Bacterial reduction of a hematite-rich natural coastal sand was studied in flow-through column reactors at flow rates which varied from 0.62 to 11 pore volumes d(-1). Sand columns were wet-packed with the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens CN32, and a PIPES-buffered, lactate-containing growth medium was pumped through the columns for over 20 days. Soluble Fe(II), acetate and lactate concentrations measured in the column effluents showed that steady-state conditions were established after a few days with every flow rate. The steady-state effluent Fe(II) concentration was directly controlled by the flow rate where [Fe(II)]ss decreased as the flow rate increased. Increased flow rate increased biologic activity based on the steady-state flux of soluble Fe(II) and total Fe(II) production (included Fe(II) extracted from sand at the conclusion of the experiment), decreased the fraction of lactate oxidized for energy that likely increased cell synthesis, and decreased the concentration of sorbed Fe(II) that, in turn, decreased the relative percentage of Fe(II) retained by the column materials. Increased biologic activity was likely promoted by greater reactant delivery (i.e., lactate, N, P) and greater advective removal of Fe(II). These results demonstrate that biologic Fe(II) reduction, cell growth, and abiotic Fe(II) sorption are all coupled to the hydrologic flow rate.

Microbe-mineral associations in regolith overlying granodiorite bedrock (4.6-4.9 m depth) 1 from the Luquillo Experimental Forest, Puerto Rico, were imaged with confocal scanning 2 laser microscopy at a novel scale of 400X magnification. After adding BacLight™ stain, 3 proportionally more surface area of minerals (quartz, biotite, and mixed opaque 4 kaolinite/goethite) emitted fluorescence from cell-impermeant propidium iodide than from cell-permeant SYTO 9, which suggested greater coverage of minerals by extracellular DNA 6 or DNA in non-intact cells than by intact cells. Microscopic observations of predominantly 7 non-intact cell material in deep saprolite were consistent with the abundance of rRNA 8 sequences related to heterotrophic bacteria in clone libraries prepared from community DNA. 9 A few sequences were affiliated with bacteria recognized to produce siderophores, oxidize 10 Fe(II), or fix N 2. Bacterial DNA in deep regolith from two boreholes 1.5 m apart yielded 11 libraries with high diversity and taxa specific for each borehole. 12

Quartz diorite bedrock underlying the Luquillo Mountains of eastern Puerto Rico undergoes weathering at one of the fastest documented rates for granitic rocks in the world. Although tropical temperatures and precipitation promote rapid weathering in this location, increased bacterial densities in the regolith immediately above the bedrock suggest that microorganisms contribute to mineral weathering as well. Deep saprolite and saprock samples were obtained at the bedrock interface in an upland location (Guaba Ridge) in the Rio Icacos watershed for examination by environmental scanning electron microscopy (ESEM). In ESEM images, mineral nanotubes were observed ro occur frequently in association with coccus-and rod-shaped structures resembling bacteria. These nanotubes (50-140-nm width and 150-2700-nm length) were identified as halloysite using transmission electron microscopy. Observations of multiple nanotubes on the surfaces of an individual cell are consistent with the cell's exterior functional groups interacting with Si in pore water to facilitate halloysite nudeation. We propose that one mechanism by which bacteria conrdbute to the rapid weathering of quartz diorite minerals in this regolith is by lowering the free energy for secondary mineral formation. The presence of bacterial surfaces may result in more rapid removal of Si from solution, thereby increasing the dissolution rates of primary minerals.Abbreviations: EDS, energy dispersive x-ray spectrometry; ESEM, environmental scanning electron microscopy; TEM, transmission electron microscopy; XRD, x-ray diffraction.

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We investigated individual toxicities of the nitrogen-based energetic materials (EMs) 2,4-dinitrotoluene (2,4-DNT); 2-amino-4,6-dinitrotoluene (2-ADNT); 4-amino-2,6-dinitrotoluene (4-ADNT); and nitroglycerin (NG) on microbial activity in Sassafras sandy loam (SSL) soil, which has physicochemical characteristics that support very high qualitative relative bioavailability for organic chemicals. Batches of SSL soil for basal respiration (BR) and substrate-induced respiration (SIR) assays were separately amended with individual EMs or acetone carrier control. Total microbial biomass carbon (biomass C) was determined from CO production increases after addition of 2500 mg/kg of glucose-water slurry to the soil. Exposure concentrations of each EM in soil were determined using US Environmental Protection Agency method 8330A. Basal respiration was the most sensitive endpoint for assessing the effects of nitroaromatic EMs on microbial activity in SSL, whereas SIR and biomass C were more sensitive endpoints for assessing the effects of NG in soil. The orders of toxicity (from greatest to least) were 4-ADNT > 2,4-DNT = 2-ADNT > NG for BR; but for SIR and biomass C, the order of toxicity was NG > 2,4-DNT > 2-ADNT = 4-ADNT. No inhibition of SIR was found up to and including the greatest concentration of each ADNT tested in SSL. These ecotoxicological data will be helpful in identifying concentrations of contaminant EMs in soil that present acceptable ecological risks for biologically mediated processes in soil. Environ Toxicol Chem 2017;36:2981-2990. Published 2017 Wiley Periodicals Inc. on behalf of SETAC.This article is a US government work and, as such, is in the public domain in the United States of America.