An Acetogen-Based Renewable Chemistry Assessing the Potential of Acetogenic Bacteria to Relieve the Petrochemical Industry of its Fossil Resource Dependence

The post-industrial global economy has grown to become a hyper extractive mechanism that exceeds many of the planetary boundaries. We extract and consume unprecedented quantities of resources to meet ever-growing needs. Primarily when it comes to fossil resources, containing vast amounts of carbon stored over long geological eras, their extraction and combustion perturbs the natural carbon cycle, leading to an accumulation of primarily carbon dioxide (CO2) in the oceans and atmosphere. This accumulation has severe consequences for the environment, most notably acidification of the oceans, and a greenhouse effect leading to global climate change. We are in the midst of a paradigm shift, pushing for a sustainable and circular use of resources. The majority of fossil resources are used as fuels, consumed for energy and power, but other industries, such as the petrochemical industry, are also large fossil resource consumers and contributors to greenhouse gas emissions. In this context, acetogenic gas fermentation is increasingly studied as a promising technology to upcycle carbon-rich waste gasses into marketable chemicals. Currently, the product range is limited, and production yields, rates, and titers for a number of interesting products do not allow for economically viable processes. This thesis aims at assessing and advancing the potential of acetogenesis as a bio-manufacturing platform of chemicals that are currently produced from petrol.

In the first chapter, a breakdown of the wicked problem of sustainable development is presented and the impact of the current petrochemical industry on the global resource utilization and greenhouse gas emissions is quantified. The mitigation potential of a transition towards a sustainable chemical industry building on acetogenic gas fermentation is assessed. The second chapter presents an engineering analysis that probes the potential of acetogenic gas fermentation to be pushed beyond the limited current practices. It specifically investigates the effect of temperature on the fermentation and the possibility of producing higher value compounds than only acetate. The third chapter is a fundamental
scientific experiment challenging the hypothesis that the Wood-Ljungdahl pathway of acetogenesis is the most ancient metabolic pathway, using evolutionary genetic analyses.

The first chapter highlight the mitigation potential of an acetogenesis-based chemical industry, under the condition that it produces long-lasting chemicals powered by renewable energy that is not taken away from potentially more impactful process replacement such as ammonia or steel production. The second chapter confirms a clear metabolic preference for ethanol and acetate production, making it challenging to push for higher value compounds, but is the first work to demonstrate the benefit for thermophilic gas fermentation. Additionally it identifies two clear bottlenecks, namely (i) the low mass transfer rates of hydrogen and carbon monoxide, and (ii) the limited In-situ evaporation of volatile
products. Lastly, chapter 3, refutes the popular claim of the Wood-Ljungdahl pathway being the most ancient form of metabolism. We demonstrate that the reduction of CO2 in Archaea and Bacteria is a product of parallel evolution. Instead, only the core enzymatic complex traces back to the universal common ancestor, which relied on various environmental methyl groups as substrate. We propose an alternative hypothesis that methylotrophic acetyl-CoA formation is the first metabolic pathway, predating life.