Rising CO₂ emissions have pushed scientists to develop new technologies for a more sustainable bio-based economy. Microbial conversion of CO₂ and CO₂-derived carbon substrates into valuable compounds can contribute to carbon neutrality and sustainability. Here, we discuss the potential of C1 carbon sources as raw materials to produce energy, materials, and food and feed using microbial cell factories. We provide an overview of potential microbes, natural and synthetic C1 utilization pathways, and compare their metabolic driving forces. Finally, we sketch a future in which C1 substrates replace traditional feedstocks and we evaluate the costs associated with such an endeavor.

The increase of CO ₂ emissions due to human activity is one of the preeminent reasons for the present climate crisis. In addition, considering the increasing demand for renewable resources, the upcycling of CO ₂ as a feedstock gains an extensive importance to establish CO ₂ -neutral or CO ₂ -negative industrial processes independent of agricultural resources. Here we assess whether synthetic autotrophic Komagataella phaffii ( Pichia pastoris ) can be used as a platform for value-added chemicals using CO ₂ as a feedstock by integrating the heterologous genes for lactic and itaconic acid synthesis. ¹³ C labeling experiments proved that the resulting strains are able to produce organic acids via the assimilation of CO ₂ as a sole carbon source. Further engineering attempts to prevent the lactic acid consumption increased the titers to 600 mg L ⁻¹ , while balancing the expression of key genes and modifying screening conditions led to 2 g L ⁻¹ itaconic acid. Bioreactor cultivations suggest that a fine-tuning on CO ₂ uptake and oxygen demand of the cells is essential to reach a higher productivity. We believe that through further metabolic and process engineering, the resulting engineered strain can become a promising host for the production of value-added bulk chemicals by microbial assimilation of CO ₂ , to support sustainability of industrial bioprocesses.

The Crabtree phenotype defines whether a yeast can perform simultaneous respiration and fermentation under aerobic conditions at high growth rates. It provides Crabtree positive yeasts an evolutionary advantage of consuming glucose faster and producing ethanol to outcompete other microorganisms in sugar rich environments. While a number of genetic events are associated with the emergence of the Crabtree effect, its evolution remains unresolved. Here we show that overexpression of a single Gal4-like transcription factor is sufficient to convert Crabtree-negative Komagataella phaffii (Pichia pastoris) into a Crabtree positive yeast. Upregulation of the glycolytic genes and a significant increase in glucose uptake rate due to the overexpression of the Gal4-like transcription factor leads to an overflow metabolism, triggering both short-term and long-term Crabtree phenotypes. This indicates that a single genetic perturbation leading to overexpression of one gene may have been sufficient as the first molecular event towards respiro-fermentative metabolism in the course of yeast evolution.

Increasing atmospheric carbon dioxide levels, a reduction of arable land area and the dependence of first and second generation biotechnology feedstocks on agricultural products, call for alternative, sustainable feedstock sources for industrial applications. The direct use of CO₂ or conversion of CO₂ into other single carbon (C1) sources have great potential as they might help to reduce carbon emissions and do not compete with agricultural land use. Here we discuss the microbial use of C1 carbon sources, their potential applications in biotechnology, and challenges towards sustainable C1-based industrial biotechnology processes. We focus on methanol, formic acid, methane, syngas, and CO₂ as feedstocks for bioprocesses, their assimilation pathways, current and emerging applications, and limitations of their application. This mini-review is intended as a first introduction for researchers who are new to the field of C1 biotechnology.