abstract: 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. / Dissertation/Thesis / Doctoral Dissertation Civil, Environmental and Sustainable Engineering 2017