Extracellular Polymeric Substances from <i>Bacillus subtilis</i> Associated with Minerals Modify the Extent and Rate of Heavy Metal Sorption

Extracellular polymeric substances (EPS) are an important source of organic matter in soil. Once released by microorganisms, a portion may be sorbed to mineral surfaces, thereby altering the mineral̀s ability to immobilize heavy metals. EPS from <i>Bacillus subtilis</i> were reacted with Ca-saturated bentonite and ferrihydrite in 0.01 M KCl at pH 5.0 to follow the preferential uptake of EPS-C, -N, and -P. The sorption kinetics of Pb²⁺, Cu²⁺, and Zn²⁺ to the resulting EPS-mineral composites was studied in single and binary metal batch experiments ([metal]_total = 50 μM, pH 5.0). Bentonite sorbed much more EPS-C (18.5 mg g^–1) than ferrihydrite (7.9 mg g^–1). During sorption, EPS were chemically and size fractionated with bentonite favoring the uptake of low-molecular weight components and EPS-N, and ferrihydrite selectively retaining high-molecular weight and P-rich components. Surface area and pore size measurements by N₂ gas adsorption at 77 K indicated that EPS altered the structure of mineral-EPS associations by inducing partial disaggregation of bentonite and aggregation of ferrihydrite. Whereas mineral-bound EPS increased the extent and rate of Pb²⁺, Cu²⁺, and Zn²⁺ sorption for bentonite, either no effect or a decrease in metal uptake was observed for ferrihydrite. The extent of sorption always followed the order Pb²⁺ > Cu²⁺ > Zn²⁺, which also prevailed in binary Pb²⁺/Cu²⁺ systems. In consequence, sorption of EPS to different minerals may have contrasting consequences for the immobilization of heavy metals in natural environments by inducing mineral-specific alterations of the pore size distribution and, thus, of available sorption sites

Retention of Sterically and Electrosterically Stabilized Silver Nanoparticles in Soils

The current study investigated the interaction of sterically stabilized OECD standard Ag ENP (AgNM-300k) and silver ions (Ag⁺) in 25 German arable soils with varying properties (organic carbon concentration of 0.4–25 mg g^–1 and clay content of <0.1–392 mg g^–1) in 24 h batch retention experiments. A soil subset (<i>n</i> = 8) was investigated to test the soil interactions with citrate-stabilized Ag ENP (AgCN30). The adsorption of Ag⁺ was consistent with the Freundlich model with high <i>K</i>_F values (mean <i>K</i>_F = 2553 L kg^–1, <i>n</i> = 25), which suggested a high retention of Ag⁺. The retention of AgNM-300k followed a linear partitioning model and generally exhibited a low retention for the majority of the investigated soils (group 1, mean <i>K</i>_r, linear = 3.7 L kg^–1, <i>n</i> = 19), and was correlated with the clay content (relation to log₁₀(<i>K</i>_r, linear), <i>r</i>² = 0.40, <i>n</i> = 19). Soils showing a high retention of AgNM-300k (group 2, mean <i>K</i>_r, linear = 1048 L kg^–1, <i>n</i> = 6) either had a low (<5.1) or high pH (>7.0) and generally contained >200 mg g^–1 clay. For the sample subset tested, AgCN30 and AgNM-300k were retained in similar dimensions regarding the same soils. The results suggest that the highest risk of long-term ENP mobilization exists when Ag ENP are applied to agricultural soils with low clay contents (<130 mg g^–1) and slightly acidic conditions

Differences in microbial community composition in different horizons in arctic soils.

<p>Principal component analysis (PCA) with relative abundances of all PFLA biomarkers. Colors indicate different horizon categories: organic topsoil (O) is dark grey, mineral topsoil (A) is light grey, mineral subsoil (B) is white, and cryoturbated material (J) is black. Symbols indicate sites: circles Cherskiy, diamonds Logata, and triangles Tazovsky. Symbols are the mean values of the coordinates for the individual categories, derived from the PCA with individual samples (n = 101). Error bars are SE. Colors of PLFA markers indicate general markers (grey), gram-positive markers (red), gram-negative markers (orange), bacterial markers (blue) and fungal markers (green).</p

Climate and Vegetation.

<p>Climate data are derived from WorldClim database including mean annual temperature (MAT), maximum temperature of the warmest month (Tmax), minimum temperature of the coldest month (Tmin) mean annual range in temperature (MART) and mean annual precipitation (MAP).</p

Extracellular enzyme activities and enzyme ratios in different horizons in arctic soils.

<p>Left panel: Extracellular enzyme activities for the C-acquiring enzyme cellobiohydrolase (CBH), the N-acquiring enzyme leucine-amino-peptidase (LAP) and the oxidative enzyme phenoloxidase (POX). Right panel: Ratios of the three enzyme activities to each other. Given are the means and standard errors for the individual horizon categories: organic topsoil (O), mineral topsoil (A), mineral subsoil (B), and cryoturbated material (J). Colors indicate different horizon categories: organic topsoil is dark grey, mineral topsoil is light grey, mineral subsoil is white, and cryoturbated material is black. Small letters indicate different statistical groups derived from ANOVA and Tukey-HSD tests.</p

Direct and indirect drivers of extracellular enzyme activities.

<p>Structural equation models for extracellular enzyme activities, cellobiohydrolase (CBH; a, d), leucine-amino-peptidase (LAP; b, e) and phenoloxidase (POX; c,f). Graphs on the left show regular soil (organic topsoil, mineral topsoil, mineral subsoil), right panel shows cryoturbated material. Black boxes and arrows indicate significant factors and paths. Boxes and arrows in grey were removed from the model because either the paths (arrows) were not significant, or the factors (boxes) had no direct or indirect effect on the enzyme activity. The boxes with C, N and Clay are the contents of organic carbon, nitrogen, and clay. Microbial biomass (MicBM) was calculated as total amount of PLFAs. PC1, PC2 and PC3 are the first three axes of PCAs with relative abundances of all PLFAs. PCAs for regular soil and cryoturbated material have been done individually. Arrow width indicates the strength of the effect and reflects the scaled estimates, which are also given as the numbers beside the respective arrows. The numbers below the boxes with the respective enzymes show R² and indicate how much of the variance in enzyme activity is explained by the model.</p

Properties of the microbial community.

<p>Total amount of PLFAs, fungi∶bacteria ratios and statistical results for the first three principal components derived from a PCA with relative abundances of all PLFA biomarkers. Values are mean values (± standard error) over all sites and for each horizon per site. Letters in parentheses indicate significantly different (P<0.05) groups between horizons derived from ANOVA and Tukey-HSD tests.</p

Map showing the three sampling sites in the Siberian Arctic.

<p>Tazovsky (TZ; 67°16′N, 78°50′E), Logata (LG; 73°25′N, 98°16′E) and Cherskiy (CH; 68°45′N, 161°20′E). The horizontal line is the polar circle.</p