Marina Héry

Arsenic release and attenuation in low organic carbon aquifer sediments from West Bengal

High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal-reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters.

Marinobacter santoriniensis sp. nov., an arsenate- respiring and arsenite-oxidizing bacterium isolated from hydrothermal sediment

A Gram-negative, arsenate-respiring and arsenite-oxidizing marine bacterium, NKSG1(T), was isolated from hydrothermal sediment at Santorini, Greece. Strain NKSG1(T) was a facultatively anaerobic, motile, non-spore-forming, rod-shaped bacterium. Growth occurred optimally at 35-40 degrees C, between pH 5.5 and 9.0 and with 0.5-16 % NaCl. Energy was conserved by the aerobic oxidation of a range of complex substrates, carbohydrates and organic acids, or anaerobically by arsenate reduction, nitrate reduction coupled to the oxidation of organic carbon or lactate fermentation. Oxidation of arsenite and anaerobic nitrate-dependent oxidation of Fe(II) were facilitated by the presence of an organic carbon source. The DNA G+C content was 58.1 mol%. The major respiratory quinone was Q-9. The significant fatty acids were 16 : 1omega9c, summed feature 3 (iso-15 : 0 2-OH/16 : 1omega7c), 16 : 0 and 18 : 1omega9c. Analysis of 16S rRNA gene sequences showed that strain NKSG1(T) fits within the phylogenetic cluster of the genus Marinobacter and is most closely related to Marinobacter koreensis DD-M3(T) (99.3 % similarity). The degree of relatedness with M. koreensis DSM 17924(T) based on DNA-DNA hybridization was 56 %. The results of a polyphasic study indicated that strain NKSG1(T) is a representative of a novel species within the genus Marinobacter, for which the name Marinobacter santoriniensis sp. nov. is proposed. The type strain is NKSG1(T) (=DSM 21262(T) =NCIMB 14441(T)=ATCC BAA-1649(T)). The capacity for arsenic reduction or oxidation has not been demonstrated previously for this genus.

Redox cycling of arsenic by the hydrothermal marine bacterium Marinobacter santoriniensis

Marinobacter santoriniensis NKSG1(T) is a mesophilic, dissimilatory arsenate-reducing and arsenite-oxidizing bacterium isolated from an arsenate-reducing enrichment culture. The inoculum was obtained from arsenic-rich shallow marine hydrothermal sediment from Santorini, Greece, with evidence of arsenic redox cycling. Growth studies demonstrated M. santoriniensis NKSG1(T) is capable of conserving energy from the reduction of arsenate [As(V)] with acetate or lactate as the electron donor, and of oxidizing arsenite [As(III)] heterotrophically with oxygen as the electron acceptor. The oxidation of As(III) coincided with the expression of the aoxB gene encoding for the catalytic molybdopterin subunit of the heterodimeric arsenite oxidase operon, indicating the reaction is enzymatically controlled, and M. santoriniensis NKSG1(T) is a heterotrophic As(III)-oxidizing bacterium. Although it is clear that this organism also performs dissimilatory As(V) reduction, no amplification of the arrA arsenate reductase gene was attained using a range of primers and PCR conditions. Marinobacter santoriniensis NKSG1(T) belongs to a genus of bacteria widely occurring in marine environments, including hydrothermal sediments, and is among the first marine bacteria shown to be capable of either anaerobic As(V) respiration or aerobic As(III) oxidation.

Microbial ecology of arsenic-mobilizing Cambodian sediments: lithological controls uncovered by stable-isotope probing.

Microbially mediated arsenic release from Holocene and Pleistocene Cambodian aquifer sediments was investigated using microcosm experiments and substrate amendments. In the Holocene sediment, the metabolically active bacteria, including arsenate-respiring bacteria, were determined by DNA stable-isotope probing. After incubation with (13) C-acetate and (13) C-lactate, active bacterial community in the Holocene sediment was dominated by different Geobacter spp.-related 16S rRNA sequences. Substrate addition also resulted in the enrichment of sequences related to the arsenate-respiring Sulfurospirillum spp. (13) C-acetate selected for ArrA related to Geobacter spp. whereas (13) C-lactate selected for ArrA which were not closely related to any cultivated organism. Incubation of the Pleistocene sediment with lactate favoured a 16S rRNA-phylotype related to the sulphate-reducing Desulfovibrio oxamicus DSM1925, whereas the ArrA sequences clustered with environmental sequences distinct from those identified in the Holocene sediment. Whereas limited As(III) release was observed in Pleistocene sediment after lactate addition, no arsenic mobilization occurred from Holocene sediments, probably because of the initial reduced state of As, as determined by X-ray Absorption Near Edge Structure. Our findings demonstrate that in the presence of reactive organic carbon, As(III) mobilization can occur in Pleistocene sediments, having implications for future strategies that aim to reduce arsenic contamination in drinking waters by using aquifers containing Pleistocene sediments.

Genome Sequence of Hydrothermal Arsenic-Respiring Bacterium Marinobacter santoriniensis NKSG1T

ABSTRACT Marinobacter santoriniensis NKSG1T originates from metalliferous marine sediment. It can respire and redox cycle arsenic species and perform mixotrophic, nitrate-dependent Fe(II) oxidation. The genome sequence, reported here, will help further elucidate the genetic mechanisms underlying these and other potential biogeochemically relevant functions, such as arsenic and mercury resistance and hydrocarbon degradation.

Selenium Mobilization by Pseudomonas aeruginosa (SNT-SG1) Isolated from Seleniferous Soils from India

Selenium (Se) is a metalloid required at trace concentrations for normal metabolic activities of the cell. The bioavailable forms viz., selenate and selenite have been found in localized high concentrations in seleniferous environments. Studies are in progress on bacterial strains that were isolated from one such location in the North-West region of Punjab, India. A facultative anaerobe, identified as Pseudomonas aeruginosa by 16S rRNA gene homology, was isolated from the rhizosphere of crop plants from this region and was examined for selenium mobilization potential in the presence of selenium oxyanions. The isolate was observed to reduce 53 and 21% of sodium selenite and selenate to elemental selenium, respectively, and volatilize 4.7 and 5.1% within 72-hour duration. This is one of the few selenium tolerant aerobic bacteria isolated and reported from tropical seleniferous soils from India, and the first to show volatilization potential. These organisms are being considered for bioaugmenting Se-impacted soils for enhanced Se mobilization and removal.

Microbial ecology of arsenic-mobilizing Cambodian sediments: lithological controls uncovered by stable-isotope probing: As mobilization in contrasting Cambodian sediments

Microbially mediated arsenic release from Holocene and Pleistocene Cambodian aquifer sediments was investigated using microcosm experiments and substrate amendments. In the Holocene sediment, the metabolically active bacteria, including arsenate-respiring bacteria, were determined by DNA stable-isotope probing. After incubation with (13) C-acetate and (13) C-lactate, active bacterial community in the Holocene sediment was dominated by different Geobacter spp.-related 16S rRNA sequences. Substrate addition also resulted in the enrichment of sequences related to the arsenate-respiring Sulfurospirillum spp. (13) C-acetate selected for ArrA related to Geobacter spp. whereas (13) C-lactate selected for ArrA which were not closely related to any cultivated organism. Incubation of the Pleistocene sediment with lactate favoured a 16S rRNA-phylotype related to the sulphate-reducing Desulfovibrio oxamicus DSM1925, whereas the ArrA sequences clustered with environmental sequences distinct from those identified in the Holocene sediment. Whereas limited As(III) release was observed in Pleistocene sediment after lactate addition, no arsenic mobilization occurred from Holocene sediments, probably because of the initial reduced state of As, as determined by X-ray Absorption Near Edge Structure. Our findings demonstrate that in the presence of reactive organic carbon, As(III) mobilization can occur in Pleistocene sediments, having implications for future strategies that aim to reduce arsenic contamination in drinking waters by using aquifers containing Pleistocene sediments.

Microbial ecology of arsenic-mobilizing Cambodian sediments

Microbially mediated arsenic release from Holocene and Pleistocene Cambodian aquifer sediments was investigated using microcosm experiments and substrate amendments. In the Holocene sediment, the metabolically active bacteria, including arsenate-respiring bacteria, were determined by DNA stable-isotope probing. After incubation with (13) C-acetate and (13) C-lactate, active bacterial community in the Holocene sediment was dominated by different Geobacter spp.-related 16S rRNA sequences. Substrate addition also resulted in the enrichment of sequences related to the arsenate-respiring Sulfurospirillum spp. (13) C-acetate selected for ArrA related to Geobacter spp. whereas (13) C-lactate selected for ArrA which were not closely related to any cultivated organism. Incubation of the Pleistocene sediment with lactate favoured a 16S rRNA-phylotype related to the sulphate-reducing Desulfovibrio oxamicus DSM1925, whereas the ArrA sequences clustered with environmental sequences distinct from those identified in the Holocene sediment. Whereas limited As(III) release was observed in Pleistocene sediment after lactate addition, no arsenic mobilization occurred from Holocene sediments, probably because of the initial reduced state of As, as determined by X-ray Absorption Near Edge Structure. Our findings demonstrate that in the presence of reactive organic carbon, As(III) mobilization can occur in Pleistocene sediments, having implications for future strategies that aim to reduce arsenic contamination in drinking waters by using aquifers containing Pleistocene sediments.