(647e) Multi-Scale Model Assisted Design of Fermentation Strategies for Efficient Microbial Production of Polyhydroxyalkanoates (PHAs)

Biocatalysis, despite the advantages regarding sustainability and selectivity of product formation, is still not viewed as a first-line alternative, but only as a last resort when other synthetic schemes have failed. The limited production of polymers using non-conventional methods (i.e., use of non-fossil resources and biocatalysts instead of chemical/metal catalysts) is a typical example that highlights the reluctance of the chemical industry to turn to biocatalysis. Thus, biopolymers, either are biodegradable and/or biobased, despite the fact that are broadly recognised for driving the evolution of plastics and contribute significantly to a sustainable society, they have apparently a niche market, considering that they represent only the 1% of the total polymer materials with a global production level of about 250 million tons.

A member of the biopolymers family that is drawing considerable attention is the polyhydroxyalkanoates (PHAs), which are completely biodegradable polymers produced by a wide variety of bacteria through the fermentation of sugars, lipids, alkanes, alkenes and alkanoic acids. PHAs hold the third position in the global ranking of biodegradable polymers with respect to their annual consumption, after the starch-polymers and polylactic acid and they stand out because of their thermoplastic and elastomeric properties and a wide range of physical and mechanical properties which are comparable to the respective one of commercial polymers like polyethylene and polypropylene. Despite the great potential of PHAs to act as alternatives to common plastics derived from petrol, their introduction to the world-wide polymer market is inconsistent with their advantageous characteristics. This is due to the fact that since several biotechnological production strategies of PHAs have barely passed the test of economic viability. The reasoning behind this is the lack of a knowledge-based methodology for the bioproduction of PHAs that will enable the control of production efficiency and product quality. Apparently, little effort has been made over the development of methods for the control of the molecular properties and the optimal fermentation process operation for the commercially efficient production of PHAs with tailored molecular properties, which are important determinants of their respective mechanical properties and end-uses.

It is known that in bacterial cultures, the accumulation of PHAs in the cells strongly depends on the culture nutritional conditions and aeration, affecting accordingly the followed pathways in the metabolic network of the cells. Thus, it is evident that there is inherited complexity over the synthesis of PHAs in microbial cell factories where a number of interactive biochemical phenomena of different length and time scale occurring simultaneously. Therefore in order to deal with such a problem, it is required extensive knowledge with respect to:

i.)        the synthesis rates of a number of enzymes and metabolites in the metabolic reaction network and the polymerization kinetic mechanism occurring in the cytoplasm of each individual cell, reflecting micro-scale phenomena;

ii.)       the growth, division and death of the cell population, the cell-cell interactions and the swelling of the cells by the accumulation of PHA granules, reflecting meso-scale phenomena and

iii.)     the fermentation substrates consumption, the increasing culture density affecting the mixing pattern in the culture and the heat and mass (nutrients, oxygen and CO₂) transfer phenomena in the culture medium, reflecting macro-scale phenomena.

It is understood that the above described cellular activities, that is the substrates consumption and their catabolism by the cells, the cells growth and respiration, the synthesis of products as well as the cells division and death are highly interactive. Hence, the use of individual sub-models focusing on the above phenomena independently can only give results of low accuracy. This nature of the problem calls for an integrated model that combines the individual sub-models in order to ensure bottom-up and top-down flow of information (i.e., from micro-scale to macro-scale model and reversely) resulting in a multi-scale modeling approach of the studied problem. It is the scope o the present work primarily to analyze the model requirements in response to the various problem dimensions. Then a combined metabolic/polymerization/macroscopic model will be developed, accounting for a highly structured biomass, for the accurate simulation and prediction of the culture growth (lag-, exponential- and stationary phase), the PHA production rate and the molecular properties of PHA and thus, for the efficient control and operation of a PHA microbial production process.