The fundamental role of pH in CH4 bioconversion into polyhydroxybutyrate in mixed methanotrophic cultures

Climate change and plastic pollution are likely the most relevant challenges for the environment in the 21st century. Developing cost-effective technologies for the bioconversion of methane (CH₄) into polyhydroxyalkanoates (PHAs) could simultaneously mitigate CH₄ emissions and boost the commercialization of biodegradable polymers. Despite the fact that the role of temperature, nitrogen deprivation, CH₄:O₂ ratio or micronutrients availability on the PHA accumulation capacity of methanotrophs has been carefully explored, there is still a need for optimization of the CH₄-to-PHA bioconversion process prior to becoming a feasible platform in future biorefineries. In this study, the influence of different cultivation broth pH values (5.5, 7, 8.5 and 10) on bacterial biomass growth, CH₄ bioconversion rate, PHA accumulation capacity and bacterial community structure was investigated in a stirred tank bioreactor under nitrogen deprivation conditions. Higher CH₄ elimination rates were obtained at increasing pH, with a maximum value of 50.4 ± 2.7 g CH₄·m^-3·h^-1 observed at pH 8.5. This was likely mediated by an increased ionic strength in the mineral medium, which enhanced the gas-liquid mass transfer. Interestingly, higher PHB accumulations were observed at decreasing pH, with the highest PHB contents recorded at a pH 5.5 (43.7 ± 3.4 %w·w^-1). The strong selective pressure of low pH towards the growth of Type II methanotrophic bacteria could explain this finding. The genus Methylocystis increased its abundance from 34 % up to 85 and 90 % at pH 5.5 and 7, respectively. On the contrary, Methylocystis was less abundant in the community enriched at pH 8.5 (14 %). The accumulation of intracellular PHB as energy and carbon storage material allowed the maintenance of high CH₄ biodegradation rates during 48 h after complete nitrogen deprivation. The results here obtained demonstrated for the first time a crucial and multifactorial role of pH on the bioconversion performance of CH₄ into PHA.