I am very happy to receive an invitation from Dr. Martin Koller to write something on my motivation for PHA research, how the knowledge about PHA and PHA-related R&D has changed in the time since I started with this topic, and what my ideas are for the future of PHA. I take this invitation as an initiative to think the past and future of these fascinating materials.
I started my PhD thesis in 1986 under the guidance of Professor Robert M Lafferty at the Graz University of Technology (TU Graz), Austria. At that time, as an organic chemist working on chemical synthesis of Nylon-6 under explosive or flammable conditions in organic solvents, I was very much attracted by the idea of using gentle conditions like aqueous phase, room temperature and normal pressure to make plastics. My initial research was on fermentation process development using Alcaligenes latus DSM 1122, 1123 and 1124 for production of copolymers PHBHV consisting of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV). At that time, ICI successfully launched the PHBHV product trademarked as BIOPOL, the whole world was enthusiastic for bioplastics as they seemed to solve the environmental and petroleum resource crisis. At the Institute of Biotechnology (today: Institute of Biotechnology and Biochemical Engineering) of TU Graz, I was able to interact not only with my supervisor Professor Lafferty but also with Dr. Gerhart Braunegg, Dr. Walter Steiner and Dr. Helmut Schwab as well as many highly talented PhD students. Dr. Gerhart Braunegg is the inventor of the gas chromatography (GC) method for qualitative and quantitative PHA determination. Although this GC method was invented and established in the 1970s, it is still the most popular method for PHA studies, as manifested by its numerous citations in the recent literature. For that all PHA researchers should pay respects to Gerhart. In addition, Gerhart has done some excellent works on using waste substrates for PHA production. He also actively promotes the concept of continuous fermentation for PHA production. This concept has an in-depth impact on my research career, which I will describe shortly.
I left Graz in 1990 to conduct my postdoc studies in University of Nottingham in UK. My postdoc study aimed to use PHA as a matrix to encapsulate drug for sustainable release. Very soon we found that PHA in vivo degradation was too slow to meet the sustainable release goal compared with other polymers like polylactide (PLA), poly(lactide-co-glycolide) (PLGA) and human serum albumin. However, the flexible mechanical property combined with biodegradability and biocompatibility makes PHA a suitable material for implant tissue engineering applications, and this forms part of my career interest in my research conducted at Tsinghua University/Beijing where I become an independent principle investigator.
PHA research has been changed dramatically since I started my PhD thesis 30 years ago. The changes can be summarized in the following areas:
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Much wider PHA diversity which evolves into “PHAome” similar to “Genome” or “Proteome”;
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Much more “Synthetic Biology” approaches for engineering PHA production strains in a more effective way;
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Open (unsterile) and continuous fermentation processes for high cell density and high PHA contents growth;
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High-value applications of PHA in areas of medical implant applications and smart material developments.
Since the breakthrough in weakening beta-oxidation in Pseudomonas spp. by our lab in 2010, it has become possible to precisely control the structures of PHA either in the form of homopolymers, random copolymers or block copolymers. This will really achieve tailor-made PHA based on property requirements. Since Pseudomonas spp. are the strains used to generate the “PHAome”, the economy to obtain diverse PHA is very important depending on the ability to reach high cell density growth for Pseudomonas spp. In this area, Dr. Bruce A Ramsay has done some excellent researches to realize high cell density growth of Pseudomonas putida containing over 50% PHA. The contribution of Bruce also contains the efficient extraction of PHA from Pseudomonas spp. In the future, PHAome produced by high cell density growth of Pseudomonas spp. will allow diverse PHA be produced in economically competitive way.
With the rapid developments of synthetic biology, many possibilities can be exploited to improve PHA production strains, including promoter engineering and ribosome-binding-site (RBS) optimization to enhance PHA content, minimized genome to, inter alia, increase substrate to PHA conversion efficiency, reprogramming cell growth pattern to accelerate cell growth, new pathway assembly for PHAome from glucose only, mixed substrate fermentation for single PHA molecules, morphology engineering the bacterial cell shapes for enhanced PHA accumulation and reduction on downstream purification cost. All of those efforts should lead to the significant reduction on PHA production costs.
Since PHA production cannot compete with the petrochemical plastic industries as it suffers from high consumptions on energy and freshwater, discontinuous processing, low product concentration and thus high product recovery cost as well as low substrate to product conversion efficiency. Therefore, to make PHA production competitive, we need to develop fermentation technology which is energy and fresh water saving and run in a continuous way instead of a batch way. To meet this need, a seawater technology based on Halomonas spp. grown in seawater in an unsterile and continuous process has been developed. Since seawater contains around 35 g/L NaCl and Halomonas spp. prefer higher pH, the process allows Halomonas spp. to grow in an open and continuous way for at least two months infection free. This seawater biotechnology achieves fresh water saving, energy saving and reduces process complexity combined with use of low cost fermentors, it significantly reduces the production cost of PHA and possibly other bioproducts. The Halomonas spp. have been engineered to allow them to become a platform for low cost production of biofuels, biomaterials and chemicals.
This vast diversity constituting the “PHAome” is waiting for exploitation. Except for very few PHAs that are commercially available from the market for application developments, such as PHB, PHBHV, P3HB4HB and PHBHHx, all other PHAs are produced by individual labs across the world in very small amounts for academic curiosity. Moving advanced materials like PHAs from the laboratory to the marketplace to address critical challenges in environment, energy, transportation, healthcare, and other areas of application concerns takes far too long, and at too high a cost. How to accelerate the pace of discovery and deployment of advanced PHA materials has been a central question for all researchers and stakeholders in this field. All these depend on the availability of the diverse PHAs in sufficient quantities and with consistent properties for studying their thermal and mechanical properties as well as other application potentials. It should be a global effort to establish platforms to supply diverse PHA based on PHAome information in sufficient quantities and material property consistency for various application developments. The understanding of the PHAome concept should allow us to construct a suitable bacterial platform that is able to provide consistent PHA molecular structures with less molecular weight variations for constant material properties. This is very important for commercial developments.
I hope this information is helpful for PHA researchers.
Thank you for your attention!
George Guo-Qiang Chen
Department of Microbiology and Biomaterials
Center for Synthetic and Systems Biology; School of Life Sciences
Tsinghua University
Beijing 100084, China
Tel: +86-10-62783844
Fax: +86-10-62794217
E-mail: chengq@mail.tsinghua.edu.cn
