Syngas Fermentation for the Production of Bio-Based Polymers: A Review

Abstract

Increasing environmental awareness among the general public and legislators has driven this modern era to seek alternatives to fossil-derived products such as fuel and plastics. Addressing environmental issues through bio-based products driven from microbial fermentation of synthetic gas (syngas) could be a future endeavor, as this could result in both fuel and plastic in the form of bioethanol and polyhydroxyalkanoates (PHA). Abundant availability in the form of cellulosic, lignocellulosic, and other organic and inorganic wastes presents syngas catalysis as an interesting topic for commercialization. Fascination with syngas fermentation is trending, as it addresses the limitations of conventional technologies like direct biochemical conversion and Fischer-Tropsch's method for the utilization of lignocellulosic biomass. A plethora of microbial strains is available for syngas fermentation and PHA production, which could be exploited either in an axenic form or in a mixed culture. These microbes constitute diverse biochemical pathways supported by the activity of hydrogenase and carbon monoxide dehydrogenase (CODH), thus resulting in product diversity. There are always possibilities of enzymatic regulation and/or gene tailoring to enhance the process's effectiveness. PHA productivity drags the techno-economical perspective of syngas fermentation, and this is further influenced by syngas impurities, gas-liquid mass transfer (GLMT), substrate or product inhibition, downstream processing, etc. Product variation and valorization could improve the economical perspective and positively impact commercial sustainability. Moreover, choices of single-stage or multi-stage fermentation processes upon product specification followed by microbial selection could be perceptively optimized.

Keywords: carbon monoxide dehydrogenase; fermentation; hydrogenase; polyhydroxyalkanoates; syngas.

Figure 1

Diagram of the conversion of lignocellulosic feedstock into alcohols, organic acids, and polymers.

Figure 2

Metabolic processes for syngas conversion to diverse products through the Wood–Ljungdahl pathway (modified from Ciliberti et al. (2020)) [24]. (a) Conversion of syngas to acetyl-CoA and (b) conversion of acetyl-CoA to fermentation products. Abbreviations: AAD, alcohol/aldehyde dehydrogenase; ACS, acetyl-CoA synthase; ADH, alcohol dehydrogenase; AK, acetate kinase; ALDC, acetolactate decarboxylase; ALS, acetolactate synthase; AOR, aldehyde:ferredoxin-oxidoreductase; BCD, butyryl-CoA dehydrogenase; BK, butyrate kinase; CODH, CO dehydrogenase; Co-Fes-P, corrinoid iron-sulfur protein; CRT, crotonase; FDH, formate dehydrogenase; FTS, formyl-THF synthetase; HBD, 3-hydroxybutyryl-CoA dehydrogenase; HYA, hydrogenase; MTC, methenyl-THF cylcohydrolase; MTD, methylene-THF dehydrogenase; MTR, methyltransferase; MTRS, methylene-THF reductase; PFOR, pyruvate:ferredoxin oxidoreductase; PTA, phosphotransacetylase; PTB, phosphotransbutyrylase; THF, tetrahydrofolate; THL, thiolase.

Figure 3

Biochemical pathway for PHA production in a microbial cell Redrawn from [52].

Figure 4

Three-stage process for PHA production through mixed microbial consortia (MMC). Modified from [85]. CSTR: continuous stirred tan reactor, SBR: sequential batch reactor.