Subsurface removal of arsenic by injection with oxygenated groundwater has been proposed as a viable technology forobtaining ‘safe’ drinking water in Bangladesh. While the oxidation of ferrous iron to solid ferric iron minerals, to whicharsenic adsorbs, is assumed to be driven by abiotic reactions, metal-cycling microorganisms may potentially affect arsenicremoval. A cultivation-independent survey covering 24 drinking water wells in several geographical regions in Bangladeshwas conducted to obtain information on microbial community structure and diversity in general, and on specific functionalgroups capable of the oxidation or reduction of arsenic or iron. Each functional group, targeted by either group-specific 16SrRNA or functional gene amplification, occurred in at least 79% of investigated samples. Putative arsenate reducers andiron-oxidizingGallionellaceaewere present at low diversity, while more variation in potentially arsenite-oxidizingmicroorganisms and iron-reducingDesulfuromonadaleswas revealed within and between samples. Relations betweencommunity composition on the one hand and hydrochemistry on the other hand were in general not evident, apart from animpact of salinity on iron-cycling microorganisms. Our data suggest widespread potential for a positive contribution ofarsenite and iron oxidizers to arsenic removal upon injection with oxygenated water, but also indicate a potential risk forarsenic re-mobilization by anaerobic arsenate and iron reducers once injection is halted.@en

Shallow geothermal systems are increasingly being used to store or harvest thermal energy for heating or cooling purposes. This technology causes temperature perturbations exceeding the natural variations in aquifers, which may impact groundwater quality. Here, we report the results of laboratory experiments on the effect of temperature variations (5–80 °C) on redox processes and associated microbial communities in anoxic unconsolidated subsurface sediments. Both hydrochemical and microbiological data showed that a temperature increase from 11 °C (in situ) to 25 °C caused a shift from iron-reducing to sulfate-reducing and methanogenic conditions. Bioenergetic calculations could explain this shift. A further temperature increase (>45 °C) resulted in the emergence of a thermophilic microbial community specialized in fermentation and sulfate reduction. Two distinct maxima in sulfate reduction rates, of similar orders of magnitude (5 × 10–10 M s–1), were observed at 40 and 70 °C. Thermophilic sulfate reduction, however, had a higher activation energy (100–160 kJ mol–1) than mesophilic sulfate reduction (30–60 kJ mol–1), which might be due to a trade-off between enzyme stability and activity with thermostable enzymes being less efficient catalysts that require higher activation energies. These results reveal that while sulfate-reducing functionality can withstand a substantial temperature rise, other key biochemical processes appear more temperature sensitive.@en

Eukaryotes may influence pollutant degradation processes in groundwater ecosystems by activities such as predation on bacteria and recycling of nutrients. Culture-independent community profiling and phylogenetic analysis of 18S rRNA gene fragments, as well as culturing, were employed to obtain insight into the sediment-associated eukaryotic community composition in an anaerobic sandy aquifer polluted with landfill leachate (Banisveld, The Netherlands). The microeukaryotic community at a depth of 1 to 5 m below the surface along a transect downgradient (21 to 68 m) from the landfill and at a clean reference location was diverse. Fungal sequences dominated most clone libraries. The fungal diversity was high, and most sequences were sequences of yeasts of the Basidiomycota. Sequences of green algae (Chlorophyta) were detected in parts of the aquifer close (<30 m) to the landfill. The bacterium-predating nanoflagellate Heteromita globosa (Cercozoa) was retrieved in enrichments, and its sequences dominated the clone library derived from the polluted aquifer at a depth of 5 m at a location 21 m downgradient from the landfill. The number of culturable eukaryotes ranged from 102 to 103 cells/g sediment. Culture-independent quantification revealed slightly higher numbers. Groundwater mesofauna was not detected. We concluded that the food chain in this polluted aquifer is short and consists of prokaryotes and fungi as decomposers of organic matter and protists as primary consumers of the prokaryotes.@en

Heterogeneity in eukaryotic and bacteria community structure in surface and subsurface sediment samples downgradient of the Banisveld landfill (The Netherlands) was studied using a culturing-independent molecular approach. Along a transect covering the part of the aquifer most polluted by landfill leachate, sediment was sampled at 1-m depth intervals, until a depth of 5.5 m, at four distances from the landfill. Two drillings were placed in a nearby clean area as a reference. Denaturing gradient gel electrophoresis banding patterns revealed high bacterial and eukaryotic diversity and complex community structures. Bacteria and eukaryotic community profiles in polluted samples grouped different from those in clean samples. Bacteria community profiles in surface samples clustered together and separately from subsurface community profiles. Subsurface bacteria profiles clustered in a location-specific manner. Eukaryotic community structure did not significantly relate to distance from the landfill or depth. No significant spatial autocorrelation of bacteria or eukaryotic communities was observed over 1-m depth intervals per sampling location. Spatial heterogeneity in sediment-associated bacterial communities appears to be much larger than in groundwater. We discuss how on the one hand, spatial heterogeneity may complicate the assessment of microbial community structure and functioning, while on the other it may provide better opportunities for natural attenuation.@en

Using culture-independent 16S rRNA gene-based methods, we previously observed that Geobacteraceae were a major component of the microbial communities in the iron-reducing aquifer polluted by the Banisveld landfill, The Netherlands. However, phylogenetic information does not tell about the functional potential of the detected Geobacteraceae, nor can phylogenetic information easily beused to establish the presence of other iron-reducers. Therefore, we enriched for iron-reducing consortia using a range of culturing media, with various electron donors and acceptors and varying incubation conditions (pH, temperature), and by applying dilution-to-extinction culturing. Enrichments and strains isolated from these enrichments were characterized by 16S rRNA gene-based methods.The number of culturable iron-reducers was less than 110 iron-reducing bacteria per gram of sediment. The Geobacterphylotypethat was previously found to constitute a major part of the microbial communities in a part of the aquifer where organic matter was attenuated at a relatively high rate, was not isolated. The isolation of another Geobacterstrain and Serratia, Clostridium, Rhodoferaxand Desulfitobacterium strains suggest the presence of a diverse iron-reducing community. Physiological capabilities of the isolatesare described and discussed in relation to the hydrogeochemistryand the high abundance ofGeobacteraceaein the aquifer pollutedby the Banisveld landfill.@en