Sustainable production of PHB by utilizing C1 compounds as carbon source

Abstract

Environmental concern about conventional plastics regarding their end-of-life management and the rapid depletion of fossil fuels, have sparked the interest in the development of biopolymer alternatives. Among the various groups of biopolymers, polyhydroxyalkanoates (PHAs) have great potential to replace their petrochemical counterparts. The biggest obstacle for PHAs is the high cost associated with their production. The factor contributing most to the overall price are commonly used carbon feedstocks. One-carbon (C1) compounds like methane and methanol are cheap and abundant potential carbon sources for the microbial production of polyhydroxybutyrate (PHB). Methane can be metabolized by microorganisms called methanotrophs, while reduced C1 compounds like methanol can be utilized by methylotrophs.Both groups of organisms accumulate PHB under nutrient-insufficient conditions. A limitation in nitrogen was chosen as PHB accumulation strategy in this work. In the first part, the production of PHB was evaluated in a bubble column reactor (BCR) setup utilizing methane as substrate. A temperature of 37 °C and an ammonium concentration of 8.5 mM were assessed as ideal for the growth of Methylocystis sp. GB25 DSMZ 7674 in serum bottles. PEG 400 was determined as suitable antifoam agent for the cultivation of M. sp. GB25DSMZ 7674. However, the PHB accumulation phase could not be reached in the BCR setup. Initial growth was visible in the cultivation of M. sp. GB25 DSMZ 7674 and Methylocystis parvus OBBP, but both strains showed a sudden stop in microbial activity after 8 to 10 hours. In contrast, a PHB content of 31 % and 11 % of cell dry weight (CDW) could be reached in the cultivation of M. sp. GB25 DSMZ 7674 in a stirred-tank reactor (STR) system. The successful production of PHB in the STR system led to the conclusion, that gas-to-liquid mass transfer might not be sufficient in the BCR setup.In a second approach, the effect of formate as auxiliary carbon source to methanol on methylotrophic growth and PHB accumulation was investigated. A PHB content of 27.4 % per CDW was reached in the cultivation of Methylorubrum extorquens DSMZ 1337 with methanol as carbon source, while a PHB content of 16.0 % per CDW could be reached with methanol and formate in combination. Even though yields could not be improved under the given experimental conditions formate as co-substrate is worth being further investigated as it has shown to support increased growth and product yields in multiple organisms and under various conditions.