Spatial changes in the bacterial community structure along a vertical oxygen gradient in flooded paddy soil cores

Inoculation with Stutzerimonas stutzeri strains decreases N₂O emissions from vegetable soil by altering microbial community composition and diversity

Inoculation with plant growth-promoting rhizobacteria (PGPR) strains has promoted plant growth and decreased nitrous oxide (N₂O) emissions from agricultural soils simultaneously. However, limited PGPR strains can mitigate N₂O emissions from agricultural soils, and the microbial ecological mechanisms underlying N₂O mitigation after inoculation are poorly understood. In greenhouse pot experiments, the effects of inoculation with Stutzerimonas stutzeri NRCB010 and NRCB025 on tomato growth and N₂O emissions were investigated in two vegetable agricultural soils with contrasting textures. Inoculation with NRCB010 and NRCB025 significantly promoted tomato growth in both soils. Moreover, inoculation with NRCB010 decreased the N₂O emissions from the fine- and coarse-textured soils by 38.7% and 52.2%, respectively, and inoculation with NRCB025 decreased the N₂O emissions from the coarse-textured soil by 76.6%. Inoculation with NRCB010 and NRCB025 decreased N₂O emissions mainly by altering soil microbial community composition and the abundance of nitrogen-cycle functional genes. The N₂O-mitigating effect might be partially explained by a decrease in the (amoA + amoB)/(nosZI + nosZII) and (nirS + nirK)/(nosZI + nosZII) ratios, respectively. Soil pH and organic matter were key variables that explain the variation in abundance of N-cycle functional genes and subsequent N₂O emission. Moreover, the N₂O-mitigating effect varied depending on soil textures and individual strain after inoculation. This study provides insights into developing biofertilizers with plant growth-promoting and N₂O-mitigating effects.

IMPORTANCE

Plant growth-promoting rhizobacteria (PGPR) have been applied to mitigate nitrous oxide (N₂O) emissions from agricultural soils, but the microbial ecological mechanisms underlying N₂O mitigation are poorly understood. That is why only limited PGPR strains can mitigate N₂O emissions from agricultural soils. Therefore, it is of substantial significance to reveal soil ecological mechanisms of PGPR strains to achieve efficient and reliable N₂O-mitigating effect after inoculation. Inoculation with Stutzerimonas stutzeri strains decreased N₂O emissions from two soils with contrasting textures probably by altering soil microbial community composition and gene abundance involved in nitrification and denitrification. Our findings provide detailed insight into soil ecological mechanisms of PGPR strains to mitigate N₂O emissions from vegetable agricultural soils.

Plutonium mobilization from contaminated estuarine sediments, Esk Estuary (UK)

Since 1952, liquid radioactive effluent containing^238-242Pu, ²⁴¹Am, ²³⁷Np, ¹³⁷Cs, and ⁹⁹Tc has been released with authorization from the Sellafield nuclear complex (UK) into the Irish Sea. This represents the largest source of plutonium (Pu) discharged in all western Europe, with 276 kg having been released. In the Eastern Irish Sea, the majority of the transuranic activity has settled into an area of sediments (Mudpatch) located off the Cumbrian coast. Radionuclides from the Mudpatch have been re-dispersed via particulate transport in fine-grained estuarine and intertidal sediments to the North-East Irish Sea, including the intertidal saltmarsh located at the mouth of the Esk Estuary. Saltmarshes are highly dynamic systems which are vulnerable to external agents (sea level change, erosion, sediment supply, and freshwater inputs), and their stability remains uncertain under current sea level rise projections and possible increases in storm activity. In this work, we examined factors affecting Pu mobility in contaminated sediments collected from the Esk Estuary by conducting leaching experiments under both anoxic and oxic conditions. Leaching experiments were conducted over a 9-month period and were periodically sampled to determine solution phase Pu via multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), and to measure redox indicators (Eh, pH and extractable Fe(II)). Microbial community composition was also characterized in the sediments, and at the beginning and end of the anoxic/oxic experiments. Results show that: 1) Pu leaching is about three times greater in solutions leached under anoxic conditions compared to oxic conditions, 2) the sediment slurry microbial communities shift as conditions change from anoxic to oxic, 3) Pu leaching is enhanced in the shallow sediments (0-10 cm depth), and 4) the magnitude of Pu leached from sediments is not correlated with total Pu, indicating that the biogeochemistry of sediment-associated Pu is spatially heterogeneous. These findings provide constraints on the stability of redox sensitive Pu in biogeochemically dynamic/transient environments on a timescale of months and suggests that anoxic conditions can enhance Pu mobility in estuarine systems.

Specific characteristics of the microbial community in the groundwater fluctuation zone

Groundwater level fluctuation is a common natural phenomenon that causes alternate changes in oxygen, moisture, and biogeochemical processes in sediments. Microbes are sensitive to these environmental changes. Therefore, a specific microbial community is proposed to form in the groundwater fluctuation zone (GFZ). The vertical distributions of microbial abundance, diversity, and functional microbes and genes in sediment profiles were investigated, focusing on the GFZ, using high-throughput 16S rRNA gene sequencing, qPCR, and the Functional Annotation of Prokaryotic Taxa (FAPROTAX) approach. The relationships between chemical variables and microbial community structure were investigated by redundancy analysis (RDA). Results showed that the microbial abundance and microbial community richness and diversity were higher in the sediments of the GFZ. The nitrate reducers prefer to stay just below the groundwater level in the GFZ. The predominant microbes in the GFZ functioned as nitrifiers and Fe-oxidizers. The specific community in the GFZ is mainly related to NO₃^- and Fe(III) in the sediment. Consequently, the biochemical processes nitrification and Fe- and Mn-oxidation sequentially happen above the nitrate-reduction zone near the groundwater level in the GFZ. These results provide new knowledge in the biogeochemistry cycle of the GFZ and its disturbance on the vertical distribution and transport of biogenic elements and contaminants.

Impacts of typical engineering nanomaterials on the response of rhizobacteria communities and rice (Oryza sativa L.) growths in waterlogged antimony-contaminated soils

The combined eco-risks of Sb (widely presented in soils, especially nearing mining areas) and the engineering nanomaterials (ENMs) (applied in agriculture and soil remediation) still remain uncovered. The current study investigated the impacts of single and combined exposure of CuO, CeO₂ nanoparticles (NPs) and multi-walled carbon nanotube (MWCNTs) with Sb on rice growths and rhizosphere bacterial communities. The results showed that co-exposure of CuO NPs (0.075 wt%) with Sb (III) posed the most adverse impacts on root biomass and branches (up to 66.59% and 70.00% compared to other treatments, respectively). Treatments containing MWCNTs showed insignificant dose-dependent effects, while CeO₂ NPs combined with Sb (III) showed significant synergistic stimulating effects on the fresh weights of root and shoot, by 68.30% and 73.48% (p < 0.05) compared to single Sb exposure, respectively. The rice planting increased the percentage of non-specifically sorbed Sb in soils by 1.50-14.49 than the no-planting stage. Analysis on microbial communities revealed that co-exposure of CuO NPs with Sb (III) induced the greatest adverse impacts on rhizobacteria abundances and community structures at both phylum and genus levels. Therein, significant decrease of Bacteroidetes, Acidobacteria and increase of Firmicutes abundance at the phylum level were observed. This study provided information about the risks of different ENMs released to Sb-contaminated soils under flooded condition on both crops and bacterial communities.

Soil redox dynamics under dynamic hydrologic regimes - A review

Electron transfer (redox) reactions, mediated by soil microbiota, modulate elemental cycling and, in part, establish the redox poise of soil systems. Understanding soil redox processes significantly improves our ability to characterize coupled biogeochemical cycling in soils and aids in soil health management. Redox-sensitive species exhibit different reactivity, mobility, and toxicity subjected to their redox state. Thus, it is crucial to quantify the redox potential (E_h) in soils and to characterize the dominant redox couples therein. Several, often coupled, external drivers, can influence E_h. Among these factors, soil hydrology dominates. It controls soil physical properties that in turn further regulates E_h. Soil spatial heterogeneity and temporally dynamic hydrologic regimes yield complex distributions of E_h. Soil redox processes have been studied under various environmental conditions, including relatively static and dynamic hydrologic regimes. Our focus here is on dynamic, variably water-saturated environments. Herein, we review previous studies on soil redox dynamics, with a specific focus on dynamic hydrologic regimes, provide recommendations on knowledge gaps, and targeted future research needs and directions. We review (1) the role of soil redox conditions on the soil chemical-species cycling of organic carbon, nitrogen, phosphorus, redox-active metals, and organic contaminants; (2) interactions between microbial activity and redox state in the near-surface and deep subsurface soil, and biomolecular methods to reveal the role of microbes in the redox processes; (3) the effects of dynamic hydrologic regimes on chemical-species cycling and microbial dynamics; (4) the experimental setups for mimicking different hydrologic regimes at both laboratory and field scales. Finally, we identify the current knowledge gaps related to the study of soil redox dynamics under different hydrologic regimes: (1) fluctuating conditions in the deep subsurface; (2) the use of biomolecular tools to understand soil biogeochemical processes beyond nitrogen; (3) limited current field measurements and potential alternative experimental setups.

Eco-friendly yield-scaled global warming potential assists to determine the right rate of nitrogen in rice system: A systematic literature review

Rice paddies are one of the largest greenhouse gases (GHGs) facilitators that are predominantly regulated by nitrogen (N) fertilization. Optimization of N uses based on the yield has been tried a long since, however, the improvement of the state-of-the-art technologies and the stiffness of global warming need to readjust N rate. Albeit, few individual studies started to, herein attempted as a systematic review to generalize the optimal N rate that minimizes global warming potential (GWP) concurrently provides sufficient yield in the rice system. To satisfy mounted food demand with inadequate land & less environmental impact, GHGs emissions are increasingly evaluated as yield-scaled basis. This systematic review (20 published studies consisting of 21 study sites and 190 observations) aimed to test the hypothesis that the lowest yield-scaled GWP would provide the minimum GWP of CH₄ and N₂O emissions from rice system at near optimal yields. Results revealed that there was a strong polynomial quadratic relationship between CH₄ emissions and N rate and strong positive correlation between N₂O emissions and N rate. Compared to control the low N dose emitted less (23%) CH₄ whereas high N dose emitted higher (63%) CH₄ emission. The highest N₂O emission observed at moderated N level. In total GWP, about 96% and 4%, GHG was emitted as CH₄ and N₂O, respectively. The mean GWP of CH₄ and N₂O emissions from rice was 5758 kg CO₂ eq ha^-1. The least yield-scaled GWP (0.7565 (kg CO₂ eq. ha^-1)) was recorded at 190 kg N ha^-1 that provided the near utmost yield. This dose could be a suitable dose in midseason drainage managed rice systems especially in tropical and subtropical climatic conditions. This yield-scaled GWP supports the concept of win-win for food security and environmental aspects through balancing between viable rice productivity and maintaining convincing greenhouse gases.

Microbial catabolism of lindane in distinct layers of acidic paddy soils combinedly affected by different water managements and bioremediation strategies

The environmental fate of the recalcitrant organic chlorine insecticide lindane and its removal from contaminated soils are still of great concern. However, the key factors influencing microbial removal of lindane from paddy soils with intermittent flooding and draining remain largely unknown. Here, we conducted laboratory experiments to investigated lindane biodegradation in different layers of typical acidic paddy soils under different water managements and bioremediation strategies, together with the changes of functional bacterial consortium, key genes and metabolic pathways. It was found that under flooded conditions, lindane spiking significantly stimulated the growth of some bacterial genera with potential anaerobic catabolic functions in both top- (0-20 cm depth) and subsoil (20-40 cm depth), leading to the shortest half-life of lindane with 7.6-9.0 d in the topsoil. In contrary, lindane spiking dramatically stimulated the growth of bacterial members with aerobic catabolic functions under drained conditions, exhibiting half-lives of lindane with 85-131 d and 18-23 d in the top- and subsoil, respectively. Overall, biostimulation coupled with flooding management would be the better combination for increased lindane bioremediation. Functional genes involved in lindane degradation and retrieved from metagenomic data further supported the anaerobic and aerobic biodegradation of lindane under flooded and drained conditions, respectively. Moreover, the integrated network analysis suggested water management and organic matter were the primary factors shaped the assembly of functional bacteria in lindane degradation, among which Clostridium and Rhodanobacter were the key anaerobic and aerobic functional genera, respectively. Taken together, our study provides a comprehensive understanding of lindane biodegradation in distinct layers of acidic paddy soils that were combinedly affected by different water managements and bioremediation strategies.

Microbial Communities Associated With Long-Term Tillage and Fertility Treatments in a Corn-Soybean Cropping System

Tillage and fertilization are common practices used to enhance soil fertility and increase yield. Changes in soil edaphic properties associated with different tillage and fertility regimes have been widely examined, yet, the microbially mediated pathways and ecological niches involved in enhancing soil fertility are poorly understood. The effects of long-term conventional tillage and no-till in parallel with three fertility treatments (No fertilization, N-only, and NPK) on soil microbial communities were investigated in a long-term field study that was established in the 1970's. Here, we used high-throughput sequencing of bacterial, fungal and oomycetes markers, followed by community-level functional and ecological assembly to discern principles governing tillage and fertility practices' influence on associated soil microbiomes. Both tillage and fertilizer significantly altered microbial community structure, but the tillage effect was more prominent than the fertilizer effect. Tillage significantly affected bacteria, fungi, fusaria, and oomycete beta-diversity, whereas fertilizer only affected bacteria and fungi beta-diversity. In our study different tillage and fertilizer regimes favored specific networks of metabolic pathways and distinct ecological guilds. No-till selected for beneficial microbes that translocate nutrients and resources and protect the host against pathogens. Notably, ecological guilds featuring arbuscular mycorrhizae, mycoparasites, and nematophagous fungi were favored in no-till soils, while fungal saprotrophs and plant pathogens dominated in tilled soils. Conventional till and fertilizer management shifted the communities toward fast growing competitors. Copiotrophic bacteria and fusarium species were favored under conventional tillage and in the presence of fertilizers. The analysis of the metagenomes revealed a higher abundance of predicted pathways associated with energy metabolism, translation, metabolism of cofactors and vitamins, glycan biosynthesis and nucleotide metabolism in no-till. Furthermore, no specific pathways were found to be enriched under the investigated fertilization regimes. Understanding how tillage and fertilizer management shift microbial diversity, structure and ecological niches, such as presented here, can assist with designing farming systems that can maintain high crop yield, while reducing soil erosion and nutrient losses.

Methane Production in Soil Environments-Anaerobic Biogeochemistry and Microbial Life between Flooding and Desiccation

Flooding and desiccation of soil environments mainly affect the availability of water and oxygen. While water is necessary for all life, oxygen is required for aerobic microorganisms. In the absence of O₂, anaerobic processes such as CH₄ production prevail. There is a substantial theoretical knowledge of the biogeochemistry and microbiology of processes in the absence of O₂. Noteworthy are processes involved in the sequential degradation of organic matter coupled with the sequential reduction of electron acceptors, and, finally, the formation of CH₄. These processes follow basic thermodynamic and kinetic principles, but also require the presence of microorganisms as catalysts. Meanwhile, there is a lot of empirical data that combines the observation of process function with the structure of microbial communities. While most of these observations confirmed existing theoretical knowledge, some resulted in new information. One important example was the observation that methanogens, which have been believed to be strictly anaerobic, can tolerate O₂ to quite some extent and thus survive desiccation of flooded soil environments amazingly well. Another example is the strong indication of the importance of redox-active soil organic carbon compounds, which may affect the rates and pathways of CH₄ production. It is noteworthy that drainage and aeration turns flooded soils, not generally, into sinks for atmospheric CH₄, probably due to the peculiarities of the resident methanotrophic bacteria.

An Assessment of Climate Induced Increase in Soil Water Availability for Soil Bacterial Communities Exposed to Long-Term Differential Phosphorus Fertilization

The fate of future food productivity depends primarily upon the health of soil used for cultivation. For Atlantic Europe, increased precipitation is predicted during both winter and summer months. Interactions between climate change and the fertilization of land used for agriculture are therefore vital to understand. This is particularly relevant for inorganic phosphorus (P) fertilization, which already suffers from resource and sustainability issues. The soil microbiota are a key indicator of soil health and their functioning is critical to plant productivity, playing an important role in nutrient acquisition, particularly when plant available nutrients are limited. A multifactorial, mesocosm study was established to assess the effects of increased soil water availability and inorganic P fertilization, on spring wheat biomass, soil enzymatic activity (dehydrogenase and acid phosphomonoesterase) and soil bacterial community assemblages. Our results highlight the significance of the spring wheat rhizosphere in shaping soil bacterial community assemblages and specific taxa under a moderate soil water content (60%), which was diminished under a higher level of soil water availability (80%). In addition, an interaction between soil water availability and plant presence overrode a long-term bacterial sensitivity to inorganic P fertilization. Together this may have implications for developing sustainable P mobilization through the use of the soil microbiota in future. Spring wheat biomass grown under the higher soil water regime (80%) was reduced compared to the constant water regime (60%) and a reduction in yield could be exacerbated in the future when grown in cultivated soil that have been fertilized with inorganic P. The potential feedback mechanisms for this need now need exploration to understand how future management of crop productivity may be impacted.

Preferential flow paths shape the structure of bacterial communities in a clayey till depth profile

Preferential flow paths in subsurface soils serve as transport routes for water, dissolved organic matter and oxygen. Little is known about bacterial communities in flow paths or in subsoils below ∼4 m. We compared communities from preferential flow paths (biopores, fractures and sand lenses) with those in adjacent matrix sediments of clayey till from the plough layer to a depth of 6 m. 16S rRNA gene-targeted community analysis showed bacterial communities of greater abundance and diversity in flow paths than in matrix sediments at all depths. Deep fracture communities contained a higher relative abundance of aerobes and plant material decomposers like Nitrospirae, Acidobacteria and Planctomycetes than adjacent matrix sediments. Similarly, analyses of the relative abundances of archaeal amoA, nirK and dsrB genes indicated transition from aerobic to anaerobic nitrogen and sulphur cycling at greater depth in preferential flow paths than in matrix sediments. Preferential flow paths in the top 260 cm contained more indicator operational taxonomic units from the plough layer community than the matrix sediments. This study indicates that the availability of oxygen and organic matter and downward transport of bacteria shape bacterial communities in preferential flow paths, and suggests that their lifestyles differ from those of bacteria in matrix communities.

Straw decreased N2O emissions from flooded paddy soils via altering denitrifying bacterial community compositions and soil organic carbon fractions

Straw return is widely applied to increase soil fertility and soil organic carbon storage. However, its effect on N2O emissions from paddy soil and the associated microbial mechanisms are still unclear. In this study, wheat straw was amended to two paddy soils (2% w/w) from Taizhou (TZ) and Yixing (YX), China, which were flooded and incubated for 30 d. Real-time PCR and Illumina sequencing were used to characterize changes in denitrifying functional gene abundance and denitrifying bacterial communities. Compared to unamended controls, straw addition significantly decreased accumulated N2O emissions in both TZ (5071 to 96 mg kg-1) and YX (1501 to 112 mg kg-1). This was mainly due to reduced N2O production with decreased abundance of major genera of nirK and nirS-bacterial communities and reduced nirK and nirS gene abundances. Further analyses showed that nirK-, nirS- and nosZ-bacterial community composition shifted mainly along the easily oxidizable carbon (EOC) arrows following straw amendment among four different soil organic carbon fractions, suggesting that increased EOC was the main driver of alerted denitrifying bacterial community composition. This study revealed straw return suppressed N2O emission via altering denitrifying bacterial community compositions and highlighted the importance of EOC in controlling denitrifying bacterial communities.

Exploring bacterial community composition in Mediterranean deep-sea sediments and their role in heavy metal accumulation

The role of microbial processes in bioaccumulation of major and trace elements has been broadly demonstrated. However, microbial communities from marine sediments have been poorly investigated to this regard. In marine environments, particularly under high anthropogenic pressure, heavy metal accumulation increases constantly, which may lead to significant environmental issues. A better knowledge of bacterial diversity and its capability to bioaccumulate metals is essential to face environmental quality assessment. The oligotrophic westernmost Mediterranean, which is highly sensitive to environmental changes and subjected to increasing anthropogenic pressure, was selected for this study. A sediment core spanning the last two millennia was sampled at two intervals, with ages corresponding to 140 (S1) and 1400 (S2) yr BP. High-throughput sequencing showed an abundance of Bacillus, Micrococcus, unclassified members of Planococcaceae, Anaerolineaceae, Planctomycetaceae, Microlunatus, and Microbacterium in both intervals, with slight differences in their abundance, along with newly detected ones in S2, i.e., Propionibacterium, Fictibacillus, Thalassobacillus, and Bacteroides. Canonical correspondence analysis (CCA) and co-occurrence patterns confirmed strong correlations among the taxa and the environmental parameters, suggesting either shared and preferred environmental conditions, or the performance of functions similar to or complementary to each other. These results were further confirmed using culture-dependent methods. The diversity of the culturable bacterial community revealed a predominance of Bacillus, and Micrococcus or Kocuria. The interaction of these bacterial communities with selected heavy metals (Cu, Cr, Zn and Pb) was also investigated, and their capacity of bioaccumulating metals within the cells and/or in the extracellular polymeric substances (EPS) is demonstrated. Interestingly, biomineralization of Pb resulted in the precipitation of Pb phosphates (pyromorphite). Our study supports that remnants of marine bacterial communities can survive in deep-sea sediments over thousands of years. This is extremely important in terms of bioremediation, in particular when considering possible environmentally friendly strategies to bioremediate inorganic contaminants.

Recovery of fen peatland microbiomes and predicted functional profiles after rewetting

Many of the world's peatlands have been affected by water table drawdown and subsequent loss of organic matter. Rewetting has been proposed as a measure to restore peatland functioning and to halt carbon loss, but its effectiveness is subject to debate. An important prerequisite for peatland recovery is a return of typical microbial communities, which drive key processes. To evaluate the effect of rewetting, we investigated 13 fen peatland areas across a wide (>1500 km) longitudinal gradient in Europe, in which we compared microbial communities between drained, undrained, and rewetted sites. There was a clear difference in microbial communities between drained and undrained fens, regardless of location. Community recovery upon rewetting was substantial in the majority of sites, and predictive functional profiling suggested a concomitant recovery of biogeochemical peatland functioning. However, communities in rewetted sites were only similar to those of undrained sites when soil organic matter quality (as expressed by cellulose fractions) and quantity were still sufficiently high. We estimate that a minimum organic matter content of ca. 70% is required to enable microbial recovery. We conclude that peatland recovery after rewetting is conditional on the level of drainage-induced degradation: severely altered physicochemical peat properties may preclude complete recovery for decades.

Similar but Not Identical Resuscitation Trajectories of the Soil Microbial Community Based on Either DNA or RNA after Flooding

Both drought and flooding are unfavorable for soil microorganisms, but nevertheless, are highly relevant to the extreme weather events that have been predicted to increase in the future. The switch of soil water status from drought to flooding can happen rapidly and microbial activity might be either stimulated or further inhibited, but we have insufficient understanding of the underlying microbial processes. Here, we tracked the changes in soil bacterial and fungal abundance and their community structures, assaying the total (DNA-based) and potentially active (RNA-based) communities in response to abrupt flooding of dry soil. Also, rates of soil respiration and enzyme activity were measured after flooding. Results showed that the bacterial community was found to be more responsive than the fungal community to flooding. The bacterial community responses were clearly classified into three distinct patterns in which the intermediate pattern displayed highly phylogenetic clustering. A transient flourish of Bacilli which belongs to Firmicutes was detected at 8–48 h of flooding, suggesting its potential importance in the microbial assemblage and subsequent ecosystem functioning. Finally, the accumulative amount of CO2 released was more closely related than enzyme activity to the change in structure of the bacterial community after flooding. In conclusion, these findings extended our understanding of the underlying soil microbial processes following abrupt water condition changes.

Molecular ecological network analysis of the response of soil microbial communities to depth gradients in farmland soils

Soil microorganisms are considered to be important indicators of soil fertility and soil quality. Most previous studies have focused solely on surface soil, but there were numerous active cells in deeper soil layers. However, studies regarding microbial communities in deeper soil layers were not comprehensive and sufficient. In this study, phylogenetic molecular ecological networks (pMENs) based on the 16S rRNA Miseq sequencing technique were applied to study the response of soil microbial communities to depth gradients and the changes of key genera along 3 meter depth gradients (0–0.2 m, 0.2–0.4 m 0.4–0.6 m, 0.6–0.8 m, 0.8–1.0 m, 1.0–1.3 m, 1.3–1.6 m, 1.6–2.0 m, 2.0–2.5 m, and 2.5–3.0 m). The results showed that the modularity of microbial communities was consistently high in all soil layers and each layer was similar, which indicated that microbial communities were more resistant to depth changes. The pMENs further demonstrated that microbial community interactions were stable as the depth increased and they cooperated well to adapt to changes in different soil gradients. This was evidenced by similar positive links, average degree, and average clustering coefficient. In addition, key genera were obtained by analyzing module hubs in the pMENs. There may be at least one dominant genus in each layer that adapted to and resisted changes in the soil environment. It seems microbial communities demonstrate a stable and strong adaptability to depth gradients in farmland soils.

The association of microbial activity with Fe, S and trace element distribution in sediment cores within a natural wetland polluted by acid mine drainage

Natural recovery and remediation of acid mine drainage (AMD) reduces the generation of acidity and transport of trace elements in the runoff. A natural wetland that receives and remediates AMD from an abandoned copper mine at Parys Mountain (Anglesey, UK) was investigated for better understanding of the remediation mechanisms. Water column concentrations of dissolved Fe and S species, trace metal (loid)s and acidity decreased markedly as the mine drainage stream passed through the wetland. The metal (loid)s were removed from the water column by deposition into the sediment. Fe typically accumulated to higher concentrations in the surface layers of sediment while S and trace metal (loid)s were deposited at higher concentration within deeper (20-50 cm) sediments. High resolution X-ray fluorescence scans of sediment cores taken at three sites along the wetland indicates co-immobilization of Zn, Cu and S with sediment depth as each element showed a similar core profile. To examine the role of bacteria in sediment elemental deposition, marker genes for Fe and S metabolism were quantified. Increased expression of marker genes for S and Fe oxidation was detected at the same location within the middle of the wetland where significant decrease in SO₄^2- and Fe²⁺ was observed and where generation of particulate Fe occurs. This suggests that the distribution and speciation of Fe and S that mediates the immobilization and deposition of trace elements within the natural wetland sediments is mediated in part by bacterial activity.

One-time nitrogen fertilization shifts switchgrass soil microbiomes within a context of larger spatial and temporal variation

Soil microbiome responses to short-term nitrogen (N) inputs remain uncertain when compared with previous research that has focused on long-term fertilization responses. Here, we examined soil bacterial/archaeal and fungal communities pre- and post-N fertilization in an 8 year-old switchgrass field, in which twenty-four plots received N fertilization at three levels (0, 100, and 200 kg N ha^-1 as NH₄NO₃) for the first time since planting. Soils were collected at two depths, 0–5 and 5–15 cm, for DNA extraction and amplicon sequencing of 16S rRNA genes and ITS regions for assessment of microbial community composition. Baseline assessments prior to fertilization revealed no significant pre-existing divergence in either bacterial/archaeal or fungal communities across plots. The one-time N fertilizations increased switchgrass yields and tissue N content, and the added N was nearly completely removed from the soil of fertilized plots by the end of the growing season. Both bacterial/archaeal and fungal communities showed large spatial (by depth) and temporal variation (by season) within each plot, accounting for 17 and 12–22% of the variation as calculated from the Sq. root of PERMANOVA tests for bacterial/archaeal and fungal community composition, respectively. While N fertilization effects accounted for only ~4% of overall variation, some specific microbial groups, including the bacterial genus Pseudonocardia and the fungal genus Archaeorhizomyces, were notably repressed by fertilization at 200 kg N ha^-1. Bacterial groups varied with both depth in the soil profile and time of sampling, while temporal variability shaped the fungal community more significantly than vertical heterogeneity in the soil. These results suggest that short-term effects of N fertilization are significant but subtle, and other sources of variation will need to be carefully accounted for study designs including multiple intra-annual sampling dates, rather than one-time “snapshot” analyses that are common in the literature. Continued analyses of these trends over time with fertilization and management are needed to understand how these effects may persist or change over time.

Microbial community structure and microbial networks correspond to nutrient gradients within coastal wetlands of the Laurentian Great Lakes

Microbial communities within the soil of Laurentian Great Lakes coastal wetlands drive biogeochemical cycles and provide several other ecosystem services. However, there exists a lack of understanding of how microbial communities respond to nutrient gradients and human activity in these systems. This research sought to address the lack of understanding through exploration of relationships among nutrient gradients, microbial community diversity, and microbial networks. Significant differences in microbial community structure were found among coastal wetlands within the western basin of Lake Erie and all other wetlands studied (three regions within Saginaw Bay and one region in the Beaver Archipelago). These diversity differences coincided with higher nutrient levels within the Lake Erie region. Site-to-site variability also existed within the majority of the regions studied, suggesting site-scale heterogeneity may impact microbial community structure. Several subnetworks of microbial communities and individual community members were related to chemical gradients among wetland regions, revealing several candidate indicator communities and taxa that may be useful for Great Lakes coastal wetland management. This research provides an initial characterization of microbial communities among Great Lakes coastal wetlands and demonstrates that microbial communities could be negatively impacted by anthropogenic activities.

Distinct rhizosphere effect on active and total bacterial communities in paddy soils

Rhizosphere microbes are critical for plant health and biogeochemical cycles. Understanding the diversity of active microorganisms in the rhizosphere is key to enhancing plant growth and productivity. We examined rhizosphere bacterial communities of rice by comparison of the 16S ribosomal subunit amplicons generated from both the total (DNA-based, 16S rRNA gene) and the active (RNA-based, 16S rRNA) soil microbiota. Analysis based on the 16S rRNA gene showed a higher microbial diversity, but with little change in bacterial populations across the growth stages of the plant. Analysis of 16S rRNA recovered much less diversity, demonstrating that much of the 16S signal was derived from free DNA, dead or inactive cells. The rRNA analysis showed a stable microbial population present in the rhizosphere, and this was distinct from that in the bulk soil, which was also stable across the growth period. Root exudates (e.g., acetate, lactate, oxalate and succinate), which are major components contributing to the rhizosphere effect, appeared to shape the bacterial community, with some taxa (e.g., Oxobacter, Lachnospiraceae, Coprococcus and α-Proteobacteria) being enhanced in the rhizosphere. Soil compartments (rhizosphere vs. bulk) had a greater effect on the bacterial communities than did the plant phenological stages, especially at the rRNA level. These results suggest that the rhizosphere effect plays a key role in structuring the bacterial communities in rhizosphere soils with a distinct effect on active and total bacterial communities.

Vertical Stratification of Sediment Microbial Communities Along Geochemical Gradients of a Subterranean Estuary Located at the Gloucester Beach of Virginia, United States

Subterranean estuaries (STEs) have been recognized as important ecosystems for the exchange of materials between the land and sea, but the microbial players of biogeochemical processes have not been well examined. In this study, we investigated the bacterial and archaeal communities within 10 cm depth intervals of a permeable sediment core (100 cm in length) collected from a STE located at Gloucester Point (GP-STE), VA, United States. High throughput sequencing of 16S rRNA genes and subsequent bioinformatics analyses were conducted to examine the composition, diversity, and potential functions of the sediment communities. The community composition varied significantly from the surface to a depth of 100 cm with up to 13,000 operational taxonomic units (OTUs) based on 97% sequence identities. More than 95% of the sequences consisted of bacterial OTUs, while the relative abundances of archaea, dominated by Crenarchaea, gradually increased with sediment core depth. Along the redox gradients of GP-STE, differential distribution of ammonia- and methane-oxidizing, denitrifying, and sulfate reducing bacteria was observed as well as methanogenic archaea based on predicted microbial functions. The aerobic-anaerobic transition zone (AATZ) had the highest diversity and abundance of microorganisms, matching with the predicted functional diversity. This indicates the AATZ as a hotspot of biogeochemical processes of STEs. The physical and geochemical gradients in different depths have attributed to vertical stratification of microbial community composition and function in the GP-STE.

Response of microbial communities to different organochlorine pesticides (OCPs) contamination levels in contaminated soils

Understanding microbial community structure and diversity in contaminated soils helps optimize the bioremediation strategies and performance. This study investigated the roles of environmental variables and contamination levels of organochlorine pesticides (OCPs) in shaping microbial community structure at an abandoned aged insecticide plant site. In total, 28 bacterial phyla were identified across soils with different physiochemical properties and OCPs levels. Proteobacteria, Bacterioidetes and Firmicutes represented the dominant lineages, and accounted for 60.2%-69.2%, 5.6%-9.7% and 6.7%-9.4% of the total population, respectively. The overall microbial diversities, in terms of phylogenetic diversity and phylotype richness, were correlated with the contents of hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs) in soils, as well as other soil properties including total nitrogen, dissolved organic carbon, pH and vegetation. The multivariate regression tree (MRT) analysis revealed that the soil microbial diversity was significantly impacted by vegetation, which explained 31.8% of the total variation, followed by OCPs level (28.3%), total nitrogen (12.4%), dissolved organic carbon (6.3%) and pH (2.4%). Our findings provide new insights and implications into the impacts on soil microbial community by OCPs contamination and other environmental variables, and offer potential strategic bioremediation for the management of OCPs contaminated sites.

Effects of Carbon Addition on Dissimilatory Fe(III) Reduction in Freshwater Marsh and Meadow Wetlands

The progress of dissimilatory iron(III) reduction is widespread in natural environments, particularly in anoxic habitats; in fact, wetland ecosystems are considered as “hotspots” of dissimilatory Fe(III) reduction. In this study, we conducted soil slurry and microbial inoculation anaerobic incubation with glucose, pyruvate, and soluble quinone anthraquinone-2,6-disulphonate (AQDS) additions in freshwater marsh and meadow wetlands in the Sanjiang Plain, to evaluate the role of carbon addition in the rates and dynamics of iron reduction. Dissimilatory Fe(III) reduction in marsh wetlands responded more quickly and showed twice the potential for Fe(III) reduction as that in meadow wetland. Fe(III) reduction rate in marsh and meadow wetlands was 76% and 30%, respectively. Glucose had a higher capacity to enhance Fe(III) reduction than pyruvate, which provides valuable information for the further isolation of Fe reduction bacteria in pure culture. AQDS could dramatically increase potential Fe(III) reduction as an electron shuttle in both wetlands. pH exhibited a negative relationship with Fe(III) reduction. In view of the significance of freshwater wetlands in the global carbon and iron cycle, further profound research is now essential and should explore the enzymatic mechanisms underlying iron reduction in freshwater wetlands.

Biogeochemical gradients and microbial communities in Winogradsky columns established with polluted wetland sediments

A Winogradsky column is a miniature ecosystem established with enriched sediments that can be used to study the relationship between biogeochemical gradients, microbial diversity and pollutant transformation. Biogeochemical processes and microbial communities changed with time and depth in Winogradsky columns incubated with heavy-metal-polluted wetland sediments for 520 days. 16S rRNA surveys were complemented by geochemical analyses, including heavy metal proportioning, to evaluate gradients in the mostly anoxic columns. Oxygen was depleted below the water-sediment interface (WSI), while NH4+, Fe2+, S2- and acetate increased by one order of magnitude at the bottom. Microbial niche differentiation occurred mainly by depth and from the light-exposed surface to the interior of the columns. Chemical gradients resulting from nutrient uptake by algae, and from iron and sulphate reduction mainly drove diversification. Heavy-metal proportioning did not significantly influence microbial diversity as Cu and Zn were immobilised at all depths. Proteobacteria were abundant in the top water and the WSI layers, whereas Firmicutes and Bacteroida dominated down-core. Together with low diversity and richness of communities at the WSI and column bottom, changes in the bacterial community coincided with algal-derived carbon sources and cellulose fermentation, respectively. We expect this study to be the starting point for the use Winogradsky columns to study microbial and geochemical dynamics in polluted sediments.

Modified RNA-seq method for microbial community and diversity analysis using rRNA in different types of environmental samples

RNA-seq-based SSU (small subunit) rRNA (ribosomal RNA) analysis has provided a better understanding of potentially active microbial community within environments. However, for RNA-seq library construction, high quantities of purified RNA are typically required. We propose a modified RNA-seq method for SSU rRNA-based microbial community analysis that depends on the direct ligation of a 5’ adaptor to RNA before reverse-transcription. The method requires only a low-input quantity of RNA (10–100 ng) and does not require a DNA removal step. The method was initially tested on three mock communities synthesized with enriched SSU rRNA of archaeal, bacterial and fungal isolates at different ratios, and was subsequently used for environmental samples of high or low biomass. For high-biomass salt-marsh sediments, enriched SSU rRNA and total nucleic acid-derived RNA-seq datasets revealed highly consistent community compositions for all of the SSU rRNA sequences, and as much as 46.4%-59.5% of 16S rRNA sequences were suitable for OTU (operational taxonomic unit)-based community and diversity analyses with complete coverage of V1-V2 regions. OTU-based community structures for the two datasets were also highly consistent with those determined by all of the 16S rRNA reads. For low-biomass samples, total nucleic acid-derived RNA-seq datasets were analyzed, and highly active bacterial taxa were also identified by the OTU-based method, notably including members of the previously underestimated genus Nitrospira and phylum Acidobacteria in tap water, members of the phylum Actinobacteria on a shower curtain, and members of the phylum Cyanobacteria on leaf surfaces. More than half of the bacterial 16S rRNA sequences covered the complete region of primer 8F, and non-coverage rates as high as 38.7% were obtained for phylum-unclassified sequences, providing many opportunities to identify novel bacterial taxa. This modified RNA-seq method will provide a better snapshot of diverse microbial communities, most notably by OTU-based analysis, even communities with low-biomass samples.

Understanding the hydrologic control of N cycle: Effect of water filled pore space on heterotrophic nitrification, denitrification and dissimilatory nitrate reduction to ammonium mechanisms in unsaturated soils

Irrigation practice will be effective if it supplies optimal water and nutrients to crops and act as a filter for contaminants leaching to ground water. There is always a scope for improving the fertilizer use efficiency and scheduling of wastewater irrigation if the fate and transport of nutrients particularly nitrogenous compounds in the soil are well understood. In the present study, nitrogen transport experiments for two different agricultural soils are performed under varying saturation 33, 57, 78% water filled pore space for sandy soil 1 and 52, 81 and 96% for loam soil 2. A HYDRUS 2D model with constructed wetland (CW2D) module could simulate aerobic nitrification and anoxic denitrification well for both soils and estimated the reaction kinetics. A hot spot of Dissimilatory Nitrate Reduction to Ammonium (DNRA) pathway has been observed at 81% moisture content for a loamy sand soil. The presence of high organic content and reductive soil environment (5.53 C/NO₃^- ratio; ORP=-125mV) results in ammonium accumulation of 16.85mg in the soil. The overall observation from this study is nitrification occurs in a wide range of saturations 33-78% with highest at 57% whereas denitrification is significant at higher water saturations 57-78% for sandy soil texture. For a loamy sand soil, denitrification is dominant at 96% saturation with least nitrification at all saturation studies. The greatest nitrogen losses (>90%) was observed for soil 2 while 30-70% for soil1. The slow dispersive subsurface transport with varying oxygen dynamics enhanced nitrogen losses from soil2 due to lesser soil permeability. This in turn, prevents NO₃^- leaching and groundwater contamination. This type of modeling study should be used before planning field experiments for designing optimal irrigation and fertigation schedules.

Temporal Dynamics of Soil Microbial Communities below the Seedbed under Two Contrasting Tillage Regimes

Agricultural productivity relies on a wide range of ecosystem services provided by the soil biota. Plowing is a fundamental component of conventional farming, but long-term detrimental effects such as soil erosion and loss of soil organic matter have been recognized. Moving towards more sustainable management practices such as reduced tillage or crop residue retention can reduce these detrimental effects, but will also influence structure and function of the soil microbiota with direct consequences for the associated ecosystem services. Although there is increasing evidence that different tillage regimes alter the soil microbiome, we have a limited understanding of the temporal dynamics of these effects. Here, we used high-throughput sequencing of bacterial and fungal ribosomal markers to explore changes in soil microbial community structure under two contrasting tillage regimes (conventional and reduced tillage) either with or without crop residue retention. Soil samples were collected over the growing season of two crops (Vicia faba and Triticum aestivum) below the seedbed (15-20 cm). Tillage, crop and growing stage were significant determinants of microbial community structure, but the impact of tillage showed only moderate temporal dependency. Whereas the tillage effect on soil bacteria showed some temporal dependency and became less strong at later growing stages, the tillage effect on soil fungi was more consistent over time. Crop residue retention had only a minor influence on the community. Six years after the conversion from conventional to reduced tillage, soil moisture contents and nutrient levels were significantly lower under reduced than under conventional tillage. These changes in edaphic properties were related to specific shifts in microbial community structure. Notably, bacterial groups featuring copiotrophic lifestyles or potentially carrying the ability to degrade more recalcitrant compounds were favored under conventional tillage, whereas taxa featuring more oligotrophic lifestyles were more abundant under reduced tillage. Our study found that, under the specific edaphic and climatic context of central Belgium, different tillage regimes created different ecological niches that select for different microbial lifestyles with potential consequences for the ecosystem services provided to the plants and their environment.

Simulating water and nitrogen loss from an irrigated paddy field under continuously flooded condition with Hydrus-1D model

Agricultural non-point source pollution is a major factor in surface water and groundwater pollution, especially for nitrogen (N) pollution. In this paper, an experiment was conducted in a direct-seeded paddy field under traditional continuously flooded irrigation (CFI). The water movement and N transport and transformation were simulated via the Hydrus-1D model, and the model was calibrated using field measurements. The model had a total water balance error of 0.236 cm and a relative error (error/input total water) of 0.23%. For the solute transport model, the N balance error and relative error (error/input total N) were 0.36 kg ha^-1 and 0.40%, respectively. The study results indicate that the plow pan plays a crucial role in vertical water movement in paddy fields. Water flow was mainly lost through surface runoff and underground drainage, with proportions to total input water of 32.33 and 42.58%, respectively. The water productivity in the study was 0.36 kg m^-3. The simulated N concentration results revealed that ammonia was the main form in rice uptake (95% of total N uptake), and its concentration was much larger than for nitrate under CFI. Denitrification and volatilization were the main losses, with proportions to total consumption of 23.18 and 14.49%, respectively. Leaching (10.28%) and surface runoff loss (2.05%) were the main losses of N pushed out of the system by water. Hydrus-1D simulation was an effective method to predict water flow and N concentrations in the three different forms. The study provides results that could be used to guide water and fertilization management and field results for numerical studies of water flow and N transport and transformation in the future.

Microbial Community and Functional Structure Significantly Varied among Distinct Types of Paddy Soils But Responded Differently along Gradients of Soil Depth Layers

Paddy rice fields occupy broad agricultural area in China and cover diverse soil types. Microbial community in paddy soils is of great interest since many microorganisms are involved in soil functional processes. In the present study, Illumina Mi-Seq sequencing and functional gene array (GeoChip 4.2) techniques were combined to investigate soil microbial communities and functional gene patterns across the three soil types including an Inceptisol (Binhai), an Oxisol (Leizhou), and an Ultisol (Taoyuan) along four profile depths (up to 70 cm in depth) in mesocosm incubation columns. Detrended correspondence analysis revealed that distinctly differentiation in microbial community existed among soil types and profile depths, while the manifest variance in functional structure was only observed among soil types and two rice growth stages, but not across profile depths. Along the profile depth within each soil type, Acidobacteria, Chloroflexi, and Firmicutes increased whereas Cyanobacteria, β-proteobacteria, and Verrucomicrobia declined, suggesting their specific ecophysiological properties. Compared to bacterial community, the archaeal community showed a more contrasting pattern with the predominant groups within phyla Euryarchaeota, Thaumarchaeota, and Crenarchaeota largely varying among soil types and depths. Phylogenetic molecular ecological network (pMEN) analysis further indicated that the pattern of bacterial and archaeal communities interactions changed with soil depth and the highest modularity of microbial community occurred in top soils, implying a relatively higher system resistance to environmental change compared to communities in deeper soil layers. Meanwhile, microbial communities had higher connectivity in deeper soils in comparison with upper soils, suggesting less microbial interaction in surface soils. Structure equation models were developed and the models indicated that pH was the most representative characteristics of soil type and identified as the key driver in shaping both bacterial and archaeal community structure, but did not directly affect microbial functional structure. The distinctive pattern of microbial taxonomic and functional composition along soil profiles implied functional redundancy within these paddy soils.

Comparison of Direct and Indirect Photolysis in Imazosulfuron Photodegradation

Imazosulfuron, a sulfonylurea herbicide used in rice cultivation, has been shown to undergo photodegradation in water, but neither the photochemical mechanism nor the role of indirect photolysis is known. The purpose of this study was to investigate the underlying processes that operate on imazosulfuron during aqueous photodegradation. Our data indicate that in the presence of oxygen, most photochemical degradation proceeds through a direct singlet-excited state pathway, whereas triplet-excited state imazosulfuron enhanced decay rates under low dissolved oxygen conditions. Oxidation by hydroxyl radical and singlet oxygen were not significant. At dissolved organic matter (DOM) concentrations representative of rice field conditions, fulvic acid solutions exhibited faster degradation than rice field water containing both humic and fulvic acid fractions. Both enhancement, via reaction with triplet-state DOM, and inhibition, via competition for photons, of degradation was observed in DOM solutions.

Logarithmic sensing in Bacillus subtilis aerotaxis

Aerotaxis, the directed migration along oxygen gradients, allows many microorganisms to locate favorable oxygen concentrations. Despite oxygen's fundamental role for life, even key aspects of aerotaxis remain poorly understood. In Bacillus subtilis, for example, there is conflicting evidence of whether migration occurs to the maximal oxygen concentration available or to an optimal intermediate one, and how aerotaxis can be maintained over a broad range of conditions. Using precisely controlled oxygen gradients in a microfluidic device, spanning the full spectrum of conditions from quasi-anoxic to oxic (60 n mol/l-1 m mol/l), we resolved B. subtilis' 'oxygen preference conundrum' by demonstrating consistent migration towards maximum oxygen concentrations ('monotonic aerotaxis'). Surprisingly, the strength of aerotaxis was largely unchanged over three decades in oxygen concentration (131 n mol/l-196 μ mol/l). We discovered that in this range B. subtilis responds to the logarithm of the oxygen concentration gradient, a rescaling strategy called 'log-sensing' that affords organisms high sensitivity over a wide range of conditions. In these experiments, high-throughput single-cell imaging yielded the best signal-to-noise ratio of any microbial taxis study to date, enabling the robust identification of the first mathematical model for aerotaxis among a broad class of alternative models. The model passed the stringent test of predicting the transient aerotactic response despite being developed on steady-state data, and quantitatively captures both monotonic aerotaxis and log-sensing. Taken together, these results shed new light on the oxygen-seeking capabilities of B. subtilis and provide a blueprint for the quantitative investigation of the many other forms of microbial taxis.

Profiling bacterial communities associated with sediment-based aquaculture bioremediation systems under contrasting redox regimes

Deposit-feeding invertebrates are proposed bioremediators in microbial-driven sediment-based aquaculture effluent treatment systems. We elucidate the role of the sediment reduction-oxidation (redox) regime in structuring benthic bacterial communities, having direct implications for bioremediation potential and deposit-feeder nutrition. The sea cucumber Holothuria scabra was cultured on sediments under contrasting redox regimes; fully oxygenated (oxic) and redox stratified (oxic-anoxic). Taxonomically, metabolically and functionally distinct bacterial communities developed between the redox treatments with the oxic treatment supporting the greater diversity; redox regime and dissolved oxygen levels were the main environmental drivers. Oxic sediments were colonised by nitrifying bacteria with the potential to remediate nitrogenous wastes. Percolation of oxygenated water prevented the proliferation of anaerobic sulphate-reducing bacteria, which were prevalent in the oxic-anoxic sediments. At the predictive functional level, bacteria within the oxic treatment were enriched with genes associated with xenobiotics metabolism. Oxic sediments showed the greater bioremediation potential; however, the oxic-anoxic sediments supported a greater sea cucumber biomass. Overall, the results indicate that bacterial communities present in fully oxic sediments may enhance the metabolic capacity and bioremediation potential of deposit-feeder microbial systems. This study highlights the benefits of incorporating deposit-feeding invertebrates into effluent treatment systems, particularly when the sediment is oxygenated.

Microbial community dynamics in soil aggregates shape biogeochemical gas fluxes from soil profiles - upscaling an aggregate biophysical model

Microbial communities inhabiting soil aggregates dynamically adjust their activity and composition in response to variations in hydration and other external conditions. These rapid dynamics shape signatures of biogeochemical activity and gas fluxes emitted from soil profiles. Recent mechanistic models of microbial processes in unsaturated aggregate-like pore networks revealed a highly dynamic interplay between oxic and anoxic microsites jointly shaped by hydration conditions and by aerobic and anaerobic microbial community abundance and self-organization. The spatial extent of anoxic niches (hotspots) flicker in time (hot moments) and support substantial anaerobic microbial activity even in aerated soil profiles. We employed an individual-based model for microbial community life in soil aggregate assemblies represented by 3D angular pore networks. Model aggregates of different sizes were subjected to variable water, carbon and oxygen contents that varied with soil depth as boundary conditions. The study integrates microbial activity within aggregates of different sizes and soil depth to obtain estimates of biogeochemical fluxes from the soil profile. The results quantify impacts of dynamic shifts in microbial community composition on CO2 and N2 O production rates in soil profiles in good agreement with experimental data. Aggregate size distribution and the shape of resource profiles in a soil determine how hydration dynamics shape denitrification and carbon utilization rates. Results from the mechanistic model for microbial activity in aggregates of different sizes were used to derive parameters for analytical representation of soil biogeochemical processes across large scales of practical interest for hydrological and climate models.

Metagenomic and PCR-Based Diversity Surveys of [FeFe]-Hydrogenases Combined with Isolation of Alkaliphilic Hydrogen-Producing Bacteria from the Serpentinite-Hosted Prony Hydrothermal Field, New Caledonia

High amounts of hydrogen are emitted in the serpentinite-hosted hydrothermal field of the Prony Bay (PHF, New Caledonia), where high-pH (~11), low-temperature (< 40°C), and low-salinity fluids are discharged in both intertidal and shallow submarine environments. In this study, we investigated the diversity and distribution of potentially hydrogen-producing bacteria in Prony hyperalkaline springs by using metagenomic analyses and different PCR-amplified DNA sequencing methods. The retrieved sequences of hydA genes, encoding the catalytic subunit of [FeFe]-hydrogenases and, used as a molecular marker of hydrogen-producing bacteria, were mainly related to those of Firmicutes and clustered into two distinct groups depending on sampling locations. Intertidal samples were dominated by new hydA sequences related to uncultured Firmicutes retrieved from paddy soils, while submarine samples were dominated by diverse hydA sequences affiliated with anaerobic and/or thermophilic submarine Firmicutes pertaining to the orders Thermoanaerobacterales or Clostridiales. The novelty and diversity of these [FeFe]-hydrogenases may reflect the unique environmental conditions prevailing in the PHF (i.e., high-pH, low-salt, mesothermic fluids). In addition, novel alkaliphilic hydrogen-producing Firmicutes (Clostridiales and Bacillales) were successfully isolated from both intertidal and submarine PHF chimney samples. Both molecular and cultivation-based data demonstrated the ability of Firmicutes originating from serpentinite-hosted environments to produce hydrogen by fermentation, potentially contributing to the molecular hydrogen balance in situ.

Microbial Community Structure in the Rhizosphere of Rice Plants

The microbial community in the rhizosphere environment is critical for the health of land plants and the processing of soil organic matter. The objective of this study was to determine the extent to which rice plants shape the microbial community in rice field soil over the course of a growing season. Rice (Oryza sativa) was cultivated under greenhouse conditions in rice field soil from Vercelli, Italy and the microbial community in the rhizosphere of planted soil microcosms was characterized at four plant growth stages using quantitative PCR and 16S rRNA gene pyrotag analysis and compared to that of unplanted bulk soil. The abundances of 16S rRNA genes in the rice rhizosphere were on average twice that of unplanted bulk soil, indicating a stimulation of microbial growth in the rhizosphere. Soil environment type (i.e., rhizosphere versus bulk soil) had a greater effect on the community structure than did time (e.g., plant growth stage). Numerous phyla were affected by the presence of rice plants, but the strongest effects were observed for Gemmatimonadetes, Proteobacteria, and Verrucomicrobia. With respect to functional groups of microorganisms, potential iron reducers (e.g., Geobacter, Anaeromyxobacter) and fermenters (e.g., Clostridiaceae, Opitutaceae) were notably enriched in the rhizosphere environment. A Herbaspirillum species was always more abundant in the rhizosphere than bulk soil and was enriched in the rhizosphere during the early stage of plant growth.

Molecular Ecology of Hypersaline Microbial Mats: Current Insights and New Directions

Microbial mats are unique geobiological ecosystems that form as a result of complex communities of microorganisms interacting with each other and their physical environment. Both the microorganisms present and the network of metabolic interactions govern ecosystem function therein. These systems are often found in a range of extreme environments, and those found in elevated salinity have been particularly well studied. The purpose of this review is to briefly describe the molecular ecology of select model hypersaline mat systems (Guerrero Negro, Shark Bay, S’Avall, and Kiritimati Atoll), and any potentially modulating effects caused by salinity to community structure. In addition, we discuss several emerging issues in the field (linking function to newly discovered phyla and microbial dark matter), which illustrate the changing paradigm that is seen as technology has rapidly advanced in the study of these extreme and evolutionally significant ecosystems.

The impact of temperature on microbial diversity and AOA activity in the Tengchong Geothermal Field, China

Using a culture-independent method that combines CARD-FISH, qPCR and 16S rDNA, we investigated the abundance, community structure and diversity of microbes along a steep thermal gradient (50–90 °C) in the Tengchong Geothermal Field. We found that Bacteria and Archaea abundance changed markedly with temperature changes and that the number of cells was lowest at high temperatures (90.8 °C). Under low-temperature conditions (52.3–74.6 °C), the microbial communities were dominated by Bacteria, which accounted for 60–80% of the total number of cells. At 74.6 °C, Archaea were dominant, and at 90.8 °C, they accounted for more than 90% of the total number of cells. Additionally, the microbial communities at high temperatures (74.6–90.8 °C) were substantially simpler than those at the low-temperature sites. Only a few genera (e.g., bacterial Caldisericum, Thermotoga and Thermoanaerobacter, archaeal Vulcanisaeta and Hyperthermus) often dominated in high-temperature environments. Additionally, a positive correlation between Ammonia-Oxidizing Archaea (AOA) activity and temperature was detected. AOA activity increased from 17 to 52 pmol of NO₂⁻ per cell d⁻¹ with a temperature change from 50 to 70 °C.

Temporal and Spatial Distribution of the Microbial Community of Winogradsky Columns

Winogradsky columns are model microbial ecosystems prepared by adding pond sediment to a clear cylinder with additional supplements and incubated with light. Environmental gradients develop within the column creating diverse niches that allow enrichment of specific bacteria. The enrichment culture can be used to study soil and sediment microbial community structure and function. In this study we used a 16S rRNA gene survey to characterize the microbial community dynamics during Winogradsky column development to determine the rate and extent of change from the source sediment community. Over a period of 60 days, the microbial community changed from the founding pond sediment population: Cyanobacteria, Chloroflexi, Nitrospirae, and Planctomycetes increased in relative abundance over time, while most Proteobacteria decreased in relative abundance. A unique, light-dependent surface biofilm community formed by 60 days that was less diverse and dominated by a few highly abundant bacteria. 67–72% of the surface community was comprised of highly enriched taxa that were rare in the source pond sediment, including the Cyanobacteria Anabaena, a member of the Gemmatimonadetes phylum, and a member of the Chloroflexi class Anaerolinea. This indicates that rare taxa can become abundant under appropriate environmental conditions and supports the hypothesis that rare taxa serve as a microbial seed bank. We also present preliminary findings that suggest that bacteriophages may be active in the Winogradsky community. The dynamics of certain taxa, most notably the Cyanobacteria, showed a bloom-and-decline pattern, consistent with bacteriophage predation as predicted in the kill-the-winner hypothesis. Time-lapse photography also supported the possibility of bacteriophage activity, revealing a pattern of colony clearance similar to formation of viral plaques. The Winogradsky column, a technique developed early in the history of microbial ecology to enrich soil microbes, may therefore be a useful model system to investigate both microbial and viral ecology.

High resolution depth distribution of Bacteria, Archaea, methanotrophs, and methanogens in the bulk and rhizosphere soils of a flooded rice paddy

The communities and abundances of methanotrophs and methanogens, along with the oxygen, methane, and total organic carbon (TOC) concentrations, were investigated along a depth gradient in a flooded rice paddy. Broad patterns in vertical profiles of oxygen, methane, TOC, and microbial abundances were similar in the bulk and rhizosphere soils, though methane and TOC concentrations and 16S rRNA gene copies were clearly higher in the rhizosphere soil than in the bulk soil. Oxygen concentrations decreased sharply to below detection limits at 8 mm depth. Pyrosequencing of 16S rRNA genes showed that bacterial and archaeal communities varied according to the oxic, oxic-anoxic, and anoxic zones, indicating that oxygen is a determining factor for the distribution of bacterial and archaeal communities. Aerobic methanotrophs were maximally observed near the oxic-anoxic interface, while methane, TOC, and methanogens were highest in the rhizosphere soil at 30–200 mm depth, suggesting that methane is produced mainly from organic carbon derived from rice plants and is metabolized aerobically. The relative abundances of type I methanotrophs such as Methylococcus, Methylomonas, and Methylocaldum decreased more drastically than those of type II methanotrophs (such as Methylocystis and Methylosinus) with increasing depth. Methanosaeta and Methanoregula were predominant methanogens at all depths, and the relative abundances of Methanosaeta, Methanoregula, and Methanosphaerula, and GOM_Arc_I increased with increasing depth. Based on contrasts between absolute abundances of methanogens and methanotrophs at depths sampled across rhizosphere and bulk soils (especially millimeter-scale slices at the surface), we have identified populations of methanogens (Methanosaeta, Methanoregula, Methanocella, Methanobacterium, and Methanosphaerula), and methanotrophs (Methylosarcina, Methylococcus, Methylosinus, and unclassified Methylocystaceae) that are likely physiologically active in situ.

Influence of rice growth on the fate of polycyclic aromatic hydrocarbons in a subtropical paddy field: a life cycle study

We measured the concentrations and profiles of polycyclic aromatic hydrocarbons (PAHs) in the soil, water, and rice tissues from a typical subtropical paddy system at various stages of rice growth over two growing seasons. Rice growth had a significant impact on the distribution and dissipation of PAHs in the paddy field. While rice was growing, the concentrations of PAHs in the soils decreased at an average decline rate of 5.3±2.9 ng PAHs g(-1) soild(-1), whereas, the concentrations of PAHs in rice tissues increased with growth time. However, the concentrations of PAHs in the rice leaves decreased during the heading stage of both two growing seasons. PAH profiles in soil, water, and different rice tissues also showed different patterns with the growing time of rice. Irrigation water was a significant source of PAHs to the paddy field. Rice growth enhanced the dissipation and transport of PAHs in the paddy system, while the sewage irrigation and straw burning after harvest added or returned PAHs to the system. For food safety precaution, sewage irrigation and straw burning should be well monitored and controlled.

Niche specificity of ammonia-oxidizing archaeal and bacterial communities in a freshwater wetland receiving municipal wastewater in Daqing, Northeast China

Ecophysiological differences between ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) enable them to adapt to different niches in complex freshwater wetland ecosystems. The community characters of AOA and AOB in the different niches in a freshwater wetland receiving municipal wastewater, as well as the physicochemical parameters of sediment/soil samples, were investigated in this study. AOA community structures varied and separated from each other among four different niches. Wetland vegetation including aquatic macrophytes and terrestrial plants affected the AOA community composition but less for AOB, whereas sediment depths might contribute to the AOB community shift. The diversity of AOA communities was higher than that of AOB across all four niches. Archaeal and bacterial amoA genes (encoding for the alpha-subunit of ammonia monooxygenases) were most diverse in the dry-land niche, indicating O2 availability might favor ammonia oxidation. The majority of AOA amoA sequences belonged to the Soil/sediment Cluster B in the freshwater wetland ecosystems, while the dominant AOB amoA sequences were affiliated with Nitrosospira-like cluster. In the Nitrosospira-like cluster, AOB amoA gene sequences affiliated with the uncultured ammonia-oxidizing beta-proteobacteria constituted the largest portion (99%). Moreover, independent methods for phylogenetic tree analysis supported high parsimony bootstrap values. As a consequence, it is proposed that Nitrosospira-like amoA gene sequences recovered in this study represent a potentially novel cluster, grouping with the sequences from Gulf of Mexico deposited in the public databases.

16S rRNA gene survey of microbial communities in Winogradsky columns

A Winogradsky column is a clear glass or plastic column filled with enriched sediment. Over time, microbial communities in the sediment grow in a stratified ecosystem with an oxic top layer and anoxic sub-surface layers. Winogradsky columns have been used extensively to demonstrate microbial nutrient cycling and metabolic diversity in undergraduate microbiology labs. In this study, we used high-throughput 16s rRNA gene sequencing to investigate the microbial diversity of Winogradsky columns. Specifically, we tested the impact of sediment source, supplemental cellulose source, and depth within the column, on microbial community structure. We found that the Winogradsky columns were highly diverse communities but are dominated by three phyla: Proteobacteria, Bacteroidetes, and Firmicutes. The community is structured by a founding population dependent on the source of sediment used to prepare the columns and is differentiated by depth within the column. Numerous biomarkers were identified distinguishing sample depth, including Cyanobacteria, Alphaproteobacteria, and Betaproteobacteria as biomarkers of the soil-water interface, and Clostridia as a biomarker of the deepest depth. Supplemental cellulose source impacted community structure but less strongly than depth and sediment source. In columns dominated by Firmicutes, the family Peptococcaceae was the most abundant sulfate reducer, while in columns abundant in Proteobacteria, several Deltaproteobacteria families, including Desulfobacteraceae, were found, showing that different taxonomic groups carry out sulfur cycling in different columns. This study brings this historical method for enrichment culture of chemolithotrophs and other soil bacteria into the modern era of microbiology and demonstrates the potential of the Winogradsky column as a model system for investigating the effect of environmental variables on soil microbial communities.

Application of Locked Nucleic Acid (LNA) oligonucleotide-PCR clamping technique to selectively PCR amplify the SSU rRNA genes of bacteria in investigating the plant-associated community structures

The simultaneous extraction of plant organelle (mitochondria and plastid) genes during the DNA extraction step is a major limitation in investigating the community structures of bacteria associated with plants because organelle SSU rRNA genes are easily amplified by PCR using primer sets that are specific to bacteria. To inhibit the amplification of organelle genes, the locked nucleic acid (LNA) oligonucleotide-PCR clamping technique was applied to selectively amplify bacterial SSU rRNA genes by PCR. LNA oligonucleotides, the sequences of which were complementary to mitochondria and plastid genes, were designed by overlapping a few bases with the annealing position of the bacterial primer and converting DNA bases into LNA bases specific to mitochondria and plastids at the shifted region from the 3' end of the primer-binding position. PCR with LNA oligonucleotides selectively amplified the bacterial genes while inhibited that of organelle genes. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that conventional amplification without LNA oligonucleotides predominantly generated DGGE bands from mitochondria and plastid genes with few bacterial bands. In contrast, additional bacterial bands were detected in DGGE patterns, the amplicons of which were prepared using LNA oligonucleotides. These results indicated that the detection of bacterial genes had been screened by the excessive amplification of the organelle genes. Sequencing of the bands newly detected by using LNA oligonucleotides revealed that their similarity to the known isolated bacteria was low, suggesting the potential to detect novel bacteria. Thus, application of the LNA oligonucleotide-PCR clamping technique was considered effective for the selective amplification of bacterial genes from extracted DNA containing plant organelle genes.

Description of Anaerobacterium chartisolvens gen. nov., sp. nov., an obligately anaerobic bacterium from Clostridium rRNA cluster III isolated from soil of a Japanese rice field, and reclassification of Bacteroides cellulosolvens Murray et al. 1984 as Pseudobacteroides cellulosolvens gen. nov., comb. nov

An obligately anaerobic bacterial strain designated T-1-35T was isolated as a dominant cultivable cellulose-degrading bacterium from soil of a Japanese rice field as an anaerobic filter-paper degrader. Cells of strain T-1-35T stained Gram-positive and were non-spore-forming rods with rounded ends, 0.8–1.0×3.5–15.0 µm, and motile by means of two to four polar flagella. Cells of strain T-1-35T exhibited pleomorphism: in aged cultures (over 90 days of incubation), almost all cells were irregularly shaped. Although no spore formation was observed, cells tolerated high temperatures, up to 90 °C for 10 min. The temperature range for growth was 15–40 °C, with an optimum at 35 °C. The pH range for growth was 5.5–9.0, with an optimum at pH 8.0–8.5 (slightly alkaliphilic). Strain T-1-35T fermented some carbohydrates to produce ethanol and lactate as the major products. Major cellular fatty acids were iso-C16 : 0 and iso-C13 : 0 3-OH. Phylogenetic analysis based on the 16S rRNA gene sequence revealed that strain T-1-35T belonged to Clostridium rRNA cluster III. The closest relative of strain T-1-35T was Bacteroides cellulosolvens WM2T, with 16S rRNA gene sequence similarity of 93.4 %. Phenotypic, physiological and molecular genetic methods demonstrated that strain T-1-35T was distinct from its phylogenetic relatives (members of Clostridium rRNA cluster III) because it predominantly produced ethanol, iso-C13 : 0 3-OH was a major cellular fatty acid and it always exhibited pleomorphism. On the basis of the results of a polyphasic taxonomic study, strain T-1-35T is considered to represent a novel genus and species, Anaerobacterium chartisolvens gen. nov., sp. nov. The type strain of Anaerobacterium chartisolvens is T-1-35T ( = DSM 27016T = NBRC 109520T). In addition, from the results of our phylogenetic analysis and its phenotypic features, the species Bacteroides cellulosolvens Murray et al. 1984 is proposed to be reclassified in the new genus Pseudobacteroides as Pseudobacteroides cellulosolvens gen. nov., comb. nov., with the type strain WM2T ( = ATCC 35603T = DSM 2933T = NRCC 2944T).