Stepwise reduction of the culture redox potential allows the analysis of microaerobic metabolism and photosynthetic membrane synthesis in Rhodospirillum rubrum.

Bacterial growth under oxygen-limited (microaerobic) conditions is often accompanied by phenomena of great interest for fundamental research and industrial application. The microaerobic lifestyle of anoxygenic photosynthetic bacteria like Rhodospirillum rubrum harbors such a phenomenon, as it allows the formation of photosynthetic membranes and related interesting products without light. However, due to the technical difficulties in process control of microaerobic cultivations and the limited sensitivity of available oxygen sensors, the analysis of microaerobic growth and physiology is still underrepresented in current research. The main focus of the present study was to establish an experimental set-up for the systematic study of physiological processes, associated with the growth of R. rubrum under microaerobic conditions in the dark. For this purpose, we introduce a robust and reliable microaerobic process control strategy, which applies the culture redox potential (CRP) for assessing different degrees of oxygen limitation in bioreactor cultivations. To describe the microaerobic growth behavior of R. rubrum cultures for each of these defined CRP reduction steps, basic growth parameters were experimentally determined. Flux variability analysis provided an insight into the metabolic activity of the TCA cycle and implied its connection to the respiratory capacity of the cells. In this context, our results suggest that microaerobic growth of R. rubrum can be described as an oxygen-activated cooperative mechanism. The present study thus contributes to the investigation of metabolic and regulatory events responsible for the redox-sensitive formation of photosynthetic membranes in facultative photosynthetic bacteria. Furthermore, the introduced microaerobic cultivation setup should be generally applicable for any microbial system of interest which can be cultivated in common stirred-tank bioreactors.