Recent Advances in Polyhydroxyalkanoate Production by Mixed Aerobic Cultures: From the Substrate to the Final Product Joa ˜o M. L. Dias, Paulo C. Lemos, Luı ´sa S. Serafim, Cristina Oliveira, Marta Eiroa, Maria G. E. Albuquerque, Ana M. Ramos, Rui Oliveira, Maria A. M. Reis * Chemistry Department, REQUIMTE/CQFB, FCT/Universidade Nova de Lisboa, Caparica 2829-516, Portugal Fax: þ35-121-294-8385; E-mail: amr@dq.fct.unl.pt Received: May 19, 2006; Revised: July 21, 2006; Accepted: July 25, 2006; DOI: 10.1002/mabi.200600112 Keywords: mixed cultures; modeling and control; operating parameters; polyhydroxyalkanoates (PHA); polymer characterization 1. Introduction Plastics occupy a high volume fraction in municipal landfills because of their relatively low density (0.9 g  cm 3 ). Substitution of synthetic plastics by biodegradable plastics can reduce almost 20% of the total waste by volume and 10% by weight. [1] Polyhydroxyalkanoates (PHA) have been recognized as good candidates for biodegradable plastics because of their similar properties to conventional plastics and their comp- lete biodegradability. Furthermore, PHA can be produced from renewable carbon sources, allowing for a sustainable and closed-cycle process for the production and use of such polymers. [2] Although the most well-studied PHA is poly(3-hydroxybutyrate) (PHB), over 150 different hydro- xyalkanoic acids are known as constituents of these bio- polymers at present. [3] From these, only the homopolymer of PHB and copolymers of 3-hydroxybutyrate (HB) and Review DOI: 10.1002/mabi.200600112 885 Summary: Numerous bacteria have been found to exhibit the capacity for intracellular polyhydroxyalkanoates (PHA) accumulation. Current methods for PHA production at the industrial scale are based on their synthesis from microbial isolates in either their wild form or by recombinant strains. High production costs are associated with these methods; thus, attempts have been made to develop more cost-effec- tive processes. Reducing the cost of the carbon substrates (e.g., through feeding renewable wastes) and increasing the efficiency of production technologies (including both fer- mentation and downstream extraction and recovery) are two such examples of these attempts. PHA production processes based on mixed microbial cultures are being investigated as a possible technology to decrease production costs, since no sterilization is required and bacteria can adapt quite well to the complex substrates that may be present in waste material. PHA accumulation by mixed cultures has been found under various operational conditions and configurations at both bench-scale and full-scale production. The process known as ‘‘feast and famine’’ or as ‘‘aerobic dynamic feeding’’seems to have a high potential for PHA production by mixed cul- tures. Enriched cultures submitted to a transient carbon sup- ply can synthesize PHA at levels comparable to those of pure cultures. Indeed, the intracellular PHA content can reach around 70% of the cell dry weight, suggesting that this pro- cess could be competitive with pure culture PHA production when fully developed. Basic and applied research of the PHA production process by mixed cultures has been carried out in the past decade, focusing on areas such as micro- bial characterization, process configuration, reactor opera- tional strategies, process modeling and control, and polymer characterization. This paper presents a review of the PHA production process with mixed cultures, encompassing the findings reported in the literature as well as our own exper- imental results in relation to each of these areas. Production of PHA by mixed cultures under feast-and- famine conditions. Macromol. Biosci. 2006, 6, 885–906 ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim