Rhodospirillum rubrum

R. rubrum cells are gram - negative , mainly spiral-shaped, with polar flagella , reaching 3-10 microns in length and 0.5-1.5 microns in width. Cells have a multilayer outer sheath. In very young cells are similar to non- photosynthetic ones . With anaerobic growth, lamellae and colored chromatophores appear in the cells in the light. Chromatophores appear in cultures after the first 12 hours of growth, and cells of cultures older than 8 days have both chromatophores and lamellae ^[2] . Cells growing under aerobic conditions have no chromatophores. Chromatophores contain a lot of protein , bacteriochlorophylls , carotenoids (for example, spirilloxanthin ^[3] ), as well as phospholipids . Chromatophores can carry out light-dependent synthesis of ATP , that is, they are necessary for photosynthesis. Since carotenoids appear only under anaerobic conditions, when the cell photosynthesizes, the cells become stained only under anaerobic conditions ^[4] .

Under anaerobic conditions in the light, R. rubrum can exist as a or photoautotroph , and in the dark it can grow under microaerophilic or aerobic conditions ^[3] . Under aerobic conditions, photosynthesis in R. rubrum cells is suppressed. The fixation of CO _{2 is} carried out using the Calvin cycle with the key enzyme Rubisco . Under photoheterotrophic conditions, the Calvin cycle plays an important role in the balance of redox equivalents and serves as their receptacle ^[5] . R. rubrum is the first known bacterium that expresses Rubisco under aerobic chemoheterotrophic conditions ^[6] .

R. rubrum is also a nitrogen-fixing bacterium , that is, it can express and regulate nitrogenase , a protein complex that catalyzes the conversion of atmospheric nitrogen to ammonia . Because of this important quality, R. rubrum was chosen by many researchers as a model organism for understanding the pattern and regulation of nitrogen fixation ^{[7] [8] [9] [10]} . It was on R. rubrum that the post-translational regulation of nitrogenase was first demonstrated. It consists in the fact that the nitrogenase protein is modified by ADP-ribosylation at the 101st arginine residue (Arg101) ^[11] . This reaction occurs in response to the presence in the environment of the so-called shutdown factors - L- glutamine or ammonia, as well as when dark. This behavior of the microorganism is understandable: it is much easier to get nitrogen by cleaving the amino groups from glutamine or by assimilating ammonia. The absence of light makes photosynthesis impossible and, as a result, the process of nitrogen fixation ^[12] .

The European Space Agency selected the Rhodospirillum rubrum S1H strain for photoheterotrophic assimilation of volatile fatty acids in the MELiSSA restorative life-supporting system ^[13] . Genetically modified R. rubrum can be used in biotechnology for the synthesis of polyhydroxyalkanoates (natural biodegradable polymers , which in many cases can replace indecomposable polymers derived from petroleum products), as well as other biopolymers ^[14] from synthesis gas (a mixture of carbon monoxide , carbon dioxide) and hydrogen ). The synthesis of gas, respectively, can be obtained using the pyrolysis of organic waste ^{[1] [15] [5]} . R. rubrum has the potential for use in wastewater treatment ^[16] .