Conversion of Volatile Fatty Acids into Carbon Dioxide by the Bacterium Rhodospirillum rubrum

The bacterium Rhodospirillum rubrum may be able to produce carbon dioxide while assimilating a mixture of VFAs. This would be beneficial to the MELiSSA loop since the volatile fatty acids, produced in previous compartment, will be processed while CO2 would benefit the growth of autotrophic compartments.
In this thesis, the conversion of volatile fatty acids into carbon dioxide by R. rubrum is evaluated in an anaerobic environment in the presence of light where a photoheterotrophic metabolism should take place. R. rubrum cultures were first generated in 50 mL penicillin flasks, then upscaled to one liter cultures in Schott bottles. Two different scenarios regarding the carbon source present for the bacterium have been considered. Firstly, acetate as the sole carbon source has been investigated to confirm previous research on the same topic. On the other hand, a 7:2:1 mixture of acetate, butyrate and propionate has been provided as carbon source for the cultures. The latter scenario simulates the mixture of VFAs coming from compartment 1 in the MELiSSA loop more closely. These cultures were performed in axenic conditions and evaluated daily on
various parameters, including the optical density, dissolved oxygen, redox potential, pH, carbon dioxide production and VFA quantification.
The findings in this thesis suggests that a net CO2 production is observed and acetate is being assimilated when R. rubrum cultures are cultivated in light anaerobic conditions with acetate as sole carbon source, confirming the result from the similar experiment that took place at SCK CEN by Moussalli (2019). When a mixture of acetate, butyrate and propionate in a ratio of 7:2:1 is provided as carbon source, butyrate was assimilated completely before acetate while CO2 production was observed. However, the CO2 concentrations that were produced were lower in comparison to when acetate as sole carbon source was provided.