Synthesis gas (syngas) fermentation is a biological carbon fixation process through which carboxydotrophic acetogenic bacteria convert CO, CO₂, and H₂ into platform chemicals. To obtain an accurate overview of the syngas fermentation research and innovation from 2012 to 2022, a systematic search was performed on Web of Science and The Lens, focusing on academic publications and patents that were published or granted during this period. Overall, the research focus was centered on process optimization, the genetic manipulation of microorganisms, and bioreactor design, in order to increase the plethora of fermentation products and expand their possible applications. Most of the published research was initially funded and developed in the United States of America. However, over the years, European countries have become the major contributors to syngas fermentation research, followed by China. Syngas fermentation seems to be developing at “two-speeds”, with a small number of companies controlling the technology that is needed for large-scale applications, while academia still focuses on low technology readiness level (TRL) research. This systematic review also showed that the fermentation of raw syngas, the effects of syngas impurities on acetogen viability and product distribution, and the process integration of gasification and fermentation are currently underdeveloped research topics, in which an investment is needed to achieve technological breakthroughs.

Biodesulfurization (BDS) is an ecofriendly process that uses microorganisms to efficiently remove sulfur from fossil fuels. To make the BDS process economically competitive with the deep hydrodesulfurization process, which is currently used in the oil industry, it is necessary to improve several factors. One crucial limitation to be overcome, common within many other biotechnological processes, is the cost of the culture medium. Therefore, an important line of work to make BDS scale-up less costly is the optimization of the culture medium composition aiming to reduce operating expenses and maximize biocatalyst production. In this context, the main goal of this study was on the minimization of inorganic key components of sulfur-free mineral (SFM) medium in order to get the maximal production of efficient desulfurizing biocatalysts. Hence, a set of assays was carried out to develop an optimal culture medium containing minimal amounts of nitrogen (N) and magnesium (Mg) sources and trace elements solution (TES). These assays allowed the design of a SFMM (SFM minimum) medium containing 85% N-source, 25% Mg-source and 25% TES. Further validation consisted of testing this minimized medium using two carbon sources: the commercial C-source (glucose + fructose) versus Jerusalem artichoke juice (JAJ) as a cheaper alternative. SFMM medium allowed microbial cells to almost duplicate their specific desulfurization rate (q_2-HBP) for both tested C-sources, namely from 2.15 to 3.39 μmoL g⁻¹ (DCW) h⁻¹ for Fru + Glu and from 1.91 to 3.58 μmoL g⁻¹ (DCW) h⁻¹ for JAJ, achieving a similar net 2-hydroxybiphenyl produced per g of consumed sugar (∼17 μmoL g⁻¹). These results point out the great advantage of using cheaper culture medium that in addition enhances the bioprocess effectiveness, paving the way to a sustainable scale-up for fossil fuel BDS.

Hybrid bio-thermochemical based technologies have the potential to ensure greater feedstock flexibility for the production of bioenergy and bioproducts. This study focused on the bioconversion of syngas produced from low grade technical lignin to C₂-/C₄-carboxylic acids by Butyribacterium methylotrophicum. The effects of pH, medium supplementation and the use of crude syngas were analyzed. At pH 6.0, B. methylotrophicum consumed CO, CO₂ and H₂ simultaneously up to 87 mol% of carbon fixation, and the supplementation of the medium with acetate increased the production of butyrate by 6.3 times. In long-term bioreactor experiments, B. methylotrophicum produced 38.3 and 51.1 mM acetic acid and 0.7 and 2.0 mM butyric acid from synthetic and lignin syngas, respectively. Carbon fixation reached 83 and 88 mol%, respectively. The lignin syngas conversion rate decreased from 13.3 to 0.9 NmL/h throughout the assay. The appearance of a grayish pellet and cell aggregates after approximately 220 h was indicative of tar deposition. Nevertheless, the stressed cells remained metabolically active and maintained acetate and butyrate production from lignin syngas. The challenge that impurities represent in the bioconversion of crude syngas has a direct impact on syngas cleaning requirements and operation costs, supporting the pursuit for more robust and versatile acetogens.

Most industrial processes depend on heat, electricity, demineralized water, and chemical inputs, which themselves are produced through energy- and resource-intensive industrial activities. In this work, acetic acid (AA) production from syngas (CO, CO₂, and H₂) fermentation is explored and compared against a thermochemical fossil benchmark and other thermochemical/biological processes across four main Key Performance Indicators (KPI)—electricity use, heat use, water consumption, and carbon footprint (CF)—for the years 2023 and 2050 in Portugal and France. CF was evaluated through transparent and public inventories for all the processes involved in chemical production and utilities. Spreadsheet-traceable matrices for hotspot identification were also developed. The fossil benchmark, with all the necessary cascade processes, was 0.64 kg CO_2-eq/kg AA, 1.53 kWh/kg AA, 22.02 MJ/kg AA, and 1.62 L water/kg AA for the Portuguese 2023 energy mix, with a reduction of 162% of the CO₂-eq in the 2050 energy transition context. The results demonstrated that industrial practices would benefit greatly from the transition from fossil to renewable energy and from more sustainable chemical sources. For carbon-intensive sectors like steel or cement, the acetogenic syngas fermentation appears as a scalable bridge technology, converting the flue gas waste stream into marketable products and accelerating the transition towards a circular economy.