Fundamental Studies on Microbial Lead Reduction and Polyhydroxyalkanoate Production

Abstract

Lead contamination threatens human life and the environment. The biological reduction of Pb(II) to metallic Pb is an attractive solution for Pb(II) pollution. Delftia acidovorans, Azonexus caeni, and Comamonas testosteroni were isolated and studied for their capabilities to utilize Pb(II) as a terminal electron acceptor. D. acidovorans strain Pb11 and A. caeni strain Pb2 cultures showed a 5.2- and 8.1-fold growth at 10.0 mg-Pb(II)/L in 3 d, respectively. Petroleum-based plastics are another emergent environmental concern. Polyhydroxyalkanoates (PHAs), a sustainable alternative to conventional plastics, are biodegradable polymers produced by PHA accumulators under autotrophic or heterotrophic conditions. In this thesis, the growth and enrichment of PHA accumulators, such as Plasticicumulans acidivorans and Cupriavidus necator, were investigated as they can accumulate 90% of their cell weight as PHA. An energetic model was developed to calculate theoretical PHA yields. The true autotrophic and heterotrophic PHB yields were estimated as 2.97 (g_PHB/ 〖mol〗_(H_2 )) and 0.66 (g_PHB/ g_acetate), respectively. Moreover, the growth of C. necator was investigated in lab-scale experiments under various autotrophic, heterotrophic, and mixotrophic conditions. When C. necator was cultivated in two-stage systems, high optical densities were attained in less than 24 h. In addition, a mathematical model for the competition between PHA and non-PHA accumulators in the feast-famine enrichments was developed. The calibrated and validated model for P. acidivorans suggested that microbial diversity in mixed cultures impacted the enrichment process. Another aspect of this thesis was to propose an innovative method for enriching PHA accumulators in mixed cultures. By applying autotrophic and autotrophic-heterotrophic enrichment strategies, C. necator dominated the mixed cultures (> 90%) in less than five days. Based on this thesis findings, it can be concluded that biotechnology applications in Pb(II) remediation and PHA production could reduce the severe impacts of Pb contamination, petrochemical plastics, and climate change due to elevated CO2 levels.