Christopher Boothman

Role of metal-reducing bacteria in arsenic release from Bengal delta sediments

The contamination of ground waters, abstracted for drinking and irrigation, by sediment-derived arsenic threatens the health of tens of millions of people worldwide, most notably in Bangladesh and West Bengal. Despite the calamitous effects on human health arising from the extensive use of arsenic-enriched ground waters in these regions, the mechanisms of arsenic release from sediments remain poorly characterized and are topics of intense international debate. We use a microscosm-based approach to investigate these mechanisms: techniques of microbiology and molecular ecology are used in combination with aqueous and solid phase speciation analysis of arsenic. Here we show that anaerobic metal-reducing bacteria can play a key role in the mobilization of arsenic in sediments collected from a contaminated aquifer in West Bengal. We also show that, for the sediments in this study, arsenic release took place after Fe(iii) reduction, rather than occurring simultaneously. Identification of the critical factors controlling the biogeochemical cycling of arsenic is one important contribution to fully informing the development of effective strategies to manage these and other similar arsenic-rich ground waters worldwide.

Microcosm depth profiles of arsenic release in a shallow aquifer, West Bengal

Abstract Arsenic mobilization and Fe(III) reduction in acetate-amended sediments collected from a range of depths from an aquifer with elevated groundwater arsenic concentrations in West Bengal were monitored over a 1 month period. Significant arsenic release was noted in sediment collected from 24 m and 45 m depth, with some Fe(III) reduction also observed in the 24 m sample. The structure of the microbial communities present in the sediments prior to incubation showed marked differences down the sediment column. Profiling of the microbial community in the 24 m and 45 m samples revealed a relatively complex make-up, with Acinetobacter species comprising the bulk of the 24 m sedimentary bacterial population, but no previously characterized As(V)-reducers were detected in either sample.

The role of indigenous microorganisms in the biodegradation of naturally occurring petroleum, the reduction of iron, and the mobilization of arsenite from west bengal aquifer sediments.

High levels of naturally occurring arsenic are found in the shallow reducing aquifers of West Bengal, Bangladesh, and other areas of Southeast Asia. These aquifers are used extensively for drinking water and irrigation by the local population. Mechanisms for its release are unclear, although increasing evidence points to a microbial control. The type of organic matter present is of vital importance because it has a direct impact on the rate of microbial activity and on the amount of arsenic released into the ground water. The discovery of naturally occurring hydrocarbons in an arsenic-rich aquifer from West Bengal provides a source of potential electron donors for this process. Using microcosm-based techniques, seven sediments from a site containing naturally occurring hydrocarbons in West Bengal were incubated with synthetic ground water for 28 d under anaerobic conditions without the addition of an external electron donor. Arsenic release and Fe(III) reduction appeared to be microbially mediated, with variable rates of arsenic mobilization in comparison to Fe(III) reduction, suggesting that multiple processes are involved. All sediments showed a preferential loss of petroleum-sourced n-alkanes over terrestrially sourced sedimentary hydrocarbons n-alkanes during the incubation, implying that the use of petroleum-sourced n-alkanes could support, directly or indirectly, microbial Fe(III) reduction. Samples undergoing maximal release of As(III) contained a significant population of Sulfurospirillum sp., a known As(V)-reducing bacterium, providing the first evidence that such organisms may mediate arsenic release from West Bengali aquifers.

Geomicrobiological Redox Cycling of the Transuranic Element Neptunium

Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O(2)(+) under oxic conditions. Here, we describe experiments to explore the biogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.

Potential role of the Fe(III)-reducing bacteria Geobacter and Geothrix in controlling arsenic solubility in Bengal delta sediments

Abstract Previous studies from our laboratory have suggested a role for indigenous metal-reducing bacteria in the reduction of sediment-bound As(V), and have also shown that a stable enrichment culture of Fe(III)-reducing bacteria was able to mobilize arsenic (as As(III)) from sediments collected from West Bengal (Islam et al, 2004). To identify the Fe(III)-reducing bacteria that may play a role in the reduction of As(V) and mobilization of As(III), we made a detailed molecular analysis of this enrichment culture. It was dominated by a close relative of Geothrix fermentans, but the type strain of this organism was unable to conserve energy for growth via the dissimilatory reduction of As(V), or reduce As(V) present in a defined medium containing fumarate as the electron acceptor. Furthermore, when the cells were grown using soluble Fe(III)-Citrate as an electron acceptor in the presence of As(V), bacterial Fe(III) reduction resulted in the precipitation of the Fe(II)-bearing mineral vivianite in 2 weeks. This was accompanied by the efficient removal of As from solution. These results demonstrate that Geothrix fermentans, in common with other key Fe(III)-reducing bacteria such as Geobacter sulfurreducens, does not reduce As(V) enzymatically, but can capture arsenic in Fe(II) minerals formed during respiration using Fe(III) as an electron acceptor. Thus, the reduction of arsenicbearing Fe(III) oxide minerals is not sufficient to mobilize arsenic, but may result in the formation of Fe(II) biominerals that could potentially act as sinks for arsenic in sediments. Additional mechanisms, including dissimilatory As(V) reduction by other specialist anaerobic bacteria, are implicated in the mobilization of arsenic from sediments.

Microbial Reduction of Fe(III) under Alkaline Conditions Relevant to Geological Disposal

ABSTRACT To determine whether biologically mediated Fe(III) reduction is possible under alkaline conditions in systems of relevance to geological disposal of radioactive wastes, a series of microcosm experiments was set up using hyperalkaline sediments (pH ∼11.8) surrounding a legacy lime working site in Buxton, United Kingdom. The microcosms were incubated for 28 days and held at pH 10. There was clear evidence for anoxic microbial activity, with consumption of lactate (added as an electron donor) concomitant with the reduction of Fe(III) as ferrihydrite (added as the electron acceptor). The products of microbial Fe(III) reduction were black and magnetic, and a range of analyses, including X-ray diffraction, transmission electron microscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism confirmed the extensive formation of biomagnetite in this system. The addition of soluble exogenous and endogenous electron shuttles such as the humic analogue anthraquinone-2,6-disulfonate and riboflavin increased both the initial rate and the final extent of Fe(III) reduction in comparison to the nonamended experiments. In addition, a soluble humic acid (Aldrich) also increased both the rate and the extent of Fe(III) reduction. These results show that microbial Fe(III) reduction can occur in conditions relevant to a geological disposal facility containing cement-based wasteforms that has evolved into a high pH environment over prolonged periods of time (>100,000 years). The potential impact of such processes on the biogeochemistry of a geological disposal facility is discussed, including possible coupling to the redox conditions and solubility of key radionuclides.

Probing the Biogeochemical Behavior of Technetium Using a Novel Nuclear Imaging Approach

Dynamic gamma-camera imaging of radiotracer technetium ((99m)Tc) was used to assess the impact of biostimulation of metal-reducing bacteria on technetium mobility at 10(-12) mol L(-1) concentrations in sediments. Addition of the electron donor acetate was used to stimulate a redox profile in sediment columns, from oxic to Fe(III)-reducing conditions. When (99m)Tc was pumped through the columns, real-time gamma-camera imaging combined with geochemical analyses showed technetium was localized in regions containing biogenic Fe(II). In parallel experiments, electron microscopy with energy-dispersive X-ray (EDX) mapping confirmed sediment-bound Tc was associated with iron, while X-ray absorption spectroscopy (XAS) confirmed reduction of Tc(VII) to poorly soluble Tc(IV). Molecular analyses of microbial communities in these experiments supported a direct link between biogenic Fe(II) accumulation and Tc(VII) reductive precipitation, with Fe(III)-reducing bacteria more abundant in technetium immobilization zones. This offers a novel approach to assessing radionuclide mobility at ultratrace concentrations in real-time biogeochemical experiments, and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilizing technetium.

Functional diversity of bacteria in a ferruginous hydrothermal sediment

A microbial community showing diverse respiratory processes was identified within an arsenic-rich, ferruginous shallow marine hydrothermal sediment (20–40 °C, pH 6.0–6.3) in Santorini, Greece. Analyses showed that ferric iron reduction with depth was broadly accompanied by manganese and arsenic reduction and FeS accumulation. Clone library analyses indicated the suboxic–anoxic transition zone sediment contained abundant Fe(III)- and sulfate-reducing Deltaproteobacteria, whereas the overlying surface sediment was dominated by clones related to the Fe(II)-oxidizing zetaproteobacterium, Mariprofundus ferroxydans. Cultures obtained from the transition zone were enriched in bacteria that reduced Fe(III), nitrate, sulfate and As(V) using acetate or lactate as electron donors. In the absence of added organic carbon, bacteria were enriched that oxidized Fe(II) anaerobically or microaerobically, sulfide microaerobically and aerobically and As(III) aerobically. According to 16S rRNA gene analyses, enriched bacteria represented a phylogenetically wide distribution. Most probable number counts indicated an abundance of nitrate-, As(V)- and Fe(III)(s,aq)-reducers, and dissolved sulfide-oxidizers over sulfate-reducers, and FeS-, As(III)- and nitrate-dependent Fe(II)-oxidisers in the transition zone. It is noteworthy that the combined community and geochemical data imply near-surface microbial iron and arsenic redox cycling were dominant biogeochemical processes.

The Synergistic Effects of High Nitrate Concentrations on Sediment Bioreduction

Groundwaters at nuclear sites can be characterized by low pH and high nitrate concentrations (10–100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction which can limit radionuclide migration. Here, sediment microcosms representative of the UK Sellafield site were used to study the influence of variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron accepting processes) progression. The rate of reduction through the terminal electron accepting cascade NO− 3 > NO− 2 > Mn(IV)/Fe(III) > SO2− 4 at low pH (∼5.5) was slower than that in bicarbonate buffered systems (pH ∼ 7.0), but in the low pH systems, denitrification and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) and sulfate-reduction. Under very high nitrate conditions (100 mM), bicarbonate buffering (pH ∼ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense dominated the microbial communities in this reduced sediment. In Fe(III)-reducing enrichment cultures from the 100 mM nitrate system, close relatives of the Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were observed. These results highlight that under certain conditions and contrary to expectations, denitrification may support bioreduction via pH conditioning for optimal metal reduction and radionuclide immobilization.

Seasonal Changes In Mineralogy, Geochemistry and Microbial Community of Bacteriogenic Iron Oxides (BIOS) Deposited in a Circumneutral Wetland

Changes in the mineralogy, microbial community structure and aquatic geochemistry of bacteriogenic iron oxides (BIOS) deposited at a circumneutral groundwater seep were recorded seasonally. X-ray diffraction and X-ray absorption spectroscopy demonstrated that 2-line ferrihydrite dominated the iron-bearing mineralogy of the BIOS (>60%). Minor proportions of the more crystalline lepidocrocite were also detected in BIOS collected throughout the year (<30%), but goethite (<15%) was only identified in sediments collected during the spring, summer and fall months. Subsurface porewater profiles generally showed an increase in dissolved equilibrium concentrations of Fe(II), indicative of microbial Fe(III) reduction, and the depth at which appreciable levels of dissolved Fe(II) (and to a lesser extent manganese) arose, and sulfate concentrations declined, become shallower as the seasons progressed from spring to fall. Clone libraries constructed from 16S rRNA gene sequence analysis of surficial BIOS showed the presence of sequences closely affiliated to known Fe(II)-oxidizing and Fe(III)-reducing prokaryotes, pointing towards a tightly coupled microbial iron cycle. Clones that showed closest identity to Fe(II)-oxidizing bacteria in culture, including Gallionella spp. and Sideroxydans spp., made up a significant portion of the clone library, but their relative proportion declined through the winter (73%), spring (26%) and summer (9%). The dominance of the putative microaerophilic Fe(II)-oxidizing chemolithoautotrophs in the winter clone library may be partly due to the lower temperature and dissolved organic carbon concentrations observed during this season, conditions that could have limited mixotrophic or organotrophic growth by mesophilic bacteria. Supplemental materials are available for this article. Go to the publishers online edition of Geomicrobiology Journal to view the free supplemental file.