Design of kinetic models for assessment of critical aspects in bioprocess development : A case study of biohydrogen

Sammanfattning: The world faces major climate challenges and extensive efforts need to be taken to combat this issue. Replacing fossil-derived fuels and chemicals with renewables are one important step on the way. Hydrogen has a great potential as a renewable energy carrier for the transport sector and as a green chemical for the industry. Today, the production of hydrogen stems primarily from fossil resources. A sustainable alternative to the current methods of hydrogen production are via biological methods using micororganisms and renewable substrates. Caldicellulosiruptor species are thermophilic bacteria able to produce hydrogen close to the theoretical maximum of 4mol H2/mol hexose. Due to economic reasons, it is preferable if the microorganism can utilize different kinds.of substrates containing both pentose and hexose sugars as well as to withstand high amounts of sugar in the feed. These two aspects were quantitatively evaluated in this research by using kinetic models. Modelling is an important tool in bioprocess development since it can contribute to an increased understanding of the process and function as a predictor for future process performance and hence strive towards in silico assessments which are more cost effective.When a microorganism is exposed to several sugars a phenomenon called diauxic-growth can occur. Caldicellulosiruptor saccharolyticus was exposed to an industrial substrate, wheat straw hydrolysate (WSH), containing glucose, xylose and arabinose, as well as to an artifical sugar mixture containing the same amount of sugars as in the WSH. It was displayed that Caldicellulosiruptor saccharolyticus expresses a diauxic-like behaviour; simultaneously taking up different sugars (hexose and pentose) but with a preference for the pentoses. When the pentoses are depleted, there is a short lag phase followed by the continued uptake of the hexoses, however, at an altered rate. This is displayed as a biphasic growth curve, most visible in the hydrogen and carbon dioxide productivity profile. We hypothesize that there are several enzyme systems involved in the uptake that are either upregulated or downregulated depending on which sugar that is preferred. By using cybernetic variables that describe which transport system that is active this phenomenon could be describedmathematically.Caldicellulosiruptor owensensis’ tolerance towards high sugar and end-product concentration (i.e., high osmolarity) were evaluated and described mathematically. The kinetic growth model was appropriate to describethe behaviour of growth when exposed to 10 and 30 g/L of glucose. At higher sugar concentration, 80 g/L, the model slightly overestimated the growth. A critical osmolarity parameter was quantified and showed a fourfold increase in value with an increasing osmolarity. This means that Caldicellulosiruptor’s tolerance to a high osmolarity had increased in the adaptive laboratory evolution experiments conducted earlier. Producing biohydrogen with microorganisms such as Caldicellulosiruptor species has great potential in the transformation from a fossil to a bio-based economy. Further efforts in constructing and tuning kinetic models for biohydrogen production would be benficial from a process development point of view.