The Opportunity for High-Performance Biomaterials from Methane

Abstract

Polyhydroxyalkanoate (PHA) biopolymers are widely recognised as outstanding candidates to replace conventional petroleum-derived polymers. Their mechanical properties are good and can be tailored through copolymer composition, they are biodegradable, and unlike many alternatives, they do not rely on oil-based feedstocks. Further, they are the only commodity polymer that can be synthesised intracellularly, ensuring stereoregularity and high molecular weight. However, despite offering enormous potential for many years, they are still not making a significant impact. This is broadly because commercial uptake has been limited by variable performance (inconsistent polymer properties) and high production costs of the raw polymer. Additionally, the main type of PHA produced naturally is poly-3-hydroxybutyrate (PHB), which has limited scope due to its brittle nature and low thermal stability, as well as its tendency to embrittle over time. Production cost is strongly impacted by the type of the feedstock used. In this article we consider: the production of PHAs from methanotrophs using methane as a cost-effective substrate; the use of mixed cultures, as opposed to pure strains; and strategies to generate a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer (PHBV), which has more desirable qualities such as toughness and elasticity.

Keywords: PHA; PHB; PHBV; biopolymer; gas fermentation; methane; syngas methanotroph.

Figure 1

Degree of reduction of methane relative to other feedstocks, metabolites, PHB (CH_1.5O_0.5: 4.5), and PHV (CH_1.6O_0.4: 4.8). The degree of reduction is of a measure of the oxidation potential, with high oxidation potential indicating higher potential for energy release upon oxidation. The figure is modified from Kracke and Kromer [29] and Pratt [30].

Figure 2

Generalised reaction scheme for methane oxidation via methanotrophs. Adapted from Kalyuzhnaya et al. [32] and Hanson and Hanson [31]; sMMO: soluble methane mono-oxygenase, pMMO: particulate methane mono-oxygenase, MDH: methanol dehydrogenase, FaDH: formaldehyde dehydrogenase, FDH: formate dehydrogenase and CytC: cytochrome C.

Figure 3

Schematic indicating carbon flow from the serine cycle with regard to balanced or unbalanced growth (adapted by combining Karthikeyan et al. [6] and Pieja et al. [54]).