Rhodospirillum rubrum is a metabolically versatile phototrophic bacterium with potential applications in sustainable hydrogen and bioplastic production. However, large-scale implementation remains limited by insufficient understanding of its growth dynamics under varying environmental conditions and the high energy costs of cultivation. This study presents a simplified, general mathematical model describing R. rubrum growth under anaerobic dark and light conditions, incorporating metabolic state transitions driven by substrate and carbon monoxide availability. The model, parameterized using literature data, captures key physiological features such as substrate inhibition and lag-phase behavior. Experimental validation under anaerobic light conditions demonstrated reasonable agreement with model predictions, while discrepancies were observed under dark conditions, that were attributed to experimental limitations rather than model deficiencies. The proposed framework provides a foundation for future model refinement and integration with other higher and lower level models.

Of all the greenhouse gases (GHGs), carbon dioxide (CO2) has been the target of most climate recovery efforts as it is the most abundantly emitted GHG by mass. In fact, in 2015 a legally binding international treaty was adopted by 196 parties in Paris, France to constrain the anthropogenic warming to 1.5-2.0˚C above the pre-industrial level. In order to meet this goal, a carbon budget was formulated as an estimate of the amount of carbon that can be emitted while limiting the anthropogenic warming to prescribed levels. However, the global CO2 emissions from industries are rapidly depleting this budget. Therefore, to mitigate the effects of climate change, CO2 emissions must be reduced by employing alternative commodities that can replace petrochemical resources. In this context, mixed culture fermentation presents an opportunity for redefining CO2 and waste streams as raw material for production of commodities traditionally derived from petrochemical resources. Previous studies by on this topic have indicated a potential association between elevated CO2 levels (pCO2) and butyrate formation from mixed culture fermentation. However, the cellular mechanism underlying this association are still poorly understood. Therefore, the principal objective of this research was to investigate the effects of initial substrate concentrations (g/L) and elevated pCO2 (bar) conditions on selectivity (moli/moltotal) of biomolecules produced from anaerobic conversion of glucose. For this purpose, a between-subject mixed factorial experimental design was developed to gauge the main and interaction effects of initial substrate concentrations (g/L) and elevated pCO2 (bar) conditions on selectivity of biomolecules. The principal findings of this research indicate that a strong positive relationship exists between the pCO2 and butyrate formation as the application of CO2 in reactor (EPBs) headspace resulted in higher butyrate selectivity compared to the control reactors (APBs). However, contrary to the conclusions reached by previous studies it was found that increasing the initial substrate concentration steered the product formation towards lactate and not butyrate. Whereas the highest recorded butyrate selectivity for EPBs was 30.41% for experimental condition with 5 g/L substrate concentration and 4 bar pCO2, the highest recorded butyrate selectivity for APBs was only 11.72% for 10 g/L substrate concentration and atmospheric pressure conditions. Conversely, the highest recorded lactate selectivity for EPBs was 15.13% for 20 g/L substrate and 3 bar pCO2 while the highest recorded lactate selectivity for APBs was 47.95% for 25 g/L substrate concentration and atmospheric pressure conditions. As a result of these investigations, theories concerning formation of butyrate and lactate were proffered in context of the role of CO2 in mixed culture fermentation. By confronting the existing understanding regarding product formation with new evidence this investigation seeks to advance theories concerning mixed culture fermentation.