Recent Advances in Polyhydroxyalkanoate Production by Mixed Aerobic Cultures: From the Substrate to the Final Product

Abstract

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-effective processes. Reducing the cost of the carbon substrates (e.g., through feeding renewable wastes) and increasing the efficiency of production technologies (including both fermentation 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 cultures. Enriched cultures submitted to a transient carbon supply 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 process 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 microbial characterization, process configuration, reactor operational 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 experimental results in relation to each of these areas.

Link between microbial composition and carbon substrate-uptake preferences in a PHA-storing community

“…When other substrates are available for cell growth (such as acetate), microorganisms can use butyrate and valerate for PHA synthesis more efficiently by not going through the b-oxidation pathways but only through direct activation followed by polymerization (Dias et al, 2006). For instance, if both butyrate and acetate are available and consumed by the microorganisms, and both are used for cell growth as well as PHA storage, the metabolic flux from butyrate toward the TCA cycle has to go through the formation of acetoacetylCoA and then acetylCoA.…”

“…Extensive research has been carried out on the impact of different SBR-operating conditions (Dias et al, 2006;Serafim et al, 2008). However, sufficient knowledge on all factors governing microbial competition has not yet been gathered to support comprehensive mathematical models that allow the design of a strategy able to couple a high selective pressure for PHA storage with a high biomassproduction rate.…”

Bacterial polyhydroxyalkanoates-eco-friendly next generation plastic: Production, biocompatibility, biodegradation, physical properties and applications

“…Recently, PHAs have attracted considerable attention due to their potential use as biodegradable thermoplastics and sources of chiral monomers (34,44,45). Thus, research is currently being performed to improve productivity, to reduce production costs and, more importantly, to produce specific functionalized PHAs (46)(47)(48). Therefore, the aim of the present study is to review the current advances and progress in biosynthesis, accumulation, extraction, chemical and physical properties, biocompatibility, degradation and potential applications of bacterial thermoplastic PHAs.…”

Composition of mixed culture PHA biopolymers and implications for downstream processing

“…In the first stage a mixed consortia is submitted to alternating periods of substrate availability and substrate depletion (a feast/famine strategy) (Dionisi et al 2004, Mengmeng et al 2009). During the famine period, the stored PHA is consumed, providing more balanced growth conditions for the PHA accumulating microorganisms, thus giving them an advantage over non-accumulating bacteria (Dias et al 2006) and enriching the culture in these organisms. In the second stage, the PHA-storing microorganisms are transferred to a nutrient limited fed-batch reactor with a high organic load maximising the polymer accumulation.…”

“…In comparison with pure cultures, it is also possible to produce copolymers with higher fractions of monomer units other than 3-hydroxybutyrate (3HB) , which could open up the 13 opportunity to produce polymers with a broader range of properties and therefore potentially novel applications. However, although high PHA yields and diverse compositions have been obtained in mixed cultures, it is still necessary to aim to: (1) increase the volumetric productivity (cell concentration is still low compared with pure cultures (Dias et al 2006)) and thus improving the yields following extraction and purification; (2) understand the effects of process parameters (feeding strategy, carbon source, dissolved oxygen concentration, etc.) and biological factors (community dynamics, cellular physiology, etc.)…”