Study of growth of Rhodospirillum rubrum and CO2 production in an anaerobic environment

We first studied the growth of R. rubrum to get familiar with its behavior. This bacterium is a versatile organism that can obtain energy through a variety of mechanisms. Respiration and photosynthetic mechanisms exist together and their activity depends upon the presence of light and energy. R. rubrum can grow in dark chemotropic environments with the presence of oxygen or can grow in a phototropic environment without oxygen. It was important to investigate the different parameters, as it helped me to improve my technics and skills in the laboratory, and gave me the knowledge to design the experiments that came next : the CO2 measurements. The main purpose of the research was to determine how we could help R. rubrum produce CO2 in light anaerobic condition using acetate as the sole carbon source (MELiSSA LAN). In the literature we found opposite views, some suggested that R. rubrum could not produce CO2 in these conditions, others stated that it could, but we could not find experimental results to back up the hypothesis. Attempting for the first time to measure the CO2 production of R. rubrum, we set the MELiSSA LAN as a negative control for CO2 production. But we were proven wrong as we measured significant amounts of CO2 produced in this very same conditions. The first objective was met, we could monitor a significant production of CO2, but we realized at the same time that our set-up was far from perfect. Measurement were only possible when the culture was depleted in oxygen, a process that took a lot of time, making CO2 measurements only possible from the end of the exponential phase onwards.. At this point the objectives of the internship shifted as we decided to investigate the effect of the carbon source on the CO2 production in light anaerobic conditions. We wanted to see if the results obtained with propionate or a mix between acetate and propionate would fit the views found in the literature. We could validate that R. rubrum required a net uptake of CO2 to metabolize propionate and we showed for the first time that a mix between acetate and propionate would led to a net CO2 production close to the CO2 with acetate as sole carbone source. These results can be very interesting for the future of the MELiSSA project as it proves that R. rubrum could be a good candidate to degrade a mix of volatile fatty acids and produces CO2 necessary for the ecosystem of the life support system. Future experiments will include the setup developed during this study to cultivate R. rubrum in light anaerobic conditions with a mixture of acetate, propionate and butyrate as carbon sources. Possible improvement of net CO2 production using microaerophilic conditions will also be tested.