Abiotic Mechanisms Research Articles

Sediment nitrogen mineralization and immobilization affected by non-native Sonneratia apetala plantation in an intertidal wetland of South China

The mineralization and immobilization of nitrogen (N) are critical biogeochemical transformations in estuarine and coastal sediments. However, the biotic and abiotic mechanisms that regulate the two processes in different aged mangrove sediments remain poorly understood. Here, we used 15N isotope dilution method to investigate the changes in sediment N mineralization (GNM) and NH4+ immobilization (GAI) of different aged mangrove habitats (including 0, 10, and 20 years Sonneratia apetala, as well as >40 years mature native Kandelia obovata) in Qi'ao Island, Guangdong Province, China. Measured GNM and GAI rates ranged from 2.69 to 17.53 μg N g−1 d−1 and 2.29–21.38 μg N g−1 d−1, respectively, which varied both spatially and seasonally. The ratio of GNM to total N (PAM%, 0.24–0.86%) also varied spatially and seasonally, but the ratio of GAI to GNM (RAI, 0.79–1.54) only varied spatially. Mangrove restoration significantly increased the N mineralization and immobilization rates, but remained lower than those of mature native Kandelia obovata habitat. The sediment bacterial abundance, labile organic matter and temperature are the dominant factors in controlling N mineralization and immobilization. Our findings suggested that exotic mangrove Sonneratia aperale plantation can enhance sediment N mineralization and immobilization rates and improve N stability through accumulated biomass rapidly. Overall, these results provide new insights into sediment N transformation processes and associated influencing mechanisms in such intertidal wetlands profoundly influenced by human activities.

Effects of algal-derived organic matter on sediment nitrogen mineralization and immobilization in a eutrophic estuary

Nitrogen (N) mineralization and immobilization are two important N transformation processes in estuarine sediments. However, the biotic and abiotic mechanisms regulating those two processes in sediments of a eutrophic estuary remains poorly understood. Here, we used 15N stable isotope dilution technique to investigate the spatial and temporal variations of gross N mineralization (GNM) and gross NH4+ immobilization (GAI) in surface sediment (0–5 cm) along the Pearl River Estuary (PRE) in 4 cruises. Results showed that GNM rates varied spatially and increased with salinity, ranging from 0.15 to 1.99 μg N g−1 d−1, while GAI varied more temporally and increased with temperature, ranging from 0 to 1.96 μg N g−1 d−1. The net N mineralization (NNM) rates ranged from −0.24 to 1.84 μg N g−1 d−1, NNM was higher in lower estuary mainly due to higher chlorophyll-induced higher GNM, while NNM was higher in low temperature season mainly due to the inhibition of GAI. As high turbidity in upper estuary reduced light penetration and hence primary production, the decrease of settling labile organic matter from the water column weaken benthic bacterial abundances, GNM and NNM rates, whereas inorganic N (especially NH4+) had greater effect on GAI than organic matter. As an NH4+ source, the contribution of sediment daily NNM to NH4+ and dissolved inorganic N (DIN) in the overlying water column increased from upstream to downstream (−5.81 to 110.74% and −0.09 to 1.27%, respectively), suggesting an increased importance of NNM for supporting primary production in the lower estuary. Our study reveals the dynamics of net NH4+ production by distinguishing N mineralization and immobilization processes, and enhances the understanding of benthic-pelagic coupling influence on N-cycling in a eutrophic estuary.

The case and context for atmospheric methane as an exoplanet biosignature

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane’s short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that—in contrast to an Earth-like biosphere—known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH4 that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO2 with comparatively little CO.

Advancing the mechanistic understanding of the priming effect on soil organic matter mineralisation

Abstract The priming effect (PE) is a key mechanism contributing to the carbon balance of the soil ecosystem. Almost 100 years of research since its discovery in 1926 have led to a rich body of scientific publications to identify the drivers and mechanisms involved. A few review articles have summarised the acquired knowledge; the last major one was published in 2010. Since then, knowledge on the soil microbial communities involved in PE and in PE + C sequestration mechanisms has been considerably renewed. This article reviews current knowledge on soil PE to state to what extent new insights may improve our ability to understand and predict the evolution of soil C stocks. We propose a framework to unify the different concepts and terms that have emerged from the international scientific community on this topic, report recent discoveries and identify key research needs. Seventy per cent of the studies on the soil PE were published in the last 10 years, illustrating a renewed interest for PE, probably linked to the increased concern about the importance of soil carbon for climate change and food security issues. Among all the drivers and mechanisms proposed along with the different studies to explain PE, some are named differently but actually refer to the same object. This overall introduces ‘artificial’ complexity for the mechanistic understanding of PE, and we propose a common, shared terminology. Despite the remaining knowledge gaps, consistent progress has been achieved to decipher the abiotic mechanisms underlying PE, together with the role of enzymes and the identity of the microbial actors involved. However, including PE into mechanistic models of SOM dynamics remains challenging as long as the mechanisms are not fully understood. In the meantime, empirical alternatives are available that reproduce observations accurately when calibration is robust. Based on the current state of knowledge, we propose different scenarios depicting to what extent PE may impact ecosystem services under climate change conditions. Read the free Plain Language Summary for this article on the Journal blog.

Fungal hyphae develop where titanomagnetite inclusions reach the surface of basalt grains

Nutrient foraging by fungi weathers rocks by mechanical and biochemical processes. Distinguishing fungal-driven transformation from abiotic mechanisms in soil remains a challenge due to complexities within natural field environments. We examined the role of fungal hyphae in the incipient weathering of granulated basalt from a three-year field experiment in a mixed hardwood-pine forest (S. Carolina) to identify alteration at the nanometer to micron scales based on microscopy-tomography analyses. Investigations of fungal-grain contacts revealed (i) a hypha-biofilm-basaltic glass interface coinciding with titanomagnetite inclusions exposed on the grain surface and embedded in the glass matrix and (ii) native dendritic and subhedral titanomagnetite inclusions in the upper 1–2 µm of the grain surface that spanned the length of the fungal-grain interface. We provide evidence of submicron basaltic glass dissolution occurring at a fungal-grain contact in a soil field setting. An example of how fungal-mediated weathering can be distinguished from abiotic mechanisms in the field was demonstrated by observing hyphal selective occupation and hydrolysis of glass-titanomagnetite surfaces. We hypothesize that the fungi were drawn to basaltic glass-titanomagnetite boundaries given that titanomagnetite exposed on or very near grain surfaces represents a source of iron to microbes. Furthermore, glass is energetically favorable to weathering in the presence of titanomagnetite. Our observations demonstrate that fungi interact with and transform basaltic substrates over a three-year time scale in field environments, which is central to understanding the rates and pathways of biogeochemical reactions related to nuclear waste disposal, geologic carbon storage, nutrient cycling, cultural artifact preservation, and soil-formation processes.

Identification and Analysis of the EIN3/EIL Gene Family in Populus × xiaohei T. S. Hwang et Liang: Expression Profiling during Stress

The ethylene-insensitive 3-like (EIN3/EIL) gene family, as a transcriptional activator in plants, not only plays an important role in the ethylene-signaling pathway in regulating plant growth and development but also participates in the defense against various biotic and abiotic stresses. However, there are few studies on the functions of EIN3/EIL genes in woody plants. Populus × xiaohei is a kind of tree species with strong drought resistance and salt-alkali tolerance and, thus, is an ideal subject for studying abiotic stress mechanisms in trees. Eight EIN3/EIL genes were cloned from Populus × xiaohei. Bioinformatic analysis showed that the PsnEIN3/EIL gene contained a highly conserved EIN3 domain, N-terminal sites rich in proline and glutamine, and other EIN3/EIL family structural characteristics. The results of a multi-species phylogenetic analysis showed that the family EIN3/EIL proteins were divided into three groups (A, B, and C). EIL3 and EIL4 belonged to groups A and B, while EIL2 and EIN3 generally belonged to group C. Analysis of tissue expression characteristics showed that PsnEIN3/EIL was expressed in different tissues and was involved in the development of stem nodes and leaves. The response analysis of the expression of PsnEIN3/EIL under abscisic acid (ABA) and abiotic stresses (salts, heavy metals, alkaline conditions, and drought) showed changes in expression, suggesting that PsnEIN3/EIL may be involved in the processes of plant hormone responses to salts, heavy metals, alkaline conditions, and drought. This study provides a foundation for further elucidation of the functions of EIN3/EIL genes in forest growth and development and abiotic stress responses.

Higher frequency of legitimate pollinators and fruit set of autotetraploid trees of Libidibia ferrea (Leguminosae) compared to diploids in a mixed tropical urban population.

In mixed-ploidy populations, newly formed polyploids initially occur at low frequencies when compared to diploids. However, polyploidy may lead to morphological and phenological changes, which promote reproductive isolation and favor polyploid establishment and reproductive success. Additionally, previous studies have shown that polyploidy can confer some adaptive advantages to organisms in stressful environments. Here, we investigate variation in reproductive phenology, floral traits and reproductive success between diploid and autotetraploid trees of Libidibia ferrea (Mart. Ex Tul.) L.P. Queiroz (Leguminosae) in a mixed tropical urban population, a stressful environment. We assessed ploidy levels, flowering and fruiting phenology, flowering synchrony, floral and reproductive biology, pollination and fruit and seed set. We tested the hypothesis that autotetraploid individuals have a higher frequency of pollinators and higher fruit and seed set per inflorescence (as a proxy of reproductive success) than diploids in an urban green space. Libidibia ferrea is a good model to test our hypothesis because it is self-incompatible (i.e. relies on pollinators to set fruits). In the urban ecosystem studied, we found that diploids flowered for 6-7 months/year and autotetraploids for 3-5 months/year. Flowering synchrony was low between and within cytotypes and even though autotetraploids and diploids exhibited some overlap in flowering period, diploids flowered alone for 2-3 months. Autotetraploids had significantly more flowers per inflorescences, larger flowers and larger pollen grains (as expected for polyploids), but also a higher frequency of visits by legitimate pollinators including two exclusive ones, and higher fruit and seed set per inflorescence when compared to diploids, despite having a shorter flowering period. Our findings reveal some advantages for polyploids over their related diploids in a tropical urban green space. Also, our results highlight the need for more studies that seek to understand abiotic mechanisms affecting reproductive output of polyploids in urban ecosystems.

Conditions for efficient alkaline storage of cover crops for biomethane production

An innovative process aiming to combine storage and alkali pretreatment of cover crops was investigated using lime as a low cost and environmental friendly reactant. Different lime loadings and Total Solid concentrations (TS) allowed to highlight the abiotic mechanisms of deacetylation during the early stages of the process. Long-term storage experiments of rye and sunflower cover crops at 100 g.kgTS−1 lime loading allowed to evaluate the fermentation kinetics and to compare performances in dry and wet conditions to classical silage storage. The dry condition allowed an efficient alkaline storage and up to a 15.7% Biochemical Methane Potential (BMP) increase, while the wet condition underwent a succession of fermentations with a high butyric acid accumulation and H2 production, leading to a 13% BMP loss. Silage experiments allowed an efficient preservation of the BMP, with no significant variation over the 6-month storage duration.

Sulfate-dependant microbially induced corrosion of mild steel in the deep sea: a 10-year microbiome study

BackgroundMetal corrosion in seawater has been extensively studied in surface and shallow waters. However, infrastructure is increasingly being installed in deep-sea environments, where extremes of temperature, salinity, and high hydrostatic pressure increase the costs and logistical challenges associated with monitoring corrosion. Moreover, there is currently only a rudimentary understanding of the role of microbially induced corrosion, which has rarely been studied in the deep-sea. We report here an integrative study of the biofilms growing on the surface of corroding mooring chain links that had been deployed for 10 years at ~2 km depth and developed a model of microbially induced corrosion based on flux-balance analysis.MethodsWe used optical emission spectrometry to analyze the chemical composition of the mooring chain and energy-dispersive X-ray spectrometry coupled with scanning electron microscopy to identify corrosion products and ultrastructural features. The taxonomic structure of the microbiome was determined using shotgun metagenomics and was confirmed by 16S amplicon analysis and quantitative PCR of the dsrB gene. The functional capacity was further analyzed by generating binned, genomic assemblies and performing flux-balance analysis on the metabolism of the dominant taxa.ResultsThe surface of the chain links showed intensive and localized corrosion with structural features typical of microbially induced corrosion. The microbiome on the links differed considerably from that of the surrounding sediment, suggesting selection for specific metal-corroding biofilms dominated by sulfur-cycling bacteria. The core metabolism of the microbiome was reconstructed to generate a mechanistic model that combines biotic and abiotic corrosion. Based on this metabolic model, we propose that sulfate reduction and sulfur disproportionation might play key roles in deep-sea corrosion.ConclusionsThe corrosion rate observed was higher than what could be expected from abiotic corrosion mechanisms under these environmental conditions. High corrosion rate and the form of corrosion (deep pitting) suggest that the corrosion of the chain links was driven by both abiotic and biotic processes. We posit that the corrosion is driven by deep-sea sulfur-cycling microorganisms which may gain energy by accelerating the reaction between metallic iron and elemental sulfur. The results of this field study provide important new insights on the ecophysiology of the corrosion process in the deep sea.

Late Triassic dinosaur tracks from Penarth, south Wales

AbstractEvidence of Late Triassic large tetrapods from the UK is rare. Here, we describe a track-bearing surface located on the shoreline near Penarth, south Wales, United Kingdom. The total exposed surface is c. 50 m long and c. 2 m wide, and is split into northern and southern sections by a small fault. We interpret these impressions as tracks, rather than abiogenic sedimentary structures, because of the possession of marked displacement rims and their relationship to each other with regularly spaced impressions forming putative trackways. The impressions are large (up to c. 50 cm in length), but poorly preserved, and retain little information about track-maker anatomy. We discuss alternative, plausible, abiotic mechanisms that might have been responsible for the formation of these features, but reject them in favour of these impressions being tetrapod tracks. We propose that the site is an additional occurrence of the ichnotaxon Eosauropus, representing a sauropodomorph trackmaker, thereby adding a useful new datum to their sparse Late Triassic record in the UK. We also used historical photogrammetry to digitally map the extent of site erosion during 2009–2020. More than 1 m of the surface exposure has been lost over this 11-year period, and the few tracks present in both models show significant smoothing, breakage and loss of detail. These tracks are an important datapoint for Late Triassic palaeontology in the UK, even if they cannot be confidently assigned to a specific trackmaker. The documented loss of the bedding surface highlights the transient and vulnerable nature of our fossil resources, particularly in coastal settings, and the need to gather data as quickly and effectively as possible.

Multilayered solid phase extraction and ultra performance liquid chromatographic method for suspect screening of halogenated pharmaceuticals and photo-transformation products in freshwater - comparison between data-dependent and data-independent acquisition mass spectrometry

Since conventional biological wastewater treatments are not admittedly effective to convert pharmaceutical active compounds (PhACs) into nontoxic products, natural abiotic mechanisms such as solar photolysis arises as an important degradation process, especially for halogenated molecules. In the present work, photolysis simulation was carried out in-lab for precursors and their respective photo-transformation products (photo-TPs), which were analyzed through reversed-phase ultra-high performance liquid chromatography coupled to high resolution mass spectrometry (RP-UHPLCHRMS). An in-house library was created in order to provide reference information for target (precursors) and suspect screening (photo-TPs) analysis of freshwater samples from impacted aquatic environments. Strategies in the use of data-dependent acquisition (DDA) and data-independent acquisition (DIA), as well as the data processing software are discussed here for the identification of 6 PhACs and photo-TPs. Because no standards of photo-TPs were available, only the target compounds, i.e. sitagliptin (398 ± 2 ng L−1), iohexol (209 ± 5 ng L−1), lamotrigine (103 ± 10 ng L−1), losartan (43 ± 10 ng L−1), ofloxacin (28 ± 7 ng L−1), and sertraline (25 ± 7 ng L−1) could be quantified through multiple standard additions.

Assessment of styrene‐divinylbenzene polymer (PPL) solid‐phase extraction and non‐targeted tandem mass spectrometry for the analysis of xenobiotics in seawater

AbstractAnthropogenic imprints have become a fundamental part of most ecosystems. Our chemical footprint is often detected using targeted approaches, whereas xenobiotics are embedded within the large pool of dissolved metabolites, altered by biotic and abiotic mechanisms. Thus, it is necessary to simultaneously study anthropogenic signals entwined with the variety of organic signatures that exist in aquatic environments. However, methods for non‐targeted analysis of natural metabolites are not always well suited for the analysis of pollutants. Here, we report the reassessment of styrene‐divinylbenzene polymer‐based Priority PolLutant (PPL) solid‐phase extraction (PPL‐SPE), which is typically used to extract marine dissolved organic matter (DOM) for biogeochemical studies, to analyze a set of xenobiotics commonly observed in coastal North Pacific seawater. After PPL extraction and analysis by nontargeted liquid chromatography tandem mass spectrometry (LC–MS/MS), we successfully detected 23 out of 25 selected pharmaceuticals, personal care products, biocides, perfluorocarbons, and polymer additives in a complex marine DOM sample using positive and negative electrospray ionization. We tested two pH conditions to mimic typical marine DOM extraction studies and found mean recovery rates of xenobiotics were approximately 10% higher in seawater pH (pH ~ 8) than in acidified samples (pH ~ 2) for both negative and positive modes, although overall, mean recovery rates were 10% lower in negative mode. Our results indicate that PPL‐SPE in combination with non‐targeted LC–MS/MS is capable of capturing the tested set of xenobiotics, thus allowing the repurposing of biogeochemical sampling strategies as well as existing DOM samples and MS data for the subsequent assessment of anthropogenic impacts in marine environments.

Understanding How Coacervates Drive Reversible Small Molecule Reactions to Promote Molecular Complexity

Liquid-liquid phase-separated coacervate droplets give rise to membraneless compartments that play an important role in the spatial organization and reactivity in cells. Due to their molecularly crowded nature and ability to sequester biomolecules, coacervate droplets create distinct environments for enzymatic reaction kinetics and reaction mechanisms that markedly differ from bulk solution. In this work, we use a combination of experiments and quantitative modeling to understand how coacervate droplets promote reversible small molecule reaction chemistry. In particular, we study a model condensation reaction generating an unstable fluorescent imine in polyacrylic acid-polyethylene glycol coacervate droplets over a range of conditions. At equilibrium, the concentration of the imine product in coacervate droplets is approximately 140-fold larger than that in bulk solution, which arises due to preferential partitioning of reactants and products into coacervate droplets and a reaction equilibrium constant that is roughly threefold larger in coacervate droplets than in solution. A reaction-diffusion model is developed to quantitatively describe how competing reaction and partitioning equilibria govern the spatial distribution of the imine product inside coacervate droplets. Overall, our results show that compartmentalization stabilizes kinetically labile reaction products, which enables larger reactant concentrations in coacervate droplets compared to bulk solution. Broadly, these results provide an improved understanding of how biomolecular condensates promote multistep reaction pathways involving unstable reaction intermediates and suggest how coacervates provide a potential abiotic mechanism to promote molecular complexity.

Vernal pool wetlands respond to livestock grazing, exclusion and reintroduction

AbstractIn disturbance‐adapted ecosystems, the removal of disturbance can lead to losses of diversity and sometimes irreversible changes in community composition. It is important to identify the thresholds at which these changes can occur, and to understand the reversibility of these shifts. We examined this question in a vernal pool ecosystem that evolved with low to moderate levels of grazing disturbance. In this system, it is not clear whether the negative effects of long‐term grazing exclusion are reversible through grazing reintroduction.We compared adjacent vernal pool wetlands in annual Mediterranean grasslands under three grazing management strategies: continuously grazed (100+ years), long‐term excluded (40+ years) and 2 years of reintroduced grazing. We also asked whether grazing treatments altered pool characteristics that are likely to influence plant community composition, and how these relationships changed with environmental conditions.Reintroducing grazing to vernal pools led to both increased diversity and native cover, but the effects on native cover were more immediate than on diversity. We identified several biotic and abiotic mechanisms related to this pattern, including changes to competitive dynamics that favour small statured native annuals and increases in hoofprint microdepressions that make soil moisture more available to plants.Synthesis and applications. Our results show that reintroduced grazing at moderate stocking rates can have significant effects on plant communities after just 2 years and can increase native cover more quickly than overall diversity. Our findings suggest that the negative effects of long‐term grazing exclusion in vernal pools may be reversible, but that land managers interested in restoring diversity should plan to monitor beyond the first two years of grazing reintroduction.

Demethylation-The Other Side of the Mercury Methylation Coin: A Critical Review.

The public and environmental health consequences of mercury (Hg) methylation have drawn much attention and considerable research to Hg methylation processes and their dynamics in diverse environments and under a multitude of conditions. However, the net methylmercury (MeHg) concentration that accumulates in the environment is equally determined by the rate of MeHg degradation, a complex process mediated by a variety of biotic and abiotic mechanisms, about which our knowledge is limited. Here we review the current knowledge on MeHg degradation and its potential pathways and mechanisms. We describe detoxification by resistant microorganisms that employ the Hg resistance (mer) system to reductively break the carbon-mercury (C-Hg) bond producing methane (CH4) and inorganic mercuric Hg(II), which is then reduced by the mercuric reductase to elemental Hg(0). Very recent research has begun to elucidate a mechanism for the long-recognized mer-independent oxidative demethylation, likely involving some strains of anaerobic bacteria as well as aerobic methane-oxidizing bacteria, i.e., methanotrophs. In addition, photochemical and chemical demethylation processes are described, including the roles of dissolved organic matter (DOM) and free radicals as well as dark abiotic demethylation in the natural environment about which little is currently known. We focus on mechanisms and processes of demethylation and highlight the uncertainties and known effects of environmental factors leading to MeHg degradation. Finally, we suggest future research directions to further elucidate the chemical and biochemical mechanisms of biotic and abiotic demethylation and their significance in controlling net MeHg production in natural ecosystems.

Genomic differentiation with gene flow in a widespread Amazonian floodplain‐specialist bird species

AbstractAimEcological, climatic and palaeogeographical processes drive biological diversification. However, the evolutionary outcomes of those mechanisms are complex and difficult to discriminate. Here, we test how alternative drivers affected connectivity along the Amazonian floodplains generating current patterns of population structure and diversity within the Striped Woodcreeper, a widespread bird species tied to forests seasonally flooded by Amazonian rivers.LocationAmazonian floodplains.TaxonXiphorhynchus obsoletus (Aves, Furnariidae).MethodsWe sequenced 2213 loci of ultraconserved elements (UCEs) and 20 exons to investigate patterns of genetic structure and connectivity across the Striped Woodcreeper distribution, using a set of spatially explicit methods. Population genetics statistics were calculated for each sampled drainage and within each identified cluster. We tested alternative evolutionary scenarios and estimated past and current demographic parameters implementing a simulation‐based framework.ResultsThree genetic clusters with high admixture proportions were identified. These clusters are distributed in the western, central and eastern parts of the Amazon Basin and are not correlated with different river water types representing ecological gradients along the floodplains. Instead, migration rates indicate two putative historical barriers along the main channel of the Amazonas‐Solimões River. Demographic model tests suggest a process of sequential differentiation, partitioned across the Amazon Basin, with stable population sizes and continuous gene flow.Main conclusionOur results support the hypothesis that late Quaternary changes in connectivity between the eastern and western Amazonian drainages were responsible for driving genetic differentiation in the Striped Woodcreeper. Genomic differentiation occurred in the presence of long‐term gene flow throughout the X. obsoletus distribution, suggesting some degree of continuous historical connectivity across Amazonian floodplains. Compared to previous studies, our results suggest that although connectivity among populations of Amazonian bird species specialized in floodplain habitats varies in response to the same abiotic mechanisms, species‐specific differences in habitat use may be a strong predictor of population divergence.

The role of oxygen in stimulating methane production in wetlands.

Methane (CH4), a potent greenhouse gas, is the second most important greenhouse gas contributor to climate change after carbon dioxide (CO2). The biological emissions of CH4 from wetlands are a major uncertainty in CH4 budgets. Microbial methanogenesis by Archaea is an anaerobic process accounting for most biological CH4 production in nature, yet recent observations indicate that large emissions can originate from oxygenated or frequently oxygenated wetland soil layers. To determine how oxygen (O2) can stimulate CH4 emissions, we used incubations of Sphagnum peat to demonstrate that the temporary exposure of peat to O2 can increase CH4 yields up to 2000‐fold during subsequent anoxic conditions relative to peat without O2 exposure. Geochemical (including ion cyclotron resonance mass spectrometry, X‐ray absorbance spectroscopy) and microbiome (16S rDNA amplicons, metagenomics) analyses of peat showed that higher CH4 yields of redox‐oscillated peat were due to functional shifts in the peat microbiome arising during redox oscillation that enhanced peat carbon (C) degradation. Novosphingobium species with O2‐dependent aromatic oxygenase genes increased greatly in relative abundance during the oxygenation period in redox‐oscillated peat compared to anoxic controls. Acidobacteria species were particularly important for anaerobic processing of peat C, including in the production of methanogenic substrates H2 and CO2. Higher CO2 production during the anoxic phase of redox‐oscillated peat stimulated hydrogenotrophic CH4 production by Methanobacterium species. The persistence of reduced iron (Fe(II)) during prolonged oxygenation in redox‐oscillated peat may further enhance C degradation through abiotic mechanisms (e.g., Fenton reactions). The results indicate that specific functional shifts in the peat microbiome underlie O2 enhancement of CH4 production in acidic, Sphagnum‐rich wetland soils. They also imply that understanding microbial dynamics spanning temporal and spatial redox transitions in peatlands is critical for constraining CH4 budgets; predicting feedbacks between climate change, hydrologic variability, and wetland CH4 emissions; and guiding wetland C management strategies.

Human landscape modification and expansion of tropical woodland in southern Arabia during the mid‐Holocene from rock hyrax middens

AbstractAimDryland ecosystems, such as those in southern Arabia, are particularly vulnerable to climate and land‐use change. Aridification and human subsistence changes at the end of the Holocene Humid Period at 6–4 ka have been used as an iconic example for evaluating such impacts to resilience of arid systems. Although records of ancient environments can provide critical insight into the biotic and abiotic mechanisms that alter ecosystems, traditional archives, such as lake deposits, are not common in southern Arabia after 6 ka; thus, we must use alternative archives.LocationYemen.TaxonPlants; Rock Hyrax (Procavia capensis).MethodsWe use fossil pollen and stable isotopes (δ15N; δ13C) of rock hyrax (P. capensis) middens from Wadi Sana, Yemen, to look at changes in ecosystem structure and function across this key transition from 6 ka to the present. A total of 17 middens were radiocarbon‐dated and stable isotopes were measured. Of these, pollen was extracted from hyraceum of 14 middens and identified using a light microscope. Fossil pollen assemblages were then compared to existing modern pollen samples from throughout Arabia.ResultsDuring the mid‐Holocene from at least 6 to 4.7 ka, the pollen flora reflects a landscape with abundant tropical trees. These included foundational woody taxa, such as Terminalia and Boswellia sacra (frankincense), which had a strong positive influence on local hydrology and the economy, respectively. Increased charcoal abundances also suggest that wildfire occurred periodically.Main conclusionsConnections with archaeological evidence suggest a strong link between human management and the presence of Terminalia woodland during the mid‐Holocene. These may have promoted increased groundwater storage and ponding as regional rainfall was decreasing. The mid‐Holocene expansion of Boswellia sacra (frankincense) may have helped support resin trade that became a critical export from the region. Fires were more common than today, suggesting semi‐arid but continuous vegetation cover across what is now bare ground. Finally, after 1 ka, increased sedge abundances at the expense of grasses and trees suggest the development of desert conditions.

Four Species Linked by Three Hybrid Zones: Two Instances of Repeated Hybridization in One Species Group (Genus Liolaemus)

Hybridization is an evolutionary process that can generate diverse outcomes, such as reinforcing species boundaries, generating new species, or facilitating the introgression of locally-adapted alleles into new genomic backgrounds.Liolaemusis a highly diverse clade of South American lizards with ~260 species and as many as ten new species are described each year. PreviousLiolaemusstudies have detected gene flow and introgression among species using phylogenetic network methods and/or through comparisons of nuclear and mitochondrial DNA patterns, yet no study has systematically studied hybrid zones betweenLiolaemusspecies. Here, we compared three hybrid zones between four species in theLiolaemus fitzingeriigroup of lizards in Central Argentina where two species,L. melanopsandL. xanthoviridis, each hybridize with two other species (L. shehuenandL. fitzingerii). We sampled three transects that were each ~120 km in length and sequenced both mitochondrial and genome-wide SNP data for 267 individuals. In our analyses of nuclear DNA, we also compared bi-allelic SNPs to phased alleles (50 bp RAD loci). Population structure analyses confirmed that boundaries separating species are sharp, and all clines are <65 km wide. Cline center estimates were consistent between SNPs and phased alleles, but cline width estimates were significantly different with the SNPs producing wider estimates. The mitochondrial clines are narrower and shifted 4–20 km southward in comparison to the nuclear clines in all three hybrid zones, indicating that either each of the species has sex-biased dispersal (males northward or females southward), the population densities are unequal, or that the hybrid zones are moving north over time. These comparisons indicate that some patterns of hybridization are similar across hybrid zones (mtDNA clines all narrower and shifted to the south), whereas cline width is variable. Hybridization in theL. fitzingeriigroup is common and geographically localized; further studies are needed to investigate whether hybrid zones act as hard species boundaries or promoters of speciation through processes such as reinforcement. Nonetheless, this study provides insights into both biotic and abiotic mechanisms helping to maintain species boundaries within the specioseLiolaemussystem.

Prokaryotic diversity and biogeochemical characteristics of field living and laboratory cultured stromatolites from the hypersaline Laguna Interna, Salar de Atacama (Chile).

Stromatolites are organo-sedimentary structures found principally in seas and saline lakes that contain sheets of sediments and minerals formed by layers of microbial communities, which trap sediments and induce the precipitation of minerals.A living stromatolite from the alkaline Laguna Interna in the Salar de Atacama was collected and one of the fragments was deposited in an experimental aquarium for 18 months. We used Illumina sequencing of PCR-amplified V4 regions of 16S rRNA genes from total extracted DNA to identify the microbial populations. The chemical structure was studied using X-Ray Diffraction (XRD) and bench chemical methods. We found that members belonging to the Proteobacteria, Planctomycetes, Chloroflexi and Bacteroidetes phyla dominated the bacterial communities of the living and aquarium cultured samples. The potential metabolic functionality of the prokaryotic community reveals that sulfur, nitrogen, methane and carbon fixation metabolism functions are present in the samples. This study is the first to provide new insights into the prokaryotic community composition from this unusual aquatic desert site. Further studies will be helpful to obtain a better understanding of the biotic and abiotic mechanisms residing in stromatolites from Laguna Interna, as well as to have better knowledge about the formation of these biosignatures.