Influence of Intensive and Super-Intensive Olive Grove Management on Soil Quality-Nutrients Content and Enzyme Activities.

Agricultural soil quality is an issue that has been widely debated in the literature in recent decades. Three olive grove areas (one in Lisbon and the others in Santarém, Portugal) with different management techniques (intensive and super-intensive) were selected. Nutrient concentrations and enzyme activities of soils were determined, as well as the C and N of litter and pruning waste (mulch) to estimate the influence of management techniques on the quality of olive grove soils and to assess the extent to which they are affected by organic covers and different cultivation intensities. Organic C and total N concentrations in soils of the intensive olive grove in Lisbon were the highest when compared with those in the intensive and super-intensive olive groves soils of Santarém. The concentrations of Ca, Mg, Na, and K were the main differences between the Lisbon olive groves and the other two from Santarém. Phosphatase, cellulase, and urease activities were related to the Na, extractable K, extractable P, Zn, Mn, organic C, and total N soil concentrations. Soil management and agricultural practices are determining factors for these enzymatic activities of Santarém olive groves, although climate conditions and soil properties play an important role in the soil enzymatic activities.

Silica colloid formation enhances performance of sediment microbial fuel cells in a low conductivity soil.

The performance of sediment microbial fuel cells (SMFCs) is usually limited by the structure, moisture, and salt content of the soil where they are allocated. Despite the influence of soil, so far most of efforts to improve SMFCs have been limited to the hardware design of the bioelectrochemical device. Our main objective was to enhance performance of SMFCs by stimulating the in situ formation of silica colloids in a low conductivity rice paddy soil. Our results have revealed that the presence of a silica colloid network, described by cryo-SEM analysis, reduced soil resistivity, enhanced ion mobility and consequently enhanced the power production by a factor of 10. Furthermore, our silica-supplemented soil showed better utilization of the electron donor, either acetate or natural rice root exudates, by electrogenic microbial populations. Sustainable manipulation of soil micromorphology using environmentally friendly reagents such as silica offers a novel approach for enhancing the performance of in situ microbial electrochemical applications in low conductivity soils, thus silica colloid geoengineering should be considered as part of future applications of SMFCs.

Subsurface clade of Geobacteraceae that predominates in a diversity of Fe(III)-reducing subsurface environments.

There are distinct differences in the physiology of Geobacter species available in pure culture. Therefore, to understand the ecology of Geobacter species in subsurface environments, it is important to know which species predominate. Clone libraries were assembled with 16S rRNA genes and transcripts amplified from three subsurface environments in which Geobacter species are known to be important members of the microbial community: (1) a uranium-contaminated aquifer located in Rifle, CO, USA undergoing in situ bioremediation; (2) an acetate-impacted aquifer that serves as an analog for the long-term acetate amendments proposed for in situ uranium bioremediation and (3) a petroleum-contaminated aquifer in which Geobacter species play a role in the oxidation of aromatic hydrocarbons coupled with the reduction of Fe(III). The majority of Geobacteraceae 16S rRNA sequences found in these environments clustered in a phylogenetically coherent subsurface clade, which also contains a number of Geobacter species isolated from subsurface environments. Concatamers constructed with 43 Geobacter genes amplified from these sites also clustered within this subsurface clade. 16S rRNA transcript and gene sequences in the sediments and groundwater at the Rifle site were highly similar, suggesting that sampling groundwater via monitoring wells can recover the most active Geobacter species. These results suggest that further study of Geobacter species in the subsurface clade is necessary to accurately model the behavior of Geobacter species during subsurface bioremediation of metal and organic contaminants.

Microbial incorporation of 13C-labeled acetate at the field scale: detection of microbes responsible for reduction of U(VI).

A field-scale acetate amendment experiment was performed in a contaminated aquifer at Old Rifle, CO to stimulate in situ microbial reduction of U(VI) in groundwater. To evaluate the microorganisms responsible for microbial uranium reduction during the experiment, 13C-labeled acetate was introduced into well bores via bio-traps containing porous activated carbon beads (Bio-Sep). Incorporation of the 13C from labeled acetate into cellular DNA and phospholipid fatty acid (PLFA) biomarkers was analyzed in parallel with geochemical parameters. An enrichment of active sigma-proteobacteria was demonstrated in downgradient monitoring wells: Geobacter dominated in wells closer to the acetate injection gallery, while various sulfate reducers were prominent in different downgradient wells. These results were consistent with the geochemical evidence of Fe(III), U(VI), and SO(4)2- reduction. PLFA profiling of bio-traps suspended in the monitoring wells also showed the incorporation of 13C into bacterial cellular lipids. Community composition of downgradient monitoring wells based on quinone and PLFA profiling was in general agreement with the 13C-DNA result. The direct application of 13C label to biosystems, coupled with DNA and PLFA analysis,

Microbiological and geochemical heterogeneity in an in situ uranium bioremediation field site.

The geochemistry and microbiology of a uranium-contaminated subsurface environment that had undergone two seasons of acetate addition to stimulate microbial U(VI) reduction was examined. There were distinct horizontal and vertical geochemical gradients that could be attributed in large part to the manner in which acetate was distributed in the aquifer, with more reduction of Fe(III) and sulfate occurring at greater depths and closer to the point of acetate injection. Clone libraries of 16S rRNA genes derived from sediments and groundwater indicated an enrichment of sulfate-reducing bacteria in the order Desulfobacterales in sediment and groundwater samples. These samples were collected nearest the injection gallery where microbially reducible Fe(III) oxides were highly depleted, groundwater sulfate concentrations were low, and increases in acid volatile sulfide were observed in the sediment. Further down-gradient, metal-reducing conditions were present as indicated by intermediate Fe(II)/Fe(total) ratios, lower acid volatile sulfide values, and increased abundance of 16S rRNA gene sequences belonging to the dissimilatory Fe(III)- and U(VI)-reducing family Geobacteraceae. Maximal Fe(III) and U(VI) reduction correlated with maximal recovery of Geobacteraceae 16S rRNA gene sequences in both groundwater and sediment; however, the sites at which these maxima occurred were spatially separated within the aquifer. The substantial microbial and geochemical heterogeneity at this site demonstrates that attempts should be made to deliver acetate in a more uniform manner and that closely spaced sampling intervals, horizontally and vertically, in both sediment and groundwater are necessary in order to obtain a more in-depth understanding of microbial processes and the relative contribution of attached and planktonic populations to in situ uranium bioremediation.

Resistance of solid-phase U(VI) to microbial reduction during in situ bioremediation of uranium-contaminated groundwater.

Speciation of solid-phase uranium in uranium-contaminated subsurface sediments undergoing uranium bioremediation demonstrated that although microbial reduction of soluble U(VI) readily immobilized uranium as U(IV), a substantial portion of the U(VI) in the aquifer was strongly associated with the sediments and was not microbially reducible. These results have important implications for in situ uranium bioremediation strategies.

Vanadium respiration by Geobacter metallireducens: novel strategy for in situ removal of vanadium from groundwater.

Vanadium can be an important contaminant in groundwaters impacted by mining activities. In order to determine if microorganisms of the Geobacteraceae, the predominant dissimilatory metal reducers in many subsurface environments, were capable of reducing vanadium(V), Geobacter metallireducens was inoculated into a medium in which acetate was the electron donor and vanadium(V) was the sole electron acceptor. Reduction of vanadium(V) resulted in the production of vanadium(IV), which subsequently precipitated. Reduction of vanadium(V) was associated with cell growth with a generation time of 15 h. No vanadium(V) was reduced and no precipitate was formed in heat-killed or abiotic controls. Acetate was the most effective of all the electron donors evaluated. When acetate was injected into the subsurface to enhance the growth and activity of Geobacteraceae in an aquifer contaminated with uranium and vanadium, vanadium was removed from the groundwater even more effectively than uranium. These studies demonstrate that G. metallireducens can grow via vanadium(V) respiration and that stimulating the activity of Geobacteraceae, and hence vanadium(V) reduction, can be an effective strategy for in situ immobilization of vanadium in contaminated subsurface environments.

Stimulating the in situ activity of Geobacter species to remove uranium from the groundwater of a uranium-contaminated aquifer.

The potential for removing uranium from contaminated groundwater by stimulating the in situ activity of dissimilatory metal-reducing microorganisms was evaluated in a uranium-contaminated aquifer located in Rifle, Colo. Acetate (1 to 3 mM) was injected into the subsurface over a 3-month period via an injection gallery composed of 20 injection wells, which was installed upgradient from a series of 15 monitoring wells. U(VI) concentrations decreased in as little as 9 days after acetate injection was initiated, and within 50 days uranium had declined below the prescribed treatment level of 0.18 micro M in some of the monitoring wells. Analysis of 16S ribosomal DNA (rDNA) sequences and phospholipid fatty acid profiles demonstrated that the initial loss of uranium from the groundwater was associated with an enrichment of Geobacter species in the treatment zone. Fe(II) in the groundwater also increased during this period, suggesting that U(VI) reduction was coincident with Fe(III) reduction. As the acetate injection continued over 50 days there was a loss of sulfate from the groundwater and an accumulation of sulfide and the composition of the microbial community changed. Organisms with 16S rDNA sequences most closely related to those of sulfate reducers became predominant, and Geobacter species became a minor component of the community. This apparent switch from Fe(III) reduction to sulfate reduction as the terminal electron accepting process for the oxidation of the injected acetate was associated with an increase in uranium concentration in the groundwater. These results demonstrate that in situ bioremediation of uranium-contaminated groundwater is feasible but suggest that the strategy should be optimized to better maintain long-term activity of Geobacter species.