Building a predictive model for PHB production from glycerol

Abstract

Poly-3-hydroxybutyrate (PHB) is a biodegradable biopolyester with plastic like properties, which on its own or as part of a heteropolymer, finds application in everyday products, competing directly with fossil fuel based plastics in terms of physical and mechanical properties. In nature, PHB is produced as an energy reservoir for the host cell, when environmental conditions limit growth. It is this inherent condition for PHB synthesis (i.e. an environment unsuitable for growth) that challenges design of conventional batch production systems. Balance between growth (driven by nitrogen availability) and PHB production (enhanced by an excess of carbon) is the critical aspect for consideration in such designs. However, selecting the best operating conditions is not obvious for this system and so a systematic approach has been used in this paper, utilising simulations based on a purpose built model to supplement experimental studies.The interaction between the carbon and nitrogen sources (glycerol and ammonium sulphate respectively) was carefully evaluated and incorporated into a low-structured model able to describe the dynamics of substrate consumption and product accumulation during C.necator DSM 545 cultivation at small scale. The kinetic parameters thus determined have been assumed to be constant, fixed accordingly, and the model used to predict the fermentation profiles for different operating conditions. Results showed good agreement with experimental data, supporting the efficacy of this approach. The dual utilization of multiple substrates suggests there is a system capacity to which both growth and PHB production contribute and that sets the maximum total biomass concentration. A logistic type term added to both growth and product rate equations enabled the effective decoupling of cell proliferation and PHB accumulation for a wide range of scenarios. In this way, the combination of predictive modelling and experimental verification potentially reduces, by a significant amount, the number of experiments required to establish operational targets such as specific growth rate and productivity as well as identifying often sought criteria such as optimum C:N ratio. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 List of symbols:Half saturation constant for glycerol (g/L)Inhibition constant for PHB production (g/L)Half saturation constant for nitrogen (g/L)Half saturation constant for PHB (g/L)Carrying capacity (g/L)Hill coefficient (-)Nitrogen concentration (g/L)Yield of PHB based on glycerol (g/g)Yield of non-PHB biomass based on nitrogen (g/g)Yield of non-PHB biomass based on glycerol (g/g)

Dynamic Metabolic Analysis of Cupriavidus necator DSM545 Producing Poly(3-hydroxybutyric acid) from Glycerol

“…Bioreactor experiments were conducted in duplicates and flask experiments were conducted in triplicates. The medium recipes for both flasks and bioreactor experiments can be found in our previous publication, Pérez Rivero et al [41]. In order to benchmark the performance of the bacterial culture, we used defined media supplemented with refined glycerol rather than crude glycerol.…”

Polyhydroxyalkanoates Production: A Challenge for the Plastic Industry

“…Glycerol, a byproduct of the production of biodiesel, has been regarded as an inexpensive raw material to produce many compounds of higher value. ,, Several microbial strains are suitable for the production of PHAs from glycerol. C. necator was able to grow using glycerol (purity of 88%) as a carbon source, yielding up to a dry cell concentration of 82.5 g/L and 62 wt % PHB .…”

From waste to wealth: upcycling of plastic and lignocellulosic wastes to PHAs

“…Nonetheless, to become a competitive alternative to fossil fuel plastics, the cost of PHA still needs to be reduced. Different approaches have been applied to address the cost reduction challenge, including strain engineering, investigation of low‐cost feedstock, and improvement of the downstream operation 17‐20 . Up to date, PHAs have been integrated in certain packaging applications but are mostly used in niche applications, including targeting delivery or biomedical use 21 .…”