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Recent rapid air temperature increases across the northern-latitude tundra have prolonged permafrost thawing and snow melting periods, resulting in increased soil temperature (Ts) and volumetric soil water content (SWC). Under prolonged soil warming at 8°C, Alaskan tundra soils were incubated in a microcosm system and examined for the SWC differential influence on the microbial decomposition activity of large molecular weight (MW) humic substances (HS). When one microcosm soil (AKC1-1) was incubated at a constant SWC of 41% for 90 days (T = 90) and then SWC was gradually decreased from 41% to 29% for another T = 90, the initial HS was partly depolymerized. In contrast, in AKC1-2 incubated at a gradually decreasing SWC from the initial 32% to 10% for T = 90 and then increasing to 27% for another T = 90, HS depolymerization was undetected. Overall, the microbial communities in AKC1-1 could maintain metabolic activity at sufficient and constant SWC during the initial T = 90 incubation. In contrast, AKC1-2 microbes may have been damaged by drought stress during the drying SWC regimen, possibly resulting in the loss of HS decomposition activity, which did not recover even after re-wetting to an optimal SWC range (20–40%). After T = 90, the CO2 production in both treatments was attributed to the increased decomposition of small-MW organic compounds (including aerobic HS-degradative products) within an optimal SWC range. We expect this study to provide new insights into the early effects of warming- and topography-induced SWC variations on the microbial contribution to CO2 emissions via HS decomposition in northern-latitude tundra soil. [ABSTRACT FROM AUTHOR]

In Arctic soils, warming accelerates decomposition of organic matter and increases emission of greenhouse gases (GHGs), contributing to a positive feedback to climate change. Although microorganisms play a key role in the processes between decomposition of organic matter and GHGs emission, the effects of warming on temporal responses of microbial activity are still elusive. In this study, treatments of warming and precipitation were conducted from 2012 to 2018 in Cambridge Bay, Canada. Soils of organic and mineral layers were collected monthly from June to September in 2018 and analyzed for extracellular enzyme activities and bacterial community structures. The activity of hydrolases was the highest in June and decreased thereafter over summer in both organic and mineral layers. Bacterial community structures changed gradually over summer, and the responses were distinct depending on soil layers and environmental factors; water content and soil temperature affected the shift of bacterial community structures in both layers, whereas bacterial abundance, dissolved organic carbon, and inorganic nitrogen did so in the organic layer only. The activity of hydrolases and bacterial community structures did not differ significantly among treatments but among months. Our results demonstrate that temporal variations may control extracellular enzyme activities and microbial community structure rather than the small effect of warming over a long period in high Arctic soil. Although the effects of the treatments on microbial activity were minor, our study provides insight that microbial activity may increase due to an increase in carbon availability, if the growing season is prolonged in the Arctic. [ABSTRACT FROM AUTHOR]

Recent increases in air temperature across the Antarctic Peninsula may prolong the thawing period and directly affect the soil temperature (Ts) and volumetric soil water content (SWC) in maritime tundra. Under an 8°C soil warming scenario, two customized microcosm systems with maritime Antarctic soils were incubated to investigate the differential influence of SWC on the bacterial community and degradation activity of humic substances (HS), the largest constituent of soil organic carbon and a key component of the terrestrial ecosystem. When the microcosm soil (KS1-4Feb) was incubated for 90 days (T = 90) at a constant SWC of ~32%, the initial HS content (167.0 mg/g of dried soil) decreased to 156.0 mg (approximately 6.6% loss, p < 0.05). However, when another microcosm soil (KS1-4Apr) was incubated with SWCs that gradually decreased from 37% to 9% for T = 90, HS degradation was undetected. The low HS degradative activity persisted, even after the SWC was restored to 30% with water supply for an additional T = 30. Overall bacterial community structure remained relatively stable at a constant SWC setting (KS1-4Feb). In contrast, we saw marked shifts in the bacterial community structure with the changing SWC regimen (KS1-4Apr), suggesting that the soil bacterial communities are vulnerable to drying and re-wetting conditions. These microcosm experiments provide new information regarding the effects of constant SWC and higher Ts on bacterial communities for HS degradation in maritime Antarctic tundra soil. [ABSTRACT FROM AUTHOR]

Over the past few decades the Arctic has warmed up more than the lower latitudes. Soil organic carbon (SOC) in the Arctic is vulnerable to climate change, and carbon dioxide (CO2) produced via SOC decomposition can amplify atmospheric temperature increase. Although SOC composition is relevant to decomposability, studies on its compositional changes with warming are scarce, particularly in the Arctic. Therefore, we investigated the responses of SOC and the bacterial community to climate manipulation under Cassiope and Salix heath vegetation communities in permafrost‐affected soil in Zackenberg, Greenland. After 8–9 years of experimental warming, we evaluated changes in SOC quantity and quality of three density fractions of soil: free light fraction (FLF), occluded light fraction (OLF) and heavy fraction (HF). The SOC content at 0–5‐cm depth was significantly reduced with warming under Cassiope, and it was accompanied by decreased FLF content, attributed to accelerated decomposition of the FLF by warming. However, SOC molecular composition and bacterial community composition were not affected by warming. By contrast, there was no warming effect on SOC under Salix, which could be partially due to smaller temperature increases caused by higher moisture levels associated with larger silt and clay contents, or to different responses of the dominant plant species to temperature. In both soils, more than 55% of SOC was associated with minerals, and its molecular composition indicated microbial decomposition. Our results suggested that long‐term warming in the high Arctic could induce the loss of SOC, particularly in the FLF; however, the response could vary with vegetation type and/or soil properties, that is, soil texture. Highlights: We show decreased SOC with long‐term (8‐year) warming of heath soils in the high ArcticParticularly, the free light fraction of SOC in topsoil decreased with warming in a Cassiope heath siteMineral‐associated SOC content was more than 55% and showed signs of microbial processingEffects of warming differed according to soil properties and/or plant community composition [ABSTRACT FROM AUTHOR]

Permafrost-underlain tundra soils in Northern Hemisphere are one of the largest reservoirs of terrestrial carbon, which are highly sensitive to microbial decomposition due to climate warming. However, knowledge about the taxonomy and functions of microbiome residing in different horizons of permafrost-underlain tundra soils is still limited. Here we compared the taxonomic and functional composition of microbiome between different horizons of soil cores from a moist tussock tundra ecosystem in Council, Alaska, using 16S rRNA gene and shotgun metagenomic sequencing. The composition, diversity, and functions of microbiome varied significantly between soil horizons, with top soil horizon harboring more diverse communities than sub-soil horizons. The vertical gradient in soil physico-chemical parameters were strongly associated with composition of microbial communities across permafrost soil horizons; however, a large fraction of the variation in microbial communities remained unexplained. The genes associated with carbon mineralization were more abundant in top soil horizon, while genes involved in acetogenesis, fermentation, methane metabolism (methanogenesis and methanotrophy), and N cycling were dominant in sub-soil horizons. The results of phylogenetic null modeling analysis showed that stochastic processes strongly influenced the composition of the microbiome in different soil horizons, except the bacterial community composition in top soil horizon, which was largely governed by homogeneous selection. Our study expands the knowledge on the structure and functional potential of microbiome associated with different horizons of permafrost soil, which could be useful in understanding the effects of environmental change on microbial responses in tundra ecosystems. [ABSTRACT FROM AUTHOR]

In an intelligent tutoring system (ITS), it can be useful to know when a student has disengaged from a task and might benefit from a particular intervention. However, predicting disengagement on a trial-by-trial basis is a challenging problem, particularly in complex cognitive domains. In the present work, data-driven methods were used to address two aspects of this problem: identification of predictive features at the single-trial level, and selection of accurate and robust models. Experiment data were collected in a middle-school classroom using a vocabulary training ITS. On each trial, the ITS presented a low-frequency (Tier 2 or frontier) word and prompted students to type in the word's meaning. Single-trial measures—including the orthographic and semantic accuracy of each response, and context-sensitive measures such as interaction patterns across trials—were computed throughout the task. There were two key findings. First, as expected, different features predicted when a student was likely to be more engaged (e.g., high semantic accuracy) or less engaged (e.g., repetition of same or similar words across consecutive trials). Second, there was added value in representing context-sensitive information, which captures patterns of performance over time, as well as trial-specific information. These findings provide useful insights into effective methods for representing and modeling temporal patterns of student engagement in an ITS, especially those related to language learning. Such models may be useful in the design and implementation of adaptive tutors in complex cognitive domains like language learning. [ABSTRACT FROM AUTHOR]

To search useful fatty acid biomarkers for cyanobacteria feeding abilities of control organisms, we compared fatty acid composition of six cultured algal species in four taxonomic groups (Cryptophyceae, Bacillariophyceae, Chlorophyceae, and Cyanophyceae) and evaluated several fatty acid biomarkers using data from three lakes that varied in trophic status. In addition, a laboratory feeding experiment was conducted to verify fatty acid markers for cyanobacteria usingSelenastrum capricornutum(Chlorophyceae) andMicrocystis aeruginosa(Cyanophyceae). Two strains ofM. aeruginosahad the highest content of 18:3ω6 whileAnabaenasp. had a relatively high content of 18:1ω7 among the six species. Both 18:1ω7 and 18:3ω6 (γ-linolenic acid, GLA) concentrations showed significant linear relationships with cyanobacteria biomass in natural systems. In the laboratory feeding experiment, a principal component analysis score plot indicate that fatty acid composition ofBranchinella kugenumaensisfed withM. aeruginosabecame similar to that of the cyanobacterial population. In S-plot of partial least square discriminant analysis, 18:3ω6 and 18:1ω7 were selected for fairy shrimps feeding onM. aeruginosa, whereas 18:3ω3 (ALA, α-linolenic acid) and 18:1ω9 were selected for those feeding onS. capricornutum. 18:3ω6 content ofB. kugenumaensisfed withM. aeruginosasignificantly increased after the third day and 18:1ω7 content was significantly different from the other feeding group after six days. In particular, 18:3ω6 appeared to be a useful fatty acid biomarker forM. aeruginosain tracing trophic relations of herbivorous organisms in freshwater ecosystems. [ABSTRACT FROM AUTHOR]

The subarctic region is highly responsive and vulnerable to climate change. Understanding the structure of subarctic soil microbial communities is essential for predicting the response of the subarctic soil environment to climate change. To determine the composition of the bacterial community and its relationship with soil properties, we investigated the bacterial community structure and properties of surface soil from the moist acidic tussock tundra in Council, Alaska. We collected 70 soil samples with 25-m intervals between sampling points from 0-10 cm to 10-20 cm depths. The bacterial community was analyzed by pyrosequencing of 16S rRNA genes, and the following soil properties were analyzed: soil moisture content (MC), pH, total carbon (TC), total nitrogen (TN), and inorganic nitrogen ( [ABSTRACT FROM AUTHOR]

We conducted a 7-d co-existence experiment to verify the inhibitory activity of an aquatic vascular plant, Myriophyllum spicatum, on the cyanobacterium Microcystis aeruginosa, and to screen for allelopathic compounds. Plants were placed on a modified solid M4 medium while M. aeruginosa was cultured in an L16 liquid medium. Our treatments included rearing Myriophyllum spicatum alone (Plant-Only; PO) or with Microcystis aeruginosa (Plant-Cyanobacteria; PCB), as well as an untreated control, or plates with the cyanobacterium alone. In the PCB treatment, M. aeruginosa became transparent in three replicates on the 6th day and in four replicates on Day 7. To make the distinction in Myriophyllum spicatum metabolites between the PO and PCB treatments, we obtained 13 m/z profiles using a 15 T Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Principle component analyses were performed on those profiles, from which we obtained accurate mass values for selected peaks that had most negative loadings on the first principal component and most positive loadings on the second principal component: 282.06983 and 283.06836. We identified those peaks as C7H17N5O3S2 and C10H12N4O6, respectively. The Dictionary of Natural Products database suggests that the latter is either oxanosine or xanthosine. [ABSTRACT FROM AUTHOR]