Brassica napus Bacterial Assembly Processes Vary with Plant Compartment and Growth Stage but Not between Lines.

Holobiont bacterial community assembly processes are an essential element to understanding the plant microbiome. To elucidate these processes, leaf, root, and rhizosphere samples were collected from eight lines of Brassica napus in Saskatchewan over the course of 10 weeks. We then used ecological null modeling to disentangle the community assembly processes over the growing season in each plant part. The root was primarily dominated by stochastic community assembly processes, which is inconsistent with previous studies that suggest of a highly selective root environment. Leaf assembly processes were primarily stochastic as well. In contrast, the rhizosphere was a highly selective environment. The dominant rhizosphere selection process leads to more similar communities. Assembly processes in all plant compartments were dependent on plant growth stage with little line effect on community assembly. The foundations of assembly in the leaf were due to the harsh environment, leading to dominance of stochastic effects, whereas the stochastic effects in the root interior likely arise due to competitive exclusion or priority effects. Engineering canola microbiomes should occur during periods of strong selection assuming strong selection could promote beneficial bacteria. For example, engineering the microbiome to resist pathogens, which are typically aerially born, should focus on the flowering period, whereas microbiomes to enhance yield should likely be engineered postflowering as the rhizosphere is undergoing strong selection. IMPORTANCE In order to harness the microbiome for more sustainable crop production, we must first have a better understanding of microbial community assembly processes that occurring during plant development. This study examines the bacterial community assembly processes of the leaf, root, and rhizosphere of eight different lines of Brassica napus over the growing season. The influence of growth stage and B. napus line were examined in conjunction with the assembly processes. Understanding what influences the assembly processes of crops might allow for more targeted breeding efforts by working with the plant to manipulate the microbiome when it is undergoing the strongest selection pressure.

Positron-emitting radiotracers spatially resolve unexpected biogeochemical relationships linked with methane oxidation in Arctic soils.

Arctic soils are marked by cryoturbic features, which impact soil-atmosphere methane (CH4 ) dynamics vital to global climate regulation. Cryoturbic diapirism alters C/N chemistry within frost boils by introducing soluble organic carbon and nutrients, potentially influencing microbial CH4 oxidation. CH4 oxidation in soils, however, requires a spatio-temporal convergence of ecological factors to occur. Spatial delineation of microbial activity with respect to these key microbial and biogeochemical factors at relevant scales is experimentally challenging in inherently complex and heterogeneous natural soil matrices. This work aims to overcome this barrier by spatially linking microbial CH4 oxidation with C/N chemistry and metagenomic characteristics. This is achieved by using positron-emitting radiotracers to visualize millimeter-scale active CH4 uptake areas in Arctic soils with and without diapirism. X-ray absorption spectroscopic speciation of active and inactive areas shows CH4 uptake spatially associates with greater proportions of inorganic N in diapiric frost boils. Metagenomic analyses reveal Ralstonia pickettii associates with CH4 uptake across soils along with pertinent CH4 and inorganic N metabolism associated genes. This study highlights the critical relationship between CH4 and N cycles in Arctic soils, with potential implications for better understanding future climate. Furthermore, our experimental framework presents a novel, widely applicable strategy for unraveling ecological relationships underlying greenhouse gas dynamics under global change.

Soil Buffering Capacity Can Be Used To Optimize Biostimulation of Psychrotrophic Hydrocarbon Remediation.

Effective bioremediation of hydrocarbons requires innovative approaches to minimize phosphate precipitation in soils of different buffering capacities. Understanding the mechanisms underlying sustained stimulation of bacterial activity remains a key challenge for optimizing bioremediation-particularly in northern regions. Positron emission tomography (PET) can trace microbial activity within the naturally occurring soil structure of intact soils. Here, we use PET to test two hypotheses: (1) optimizing phosphate bioavailability in soil will outperform a generic biostimulatory solution in promoting hydrocarbon remediation and (2) oligotrophic biostimulation will be more effective than eutrophic approaches. In so doing, we highlight the key bacterial taxa that underlie aerobic and anaerobic hydrocarbon degradation in subarctic soils. In particular, we showed that (i) optimized phosphate bioavailability outperformed generic biostimulatory solutions in promoting hydrocarbon degradation, (ii) oligotrophic biostimulation is more effective than eutrophic approaches, and (iii) optimized biostimulatory solutions stimulated specific soil regions and bacterial consortia. The knowledge gleaned from this study will be crucial in developing field-scale biodegradation treatments for sustained stimulation of bacterial activity in northern regions.

From the Outside in: An Overview of Positron Imaging of Plant and Soil Processes.

Positron-emitting nuclides have long been used as imaging agents in medical science to spatially trace processes non-invasively, allowing for real-time molecular imaging using low tracer concentrations. This ability to non-destructively visualize processes in real time also makes positron imaging uniquely suitable for probing various processes in plants and porous environmental media, such as soils and sediments. Here, we provide an overview of historical and current applications of positron imaging in environmental research. We highlight plant physiological research, where positron imaging has been used extensively to image dynamics of macronutrients, signalling molecules, trace elements, and contaminant metals under various conditions and perturbations. We describe how positron imaging is used in porous soils and sediments to visualize transport, flow, and microbial metabolic processes. We also address the interface between positron imaging and other imaging approaches, and present accompanying chemical analysis of labelled compounds for reviewed topics, highlighting the bridge between positron imaging and complementary techniques across scales. Finally, we discuss possible future applications of positron imaging and its potential as a nexus of interdisciplinary biogeochemical research.

Assessing Space, Time, and Remediation Contribution to Soil Pollutant Variation near the Detection Limit Using Hurdle Models to Account for a Large Proportion of Nondetectable Results.

Many emerging, and some legacy, pollutants pose risks to humans and ecosystems near the detection limits (DL) of existing analytical systems. As a result, site assessments and management options are often presented with data sets that are sparse, highly skewed, and left-censored. Existing analysis methods are unable to differentiate effects of treatment from covariates, such as space, obscuring influences of site management. As a case study, we computed the mean and variance of censored soil benzene data across four sites over a three year period by gamma distribution with a maximum likelihood. Further, a combined hurdle model to accommodate left-censored concentrations was applied to analyze factors affecting benzene variation. This approach allowed us to assess the success and spatial dependency of a biostimulatory solution in reducing benzene concentrations at very low concentrations. Benzene concentrations decreased due to the addition of biostimulatory solution and spatial effects, but the detection of soil benzene after biostimulation was highly spatially dependent. By combining computed values for censored observations estimated by the hurdle-gamma model and uncensored observations, we can get the pseudocomplete data sets. The combined model is ideally suited to evaluate existing and emerging pollutants, that pose risks to humans and ecosystems but are typically at or near analytical detection limits.

Chemical speciation and fate of tripolyphosphate after application to a calcareous soil.

Adsorption and precipitation reactions often dictate the availability of phosphorus in soil environments. Tripolyphosphate (TPP) is considered a form of slow release P fertilizer in P limited soils, however, investigations of the chemical fate of TPP in soils are limited. It has been proposed that TPP rapidly hydrolyzes in the soil solution before adsorbing or precipitating with soil surfaces, but in model systems, TPP also adsorbs rapidly onto mineral surfaces. To study the adsorption behavior of TPP in calcareous soils, a short-term (48 h) TPP spike was performed under laboratory conditions. To determine the fate of TPP under field conditions, two different liquid TPP amendments were applied to a P limited subsurface field site via an in-ground injection system. Phosphorus speciation was assessed using X-ray absorption spectroscopy, total and labile extractable P, and X-ray diffraction. Adsorption of TPP to soil mineral surfaces was rapid (< 48 h) and persisted without fully hydrolyzing to ortho-P. Linear combination fitting of XAS data indicated that the distribution of adsorbed P was highest (~ 30-40%) throughout the site after the first TPP amendment application (high water volume and low TPP concentrations). In contrast, lower water volumes with more concentrated TPP resulted in lower relative fractions of adsorbed P (15-25%), but a significant increase in total P concentrations (~ 3000 mg P kg soil) and adsorbed P (60%) directly adjacent to the injection system. This demonstrates that TPP application increases the adsorbed P fraction of calcareous soils through rapid adsorption reactions with soil mineral surfaces.

Who Is the Rock Miner and Who Is the Hunter? The Use of Heavy-Oxygen Labeled Phosphate (P18O4) to Differentiate between C and P Fluxes in a Benzene-Degrading Consortium.

Phosphorus availability and cycling in microbial communities is a key determinant of bacterial activity. However, identifying organisms critical to P cycling in complex biodegrading consortia has proven elusive. Here we assess a new DNA stable isotope probing (SIP) technique using heavy oxygen-labeled phosphate (P18O4) and its effectiveness in pure cultures and a nitrate-reducing benzene-degrading consortium. First, we successfully labeled pure cultures of Gram-positive Micrococcus luteus and Gram-negative Bradyrhizobium elkanii and separated isotopically light and heavy DNA in pure cultures using centrifugal analyses. Second, using high-throughput amplicon sequencing of 16S rRNA genes to characterize active bacterial taxa (13C-labeled), we found taxa like Betaproteobacteria were key in denitrifying benzene degradation and that other degrading (nonhydrocarbon) inactive taxa (P18O4-labeled) like Staphylococcus and Corynebacterium may promote degradation through production of secondary metabolites (i.e., "helper" or "rock miner" bacteria). Overall, we successfully separated active and inactive taxa in contaminated soils, demonstrating the utility of P18O4-DNA SIP for identifying actively growing bacterial taxa. We also identified potential "miner" bacteria that choreograph hydrocarbon degradation by other microbes (i.e., the "hunters") without directly degrading contaminants themselves. Thus, while several taxa degrade benzene under denitrifying conditions, microbial benzene degradation may be enhanced by both direct degraders and miner bacteria.

Linking Herbicide Dissipation to Soil Ecological Risk along Transmission Rights-of-Way in the Yukon Territory, Canada.

In the Yukon Territory, transmission rights-of-way (ROWs) are managed using brushing and mowing techniques alone. When cut, target species such as Michx. and spp. grow rapidly shortening maintenance cycles. Long-term vegetation control may be improved by integrating herbicide application. However, prior to implementation, the dissipation and toxicity of herbicides in northern latitudes needed to be assessed. The dissipation of Garlon XRT (triclopyr) and Arsenal Powerline (imazapyr) in soils was assessed at five ROW locations representative of the main ecoregion types where ROWs occur within the Yukon Territory. Soils from four sites were collected at 1, 30, and 365 d after treatment to determine persistence of herbicides for each of three application methods (backpack spraying, cut stump, and point injection). Increased sampling intervals were added to better determine the dissipation rate of each herbicide in Yukon Territory soils. Soil dissipation data were linked to a series of standardized toxicity tests, including three soil invertebrates (, , and ). Additionally, the dissipation of both herbicides from the target species L. was assessed at one site. Herbicide residues persisted in soils for longer than 365 d after treatment and longer than 30 d after treatment in . However, concentrations were below the concentration that would affect 25% of the invertebrate species tested. Weight of evidence and toxic exposure ratios were used to characterize the risks associated with herbicide application in northern latitudes and provided both qualitative and quantitative means to communicate the results to the public.

Salix arctica changes root distribution and nutrient uptake in response to subsurface nutrients in High Arctic deserts.

Moisture is critical for plant success in polar deserts but not by the obvious pathway of reduced water stress. We hypothesized that an indirect, nutrient-linked, pathway resulting from unique water/frozen soil interactions in polar deserts creates nutrient-rich patches critical for plant growth. These nutrient-rich patches (diapirs) form deep in High Arctic polar deserts soils from water accumulating at the permafrost freezing front and ultimately rising into the upper soil horizons through cryoturbated convective landforms (frost boils). To determine if diapirs provide an enhanced source of plant-available N for Salix arctica (Arctic willow), we characterized soil, root, stem, and leaf 15 N natural abundance across 24 diapir and non-diapir frost boils in a High Arctic granitic semi-desert. When diapir horizons were available, S. arctica increased its subsurface (i.e., diapir) N uptake and plant root biomass doubled within diapir. Plant uptake of enriched 15 N injected into organic rich soil patches was 2.5-fold greater in diapir than in non-diapir frost boils. S. arctica percent cover was often higher (7.3 ± 1.0 [mean ± SE]) on diapiric frost boils, compared to frost boils without diapirs (4.4 ± 0.7), potentially reflecting the additional 20% nitrogen available in the subsurface of diapiric frost boils. Selective N acquisition from diapirs is a mechanism by which soil moisture indirectly enhances plant growth. Our work suggests that diapirs may be one mechanism contributing to Arctic greening by shrub expansion.

Application Method and Biochar Type Affect Petroleum Hydrocarbon Degradation in Northern Landfarms.

To integrate biochar as a practical and successful remedial amendment at northern landfarms, components of its formulation and application must be optimized for site-specific environmental conditions. Different biochar amendments were applied to petroleum hydrocarbon (PHC)-contaminated soil at two northern field sites (Iqaluit and Whitehorse) and in a laboratory study at -5°C to determine the effects of application method (injection or incorporation) and biochar type (wood, fishmeal, bonemeal, and/or compost) on PHC degradation and associated soil properties. Incorporation decreased F2 (equivalent C-C) and F3 (equivalent C-C) PHC concentrations in soil after 31 d, whereas injection did not decrease PHC concentrations until Day 334. Bonemeal-derived biochar selectively stimulated F3-PHC degradation in frozen soil over 90 d under controlled laboratory conditions. In the field, there was little difference in PHC degradation between biochar types and the fertilizer control. Incorporation also increased NO availability, and in field trials, all biochars increased NO availability relative to the fertilizer control, whereas the effects of biochars on NH and PO were variable. Aromatic functional gene abundance was enhanced when treatments were incorporated, compared with when injected. In field trials, 6% Zakus wood plus fertilizer inhibited aliphatic and aromatic gene abundance. Liquid water content increased in incorporated treatments, specifically those amended with fishmeal biochar. Incorporation was the most successful application method for these northern soils, and although biochar amendments are not clearly effective in reducing PHC concentrations, there is evidence to suggest it can beneficially influence soil properties and PHC degradation under specific environmental conditions.

Citrate Addition Increased Phosphorus Bioavailability and Enhanced Gasoline Bioremediation.

Phosphorus (P) bioavailability often limits gasoline biodegradation in calcareous cold-region soils. One possible method to increase P bioavailability in such soils is the addition of citrate. Citrate addition at the field scale may increase hydrocarbon degradation by: (i) enhancing inorganic and organic P dissolution and desorption, (ii) increasing hydrocarbon bioavailability, and/or (iii) stimulating microbial activity. Alternatively, citrate addition may inhibit activity due to competitive effects on carbon metabolism. Using a field-scale in situ biostimulation study, we evaluated if citrate could stimulate gasoline degradation and what the dominant mechanism of this stimulation will be. Two large bore injectors were constructed at a site contaminated with gasoline, and a biostimulation solution of 11 mM MgSO, 1 mM HPO, and 0.08 mM HNO at pH 6.5 in municipal potable water was injected at ∼5000 L d for about 4 mo. Following this, 10 mM citric acid was incorporated into the existing biostimulation solution and the site continued to be stimulated for 8 mo. After citrate addition, the bioavailable P fraction in groundwater and soil increased. Iron(II) groundwater concentrations increased and corresponded to decreases in benzene, toluene, ethylbenzene, xylenes (BTEX) in groundwater, as well as a decrease in F1 in the soil saturated zone. Overall, citrate addition increased P bioavailability and may stimulate anaerobic microbial activity, resulting in accelerated anaerobic gasoline bioremediation in cold-region calcareous soils.

Geological connectivity drives microbial community structure and connectivity in polar, terrestrial ecosystems.

Landscape heterogeneity impacts community assembly in animals and plants, but it is not clear if this ecological concept extends to microbes. To examine this question, we chose to investigate polar soil environments from the Antarctic and Arctic, where microbes often form the major component of biomass. We examined soil environments that ranged in connectivity from relatively well-connected slopes to patchy, fragmented landforms that comprised isolated frost boils. We found landscape connectedness to have a significant correlation with microbial community structure and connectivity, as measured by co-occurrence networks. Soils from within fragmented landforms appeared to exhibit less local environmental heterogeneity, harboured more similar communities, but fewer biological associations than connected landforms. This effect was observed at both poles, despite the geographical distances and ecological differences between them. We suggest that microbial communities inhabiting well-connected landscape elements respond consistently to regional-scale gradients in biotic and edaphic factors. Conversely, the repeated freeze thaw cycles that characterize fragmented landscapes create barriers within the landscape and act to homogenize the soil environment within individual frost boils and consequently the microbial communities. We propose that lower microbial connectivity in the fragmented landforms is a function of smaller patch size and continual disturbances following soil mixing.

Do biomarkers of exposure and effect correlate with internal exposure to PAHs in swine?

Humans are commonly exposed to polycyclic aromatic hydrocarbons (PAHs), a family of compounds present as mixtures in the environment. This study exposed swine to PAH mixtures in single and subacute dose regimens and collected liver and ileum tissue to measure cytochrome P450 mRNA expression and enzyme activity as biomarkers of exposure and DNA adducts and oxidized proteins as biomarkers of effect. Micronucleated reticulocytes were measured as systemic biomarkers of effect. Duration of exposure did not influence biomarkers of exposure, though exposure duration produced significant increases in DNA adducts and oxidative stress. Micronucleated reticulocyte numbers were not affected by exposure length.

Predicting Polycyclic Aromatic Hydrocarbon Bioavailability to Mammals from Incidentally Ingested Soils Using Partitioning and Fugacity.

Soil and dust ingestion is one of the major human exposure pathways to contaminated soil; however, pollutant transfer from ingested substances to humans cannot currently be confidently predicted. Soil polycyclic aromatic hydrocarbon (PAH) bioavailability is likely dependent upon properties linked to chemical potential and partitioning such as fugacity, fugacity capacity, soil organic carbon, and partitioning to simulated intestinal fluids. We estimated the oral PAH bioavailability of 19 historically contaminated soils fed to juvenile swine. Between soils, PAH blood content, with the exception of benzo(a)pyrene, was not linked to fugacity. In contrast, between individual PAHs, using partitioning explained PAH blood content (area under the curve = 0.47 log fugacity + 0.34, r(2) = 0.68, p < 0.005, n = 14). Soil fugacity capacity predicts PAH soil concentration with an average slope of 0.30 (µg PAH g(-1) soil) Pa(-1) and r(2)'s of 0.61-0.73. Because PAH blood content was independent of soil concentration, soil fugacity correlated to PAH bioavailability via soil fugacity's link to soil concentration. In conclusion, we can use fugacity to explain PAH uptake from a soil into blood. However, something other than partitioning is critical to explain the differences in PAH uptake into blood between soils.

Total Phosphate Influences the Rate of Hydrocarbon Degradation but Phosphate Mineralogy Shapes Microbial Community Composition in Cold-Region Calcareous Soils.

Managing phosphorus bioaccessibility is critical for the bioremediation of hydrocarbons in calcareous soils. This paper explores how soil mineralogy interacts with a novel biostimulatory solution to both control phosphorus bioavailability and influence bioremediation. Two large bore infiltrators (1 m diameter) were installed at a PHC contaminated site and continuously supplied with a solution containing nutrients and an electron acceptor. Soils from eight contaminated sites were prepared and pretreated, analyzed pretrial, spiked with diesel, placed into nylon bags into the infiltrators, and removed after 3 months. From XAS, we learned that three principal phosphate phases had formed: adsorbed phosphate, brushite, and newberyite. All measures of biodegradation in the samples (in situ degradation estimates, mineralization assays, culturable bacteria, catabolic genes) varied depending upon the soil's phosphate speciation. Notably, adsorbed phosphate increased anaerobic phenanthrene degradation and bzdN catabolic gene prevalence. The dominant mineralogical constraints on community composition were the relative amounts of adsorbed phosphate, brushite, and newberyite. Overall, this study finds that total phosphate influences microbial community phenotypes whereas relative percentages of phosphate minerals influences microbial community genotype composition.

Archaeal ammonia oxidizers respond to soil factors at smaller spatial scales than the overall archaeal community does in a high Arctic polar oasis.

Archaea are ubiquitous and highly abundant in Arctic soils. Because of their oligotrophic nature, archaea play an important role in biogeochemical processes in nutrient-limited Arctic soils. With the existing knowledge of high archaeal abundance and functional potential in Arctic soils, this study employed terminal restriction fragment length polymorphism (t-RFLP) profiling and geostatistical analysis to explore spatial dependency and edaphic determinants of the overall archaeal (ARC) and ammonia-oxidizing archaeal (AOA) communities in a high Arctic polar oasis soil. ARC communities were spatially dependent at the 2-5 m scale (P < 0.05), whereas AOA communities were dependent at the ∼1 m scale (P < 0.0001). Soil moisture, pH, and total carbon content were key edaphic factors driving both the ARC and AOA community structure. However, AOA evenness had simultaneous correlations with dissolved organic nitrogen and mineral nitrogen, indicating a possible niche differentiation for AOA in which dry mineral and wet organic soil microsites support different AOA genotypes. Richness, evenness, and diversity indices of both ARC and AOA communities showed high spatial dependency along the landscape and resembled scaling of edaphic factors. The spatial link between archaeal community structure and soil resources found in this study has implications for predictive understanding of archaea-driven processes in polar oases.

Validating the scalability of soft X-ray spectromicroscopy for quantitative soil ecology and biogeochemistry research.

Synchrotron-based soft-X-ray scanning transmission X-ray microscopy (STXM) has the potential to provide nanoscale resolution of the associations among biological and geological materials. However, standard methods for how samples should be prepared, measured, and analyzed to allow the results from these nanoscale imaging and spectroscopic tools to be scaled to field scale biogeochemical results are not well established. We utilized a simple sample preparation technique that allows one to assess detailed mineral, metal, and microbe spectroscopic information at the nano- and microscale in soil colloids. We then evaluated three common approaches to collect and process nano- and micronscale information by STXM and the correspondence of these approaches to millimeter scale soil measurements. Finally, we assessed the reproducibility and spatial autocorrelation of nano- and micronscale protein, Fe(II) and Fe(III) densities in a soil sample. We demonstrate that linear combination fitting of entire spectra provides slightly different Fe(II) mineral densities compared to image resonance difference mapping but that difference mapping results are highly reproducible between among sample replicates. Further, STXM results scale to the mm scale in complex systems with an approximate geospatial range of 3 µm in these samples.

Combined exposure to lead, inorganic mercury and methylmercury shows deviation from additivity for cardiovascular toxicity in rats.

Environmental exposure to metal mixtures in the human population is common. Mixture risk assessments are often challenging because of a lack of suitable data on the relevant mixture. A growing number of studies show an association between lead or mercury exposure and cardiovascular effects. We investigated the cardiovascular effects of single metal exposure or co-exposure to methylmercury [MeHg(I)], inorganic mercury [Hg(II)] and lead [Pb(II)]. Male Wistar rats received four different metal mixtures for 28 days through the drinking water. The ratios of the metals were based on reference and environmental exposure values. Blood and pulse pressure, cardiac output and electrical activity of the heart were selected as end-points. While exposure to only MeHg(I) increased the systolic blood pressure and decreased cardiac output, the effects were reversed with combined exposures (antagonism). In contrast to these effects, combined exposures negatively affected the electrical activity of the heart (synergism). Thus, it appears that estimates of blood total Hg levels need to be paired with estimates of what species of mercury dominate exposure as well as whether lead co-exposure is present to link total blood Hg levels to cardiovascular effects. Based on current human exposure data and our results, there may be an increased risk of cardiac events as a result of combined exposures to Hg(II), MeHg(I) and Pb(II). This increased risk needs to be clarified by analyzing lead and Hg exposure data in relation to cardiac electrical activity in epidemiological studies.

Bacterial targets as potential indicators of diesel fuel toxicity in subantarctic soils.

Appropriate remediation targets or universal guidelines for polar regions do not currently exist, and a comprehensive understanding of the effects of diesel fuel on the natural microbial populations in polar and subpolar soils is lacking. Our aim was to investigate the response of the bacterial community to diesel fuel and to evaluate if these responses have the potential to be used as indicators of soil toxicity thresholds. We set up short- and long-exposure tests across a soil organic carbon gradient. Utilizing broad and targeted community indices, as well as functional genes involved in the nitrogen cycle, we investigated the bacterial community structure and its potential functioning in response to special Antarctic blend (SAB) diesel fuel. We found the primary effect of diesel fuel toxicity was a reduction in species richness, evenness, and phylogenetic diversity, with the resulting community heavily dominated by a few species, principally Pseudomonas. The decline in richness and phylogenetic diversity was linked to disruption of the nitrogen cycle, with species and functional genes involved in nitrification significantly reduced. Of the 11 targets we evaluated, we found the bacterial amoA gene indicative of potential ammonium oxidation, the most suitable indicator of toxicity. Dose-response modeling for this target generated an average effective concentration responsible for 20% change (EC20) of 155 mg kg(-1), which is consistent with previous Macquarie Island ecotoxicology assays. Unlike traditional single-species tolerance testing, bacterial targets allowed us to simultaneously evaluate more than 1,700 species from 39 phyla, inclusive of rare, sensitive, and functionally relevant portions of the community.

Microbial community responses to anthropogenically induced environmental change: towards a systems approach.

The soil environment is essential to many ecosystem services which are primarily mediated by microbial communities. Soil physical and chemical conditions are altered on local and global scales by anthropogenic activity and which threatens the provision of many soil services. Despite the importance of soil biota for ecosystem function, we have limited ability to predict and manage soil microbial community responses to change. To better understand causal relationships between microbial community structure and ecological function, we argue for a systems approach to prediction and management of microbial response to environmental change. This necessitates moving beyond concepts of resilience, resistance and redundancy that assume single optimum stable states, to ones that better reflect the dynamic and interactive nature of microbial systems. We consider the response of three soil groups (ammonia oxidisers, denitrifiers, symbionts) to anthropogenic perturbation to motivate our discussion. We also present a network re-analysis of a saltmarsh microbial community which illustrates how such approaches can reveal ecologically important connections between functional groups. More generally, we suggest the need for integrative studies which consider how environmental variables moderate interactions between functional groups, how this moderation affects biogeochemical processes and how these feedbacks ultimately drive ecosystem services provided by soil biota.