Coloured scanning electron micrograph (SEM) of Rhodospirillum rubrum, Gram-negative, purple nonsulfur prokaryote (bacterium). R. rubrum is a purple-colored Proteobacterium from the family Rhodospirillaceae. Its colour comes from carotenoid pigments, rhodovibrin and spirilloxanthin. It is a spiral-shaped bacterium that has polar flagella and can live both anaerobically and aerobically. Under anaerobic growth conditions (without oxygen) it can photosynthesize using light (phototroph). As a phototroph, R. rubrum can grow autotrophically or heterotrophically. Photosynthesis by R. rubrum differs from that of plants, it does not have chlorophyll a, but bacteriochlorophyll. The carotenoids also help gather light energy for photosynthesis. Under aerobic growth conditions (with oxygen) it is called a chemotroph. R. rubrum is considered both heterotrophic and autotrophic. It is found in mud, pond water, and sewage samples. Magnification: x2,600 when shortest.
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