An open mixed culture was enriched with glycogen-accumulating organisms (GAOs) by using a sequencing batch reactor and treating an agroindustrial waste (sugar cane molasses) under cyclic anaerobic-aerobic conditions. Over a 1-year operating period, the culture exhibited a very stable GAO phenotype with an average polyhydroxyalkanoate (PHA) content of 17% total suspended solids. However, the GAO microbial community evolved over the course of operation to a culture exhibiting unusual characteristics in producing PHAs comprised of short-chain-length monomers, namely, 3-hydroxybutyrate, 3-hydroxy-2-methylbutyrate, 3-hydroxyvalerate, and 3-hydroxy-2-methylvalerate, and also, up to 31 mol% of the medium-chain-length (MCL) monomer 3-hydroxyhexanoate (3HHx). Microbial community analysis by fluorescence in situ hybridization revealed a concurrent long-term drift in the GAO community balance, from mainly "Candidatus Competibacter phosphatis" to mainly Defluviicoccus vanus-related organisms. The production of 3HHx was confirmed by 13 C nuclear magnetic resonance (NMR) and appeared to be related to the increased presence of D. vanus-related GAOs. These results suggest a broadened spectrum of material, chemical, and mechanical properties that can be achieved for biopolymers produced by open mixed cultures from fermented waste. The increased spectrum of polymer properties brings a wider scope of potential applications.Polyhydroxyalkanoates (PHAs) are biopolymers synthesized by bacteria as intracellular reserves of carbon, energy, and reducing power, with physical properties that make them an attractive source of biobased thermoplastics. These polymers are completely biodegradable and biocompatible and show promise for a broad range of engineering applications, from commodity to high-value-added specialty niche products (27).The most common PHAs are homopolymers of 3-hydroxybutyrate [P(3HB)] and copolymers with, for example, 3-hydroxyvalerate (3HV) [P(3HB-co-3HV)]. The homopolymer P(3HB) is highly crystalline with low impact strength. The copolymer P(3HB-co-3HV) exhibits contrasting mechanical properties, such as lower crystallinity and an increased impact strength and flexibility due to the incorporation of 3HV units. The copolymers with P(3HB) thus exhibit properties that are strongly dependent on the type and content of additional monomer elements. The incorporation of monomers other than 3HV may further extend the scope for PHA applications by tailoring the polymer's mechanical properties (8,17).The industrial production of PHAs is currently conducted using bacterial cultures of pure or recombinant strains that require sterile production conditions and strict process control. This state of the art has a high energy demand and, consequently, high production costs relative to those of, for example, polypropylene. Furthermore, well-defined and expensive substrates, such as glucose or propionic acid, are used as feedstock, contributing even further to the relatively high costs of production (10). As a result and notwithstandin...
