Enrichment of mixed methanotrophic cultures producing polyhydroxyalkanoates (PHAs) from various environmental sources

Methanotrophic bacteria can use atmospheric methane (CH4) as a sole carbon source for the growth and pro-duction of polyhydroxyalkanoates (PHA). The development of CH4 bioconversion processes relies heavily on the selection of an efficient methanotrophic culture. This research assessed the effect of selected growth conditions, such as nitrogen sources on the enrichment of methanotrophic cultures from various environments for PHA accumulation. Nitrate-based medium favoured the culture growth and selection for PHA-producing methano-trophic cultures with Methylocystis sp. as a major genus and accumulation of up to 27 % polyhydroxybutyrate (PHB) in the biomass. Three PHB-producing cultures: enriched from waste activated sludge (AS), peat bog soil (PB) and landfill biocover soil (LB) were then tested for their ability to produce PHA copolymer at different CH4: O2 ratios. All enriched cultures were able to utilise valeric acid as a cosubstrate for the accumulation of PHA with a 3-hydroxyvaleric (3HV) fraction of 21-41 mol% depending on the inoculum source and CH4 concentration. The process performance of selected cultures was evaluated and compared to the culture of reference strain Meth-ylocystis hirsuta DSM 18500. All mixed cultures irrespective of their inoculum source had similar levels of 3HV fraction in the PHA (38 +/- 2 mol%). The highest poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) pro-duction was observed for AS culture at 10 % CH4 with an accumulation of 27 +/- 3 % of dry cell weight (DCW), 3HV fraction of 39 +/- 2 mol% and yield of 0.42 +/- 0.02 g-PHA/g-substrate.