera japonica (Ephemeroptera) was projected to lose rear-edge habitats (i.e., downstream) but retain the adaptive genetic diversity owing to evolutionary rescue. In contrast, the habitat range of the upstream-dwelling Hydropsyche albicephala (Trichoptera) was found to remarkably decline, resulting in decreases in the watershed genetic diversity. While the other two Trichoptera species expanded their habitat ranges, the genetic structures were homogenized over the watershed and experienced moderate decreases in gamma diversity. The findings emphasize the evolutionary rescue potential, depending on the extent of species-specific local adaptation.

Long-term changes in macroinvertebrate communities across high-latitude streams

Long-term records of benthic macroinvertebrates in high-latitude streams are essential for understanding climatic changes, including extreme events (e.g. floods). Data extending over multiple decades are typically scarce. Here, we investigated macroinvertebrate community structural change (including alpha and beta diversity and gain and loss of species) over 22 years (1994-2016) in 10 stream systems across Denali National Park (Alaska, USA) in relation to climatological and meteorological drivers (e.g. air temperature, snowpack depth, precipitation). We hypothesised that increases in air temperature and reduced snowpack depth, due to climatic change, would reduce beta and gamma diversity but increase alpha diversity. Findings showed temporal trends in alpha diversity were variable across streams, with oscillating patterns in many snowmelt- and rainfall runoff-fed streams linked to climatic variation (temperature and precipitation), but increased over time in several streams supported by a mixture of water sources, including more stable groundwater-fed streams. Beta-diversity over the time series was highly variable, yet marked transitions were observed in response to extreme snowpack accumulation (1999-2000), where species loss drove turnover. Gamma diversity did not significantly increase or decrease over time. Investigating trends in individual taxa, several taxa were lost and gained during a relative constrained time period (2000-2006), likely in response to climatic variability and significant shifts in instream environmental conditions. Findings demonstrate the importance of long-term biological studies in stream ecosystems and highlight the vulnerability of high-latitude streams to climate change.

Rapid adaptation in a fast-changing world: Emerging insights from insect genomics

Many researchers have questioned the ability of biota to adapt to rapid anthropogenic environmental shifts. Here, we synthesize emerging genomic evidence for rapid insect evolution in response to human pressure. These new data reveal diverse genomic mechanisms (single locus, polygenic, structural shifts; introgression) underpinning rapid adaptive responses to a variety of anthropogenic selective pressures. While the effects of some human impacts (e.g. pollution; pesticides) have been previously documented, here we highlight startling new evidence for rapid evolutionary responses to additional anthropogenic processes such as deforestation. These recent findings indicate that diverse insect assemblages can indeed respond dynamically to major anthropogenic evolutionary challenges. Our synthesis also emphasizes the critical roles of genomic architecture, standing variation and gene flow in maintaining future adaptive potential. Broadly, it is clear that genomic approaches are essential for predicting, monitoring and responding to ongoing anthropogenic biodiversity shifts in a fast-changing world.

On the delay between water temperature and invertebrate community response to warming climate

We evaluated the effect of global warming on invertebrate communities at high altitudes using data from the Careser system. We procured data on air temperature, which was obtained over 50 years at altitudes above 2600 m a.s.l., and data on water temperature, which was available for approximately 30 years. We sampled thrice in the past 20 years (2001, 2014, 2018) at three sampling sites (CR0-metakryal, CR1-hypokryal, CR2-glacio-rhithral) of the Careser glacier-fed stream and its main non-glacial tributary (CR1bis-krenal). Warmer climates were observed in the last decade compared to the 1980s, with a mean maximum summer air temperature (mTmax) increase of 1.7 °C at 2642 m a.s.l. and 1.8 °C at 2858 m a.s.l. Compared to air temperatures, the rise in water temperature was delayed by approximately 20 years; water mTmax started to increase in 2003, reaching 8.1 °C at 2642 m a.s.l. and 2.4 °C at 2858 m a.s.l in the year 2020. The invertebrate community exhibited a delayed response approximately 13 years from the water warming; there was a sequential increase in the number of taxa, Shannon diversity, and after 17 years, functional diversity. In the kryal sites, taxonomical and functional diversity changed more consistently than in the glacio-rhithral site in the same period, due to the arrival of taxa that were previously absent upstream and bearers of entirely new traits. Progressive taxonomical homogenisation was evident with decreasing glacial influence, mainly between glacio-rhithral and krenal sites. The numbers of Diamesa steinboecki, an insect that was adapted to the cold, declined in summer (water mTmax >6 °C and air mTmax >12 °C). This study highlights the mode and time of response of stream invertebrate communities to global warming in alpine streams and provides guidelines for analysing changes in the stream invertebrate communities of other glacial systems in alpine regions.

Effects of thermal acclimation on the proteome of the planarian Crenobia alpina from an alpine freshwater spring

Species' acclimation capacity and their ability to maintain molecular homeostasis outside ideal temperature ranges will partly predict their success following climate change-induced thermal regime shifts. Theory predicts that ectothermic organisms from thermally stable environments have muted plasticity, and that these species may be particularly vulnerable to temperature increases. Whether such species retained or lost acclimation capacity remains largely unknown. We studied proteome changes in the planarian Crenobia alpina, a prominent member of cold-stable alpine habitats that is considered to be a cold-adapted stenotherm. We found that the species' critical thermal maximum (CT_max) is above its experienced habitat temperatures and that different populations exhibit differential CT_max acclimation capacity, whereby an alpine population showed reduced plasticity. In a separate experiment, we acclimated C. alpina individuals from the alpine population to 8, 11, 14 or 17°C over the course of 168 h and compared their comprehensively annotated proteomes. Network analyses of 3399 proteins and protein set enrichment showed that while the species' proteome is overall stable across these temperatures, protein sets functioning in oxidative stress response, mitochondria, protein synthesis and turnover are lower in abundance following warm acclimation. Proteins associated with an unfolded protein response, ciliogenesis, tissue damage repair, development and the innate immune system were higher in abundance following warm acclimation. Our findings suggest that this species has not suffered DNA decay (e.g. loss of heat-shock proteins) during evolution in a cold-stable environment and has retained plasticity in response to elevated temperatures, challenging the notion that stable environments necessarily result in muted plasticity.

Summary: The proteome of an alpine Crenobia alpina population shows plasticity in response to acclimation to warmer temperatures.

Past climate cooling promoted global dispersal of amphipods from Tian Shan montane lakes to circumboreal lakes

Climate changes have substantial impacts on the geographic distribution of montane lakes and evolutionary dynamics of cold-adapted species. Past climate cooling is hypothesized to have promoted the dispersal of cold-adapted species via montane lakes, while future climate warming is thought to constrain their distributions. We test this hypothesis by using phylogeographic analysis and niche modeling of the Holarctic crustacean Gammarus lacustris with global sampling comprised of 567 sequenced individuals and 3180 occurrence records. We found that the species arose in Tian Shan in Central Asia and dispersed into montane lakes along the Alps, Himalayas, Tibet, East Asia, and the North American Rocky Mountain ranges, with accelerated diversification rates outside Tian Shan. Climatically suitable regions for geographic lineages of G. lacustris were larger during cooling periods (LGM), but smaller during warming periods (Mid-Holocene). In the future (2070) scenario, potential distributions in the Himalayas, North Tibet, South Tibet and North America are predicted to expand, whereas ranges in East Asia, Europe and Tian Shan will decline. Our results suggest that Mid-Miocene-to-Pleistocene continuous cooling promoted multiple independent dispersal events out of Tian Shan due to increased availability of montane lakes via "budding" of lineages. Montane lakes are conduits through which cold-adapted amphipods globally dispersed, dominating circumboreal lakes. However, future climate warming is likely to force organisms to shift upward in altitude and northward in latitude, leading to a future change in local populations. These findings highlight the importance of conservation of montane lakes, especially in the context of climate change.

Taxonomy and natural history of Cavernocypris hokkaiensis sp. nov., the first ostracod reported from alpine streams in Japan

We describe the cypridoidean ostracod Cavernocypris hokkaiensis sp. nov. from riverbed sediments in an alpine stream at an elevation of ca. 1850 m in the Taisetsu Mountains, Hokkaido, Japan. This species differs from congeners in having (1) the outer surface of the carapace smooth, with sparse, tiny setae, but without shallow pits; (2) the carapace elongate rather than triangular in lateral view; (3) the antennula consisting of seven podomeres; (4) first palpal podomere of maxillula with five dorsodistal and one ventro-subdistal setae; (5) the fifth limb lacking setae b and d; and (6) the fifth limb lacking a vibratory plate. We provided the key to the Cavernocypris species. We determined partial sequences for the cytochrome c oxidase subunit I (COI; cox1) and 18S rRNA (18S) genes in C. hokkaiensis. Our sample contained only females, and we obtained a partial 16S rRNA sequence for the endosymbiotic bacterium Cardinium from C. hokkaiensis, indicating the possibility that this ostracod species reproduces parthenogenetically. Our field survey and observations of captive individuals suggested that C. hokkaiensis may be endemic to the Taisetsu Mountains, with a low population density, narrow distributional range, and slow maturation to sexual maturity.

Life from a Snowflake: Diversity and Adaptation of Cold-Loving Bacteria among Ice Crystals

Incredible as it is, researchers have now the awareness that even the most extreme environment includes special habitats that host several forms of life. Cold environments cover different compartments of the cryosphere, as sea and freshwater ice, glaciers, snow, and permafrost. Although these are very particular environmental compartments in which various stressors coexist (i.e., freeze–thaw cycles, scarce water availability, irradiance conditions, and poorness of nutrients), diverse specialized microbial communities are harbored. This raises many intriguing questions, many of which are still unresolved. For instance, a challenging focus is to understand if microorganisms survive trapped frozen among ice crystals for long periods of time or if they indeed remain metabolically active. Likewise, a look at their site-specific diversity and at their putative geochemical activity is demanded, as well as at the equally interesting microbial activity at subzero temperatures. The production of special molecules such as strategy of adaptations, cryoprotectants, and ice crystal-controlling molecules is even more intriguing. This paper aims at reviewing all these aspects with the intent of providing a thorough overview of the main contributors in investigating the microbial life in the cryosphere, touching on the themes of diversity, adaptation, and metabolic potential.

Fate of glacier surface snow-originating bacteria in the glacier-fed hydrologic continuums

Glaciers represent important biomes of Earth and are recognized as key species pools for downstream aquatic environments. Worldwide, rapidly receding glaciers are driving shifts in hydrology, species distributions and threatening microbial diversity in glacier-fed aquatic ecosystems. However, the impact of glacier surface snow-originating taxa on the microbial diversity in downstream aquatic environments has been little explored. To elucidate the contribution of glacier surface snow-originating taxa to bacterial diversity in downstream aquatic environments, we collected samples from glacier surface snows, downstream streams and lakes along three glacier-fed hydrologic continuums on the Tibetan Plateau. Our results showed that glacier stream acts as recipients and vectors of bacteria originating from the glacier environments. The contributions of glacier surface snow-originating taxa to downstream bacterial communities decrease from the streams to lakes, which was consistently observed in three geographically separated glacier-fed ecosystems. Our results also revealed that some rare snow-originating bacteria can thrive along the hydrologic continuums and become dominant in downstream habitats. Finally, our results indicated that the dispersal patterns of bacterial communities are largely determined by mass effects and increasingly subjected to local sorting of species along the glacier-fed hydrologic continuums. Collectively, this study provides insights into the fate of bacterial assemblages in glacier surface snow following snow melt and how bacterial communities in aquatic environments are affected by the influx of glacier snow-originating bacteria.

Microbial communities in floodplain ecosystems in relation to altered flow regimes and experimental flooding

River floodplains are spatially diverse ecosystems that respond quickly to flow variations and disturbance. However, it remains unclear how flow alteration and hydrological disturbance impacts the structure and biodiversity of complex microbial communities in these ecosystems. Here, we examined the spatial and seasonal dynamics of microbial communities in aquatic (benthic) and terrestrial habitats of three hydrologically contrasting (natural flow, residual flow, hydropeaking flow) floodplain systems. Microbial communities (alpha and beta diversity) differed more among floodplain habitats than between riverine floodplains. Microbial communities in all systems displayed congruent seasonal effects. In the residual and hydropeaking systems, an experimental flood was released from a reservoir to mimic a natural high flow event causing hydromorphological disturbance. The experimental flood caused a temporary shift in microbial communities by releasing microbes from the reservoir as well as redistributing communities among floodplain habitats. The flood-mediated shift in community structures had only a transient impact as pelagic bacteria did not persist within floodplain habitats over time after the flood. More frequent pulse disturbances might lead to an alternate structure of bacterial communities in floodplains over time.

Rock glaciers and related cold rocky landforms: Overlooked climate refugia for mountain biodiversity

Mountains are global biodiversity hotspots where cold environments and their associated ecological communities are threatened by climate warming. Considerable research attention has been devoted to understanding the ecological effects of alpine glacier and snowfield recession. However, much less attention has been given to identifying climate refugia in mountain ecosystems where present-day environmental conditions will be maintained, at least in the near-term, as other habitats change. Around the world, montane communities of microbes, animals, and plants live on, adjacent to, and downstream of rock glaciers and related cold rocky landforms (CRL). These geomorphological features have been overlooked in the ecological literature despite being extremely common in mountain ranges worldwide with a propensity to support cold and stable habitats for aquatic and terrestrial biodiversity. CRLs are less responsive to atmospheric warming than alpine glaciers and snowfields due to the insulating nature and thermal inertia of their debris cover paired with their internal ventilation patterns. Thus, CRLs are likely to remain on the landscape after adjacent glaciers and snowfields have melted, thereby providing longer-term cold habitat for biodiversity living on and downstream of them. Here, we show that CRLs will likely act as key climate refugia for terrestrial and aquatic biodiversity in mountain ecosystems, offer guidelines for incorporating CRLs into conservation practices, and identify areas for future research.

Community assembly processes underlying the temporal dynamics of glacial stream and lake bacterial communities

Community assembly processes are important in structuring aquatic microbial communities; however, the influence of these processes on the dynamics of bacterial communities in glacial streams and lakes remains largely unstudied. To investigate the assembly processes underlying the temporal variation of the bacterial community, we collected 50 water samples over five months in an ephemeral glacial stream and its downstream lake at the terminus of the Qiangyong glacier on the Tibetan Plateau. Using the V4 hypervariable region of the bacterial 16S rRNA gene combined with environmental measurements, such as water temperature, pH, total nitrogen (TN), dissolved organic carbon (DOC) and water conductivity, we found that temporal variation in the environmental factors promoted the shift in the proglacial stream and the lake bacterial communities. The quantification of ecological processes showed that the stream microbial communities were influenced by the ecological drift (40%) in June, then changed to homogeneous selection (40%) in July and variable selection (60%) in September, while the dynamic pattern of proglacial lake bacterioplankton was governed by homogeneous selection (≥ 50%) over the time. Overall, the dynamic of bacterial community in the proglacial stream and lake water is influenced by environmental factors, and the community composition assembly of the Qiangyong glacial stream and lake could be dynamic and primarily governed by deterministic processes.

Beta diversity of aquatic macroinvertebrate assemblages associated with leaf patches in neotropical montane streams

Over 70% of the total channel length in all river basins is formed by low order streams, many of which originate on mountaintops. Headwater streams play fundamental roles in processing and transporting terrestrial and aquatic organic matter, often harboring high biodiversity in bottom leaf patches deposited from riparian vegetation. The objective of this study was to assess the variation in taxonomic composition (measured by beta diversity of aquatic macroinvertebrates) among stream sites located in the Espinhaço Meridional Mountain Range, part of a UNESCO Biosphere Reserve in eastern Brazil. We tested two hypotheses. (a) Taxa turnover is the main reason for differences in aquatic insect assemblages within stream sites; we predicted that turnover would be higher than nestedness in all stream sites. (b) Stream site altitude and catchment elevation range are the main explanatory variables for the differences in beta diversity; we predicted that local stream site variables would account for only minor amounts of variation. In both dry and wet seasons, we sampled twice in two habitat types (five leaf patches in pools and five in riffles) in each of nine stream sites distributed in three different river basins. We computed average pairwise beta diversity among sampling stations and seasons in each stream site by using Jaccard and Bray-Curtis indices, and calculated the percentages of diversity resulting from turnover and nestedness. Finally, we tested the degree that local- or catchment-level predictor variables explained beta diversity. We found that turnover was the main component of beta diversity and that both dissolved oxygen and elevation range best explained Bray-Curtis beta diversity. These results reinforce the importance of leaf patches in montane (sky islands) Neotropical savanna streams as biodiversity hotbeds for macroinvertebrates, and that both local and landscape variables explained beta diversity.

Climate- versus geographic-dependent patterns in the spatial distribution of macroinvertebrate assemblages in New World depressional wetlands

Analyses of biota at lower latitudes may presage impacts of climate change on biota at higher latitudes. Macroinvertebrate assemblages in depressional wetlands may be especially sensitive to climate change because weather-related precipitation and evapotranspiration are dominant ecological controls on habitats, and organisms of depressional wetlands are temperature-sensitive ectotherms. We aimed to better understand how wetland macroinvertebrate assemblages were structured according to geography and climate. To do so, we contrasted aquatic-macroinvertebrate assemblage structure (family level) between subtropical and temperate depressional wetlands of North and South America using presence-absence data from 264 of these habitats across the continents and more-detailed relative-abundance data from 56 depressional wetlands from four case-study locations (North Dakota and Georgia in North America; southern Brazil and Argentinian Patagonia in South America). Both data sets roughly partitioned wetland numbers equally between the two climatic zones and between the continents. We used ordination methods (PCA and NMDS) and tests of multivariate dispersion (PERMDISP) to assess the distribution and the homogeneity in variation in the composition of macroinvertebrate assemblages across climates and continents, respectively. We found that macroinvertebrate assemblage structures in the subtropical depressional wetlands of North and South America were similar to each other (at the family level), while assemblages in the North and South American temperate wetlands were unique from the subtropics, and from each other. Tests of homogeneity of multivariate dispersion indicated that family-level assemblage structures were more homogeneous in wetlands from the subtropical than the temperate zones. Our study suggests that ongoing climate change may result in the homogenization of macroinvertebrate assemblage structures in temperate zones of North and South America, with those assemblages becoming enveloped by assemblages from the subtropics. Biotic homogenization, more typically associated with other kinds of anthropogenic factors, may also be affected by climate change.

Key rules of life and the fading cryosphere: Impacts in alpine lakes and streams

Alpine regions are changing rapidly due to loss of snow and ice in response to ongoing climate change. While studies have documented ecological responses in alpine lakes and streams to these changes, our ability to predict such outcomes is limited. We propose that the application of fundamental rules of life can help develop necessary predictive frameworks. We focus on four key rules of life and their interactions: the temperature dependence of biotic processes from enzymes to evolution; the wavelength dependence of the effects of solar radiation on biological and ecological processes; the ramifications of the non-arbitrary elemental stoichiometry of life; and maximization of limiting resource use efficiency across scales. As the cryosphere melts and thaws, alpine lakes and streams will experience major changes in temperature regimes, absolute and relative inputs of solar radiation in ultraviolet and photosynthetically active radiation, and relative supplies of resources (e.g., carbon, nitrogen, and phosphorus), leading to nonlinear and interactive effects on particular biota, as well as on community and ecosystem properties. We propose that applying these key rules of life to cryosphere-influenced ecosystems will reduce uncertainties about the impacts of global change and help develop an integrated global view of rapidly changing alpine environments. However, doing so will require intensive interdisciplinary collaboration and international cooperation. More broadly, the alpine cryosphere is an example of a system where improving our understanding of mechanistic underpinnings of living systems might transform our ability to predict and mitigate the impacts of ongoing global change across the daunting scope of diversity in Earth's biota and environments.

Insects in high-elevation streams: Life in extreme environments imperiled by climate change

Climate change is altering conditions in high-elevation streams worldwide, with largely unknown effects on resident communities of aquatic insects. Here, we review the challenges of climate change for high-elevation aquatic insects and how they may respond, focusing on current gaps in knowledge. Understanding current effects and predicting future impacts will depend on progress in three areas. First, we need better descriptions of the multivariate physical challenges and interactions among challenges in high-elevation streams, which include low but rising temperatures, low oxygen supply and increasing oxygen demand, high and rising exposure to ultraviolet radiation, low ionic strength, and variable but shifting flow regimes. These factors are often studied in isolation even though they covary in nature and interact in space and time. Second, we need a better mechanistic understanding of how physical conditions in streams drive the performance of individual insects. Environment-performance links are mediated by physiology and behavior, which are poorly known in high-elevation taxa. Third, we need to define the scope and importance of potential responses across levels of biological organization. Short-term responses are defined by the tolerances of individuals, their capacities to perform adequately across a range of conditions, and behaviors used to exploit local, fine-scale variation in abiotic factors. Longer term responses to climate change, however, may include individual plasticity and evolution of populations. Whether high-elevation aquatic insects can mitigate climatic risks via these pathways is largely unknown.

Evaluación de la calidad del agua en el río Alambrado utilizando macroinvertebrados bentónicos como bioindicadores en la zona del embalse de la Laguna de la Mica

Se evaluó la calidad del agua del río Alambrado usando macroinvertebrados bentónicos y parámetros físico-químicos. Se midieron los índices: Índice Multimétrico de estado ecológico de Ríos Altoandinos (IMEERA), Índice Ephemeroptera, Plecoptera y Trichoptera (EPT), Índice Biótico Andino (ABI) y el índice piloto Biological Monitoring Working Party / Ecuador (BMWP/ECU), Análisis de Componentes Principales (ACP) y Análisis de Correspondencia Canónica (ACC), durante tres meses (enero a marzo, 2019). Se colectaron 1993 individuos; la familia Chironomidae fue la más abundante (80%) y las menos abundantes fueron: Gerridae, Simulidae y Leptoceridae (0,05%). Más del 90% fueron macroinvertebrados colectores. El ACP y ACC determinó que los sólidos suspendidos totales tuvieron una mayor relación con la gran mayoría de taxones. Se determinó que la calidad del agua se encuentra en un rango de malo a moderado y los resultados de los índices biológicos son sensibles a los parámetros físico-químicos de oxígeno disuelto, Demanda Bioquímica de Oxígeno 5 (DBO5), Demanda Química de Oxígeno (DQO); los cuales varían según la temporada climática.

Mountain stoneflies may tolerate warming streams: Evidence from organismal physiology and gene expression

Rapid glacier recession is altering the physical conditions of headwater streams. Stream temperatures are predicted to rise and become increasingly variable, putting entire meltwater-associated biological communities at risk of extinction. Thus, there is a pressing need to understand how thermal stress affects mountain stream insects, particularly where glaciers are likely to vanish on contemporary timescales. In this study, we measured the critical thermal maximum (CT_MAX ) of stonefly nymphs representing multiple species and a range of thermal regimes in the high Rocky Mountains, USA. We then collected RNA-sequencing data to assess how organismal thermal stress translated to the cellular level. Our focal species included the meltwater stonefly, Lednia tumana, which was recently listed under the U.S. Endangered Species Act due to climate-induced habitat loss. For all study species, critical thermal maxima (CT_MAX  > 20°C) far exceeded the stream temperatures mountain stoneflies experience (<10°C). Moreover, while evidence for a cellular stress response was present, we also observed constitutive expression of genes encoding proteins known to underlie thermal stress (i.e., heat shock proteins) even at low temperatures that reflected natural conditions. We show that high-elevation aquatic insects may not be physiologically threatened by short-term exposure to warm temperatures and that longer-term physiological responses or biotic factors (e.g., competition) may better explain their extreme distributions.

High elevation insect communities face shifting ecological and evolutionary landscapes

Climate change is proceeding rapidly in high mountain regions worldwide. Rising temperatures will impact insect physiology and associated fitness and will shift populations in space and time, thereby altering community interactions and composition. Shifts in space are expected as insects move upslope to escape warming temperatures and shifts in time will occur with changes in phenology of resident high-elevation insects. Clearly, spatiotemporal shifts will not affect all species equally. Terrestrial insects may have more opportunities than aquatic insects to exploit microhabitats, potentially buffering them from warming. Such responses of insects to warming may also fuel evolutionary change, including hitchhiking of maladaptive alleles and genetic rescue. Together, these considerations suggest a striking restructuring of high-elevation insect communities that remains largely unstudied.

Specialized meltwater biodiversity persists despite widespread deglaciation

Glaciers are important drivers of environmental heterogeneity and biological diversity across mountain landscapes. Worldwide, glaciers are receding rapidly due to climate change, with important consequences for biodiversity in mountain ecosystems. However, the effects of glacier loss on biodiversity have never been quantified across a mountainous region, primarily due to a lack of adequate data at large spatial and temporal scales. Here, we combine high-resolution biological and glacier change (ca. 1850-2015) datasets for Glacier National Park, USA, to test the prediction that glacier retreat reduces biodiversity in mountain ecosystems through the loss of uniquely adapted meltwater stream species. We identified a specialized cold-water invertebrate community restricted to the highest elevation streams primarily below glaciers, but also snowfields and groundwater springs. We show that this community and endemic species have unexpectedly persisted in cold, high-elevation sites, even in catchments that have not been glaciated in ∼170 y. Future projections suggest substantial declines in suitable habitat, but not necessarily loss of this community with the complete disappearance of glaciers. Our findings demonstrate that high-elevation streams fed by snow and other cold-water sources continue to serve as critical climate refugia for mountain biodiversity even after glaciers disappear.

Alpine headwaters emerging from glaciers and rock glaciers host different bacterial communities: Ecological implications for the future

Mountain glacier shrinkage represents a major effect of the current global warming and 80-100% of the Alpine glaciers are predicted to vanish within the next few decades. As the thawing rate of mountain permafrost ice is much lower than for glacier ice, a shift from glacial to periglacial dynamics is predicted for Alpine landscapes during the 21st century. Despite the growing literature on the impacts of deglaciation on Alpine hydrology and ecosystems, chemical and biological features of waters emerging from Alpine rock glaciers (i.e. permafrost landforms composed by a mixture of ice and debris) have been poorly investigated so far, and knowledge on microbial biodiversity of headwaters is still sparse. A set of glacier-, rock glacier- and groundwater/precipitation-fed streams was investigated in the Italian Central Alps in late summer 2016, aiming at exploring bacterial community composition and diversity in epilithic and surface sediment biofilm and at verifying the hypothesis that rock glacier-fed headwaters represent peculiar ecosystems from both a chemical and biological point of view. Rock glacier-fed waters showed high values of electrical conductivity and trace elements related to their bedrock lithology, and their highly diverse bacterial assemblages significantly differed from those detected in glacier-fed streams. Bacterial taxonomic composition appeared to be mainly related to water and substrate type, as well as to water chemistry, the latter including concentrations of nutrients and trace metals. The results of this study confirm the chemical and biological peculiarity of rock glacier-fed waters compared to glacial waters, and suggest a potential driving role of thawing permafrost in modulating future ecological traits of Alpine headwaters within the context of progressing deglaciation.

Patterns in Microbial Assemblages Exported From the Meltwater of Arctic and Sub-Arctic Glaciers

Meltwater streams connect the glacial cryosphere with downstream ecosystems. Dissolved and particulate matter exported from glacial ecosystems originates from contrasting supraglacial and subglacial environments, and exported microbial cells have the potential to serve as ecological and hydrological indicators for glacial ecosystem processes. Here, we compare exported microbial assemblages from the meltwater of 24 glaciers from six (sub)Arctic regions – the southwestern Greenland Ice Sheet, Qeqertarsuaq (Disko Island) in west Greenland, Iceland, Svalbard, western Norway, and southeast Alaska – differing in their lithology, catchment size, and climatic characteristics, to investigate spatial and environmental factors structuring exported meltwater assemblages. We found that 16S rRNA gene sequences of all samples were dominated by the phyla Proteobacteria, Bacteroidetes, and Actinobacteria, with Verrucomicrobia also common in Greenland localities. Clustered OTUs were largely composed of aerobic and anaerobic heterotrophs capable of degrading a wide variety of carbon substrates. A small number of OTUs dominated all assemblages, with the most abundant being from the genera Polaromonas, Methylophilus, and Nitrotoga. However, 16–32% of a region’s OTUs were unique to that region, and rare taxa revealed unique metabolic potentials and reflected differences between regions, such as the elevated relative abundances of sulfur oxidizers Sulfuricurvum sp. and Thiobacillus sp. at Svalbard sites. Meltwater alpha diversity showed a pronounced decrease with increasing latitude, and multivariate analyses of assemblages revealed significant regional clusters. Distance-based redundancy and correlation analyses further resolved associations between whole assemblages and individual OTUs with variables primarily corresponding with the sampled regions. Interestingly, some OTUs indicating specific metabolic processes were not strongly associated with corresponding meltwater characteristics (e.g., nitrification and inorganic nitrogen concentrations). Thus, while exported assemblage structure appears regionally specific, and probably reflects differences in dominant hydrological flowpaths, OTUs can also serve as indicators for more localized microbially mediated processes not captured by the traditional characterization of bulk meltwater hydrochemistry. These results collectively promote a better understanding of microbial distributions across the Arctic, as well as linkages between the terrestrial cryosphere habitats and downstream ecosystems.

A long noncoding RNA acts as a post-transcriptional regulator of heat shock protein (HSP70) synthesis in the cold hardy Diamesa tonsa under heat shock

Cold stenothermal insects living in glacier-fed streams are stressed by temperature variations resulting from glacial retreat during global warming. The molecular aspects of insect response to environmental stresses remain largely unexplored. The aim of this study was to expand our knowledge of how a cold stenothermal organism controls gene expression at the transcriptional, translational, and protein level under warming conditions. Using the chironomid Diamesa tonsa as target species and a combination of RACE, qPCR, polysomal profiling, western blotting, and bioinformatics techniques, we discovered a new molecular pathway leading to previously overlooked adaptive strategies to stress. We obtained and characterized the complete cDNA sequences of three heat shock inducible 70 (hsp70) and two members of heat-shock cognate 70 (hsc70). Strikingly, we showed that a novel pseudo-hsp70 gene encoding a putative long noncoding RNA (lncRNA) which is transcribed during thermal stress, acting as a ribosome sponge to provide post-transcriptional control of HSP70 protein levels. The expression of the pseudo-hsp70 gene and its function suggest the existence of a new and unexpected mechanism to cope with thermal stress: lowering the pace of protein production to save energy and optimize resources for recovery.

Linkages between flow regime, biota, and ecosystem processes: Implications for river restoration

River ecosystems are highly biodiverse, influence global biogeochemical cycles, and provide valued services. However, humans are increasingly degrading fluvial ecosystems by altering their streamflows. Effective river restoration requires advancing our mechanistic understanding of how flow regimes affect biota and ecosystem processes. Here, we review emerging advances in hydroecology relevant to this goal. Spatiotemporal variation in flow exerts direct and indirect control on the composition, structure, and dynamics of communities at local to regional scales. Streamflows also influence ecosystem processes, such as nutrient uptake and transformation, organic matter processing, and ecosystem metabolism. We are deepening our understanding of how biological processes, not just static patterns, affect and are affected by stream ecosystem processes. However, research on this nexus of flow-biota-ecosystem processes is at an early stage. We illustrate this frontier with evidence from highly altered regulated rivers and urban streams. We also identify research challenges that should be prioritized to advance process-based river restoration.

Microbial assemblages reflect environmental heterogeneity in alpine streams

Alpine streams are dynamic habitats harboring substantial biodiversity across small spatial extents. The diversity of alpine stream biota is largely reflective of environmental heterogeneity stemming from varying hydrological sources. Globally, alpine stream diversity is under threat as meltwater sources recede and stream conditions become increasingly homogeneous. Much attention has been devoted to macroinvertebrate diversity in alpine headwaters, yet to fully understand the breadth of climate change threats, a more thorough accounting of microbial diversity is needed. We characterized microbial diversity (specifically Bacteria and Archaea) of 13 streams in two disjunct Rocky Mountain subranges through 16S rRNA gene sequencing. Our study encompassed the spectrum of alpine stream sources (glaciers, snowfields, subterranean ice, and groundwater) and three microhabitats (ice, biofilms, and streamwater). We observed no difference in regional (γ) diversity between subranges but substantial differences in diversity among (β) stream types and microhabitats. Within-stream (α) diversity was highest in groundwater-fed springs, lowest in glacier-fed streams, and positively correlated with water temperature for both streamwater and biofilm assemblages. We identified an underappreciated alpine stream type-the icy seep-that are fed by subterranean ice, exhibit cold temperatures (summer mean <2°C), moderate bed stability, and relatively high conductivity. Icy seeps will likely be important for combatting biodiversity losses as they contain similar microbial assemblages to streams fed by surface ice yet may be buffered against climate change by insulating debris cover. Our results show that the patterns of microbial diversity support an ominous trend for alpine stream biodiversity; as meltwater sources decline, stream communities will become more diverse locally, but regional diversity will be lost. Icy seeps, however, represent a source of optimism for the future of biodiversity in these imperiled ecosystems.

Ecosystem shifts in Alpine streams under glacier retreat and rock glacier thaw: A review

This review provides a detailed synthesis of the effects of glacier retreat and permafrost thaw on stream ecosystems in the European Alps. As a working framework, we present a conceptual model developed from an integration of current knowledge and understanding of the habitat and ecological shifts in Alpine streams caused by deglaciation. In our work, we depict how climate change and the loss of cryosphere trigger complex cascading effects on Alpine hydrology, as the main water sources shift from snow and glaciers to rock glaciers, groundwater, and precipitation. The associated changes in habitat conditions, such as channel stability, turbidity, temperature, nutrient loadings, and concentrations of legacy pollutants and trace elements are identified. These changes are followed by complex ecological shifts in the stream communities (microbial community, primary producers, invertebrates) and food webs, with a predicted loss of biotic diversity. Corresponding increases in taxa abundances, biomass, functional diversity, and in the complexity of food webs, are predicted to occur in the upper reaches of Alpine catchments in response to ameliorating climatic and habitat conditions. Finally, current knowledge gaps are highlighted as a basis for framing future research agendas. In particular, we call for an improved understanding of permafrost influence on Alpine headwaters, including the ecology of rock-glacier fed streams, as these streams are likely to become increasingly important for water supply in many glacier-free Alpine valleys in the near future.

Structural and functional responses of invertebrate communities to climate change and flow regulation in alpine catchments

Understanding and predicting how biological communities respond to climate change is critical for assessing biodiversity vulnerability and guiding conservation efforts. Glacier- and snow-fed rivers are one of the most sensitive ecosystems to climate change, and can provide early warning of wider-scale changes. These rivers are frequently used for hydropower production but there is minimal understanding of how biological communities are influenced by climate change in a context of flow regulation. This study sheds light on this issue by disentangling structural (water temperature preference, taxonomic composition, alpha, beta and gamma diversities) and functional (functional traits, diversity, richness, evenness, dispersion and redundancy) effects of climate change in interaction with flow regulation in the Alps. For this, we compared environmental and aquatic invertebrate data collected in the 1970s and 2010s in regulated and unregulated alpine catchments. We hypothesized a replacement of cold-adapted species by warming-tolerant ones, high temporal and spatial turnover in taxa and trait composition, along with reduced taxonomic and functional diversities in consequence of climate change. We expected communities in regulated rivers to respond more drastically due to additive or synergistic effects between flow regulation and climate change. We found divergent structural but convergent functional responses between free-flowing and regulated catchments. Although cold-adapted taxa decreased in both of them, greater colonization and spread of thermophilic species was found in the free-flowing one, resulting in higher spatial and temporal turnover. Since the 1970s, taxonomic diversity increased in the free flowing but decreased in the regulated catchment due to biotic homogenization. Colonization by taxa with new functional strategies (i.e. multivoltine taxa with small body size, resistance forms, aerial dispersion and reproduction by clutches) increased functional diversity but decreased functional redundancy through time. These functional changes could jeopardize the ability of aquatic communities facing intensification of ongoing climate change or new anthropogenic disturbances.

Bacterial Diversity in Replicated Hydrogen Sulfide-Rich Streams

Extreme environments typically require costly adaptations for survival, an attribute that often translates to an elevated influence of habitat conditions on biotic communities. Microbes, primarily bacteria, are successful colonizers of extreme environments worldwide, yet in many instances, the interplay between harsh conditions, dispersal, and microbial biogeography remains unclear. This lack of clarity is particularly true for habitats where extreme temperature is not the overarching stressor, highlighting a need for studies that focus on the role other primary stressors (e.g., toxicants) play in shaping biogeographic patterns. In this study, we leveraged a naturally paired stream system in southern Mexico to explore how elevated hydrogen sulfide (H₂S) influences microbial diversity. We sequenced a portion of the 16S rRNA gene using bacterial primers for water sampled from three geographically proximate pairings of streams with high (> 20 μM) or low (~ 0 μM) H₂S concentrations. After exploring bacterial diversity within and among sites, we compared our results to a previous study of macroinvertebrates and fish for the same sites. By spanning multiple organismal groups, we were able to illuminate how H₂S may differentially affect biodiversity. The presence of elevated H₂S had no effect on overall bacterial diversity (p = 0.21), a large effect on community composition (25.8% of variation explained, p < 0.0001), and variable influence depending upon the group-whether fish, macroinvertebrates, or bacteria-being considered. For bacterial diversity, we recovered nine abundant operational taxonomic units (OTUs) that comprised a core H₂S-rich stream microbiome in the region. Many H₂S-associated OTUs were members of the Epsilonproteobacteria and Gammaproteobacteria, which both have been implicated in endosymbiotic relationships between sulfur-oxidizing bacteria and eukaryotes, suggesting the potential for symbioses that remain to be discovered in these habitats.