Use of condensed corn solubles (CCS) and syngas for the production of poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) by Rhodospirillum rubrum.

摘要

Polyhydroxyalkanoate (PHA) is a naturally-occurring, biodegradable polymer with plasticlike characteristics. PHA has potential applications in the manufacturing of biodegradable plastics and providing an alternative to petroleum-based plastic products. Rhodospirillum rubrum produces PHA as a means of storing carbon and energy when excess carbon is present and another nutrient (N, S, phosphate, Fe, Mg, K or oxygen) is limiting. R. rubrum is typically cultured on an expensive medium (SMN), and our objective was to develop a less expensive alternative using condensed corn solubles (CCS), a byproduct of ethanol fermentation. In serum bottle trials a basal medium, composed of 240 g/L CCS (wet basis), performed as well as SMN and tryptic soy broth, achieving a maximum cell density of 1.6E9 CFU/ml and growth rate of 0.95 h-1. Illumination had no effect on growth, and supplementation of CCS with nickel or biotin also had no stimulatory effect. CCS concentrations of 80-320 g/L also supported similar growth. The presence of a small amount of oxygen in the headspace resulted in significantly higher (P0.05) maximum cell densities and growth rates, compared to strictly anaerobic cultures. In bioreactor trials growth was significantly better in 240 g/L CCS (2.3E9 CFU/ml) than in the defined mSMN medium (1.9E9 CFU/ml). Active aeration of the CCS medium greatly increased maximum cell density to 1.3E10 CFU/ml. Nitrogen supplementation of 240 g/L CCS increased the C:N ratio from 6.7:1 to 5.7:1 which dramatically increased maximum cell density (9.6E10 CFU/ml) and carbon utilization when the pH of the media was controlled. Decreasing the concentration of the CCS to 80 g/L supplemented with nitrogen allowed good cell growth with complete utilization of carbon and nitrogen in the growth phase, not P(HB-HV) production phase. This carbon and nitrogen utilization would ensure that P(HB-HV) would be produced under nitrogen-limiting conditions with an additional source of carbon at that time. Syngas (produced from the gasification of dry distillers grain) was tested as a potential source of carbon for autotrophic P(HB-HV) synthesis after initial growth in CCS. Optimal timing of syngas addition was found to be between 6 h and 17 h when most of the nutrients are used up in the CCS and the culture has reached a high density so it starts using the syngas carbon for P(HB-HV) synthesis. Centrifuging and switching to autotrophic growth conditions with a basal medium was not a sufficient trigger to induce R. rubrum to accumulate a significant amount of P(HB-HV). Utilization of CCS and syngas for P(HB-HV) production would provide the ethanol industry with an additional high value product.