Enrichment Strategies for Polyhydroxyalkanotaes (Pha) Production from Fermented Crude Glycerol Using Microbial Mixed Cultures

Polyhydroxyalkanoates (PHA) are biopolymers produced as internal carbon storage in bacteria. They are a potential alternative to petroleum derived plastics, although their competitiveness is still limited by their productions costs. These could be lowered by the use of waste substrates - such as crude glycerol, a major by-product of the biodiesel industry- and microbial mixed cultures - which do not require sterilisation 1.An important limitation for the PHA production from glycerol is the side accumulation of glycogen, a polymer with no market value. This can be circumvented by submitting first the substrate to anaerobic fermentation to produce volatile fatty acids (VFA), which are preferred substrates for PHA production2. However, the fermentation also leads to 1,3-propanediol (1,3-PDO), a compound not previously described for PHA production.Initial experiments aimed at the transformation of both VFA and 1,3-PDO to PHA. Using the classical feast-famine enrichment strategy, PHA production from 1,3-PDO was not observed. Conversely, the transformation was achieved by using a new strategy that limited the nitrogen during the feast phase. Nevertheless, the PHA yield obtained form 1,3-PDO was very low compared to the one obtained from VFA (0.24 vs 0.74 Cmol PHA/Cmol S).Given that 1,3-PDO is also a high value product, a third enrichment strategy was applied where only VFA were converted into PHA (up to 70% of the cells dry matter with a yield of 0.9 Cmol PHA/Cmol S) and 1,3-PDO was recovered from the process (97% recovery) in a biorefinery scheme. This strategy, based on the development of substrate preferences in mixed cultures, has the potential of being applied in other downstream processes of biological conversions, where fermentation side-products could be transformed into a high value product (PHA).