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

Main characteristics of the experimental trees.

*<p>labeled tree;</p>†<p>control tree.</p

Time course of cumulative excess of ¹³C in soil CO₂ efflux under C<i>roton macrostachyus</i> and <i>Podocarpus falcatus</i> during the one year chasing period.

<p>Data are means ± standard deviation (n = 5). The curves are fitted with a double exponential function for <i>C. macrostachyus</i> and a single exponential function for <i>P. falcatus</i>. Parameters are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045540#pone-0045540-t002" target="_blank">Table 2</a>.</p

Time course of excess of ¹³C in total foliage biomass of <i>Croton macrostachyus</i> and <i>Podocarpus falcatus</i> during the one year chasing period.

<p>Data are means ± standard deviation (n = 5). Parameters are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045540#pone-0045540-t002" target="_blank">Table 2</a>.</p

Time course of the ¹³C label in plant-soil compartments of <i>Croton macrostachyus</i> and <i>Podocarpus falcatus</i> during the one year chasing period.

<p>Shown are <b>(a)</b> leaves of <i>C. macrostachyus</i>, <b>(b)</b> leaves of <i>P. falcatus</i>, <b>(c)</b> weighed sum of soluble mono and disaccharides in tree phloem sap at 1.3 m and 0.5 m above ground of <i>C. macrostachyus</i>, <b>(d)</b> weighed sum of soluble mono and disaccharides in tree phloem sap at 1.3 m and 0.5 m above ground of <i>P. falcatus</i>, <b>(e)</b> 16∶1ω5 NLFA in adhering and bulk soil under <i>C. macrostachyus</i>, <b>(f)</b> 16∶1ω5 NLFA in adhering and bulk soil under <i>P. falcatus</i>, <b>(g)</b> soil CO₂ efflux under <i>C. macrostachyus</i>, <b>(h)</b> soil CO₂ efflux under <i>P. falcatus</i> For the sake of clarity of the figure, we omitted to show the δ¹³C values for the control of soluble sugars in tree phloem sap and the 16∶1ω5 NLFA in adhering and bulk soil. The former was −25.53±0.85‰ for <i>C. macrostachyus</i> and −25.36±0.33 ‰ for <i>P. falcatus</i>, and the latter was −27.7±1.4‰ for <i>C. macrostachyus</i> and −29.3±1.1‰ for <i>P. falcatus.</i> For leaves and soil CO₂ efflux data are means ± standard deviation (n = 5). No replicates were taken for phloem sap extraction and soil cores for analysis of 16∶1ω5 NLFA in adhering and bulk soil to keep impact of destructive sampling to the plant-soil system to a minimum.</p

Change in the apparent amount of CO₂ (mol) in chambers of <i>Croton macrostachyus</i> and <i>Podocarpus falcatus</i> during the ¹³CO₂-labeling period.

<p>The decline in the amount of CO₂ reflects the predomination of photosynthesis over leaf and stem respiration. The arrow shows the release of 12.3 mmol ¹³CO₂ m⁻³ chamber volume.</p

Results of the fit of exponential functions on the excess of ¹³C in leaves and the cumulative excess of ¹³C in soil CO₂ efflux related to time after labeling.

<p>Shown are the amount of labeled carbon that was recovered in a given compartment as parameters <i>a</i> and <i>c</i>, being expressed as the relative size of a fast and a slow pool, the mean residence time (MRT) and the half life of both pools, and the coefficient of determination (<i>R²</i>). Please note that the size of the pools refer to the percentage of the overall assimilated ¹³C.</p>†<p>No separation between fast and slow pool could be made.</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

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