Sediment arsenic remediation by submerged macrophytes via root-released O2 and microbe-mediated arsenic biotransformation.

Arsenic (As)-contaminated water restoration is extremely challenging because As remobilization from sediments can result in episodic or long-term release of As to the overlying water. In this study, by combining high-resolution imaging techniques with microbial community profiling, we examined the feasibility of utilizing the rhizoremediation of submerged macrophytes (Potamogeton crispus) to decrease As bioavailability and regulate its biotransformation in sediments. Results showed that P. crispus considerably decreased the rhizospheric labile As flux to lower than 4 pg cm-2 s-1 from larger than 7 pg cm-2 s-1, suggesting its effectiveness in promoting As retention in sediments. Iron plaques induced by radial oxygen loss from roots decreased the mobility of As by sequestering it. Additionally, Mn-oxides may act as an oxidizer for the oxidation of As(III) to As(V) in the rhizosphere, which can further increase the As adsorption owing to the strong binding affinity between As(V) and Fe-oxides. Furthermore, microbially mediated As oxidation and methylation were intensified in the microoxic rhizosphere, which decreased the mobility and toxicity of As by changing its speciation. Our study demonstrated that root-driven abiotic and biotic transformation contribute to As retention in sediments, which lays a foundation for applying macrophytes to the remediation of As-contaminated sediments.

Decrease in macrofauna density increases the sediment phosphorus release and maintains the high phosphorus level of water column in Lake Taihu: A case study on Grandidierella taihuensis.

Internal phosphorus (P) loading can increase the P level in the water column and further sustains cyanobacterial blooms. This study focused on the role of benthic fauna bioturbation in affecting the sediment P release and the P level of water column in a eutrophic lake, Lake Taihu. The macrofauna density decreased from 4766.56 ± 10541.80 ind/m2 in 2007 to 345 ± 447.63 ind/m2 in 2020 due to the frequent bottom-water hypoxia in Lake Taihu. The reduced macrofauna density majorly resulted from Grandidierella taihuensis, Limnodrilus hoffmeisteri, and Tanypus chinensis larvae, and their total density decreased by approximately 97% in 2020 compared to 2007. G. taihuensis, one of the major benthic faunas, was further used as a representative to investigate the effects of bioturbation on sediment P release using high-resolution sampling and imaging techniques. The results show that G. taihuensis can increase the O2 penetration depth by more than 20 mm through bio-irrigation, and causes the redox conditions in burrows and surrounding sediments to change dramatically within a few minutes due to the intermittent ventilation. Subsequent oxidation of the soluble Fe(II) led to the formation of Fe-oxide bound P in the surface sediments, thereby increasing the P retention in the sediments. When the G. taihuensis density was 1563 ind/m2 at the sampling site, approximately 0.12 g m-2 yr-1 P can be retained in sediments. As previous studies have shown that L. hoffmeisteri and T. chinensis played a similar role in increasing the P retention in sediments through their bioturbation activities, the sharp decline in benthic fauna density and burrowing activities in Lake Taihu should be an important reason for maintaining the high P level in the water column by decreasing the P retention in sediments.

Stimulation of high-concentration dissolved nitrogen and reactive phosphorus in Lake Taihu sediments on the initiation and maintenance of cyanobacterial blooms.

The spatio-temporal variation of dissolved inorganic nitrogen (DIN: NH4+-N, NO2--N, and NO3--N) and dissolved reactive phosphorus (DRP) in Meiliang Bay of Lake Taihu sediments and their potential effects on the cyanobacterial blooms were studied. Monthly sampling was performed using high-resolution dialysis sampling devices (HR-Peeper) and two important results were observed in April (the initiation period of cyanobacterial bloom) and June-August (the maintenance period of cyanobacterial blooms). In April, high concentrations of dissolved NO2--N and NO3--N, probably caused by the groundwater influx, were observed in deep anoxic sediments (below 110 mm). NO2--N and NO3--N are good electron acceptors for the mineralization of organic P under anaerobic conditions and should lead to an increase in DRP concentrations in sediments, DRP released from sediments thus further stimulating the cyanobacterial growth and the outbreak of severe cyanobacterial blooms in May due to the extremely low concentrations of DRP in the water body. From June to August, high concentrations of NO2--N, NO3--N, and DRP were observed in the surface sediment, which was caused by the release of NH4+-N from the mineralization of cyanobacterial debris. This should play an important role in maintaining cyanobacterial growth, especially in stimulating the occurrence of cyanobacterial blooms during September, when N and P were co-limited. This study revealed high-concentration DIN and DRP in Lake Taihu sediments potentially stimulated the initiation and maintenance of cyanobacterial blooms.

Enhancing the retention of phosphorus through bacterial oxidation of iron or sulfide in the eutrophic sediments of Lake Taihu.

Microbial activity can enhance the sequestration of phosphorus (P) in sediments, but little is known about the mechanisms behind it. In this study, sediment cores were sampled from the most eutrophic Meiliang Bay of Lake Taihu, and three treatments were set up in a laboratory incubation experiment, involving (a) the non-treated sediment cores, (b) inoculation, and (c) sterilization. The dissolved and labile iron (Fe) and P were obtained by high-resolution dialysis and the diffusive gradients in thin films (DGT) technique, respectively. AgI-based DGT was used for measuring the 2D distribution of labile sulfide. The bacterial community was investigated using a scanning electron microscope and 16S rRNA high throughput sequencing technique. The results showed that sterilization reduced the capacity of sediment to immobilize P, and that the critical sediment depth layer for microbial P sequestration was 0-10 mm. In addition, sterilization or inoculation significantly changes the structure of bacterial communities. Fe or S oxidation under micro-aerobic or anaerobic conditions played an important role in bacterial retention of P in the sediments. Nitrate-reducing coupling Fe(II)-oxidizing bacteria (Acidovorax) in the inoculated sediment and electrogenic sulfur-oxidizing bacteria (Candidatus Electronema) in the non-treated sediment were identified as the key bacterial genera responsible for the retention of P in sediments. This implies that bacterial communities could quickly establish the ability for negative feedback regulation by inoculation once the function and structure of indigenous sediment bacteria are seriously impaired, although this needs further validation in the field.

A holistic understanding of cobalt cycling and limiting roles in the eutrophic Lake Taihu.

Cobalt (Co) cycling is often dominated by its role as a micronutrient in marine, while little is known on its cycling in a shallow eutrophic lake. Monthly sampling was performed in eutrophic Meiliang Bay of Lake Taihu, combining two laboratory control experiments and in situ Co limitation bioassay experiments. The high-resolution dialysis and the diffusive gradients in thin films technique were used to detect dissolved and labile Co, respectively. The positive correlations between dissolved/labile Co and Mn in the sediments for 6 or 7 months demonstrated that the mobility of Co in the sediments was primarily controlled by Mn redox cycling in the field. However, it is unexpected that the dissolved and labile Co only showed a small change over one year irrespective of the significant fluctuation in dissolved/labile Mn, with the concentrations being as low as 1.08 ± 0.22 µg/L and 0.246 ± 0.091 µg/L for dissolved and labile Co in the surface 20 mm sediment, respectively. Cyanobacterial bloom simulation and aerobic-anaerobic-cyanobacterial addition experiments indicated that the level of Co in the sediment-overlying water system was strongly regulated by cyanobacterial uptake, followed by the degradation of Co-enriched cyanobacterial biomass, which offset the influence of Mn redox cycling on Co mobility in the sediment. The significant enhancement of Microcystis spp. biomass by Co addition further indicated that Co was the potential limiting nutrient for cyanobacterial blooms. This work provides new ideas for better management strategies of eutrophication in shallow lakes.

High cadmium pollution from sediments in a eutrophic lake caused by dissolved organic matter complexation and reduction of manganese oxide.

Eutrophication and metal pollution are global environmental problems. The risk of metal pollution is high in the eutrophic lakes because of high mobility of metal in sediments. However, the mechanism of cadmium (Cd) mobility in sediments is still unclear. Here we study the mobilization of Cd in sediments from the eutrophic Lake Taihu via monthly field monitoring of mobile Cd using diffusive gradient in thin films (DGT) and high resolution dialysis (HR-Peeper) techniques. We found a high mobility of Cd in sediments in February and March, resulting from reductive dissolution of Mn oxide mediation by high microbial activities, as shown by the similarities in distribution patterns of DGT-labile Cd and Mn. A two orders of magnitude increase in dissolved Cd concentrations (about 28 µg L-1) was observed in May and June, with dissolved Cd concentrations in overlying water about 110 times higher than the criteria continuous concentration set by Environmental Protection Agency. Hourly changes were found to coincide and correlate between dissolved Cd and dissolved organic matter (DOM) under simulated anaerobic conditions, strongly suggesting that the sudden outbreak of Cd pollution observed in the field resulted from the complexation of DOM with Cd in sediments. This was further supported by the NICA-Donnan model that more than 71% of dissolved Cd in the pore water in May and June was present as Cd-DOM complexes. Three components of DOM including humic-, tryptophan-, and tyrosine-like components in the sediments in June was identified using the fluorescence excitation emission matrix-parallel factor analysis. We found that Cd was stable complexed with tyrosine-like component. The Fourier transform infrared and two-dimensional correlation spectroscopy further revealed that Cd was bound to phenolic OH, alkene CC, alcoholic CO, aromatic CH, and alkene CH groups. Our study effectively promotes the understanding of Cd mobilization in sediments and highlights the risk of sudden Cd pollution events in the eutrophic lakes.

Mobility of chromium in sediments dominated by macrophytes and cyanobacteria in different zones of Lake Taihu.

To study the mechanisms of chromium (Cr) mobilization in sediments of lakes with different ecotypes, seasonal sampling was performed in the macrophyte-dominated East Taihu (MDET) and cyanobacteria-dominated Meiliang Bay (CDMB) in Lake Taihu. Concentrations of labile Cr(VI) and dissolved Cr were assessed using diffusive gradients in thin films (DGT) and high-resolution dialysis passive sampling devices, respectively. Results indicated that in pore water the dissolved Cr concentrations and in sediments total Cr and Cr fractions concentrations (dissolved, exchangeable and carbonate fraction (F1), Fe-Mn oxide fraction (F2), organic/sulfide fraction (F3)) were lower in MDET than in CDMB. However, the highly toxic DGT-labile Cr(VI) concentrations were higher in MDET sediments than in CDMB sediments. In the two lake zones, the seasonal variations in concentrations of dissolved Cr and DGT-labile Cr(VI) were significant, while total Cr and Cr fractions showed negligible changes (except F1 fraction). In summer, the high mobility of dissolved Cr (MDET: 103.42±10.82µg/L; CDMB: 108.99±4.24µg/L) were mainly caused by dissolved organic matter complexing with Cr(III). In winter the high mobility of dissolved Cr (MDET: 100.27±22.04µg/L; CDMB: 102.01±8.81µg/L) and DGT-labile Cr(VI) (MDET: 28.26±3.73µg/L; CDLZ: 25.82±2.26µg/L) were primarily caused by the oxidization of Cr (III) by Mn(III/IV) oxides. This study establishes the mechanisms for seasonal variation of Cr mobilization in different lake ecological zones, highlighting the urgent need for remediation of Cr pollution, especially in macrophyte-dominated lake zones.

Zinc pollution in zones dominated by algae and submerged macrophytes in Lake Taihu.

Zinc (Zn) contamination in lake zones dominated by algae and macrophytes in Lake Taihu was analyzed through diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) methods. It was found that in both zones Zn contamination varied by season. In July and October, dissolved Zn was present in high concentrations, and in July, high concentrations of labile Zn were found in sediments. In July, reductive dissolutions of Fe/Mn oxides likely played a key role in the release of Zn, which was confirmed by both zones having the lowest percentage of the reducible fraction of Zn in July. Complexation of dissolved organic matter (DOM) with Zn may be responsible for the observed increase in the dissolved Zn concentration in October. This conclusion was supported by noting that October had the highest percentages of Zn-DOM complexes (25.3% and 34.4%) in the algae- and macrophytes-dominated zones, respectively. However, in January, low dissolved and labile Zn contents were observed in sediments in the two zones, suggesting that the decrease of Zn in sediments was caused by the adsorption of Fe/Mn oxides.

Peak Chromium Pollution in Summer and Winter Caused by High Mobility of Chromium in Sediment of a Eutrophic Lake: In Situ Evidence from High Spatiotemporal Sampling.

To study the mechanisms of chromium (Cr) mobilization in sediments of eutrophic lakes, monthly sampling was performed in the Meiliang Bay of Lake Taihu, China, combined with laboratory experiments. High-resolution dialysis and diffusive gradients in thin film (DGT) sampling techniques were used. Results indicated that in July 2016 and January 2017, the concentrations of soluble Cr and DGT-labile Cr(VI) in the overlying water exceeded both drinking and fishery water quality standards, resulting from the high mobility of Cr in sediments. In July (summer), the high concentration of soluble Cr (134.04 ± 7.20 µg/L) detected in the anaerobic sediments was primarily caused by the complexation of Cr(III) with dissolved organic matter (DOM). This mechanism was supported by an observed simultaneous increase of soluble Cr and DOM under simulated anaerobic conditions. In January (winter), the high concentrations of soluble Cr (97.55 ± 9.65 µg/L) and DGT-labile Cr(VI) (25.83 ± 1.25 µg/L) in aerobic sediments were primarily caused by reoxidation of Cr(III) by Mn(III/IV) oxides as evidenced by the lowest concentrations of soluble and DGT-labile Mn(II). This study sheds light on the full-year variation and mechanisms of Cr mobilization in eutrophic lake sediments and suggests the urgent need for remediation of Cr pollution especially for winter.

Seasonal changes of lead mobility in sediments in algae- and macrophyte-dominated zones of the lake.

This study examined lead (Pb) pollution in algae- and macrophytes-dominated sediments, using diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques. Lead pollution varied by season in the two different ecotype sediments. In the algae-dominated zone, the highest concentrations of DGT-labile Pb and dissolved Pb occurred in April and July, respectively. The reductive dissolution of Fe/Mn oxides was identified as an important driver for Pb releases in April and July. This was supported by the decrease of the reducible fraction of Pb in sediments during those sampling periods. Furthermore, dissolved organic matter (DOM) complexation with Pb in sediments also significantly increased the dissolved Pb concentrations in July. The Pb-DOM complexes accounted for 95% of the total chemical species of Pb in pore water, calculated by Visual MINTEQ 3.1 model. Low concentrations of labile and dissolved Pb were observed in October and January; these resulted from the formation of Pb-sulfide precipitates and adsorption by Fe/Mn oxides. It was supported by the high rate of Pb(HS)2 precipitation (saturation index > 0), at 36%, in October samples and the high reducible fraction of Pb in sediments in January samples. In the macrophytes-dominated region, there was a decrease of labile and dissolved Pb concentrations in April and July. It is likely because of the uptake of Pb by submerged macrophyte roots and the Fe/Mn plaques in the root surface. High concentrations of labile and dissolved Pb were observed in October and January, likely resulting from the DOM complexation with Pb in sediments. This was supported by the fact that the Pb-DOM complexes accounted for 90% and 87% of the total chemical species of Pb in October and January, respectively.

Phosphorus mobilization in lake sediments: Experimental evidence of strong control by iron and negligible influences of manganese redox reactions.

Iron (Fe) and manganese (Mn) reactions have been regarded as the primary factors responsible for the mobilization of phosphorus (P) in lake sediments, although their individual roles are hard to distinguish. In this study, in situ mobilization of P, Fe and Mn in sediments was assessed by high resolution spatio-temporal sampling of their labile forms using diffusive gradient in thin films (DGT) and suction device (Rhizon) techniques. It was found that the monthly concentration distributions showed greater agreement and better correlation coefficients between labile P and labile Fe, than those between labile P and labile Mn, implying that Fe plays a key role in controlling P release in sediments. Furthermore, better correlations were observed between hourly changes in concentrations of soluble reactive phosphorus (SRP) and soluble Fe(II), than those between SRP and soluble Mn. Changes were observed under simulated anaerobic incubation conditions, suggesting that P release was caused by the reductive dissolution of Fe oxides. This was supported by the lack of influences on P release from reductive dissolution of Mn oxides in the sediment-water interface and top sediment layers under the anaerobic incubations. In simulated algal bloom experiments, positive correlations and consistent changes were observed between SRP and soluble Fe(II) concentrations, but not between SRP and soluble Mn concentrations. This further demonstrated the Fe-dependent and Mn-independent release of P in sediments. Therefore, Fe redox reactions have a high impact on P mobilization in sediments, while Mn redox reactions appear to have negligible influences.

Different Influences of Bacterial Communities on Fe (III) Reduction and Phosphorus Availability in Sediments of the Cyanobacteria- and Macrophyte-Dominated Zones.

Little is known about the effects of bacterial community on iron (Fe) and phosphorus (P) cycles in sediments under different primary producer habitats in different seasons. Lake Taihu has both the cyanobacteria- and macrophyte-dominated lake zones. In this work, the abundance and structure of bacterial community was investigated using qPCR and 16S rRNA gene high throughput sequencing, respectively. Compared with the sediments in the cyanobacteria-dominated lake zone, sediments in the macrophyte-dominated lake zone had higher TP, TOC and TN contents but lower DO and Eh values. Dissolved reactive P, dissolved Fe, and their molar ratios (Fe/P) were lower in the sediments of the cyanobacteria-dominated lake zone than those in the macrophyte-dominated lake zone. Consistent with this was the significantly lower abundance of total and typical Fe redox transforming bacteria in the sediment of the cyanobacteria-dominated lake zone than those in the macrophyte-dominated lake zone. Correlation analyses also revealed positive influence of abundances of total bacteria and typical Fe reducing bacteria on dissolved Fe and Fe/P ratio. The results showed that, in the cyanobacteria-dominated open water zone, Acidimicrobiaceae was capable of Fe metabolism, contributing to higher P flux in summer. In the cyanobacteria-dominated bay, Sva0081 sediment group and Desulfobulbaceae could transform sulfate to sulfide, which resulted in the reduction of Fe (III), while in the macrophyte-dominated zones, Clostridium sensu stricto 1 could couple oxidation of organic carbon with the reduction of Fe (III). The present study adds new knowledge linking the bacterial communities with the physicochemical cycles of Fe and P in sediments under different primary producer habitats.

Mechanisms driving phosphorus release during algal blooms based on hourly changes in iron and phosphorus concentrations in sediments.

Algal growth causes a drastic change in aquatic conditions over a diel cycle, which may induce sensitive feedback systems in sediments, causing P release. In this study, a microcosm experiment was performed using a suction sampler (Rhizon) to observe changes in soluble reactive phosphorus (SRP) and soluble Fe(II) concentrations in the top 20 mm sediment layer on a 3-h time interval, at different phases of harmful algal bloom (HAB) development. The results showed that the algal blooms prevailed up to 15 days after incubation, after which the process of bloom collapse proceeded until the 70th day. The concentrations of pore-water soluble Fe(II) and SRP increased throughout the incubation period. Compared to day 1, maximum increases of 214% in soluble Fe(II) and 387% in SRP were observed at night during the bloom and collapse periods, respectively. The diffusive fluxes of Fe and P at the sediment-water interface (SWI) generally corresponded to their changes in concentrations. Hourly fluctuation in soluble Fe(II) and SRP concentrations were observed with two distinct concentration peaks occurred at 21:00 p.m. and 06:00 a.m. (or 03:00 a.m.), respectively. These findings suggest that Fe-P coupling mechanisms are responsible for the release of P from sediments. During the collapse period, soluble Fe(II) concentrations were suppressed by the increase of labile S(-II) at night. Meanwhile, SRP concentrations were decoupled from Fe cycling with small fluctuations (<11% RSD) on an hourly timescale, and the decomposition of algae was a dominant source contributing to the release of P from sediments. These results significantly improved the understanding of processes and mechanisms behind the stimulated release of P from sediments during HABs.

Internal phosphorus loading from sediments causes seasonal nitrogen limitation for harmful algal blooms.

It is proposed that the internal loading of phosphorus (P) from sediments plays an important role in seasonal nitrogen (N) limitation for harmful algal blooms (HABs), although there is a lack of experimental evidence. In this study, an eutrophic bay from the large and shallow Lake Taihu was studied for investigating the contribution of internal P to N limitation over one-year field sampling (February 2016 to January 2017). A prebloom-bloom period was identified from February to August according to the increase in Chla concentration in the water column, during which the ratio of total N to total P (TN/TP) exponentially decreased with month from 43.4 to 7.4. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) analysis showed large variations in the vertical distribution of mobile P (SRP and DGT-labile P) in sediments, resulting in the SRP diffusion flux at the sediment-water interface ranging from -0.01 to 6.76mg/m2/d (minus sign denotes downward flux). Significant and linear correlations existed between SRP and soluble Fe(II) concentrations in pore water, reflecting that the spatial-temporal variation in mobile P was controlled by microbe-mediated Fe redox cycling. Mass estimation showed that the cumulative flux of SRP from sediments accounted for 54% of the increase in TP observed in the water column during the prebloom-bloom period. These findings are supported by the significantly negative correlation (p<0.01) observed between sediment SRP flux and water column TN/TP during the same period. Overall, these results provide solid evidence for the major role of internal P loading in causing N limitation during the prebloom-bloom period.

Submillimeter-scale heterogeneity of labile phosphorus in sediments characterized by diffusive gradients in thin films and spatial analysis.

Sediments have a heterogeneous distribution of labile redox-sensitive elements due to a drastic downward transition from oxic to anoxic condition as a result of organic matter degradation. Characterization of the heterogeneous nature of sediments is vital for understanding of small-scale biogeochemical processes. However, there are limited reports on the related specialized methodology. In this study, the monthly distributions of labile phosphorus (P), a redox-sensitive limiting nutrient, were measured in the eutrophic Lake Taihu by Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) on a two-dimensional (2D) submillimeter level. Geographical information system (GIS) techniques were used to visualize the labile P distribution at such a micro-scale, showing that the DGT-labile P was low in winter and high in summer. Spatial analysis methods, including semivariogram and Moran's I, were used to quantify the spatial variation of DGT-labile P. The distribution of DGT-labile P had clear submillimeter-scale spatial patterns with significant spatial autocorrelation during the whole year and displayed seasonal changes. High values of labile P with strong spatial variation were observed in summer, while low values of labile P with relatively uniform spatial patterns were detected in winter, demonstrating the strong influences of temperature on the mobility and spatial distribution of P in sediment profiles.

Mobile phosphorus stratification in sediments by aluminum immobilization.

There is a great heterogeneity in the distribution of mobile phosphorus (P) in natural sediments, while the assessment of P immobilization by amendment agents has mostly neglected this feature. In this study, the effects of aluminum sulfate (ALS) on P immobilization were investigated via a 60-day microcosm experiment using sampled sediment cores. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were first employed to establish the profiles of soluble reactive P (SRP) and DGT-labile P in aluminum-amended sediments at 2 and 1 mm resolutions, respectively. Both concentrations of two mobile P forms decreased with increasing ALS dosages. The static layers with extremely low P concentrations (≤0.060 mg L-1 for SRP and ≤0.024 mg L-1 for DGT-labile P) were observed in the upper 6-16 mm sediment layers at 6-15 ALS/Pmobile treatments (mobile P is the sum of NH4Cl-P, BD-P, and NaOH-nrP; mol mol-1). The static layer was followed by an active layer in which the upward release potential (RAL) decreased from 33.4 to 21.9 for SRP and from 92.2 to 51.0 for DGT-labile P, respectively. As the formation of the static layer is a key to sustaining P immobilization in sediments, the minimal dosage of 9 ALS/Pmobile is required for ALS capping. Modeling with DGT Induced Fluxes in Sediments (DIFS) showed a greater increase in adsorption rate constant (k1, maximum 7.2-fold) compared to adsorption rate constant (k1, maximum 2.2-fold), demonstrating that the release of P from sediment solids became increasingly difficult after ALS amendment.

Differences in the Composition of Archaeal Communities in Sediments from Contrasting Zones of Lake Taihu.

In shallow lakes, different primary producers might impact the physiochemical characteristics of the sediment and the associated microbial communities. Until now, little was known about the features of sediment Archaea and their variation across different primary producer-dominated ecosystems. Lake Taihu provides a suitable study area with cyanobacteria- and macrophyte-dominated zones co-occurring in one ecosystem. The composition of the sediment archaeal community was assessed using 16S rRNA gene amplicon sequencing technology, based on which the potential variation with respect to the physiochemical characteristics of the sediment was analyzed. Euryarchaeota (30.19% of total archaeal sequences) and Bathyarchaeota (28.00%) were the two most abundant phyla, followed by Crenarchaeota (11.37%), Aigarchaeota (10.24%) and Thaumarchaeota (5.98%). The differences found in the composition of the archaeal communities between the two zones was significant (p = 0.005). Sediment from macrophyte-dominated zones had high TOC and TN content and an abundance of archaeal lineages potentially involved in the degradation of complex organic compounds, such as the order Thermoplasmatales. In the area dominated by Cyanobacteria, archaeal lineages related to sulfur metabolism, for example, Sulfolobales and Desulfurococcales, were significantly enriched. Among Bathyarchaeota, subgroups MCG-6 and MCG-15 were significantly accumulated in the sediment of areas dominated by macrophytes whereas MCG-4 was consistently dominant in both type of sediments. The present study contributes to the knowledge of sediment archaeal communities with different primary producers and their possible biogeochemical functions in sediment habitats.

Carbon isotope fractionation reveals distinct process of CH4 emission from different compartments of paddy ecosystem.

Carbon isotopic fractionations in the processes of CH4 emission from paddy field remain poorly understood. The δ(13)C-values of CH4 in association with production, oxidation and transport of CH4 in different pools of a paddy field were determined, and the stable carbon isotope fractionations were calibrated to assess relative contribution of acetate to CH4 production (fac) and fraction of CH4 oxidized (fox) by different pathways. The apparent isotope fractionation for CO2 conversion to CH4 (αapp) was 1.041-1.056 in the soil and 1.046-1.080 on the roots, indicating that fac was 10-60% and 0-50%, respectively. Isotope fractionation associated with CH4 oxidation (αox) was 1.021 ± 0.007 in the soil and 1.013 ± 0.005 on the roots, and the transport fractionation (εtransport) by rice plants was estimated to be -16.7‰ ~ -11.1‰. Rhizospheric fox was about 30-100%, and it was more important at the beginning but decreased fast towards the end of season. Large value of fox was also observed at the soil-water interface and soil and roots surfaces, respectively. The results demonstrate that carbon isotopic fractionations which might be different in different conditions were sensitive to the estimations of fac and fox in paddy field.

The Vertical Distribution of Sediment Archaeal Community in the "Black Bloom" Disturbing Zhushan Bay of Lake Taihu.

Using the Illumina sequencing technology, we investigated the vertical distribution of archaeal community in the sediment of Zhushan Bay of Lake Taihu, where the black bloom frequently occurred in summer. Overall, the Miscellaneous Crenarchaeotal Group (MCG), Deep Sea Hydrothermal Vent Group 6 (DHVEG-6), and Methanobacterium dominated the archaeal community. However, we observed significant difference in composition of archaeal community among different depths of the sediment. DHVEG-6 dominated in the surface layer (0-3 cm) sediment. Methanobacterium was the dominating archaeal taxa in the L2 (3-6 cm) and L3 (6-10) sediment. MCG was most abundant in the L4 (10-15 cm) and L5 (15-20 cm) sediment. Besides, DHVEG-6 was significantly affected by the concentration of total phosphorus (TP). And loss on ignition (LOI) was an important environmental factor for Methanobacterium. As the typical archaeal taxa in the surface layer sediment, DHVEG-6 and Methanobacterium might be more adapted to abundant substrate supply from cyanobacterial blooms and take active part in the biomass transformation. We propose that DHVEG-6 and Methanobacterium could be the key archaeal taxa correlated with the "black bloom" formation in Zhushan Bay.

Effects of fertilization on microbial abundance and emissions of greenhouse gases (CH4 and N2O) in rice paddy fields.

This study is to explore effects of nitrogen application and straw incorporation on abundance of relevant microbes and CH 4 and N2O fluxes in a midseason aerated rice paddy field. Fluxes of CH 4 and N2O were recorded, and abundance of relevant soil microbial functional genes was determined during rice-growing season in a 6-year-long fertilization experiment field in China. Results indicate that application of urea significantly changed the functional microbial composition, while the influence of straw incorporation was not significant. Application of urea significantly decreased the gene abundances of archaeal amoA and mcrA, but it significantly increased the gene abundances of bacterial amoA. CH 4 emission was significantly increased by fresh straw incorporation. Incorporation of burnt straw tended to increase CH 4 emission, while the urea application had no obvious effect on CH 4 emission. N2O emission was significantly increased by urea application, while fresh or burnt straw incorporation tended to decrease N2O emission. The functional microbial composition did not change significantly over time, although the abundances of pmoA, archaeal amoA, nirS, and nosZ genes changed significantly. The change of CH 4 emission showed an inverse trend with the one of the N2O emissions over time. To some extent, the abundance of some functional genes in this study can explain CH 4 and N2O emissions. However, the correlation between CH 4 and N2O emissions and the abundance of related functional genes was not significant. Environmental factors, such as soil Eh, may be more related to CH 4 and N2O emissions.