Cells curved rods or spirals (sigmoid), dividing by binary fission, motile by (multi trichous) polar flagella. Cells may be immobile and grow in irregular aggregates surrounded by slime. Gram negative, belong to the Gammaproteobacteria, and contain internal photosynthetic membranes of vesicular type in which the pho tosynthetic pigments bacteriochlorophyll a and carotenoids are located.