Matching Items (2)
Description
On average, our society generates ~0.5 ton of municipal solid waste per person annually. Biomass waste can be gasified to generate synthesis gas (syngas), a gas mixture consisting predominantly of CO, CO2, and H2. Syngas, rich in carbon and electrons, can fuel the metabolism of carboxidotrophs, anaerobic microorganisms that metabolize CO (a toxic pollutant) and produce biofuels (H2, ethanol) and commodity chemicals (acetate and other fatty acids). Despite the attempts for commercialization of syngas fermentation by several companies, the metabolic processes involved in CO and syngas metabolism are not well understood. This dissertation aims to contribute to the understanding of CO and syngas fermentation by uncovering key microorganisms and understanding their metabolism. For this, microbiology and molecular biology techniques were combined with analytical chemistry analyses and deep sequencing techniques. First, environments where CO is commonly detected, including the seafloor, volcanic sand, and sewage sludge, were explored to identify potential carboxidotrophs. Since carboxidotrophs from sludge consumed CO 1000 faster than those in nature, mesophilic sludge was used as inoculum to enrich for CO- and syngas- metabolizing microbes. Two carboxidotrophs were isolated from this culture: an acetate/ethanol-producer 99% phylogenetically similar to Acetobacterium wieringae and a novel H2-producer, Pleomorphomonas carboxidotrophicus sp. nov. Comparison of CO and syngas fermentation by the CO-enriched culture and the isolates suggested mixed-culture syngas fermentation as a better alternative to ferment CO-rich gases. Advantages of mixed cultures included complete consumption of H2 and CO2 (along with CO), flexibility under different syngas compositions, functional redundancy (for acetate production) and high ethanol production after providing a continuous supply of electrons. Lastly, dilute ethanol solutions, typical of syngas fermentation processes, were upgraded to medium-chain fatty acids (MCFA), biofuel precursors, through the continuous addition of CO. In these bioreactors, methanogens were inhibited and Peptostreptococcaceae and Lachnospiraceae spp. most likely partnered with carboxidotrophs for MCFA production. These results reveal novel microorganisms capable of effectively consuming an atmospheric pollutant, shed light on the interplay between syngas components, microbial communities, and metabolites produced, and support mixed-culture syngas fermentation for the production of a wide variety of biofuels and commodity chemicals.
ContributorsEsquivel Elizondo, Sofia Victoria (Author) / Krajmalnik-Brown, Rosa (Thesis advisor) / Rittmann, Bruce E. (Committee member) / Delgado, Anca G. (Committee member) / Torres, Cesar I. (Committee member) / Arizona State University (Publisher)
Created2017
Description
The increasing concentrations of greenhouse gases into the atmosphere call for urgent measures to use non-fossil feedstock for fuels and chemicals. Synthesis gas (or syngas) is a mixture of three gases: hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2). Syngas already is widely used as a non-fossil fuel and a building block for a variety of chemicals using the Fischer-Tropsch process. Recently, syngas fermentation has attracted attention as a more sustainable way for the conversion of syngas to chemicals, since its biocatalysts are self-generating, are resilient, and can utilize a wide range of syngas compositions. However, syngas fermentation has technical and economic limitations. This dissertation, by contributing to the understanding of syngas fermentation, helps to overcome the limitations. A bibliometric analysis showed the topic’s landscape and identified that mass transfer is the biggest challenge for the process. One means to improve syngas mass transfer is to use the membrane biofilm reactor, or MBfR, to deliver syngas to the microorganisms. MBfR experiments delivering pure H2 demonstrated that the H2:IC ratio (IC is inorganic carbon) controlled the overall production rate of organic compounds and their carbon-chain length. Organic chemicals up to eight carbons could be produced with a high H2:IC ratio. A novel asymmetric membrane dramatically improved mass transfer rates for all syngas components, and its low selectivity among them made it ideal for high-rate syngas fermentation. MBfR experiments using syngas and the asymmetric membrane, as well as a conventional symmetric membrane, confirmed that the key parameter for generating long-chain products was a high H2:IC ratio. The fast mass transfer rate of the asymmetric membrane allowed a very high areal production rate of acetate: 253 g.m-2.d-1, the highest reported to date. Since the membrane delivered H2 and C from the syngas feed, the relatively low selectivity of the asymmetric membrane favored acetogenesis over microbial chain elongation. A techno-economic analysis of the MBfR showed that the cost to produce acetate was less than its market price. All results presented in this dissertation support the potential of syngas fermentation using the MBfR as a means to produce commodity chemicals and biofuels from syngas.
ContributorsCalvo Martinez, Diana Carolina (Author) / Rittmann, Bruce E (Thesis advisor) / Torres, César I (Thesis advisor) / Kralmajnik-Brown, Rosa (Committee member) / Arizona State University (Publisher)
Created2021