Biopolymers made from methane in bioreactors

Methane (CH₄) is a potent greenhouse gas and mitigation is important to reduce global warming impacts. In this study, we aimed to convert CH₄ to polyhydroxybutyrate (PHB; a biopolymer) by enrichment of methanotrophic consortia in bioreactors. Two different methanotrophic consortia were established form landfill top-cover (landfill biomass [LB]) and compost soils (compost biomass [CB]), through cultivation under CH₄:CO₂:air (30:10:60) in batch systems. The established cultures were then used as inoculi (0.5 g LB or CB·L^-1) in continuous stirred tank reactors (CSTRs) aerated with CH₄:CO₂:air at 0.25 L·min^-1. Under stable CSTRs operating conditions, the effect of spiking with 1:1 copper:iron (final concentrations of 5μM) was tested. Methane oxidation capacity (MOC), biomass dry weight (DW_biomass), PHB, and fatty acid methyl esters (FAMEs) contents were used as effect parameters. A maximum MOC of 481.9 ± 8.9 and 279.6 ± 11.3 mg CH₄·g^-1 DW_biomass·h^-1 was recorded in LB-CSTR and CB-CSTR, respectively, but PHB production was similar for both systems, that is 37.7 mg·g^-1 DW_biomass. Treatment with copper and iron improved PHB production (22.5% of DW_biomass) in LB-CSTR, but a reduction of 13.6% was observed in CB-CSTR. The results indicated that CH₄ to PHB conversion is feasible using LB-CSTRs and addition of copper and iron is beneficial.