Biogas-based polyhydroxyalkanoates production by Methylocystis hirsuta: A step further in anaerobic digestion biorefineries

The potential of biogas (with and without H2S) and volatile fatty acids (VFAs) to support microbial growth and accumulation of polyhydroxyalkanoates (PHAs) in type II methanotrophs was evaluated batchwise under aerobic conditions. Methylocystis hirsuta was able to grow on artificial biogas (70% CH4, 29.5% CO2, 0.5% H2S) and accumulate PHA up to 45 +/- 1% (wt.%) under N-limited conditions. The presence of CO2 and H2S did not significantly influence the growth and PHA synthesis in M. hirsuta compared to control tests provided with pure CH4 at similar concentrations. Likewise, the addition of VFAs to the cultivation broth at initial concentrations of 100-200 mg L-1 did not hamper the growth of this strain on artificial biogas. Indeed, the addition of 10% extra carbon in the form of individual VFAs resulted in an increase in the maximum PHA yield and final PHA content up to 0.45-0.63 gPHA gSubstrate(1) and 48-54% (wt.%), respectively, at the expense of a higher energy demand. Valeric acid supplementation supported the highest 3-hydroxyvalerate content (13.5%) within the biocomposite. In this context, this study demonstrated for the first time that 3-hydroxyvalerate synthesis by M. hirsuta did not depend on CH4 assimilation.