Synthesis of intracellular polyhydroxyalkanoates (PHA) from mixed phenolic substrates in an acclimated consortium and the mechanisms of toxicity

Abstract

Polyhydroxyalkanoates (PHA) are a class of intracellular storage polymers with the potential to supplement/replace petroleum-based plastics. Production of PHA from waste organics can reduce its cost, but toxic carbon sources such as simple aromatics are seldom used. This study investigates the feasibility of mixed toxic phenolics as substrates, utilizing a phenol-acclimated consortium of high PHA accumulation capability. Phenol was employed as the main feedstock, and four kinds of mono- and di-chlorinated phenols (CPs) were individually mixed with it as possible impurities. The inclusion of CPs was found to reduce substrate removal and PHA accumulation rates, compared to phenol alone. The degree of inhibition was positively-correlated with CP concentration, but the mechanisms differed. 2- and 3-chlorinated phenols formed dark, toxic, dead-end products; 4-chlorinated phenol (4-CP) appeared to exert competitive inhibition; and 2,4-dichlorinated phenol acted like an inert component. Transformation pathway study discovered that phenol was degraded via meta-cleavage, while 2- and 3-CPs were transformed into 3-chlorocatechol as the intermediate. FTIR, DSC, NMR, HPLC and elemental analysis were carried out to characterize the extracted polymer. The results show that phenol was converted to poly(3-hydroxybutyrate), while the inclusion of mono-chlorinated phenol promoted the production of PHA containing 3-hydroxyvalerate (3HV) monomers. A recovery test after mild CP exposure found that the affected biomass was able to resume phenol degradation and PHA synthesis immediately after simple aeration, indicating its robustness and applicability in a real industrial setting.