Carbon Monoxide as an Electron Donor for the Biological Reduction of Sulphate

Several strains of Gram-negative and Gram-positive sulphate-reducing bacteria (SRB) are able to use carbon monoxide (CO) as a carbon source and electron donor for biological sulphate reduction. These strains exhibit variable resistance to CO toxicity. The most resistant SRB can grow and use CO as an electron donor at concentrations up to 100%, whereas others are already severely inhibited at CO concentrations as low as 1-2%. Here, the utilization, inhibition characteristics, and enzymology of CO metabolism as well as the current state of genomics of CO-oxidizing SRB are reviewed. Carboxydotrophic sulphate-reducing bacteria can be applied for biological sulphate reduction with synthesis gas (a mixture of hydrogen and carbon monoxide) as an electron donor

Carbon monoxide conversion by thermophilic sulfate-reducing bacteria in pure culture and in co-culture with Carboxydothermus hydrogenoformans

Biological sulfate (SO4) reduction with carbon monoxide (CO) as electron donor was investigated. Four thermophilic SO4-reducing bacteria, Desulfotomaculum thermoacetoxidans (DSM 5813), Thermodesulfovibrio yellowstonii (ATCC 51303), Desulfotomaculum kuznetsovii (DSM 6115; VKM B-1805), and Desulfotomaculum thermobenzoicum subsp. thermosyntrophicum (DSM 14055), were studied in pure culture and in co-culture with the thermophilic carboxydotrophic bacterium Carboxydothermus hydrogenoformans (DSM 6008). D. thermoacetoxidans and T. yellowstonii were extremely sensitive to CO: their growth on pyruvate was completely inhibited at CO concentrations above 2% in the gas phase. D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum were less sensitive to CO. In pure culture, D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum were able to grow on CO as the only electron donor and, in particular in the presence of hydrogen/carbon dioxide, at CO concentrations as high as 50-70%. The latter SO4 reducers coupled CO oxidation to SO4 reduction, but a large part of the CO was converted to acetate. In co-culture with C. hydrogenoformans, D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum could even grow with 100% CO (P CO=120 kPa)

Desulfotomaculum carboxydivorans sp.nov., a novel sulfate-reducing bacterium capable of growth at 100% CO

A moderately thermophilic, anaerobic, chemolithoheterotrophic, sulfate-reducing bacterium, strain CO-1-SRBT, was isolated from sludge from an anaerobic bioreactor treating paper mill wastewater. Cells were Gram-positive, motile, spore-forming rods. The temperature range for growth was 30¿68 °C, with an optimum at 55 °C. The NaCl concentration range for growth was 0¿17 g l¿1; there was no change in growth rate until the NaCl concentration reached 8 g l¿1. The pH range for growth was 6·0¿8·0, with an optimum of 6·8¿7·2. The bacterium could grow with 100 % CO in the gas phase. With sulfate, CO was converted to H2 and CO2 and part of the H2 was used for sulfate reduction; without sulfate, CO was completely converted to H2 and CO2. With sulfate, strain CO-1-SRBT utilized H2/CO2, pyruvate, glucose, fructose, maltose, lactate, serine, alanine, ethanol and glycerol. The strain fermented pyruvate, lactate, glucose and fructose. Yeast extract was necessary for growth. Sulfate, thiosulfate and sulfite were used as electron acceptors, whereas elemental sulfur and nitrate were not. A phylogenetic analysis of 16S rRNA gene sequences placed strain CO-1-SRBT in the genus Desulfotomaculum, closely resembling Desulfotomaculum nigrificans DSM 574T and Desulfotomaculum sp. RHT-3 (99 and 100 % similarity, respectively). However, the latter strains were completely inhibited above 20 and 50 % CO in the gas phase, respectively, and were unable to ferment CO, lactate or glucose in the absence of sulfate. DNA¿DNA hybridization of strain CO-1-SRBT with D. nigrificans and Desulfotomaculum sp. RHT-3 showed 53 and 60 % relatedness, respectively. On the basis of phylogenetic and physiological features, it is suggested that strain CO-1-SRBT represents a novel species within the genus Desulfotomaculum, for which the name Desulfotomaculum carboxydivorans is proposed. This is the first description of a sulfate-reducing micro-organism that is capable of growth under an atmosphere of pure CO with and without sulfate. The type strain is CO-1-SRBT (=DSM 14880T=VKM B-2319T

Effect of methanogenic substrates on anaerobic oxidation of methane and sulfate reduction by an anaerobic methanotrophic enrichment

Anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) is assumed to be a syntrophic process, in which methanotrophic archaea produce an interspecies electron carrier (IEC), which is subsequently utilized by sulfate-reducing bacteria. In this paper, six methanogenic substrates are tested as candidate-IECs by assessing their effect on AOM and SR by an anaerobic methanotrophic enrichment. The presence of acetate, formate or hydrogen enhanced SR, but did not inhibit AOM, nor did these substrates trigger methanogenesis. Carbon monoxide also enhanced SR but slightly inhibited AOM. Methanol did not enhance SR nor did it inhibit AOM, and methanethiol inhibited both SR and AOM completely. Subsequently, it was calculated at which candidate-IEC concentrations no more Gibbs free energy can be conserved from their production from methane at the applied conditions. These concentrations were at least 1,000 times lower can the final candidate-IEC concentration in the bulk liquid. Therefore, the tested candidate-IECs could not have been produced from methane during the incubations. Hence, acetate, formate, methanol, carbon monoxide, and hydrogen can be excluded as sole IEC in AOM coupled to SR. Methanethiol did inhibit AOM and can therefore not be excluded as IEC by this study

Biomethanation potential of biological and other wastes

Anaerobic technology has been traditionally applied for the treatment of carbon rich wastewater and organic residues. Anaerobic processes can be fully integrated in the biobased economy concept for resource recovery. After a brief introduction about applications of anaerobic processes to industrial wastewater treatment, agriculture feedstock and organic fraction of municipal solid waste, the position of anaerobic processes in biorefinery concepts is presented. Integration of anaerobic digestion with these processes can help in the maximisation of the economic value of the biomass used, while reducing the waste streams produced and mitigating greenhouse gases emissions. Besides the integration of biogas in the existing full-scale bioethanol and biodiesel production processes, the potential applications of biogas in the second generation lignocellulosic, algae and syngas-based biorefinery platforms are discussed.(undefined

Microbial hydrogenogenic CO conversions: applications in synthesis gas purification and biodesulfurization

Hydrogen gas attracts great interest as a potential clean future fuel and it is an excellent electron donor in biotechnological reductive processes, e.g. in biodesulfurization. Bulk production of H 2 relies on the conversion of organic matter into synthesis gas, a mixture of H 2 , CO and CO 2 . The relative abundance of CO restricts its applicability, due to toxicity to hydrogenotrophic microorganisms and poisoning of chemical catalysts in low temperature fuel cells. Currently, synthesis gas purification, i.e. CO conversion to H 2 , is performed in chemical catalytic systems. A recently discovered group of thermophilic anaerobic bacteria is able to grow by converting CO with water to H 2 and CO 2 . This feature makes thesehydrogenogensinteresting for cost effective hydrogen production.Several anaerobicwastewatertreatingsludgesharbor CO utilizing moderately thermophilic (55°C) hydrogenogenic microorganisms. CO conversion at 30°C resulted in the production of acetate, whereas at 55°C it proceeded via H 2 /CO 2 .One of the tested sludge samples could even reduce sulfate with the CO-derived H 2 , tolerating and using high CO concentrations (P CO >160kPa). From this sludge amoderately thermophilic, anaerobic, sulfate-reducing bacterium was isolated, i.e.Desulfotomaculumcarboxydivorans , capable of growth on CO as sole energy and carbon source both in the presence and absence of sulfate as electron acceptor. D. carboxydivorans grows rapidly at 200kPaCO, pH 7.0 and 55ºC (t d of 100 minutes), producing nearlyequimolaramounts of H 2 and CO 2 from CO revealing a high specific CO conversion rate of 0.8 mol CO.(g protein) -1 .hour -1 . Furthermore, D. carboxydivorans is capable of hydrogenotrophic sulfate reduction at partial CO pressures exceeding 100kPa, at a maximal specific sulfate reduction rate of 32mmol.(g protein) -1 .hour -1 . These characteristics make it an interesting candidate for synthesis gas purification as well as for the direct use of synthesis gas in biodesulfurization at elevated temperatures. Although in the latter case, the low sulfide tolerance of D. carboxydivorans , i.e. total inhibition at 5mMand 9mMat pH 6.5 and 7.2, respectively, may require special features to maintain sufficient low sulfide concentrations.Thermophilicsulfate reduction using CO as electron donor with anaerobic granular sludge, from which D. carboxydivorans originated, showed that despite the high CO conversion capacity of the biomass present, the sulfate reduction capacity was limited due to strong competition for the produced H 2 . Operation at HRT >9 hours resulted in a predominant consumption of the CO-derived H 2 by methanogens (up to 90%) and thus in a poor sulfate reduction efficiency (<15%). Although, the methanogens appeared to be more sensitive to pH and temperature shocks imposed to the reactor, they were not eliminated by these treatments. The high growth rates of the methanogens (t d of 4.5 hours) resulted in fast recovery and domination of the consumption of CO-derived H 2 by methanogens. At HRT <4 hours, the consumption of CO-derived H 2 was dominated by the sulfate reducing bacteria (up to 95%). The highest sulfate reduction rates achieved were17 mmol.L -1 .day -1 at a HRT of 3 hours (87% of the H 2 used by sulfate reducers). These rates were limited by the amount of CO supplied and the CO conversion efficiency (85%) at higher CO loads (106 mmol.L -1 .day -1 ), probably as a result of limited biomass retention in the reactor.Elimination of methanogenesis is a prerequisite for practical application of both synthesis gas utilization as electron donor for thermophilic sulfate reduction processes and synthesis gas purification. Short-term (90 minutes) pretreatment of the sludge at95°Celiminatesmethanogenesis, but nothomoacetogenesis. Although, homoacetogens did not seem to reduce the electron flow towards sulfate reduction much, their activity represents an unwanted loss of H 2 . For practical applications a complete heat-treatment of the sludge at temperatures exceeding at least 85ºC and treatment of the (empty) reactor system using steam as well as additional measures to prevent introduction of amethanogenicpopulation could be considered