Potential for Methanosarcina to contribute to uranium reduction during acetate-promoted groundwater bioremediation

TLDR: It is demonstrated that Methanosarcina species could play an important role in the long-term bioremediation of uranium-contaminated aquifers after depletion of Fe(III) oxides limits the growth of Geobacter species and suggested that Methosarcina have the potential to influence uranium geochemistry in a diversity of anaerobic sedimentary environments.

Abstract: Previous studies of in situ bioremediation of uranium-contaminated groundwater with acetate injections have focused on the role of Geobacter species in U(VI) reduction because of a lack of other abundant known U(VI)-reducing microorganisms. Monitoring the levels of methyl CoM reductase subunit A (mcrA) transcripts during an acetate-injection field experiment demonstrated that acetoclastic methanogens from the genus Methanosarcina were enriched after 40 days of acetate amendment. The increased abundance of Methanosarcina corresponded with an accumulation of methane in the groundwater. An enrichment culture dominated by a Methanosarcina species with the same Methanosarcina mcrA sequence that predominated in the field experiment could effectively convert acetate to methane. In order to determine whether Methanosarcina species could be participating in U(VI) reduction in the subsurface, cell suspensions of M. barkeri were incubated in the presence of U(VI) with acetate provided as the electron donor. U(VI) was reduced by metabolically active M. barkeri cells, however, no U(VI) reduction was observed in inactive controls. These results demonstrate that Methanosarcina species could play an important role in the long-term bioremediation of uranium-contaminated aquifers after depletion of Fe(III) oxides limits the growth of Geobacter species. The results also suggest that Methanosarcina have the potential to influence uranium geochemistry in a diversity of anaerobic sedimentary environments.

Figures

Fig. 2 Phylogenetic tree generated with the maximum likelihood algorithm comparing translated mcrA mRNA transcript sequences to McrA protein sequences from known methanogenic archaea. Bootstrap values were generated with 100 replicates and Methanobacterium formicum, Methanothermobacter marburgensis, and Methanobrevibacter ruminantium were used as outgroups

Fig. 1 The injection of acetate into a uranium-contaminated aquifer, triggered acetate utilization coupled with iron reduction, sulfate reduction, and methanogenesis. a Quantitative RT-PCR of Methanosarcina mcrA mRNA transcripts normalized against Methanosarcina mcrA gene copy numbers recovered in the groundwater over the course of 100 days. b Concentrations of hydrogen sulfide (μM) and sulfate (mM) detected in the groundwater. cConcentrations of acetate

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Potential for methanosarcina to contribute to uranium reduction during acetate-promoted groundwater bioremediation" ?

In order to determine whetherMethanosarcina species could be participating in U ( VI ) reduction in the subsurface, cell suspensions of Methanosarcina barkeri were incubated in the presence of U ( VI ) with acetate provided as the electron donor. The results also suggest that Methanosarcina have the potential to influence uranium geochemistry in a diversity of anaerobic sedimentary environments. 

Q2. What is the way to reduce U(VI) in uranium contaminated soil?

Although growth with acetate as the electron donor and U(VI) as the electron acceptor is possible [4], the low concentrations of U(VI), even in heavily contaminated subsurface environments requires that microbes use other forms of respiration as their primary means of energy conservation [10]. 

Q3. What is the way to reduce U(VI) in uranium contaminated ?

Injection of acetate into the groundwater of uraniumcontaminated aquifers has been shown to be an effective way to stimulate microbially mediated reductive precipitation of soluble U(VI) to poorly soluble U(IV) [1–3]. 

Q4. What was the qPCR amplification and detection process?

Quantitative PCR amplification and detection was performed with the 7500 Real Time System (Applied Biosystems) using cDNA made by reverse transcription from total RNA extracted from groundwater collected during the bioremediation experiment. 

Q5. What was the first step in the process of concentrating the acetate on the groundwater?

For nucleic acid extraction, it was first necessary to concentrate 50 L of groundwater by impact filtration on 293 mm diameter Supor membrane disc filters with pore sizes of 1.2 and 0.2 μm (Pall Life Sciences). 

Q6. What was the thermal cycling parameters for mcrA?

Optimal thermal cycling parameters consisted of an activation step at 50 °C for 2 min, an initial 10 min denaturation step at 95 °C, and 50 cycles of 95 °C for 15 s and 60 °C for 1 min. 

Q7. What grants have been awarded to AER?

AER has been supported by grants from the Novo Nordisk Foundation, Innovationsfonden (grant no. 410600017) and Danish Research Council (418100203). 

Q8. What is the common species of M. barkeri?

Although M. barkeri was isolated from an anaerobic sewage digester [42], it grows in freshwater medium and can utilize acetate as a substrate for methanogenesis, similar to methanogens likely to be enriched from the acetate-amended Rifle aquifer. 

Q9. What is the way to reduce U(VI)?

Awide diversity of microorganisms are capable of U(VI) reduction [4–9] but only Geobacter species have been shown to reduce U(VI) with acetate as an electron donor. 

Q10. What was the synthesis kit used to generate cDNA from RNA?

A DuraScript enhanced avian RT single-strand synthesis kit (Sigma, Sigma-Aldrich, St Louis, MO, USA) was used to generate cDNA from RNA, as previously described [32]. 

Q11. What was the effect of the acetate injections?

The increase in abundance of Methanosarcinales was later followed by a decline, which coincided with acetate limitation associated with the halt in acetate injections on day 68. 

Q12. What is the role of Methanosarcina in uranium removal?

in long-term in situ uranium bioremediation, Methanosarcina may emerge as an important microbial catalyst for uranium removal. 

Q13. What was the effect of the acetate reduction on the methane concentrations?

Methane concentrations steeply declined over time coincident with the steep decline in acetate availability.0 1 2 3 4 5 6 7 8 9 10 0 50 100 150 200 250 3000 20 40 60 80 100 

Q14. What is the reason for the low number of Methanosarcinales in the sub?

The lack of sufficient reducing conditions coupled with the slow growth rate of Methanosarcinales may explain the finding that although acetate concentrations were high during the Fe(III) reducing phase of the experiment (days 0– 33) (Fig. 1c), the number of Methanosarcinales sequences stayed low until sulfate reduction became the primary subsurface metabolism (Fig. 1a, d). 

Q15. What is the effect of acetate on the growth of Methanosarcinales?

Although sulfate reducers and methanogens compete for acetate [47, 48], high concentrations of acetate in the groundwater (Fig. 1c) made it unlikely that growth of Methanosarcinales in the subsurface was being restricted by competition for acetate. 

Q16. What is the abundant Methanosarcina mcrA sequences?

More than half of these sequences clustered with acetoclastic Methanosarcina that are unable to use formate or hydrogen as substrates forFig. 3 UraniumU(VI) reduction by metabolically activeMethanosarcina cells. 

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In te rna t ional License (h t tp : / / creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.