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Phototrophic iron-oxidizing bacteria, coloured scanning electron micrograph (SEM). The bacterium Rhodopseudomonas palustris TIE-1 is grown with acetate. This strain, which requires light to grow, is a rod-shaped gram-negative bacterium that can use acetate as an electron donor and carbon substrate. R. palustris can switch between different types of metabolism and grow on either acetate, hydrogen and ferrous iron. Magnification: x8000 when printed at 10 centimetres across. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)