Single carbon metabolism - A new paradigm for microbial bioprocesses?

Abstract

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.

Keywords: Bioeconomy; Carbon dioxide; Formate; Methane; Methanol; Sustainability.

Fig. 1

Overview of the major native single carbon assimilation pathways. Methanol assimilation via formaldehyde is achieved by the RuMP cycle (orange) or the XuMP cycle (blue). These cycles bear high similarity to the CBB cycle for CO₂ assimilation (green). Formaldehyde can be further oxidized to formate and assimilated via the serine cycle (yellow) or the reductive glycine pathway (purple). The latter is not cyclic but a linear pathway, same as the Wood Ljungdahl pathway (brown). Abbreviations: AcCoA – acetyl coenzyme A, CH₂-THF – methylene tetrahydrofolate, DHA – dihydroxyacetone, DHAP – dihydroxyacetone phosphate, E4P – erythrose 4-phosphate, Fald – formaldehyde, FBP – fructose bisphosphate, For – formate, G3P – glyceraldehyde 3-phosphate, Gly – glycine, Glyc – glycerate, Glx – glyoxylate, H6P – hexulose 6-phosphate, HPyr – hydroxypyruvate, Mal – malate, MeOH – methanol, OAA – oxaloacetate, PEP – phosphoenolpyruvate, Pyr – pyruvate, Ru5P – ribulose 5-phosphate, RuBP – ribulose bisphosphate, S7P – sedoheptulose 7-phosphate, SBP – sedoheptulose bisphosphate, Ser – serine, Xu5P – xylulose 5-phosphate.

Fig. 2

Single carbon substrates for biotechnology. The main gaseous and liquid single carbon substrates and their advantages and disadvantages are highlighted. Tank sizes illustrate the respective relative annual production to date.