Rhodobacter sphaeroides showed chemotaxis to the terminal electron acceptors oxygen and dimethyl sulfoxide, and the responses to these effectors were shown to be influenced by the relative activities of the different electron transport pathways. R. sphaeroides cells tethered by their flagella showed a step-down response to a decrease in the oxygen or dimethyl sulfoxide concentration when using them as terminal acceptors. Bacteria using photosynthetic electron transport, however, showed a step-down response to oxygen addition. Addition of the proton ionophore carbonyl cyanide 4-trifluoromethoxyphenylhydrazone did not cause a transient behavioral response, although it decreased the electrochemical proton gradient (⌬p) and increased the rate of electron transport. However, removal of the ionophore, which caused an increase in ⌬p and a decrease in the electron transport rate, resulted in a step-down response. Together, these data suggest that behavioral responses of R. sphaeroides to electron transport effectors are caused by changes in the rate of electron transport rather than changes in ⌬p.The purple nonsulfur phototroph Rhodobacter sphaeroides is a motile, chemotactic bacterium capable of growth and division under a wide range of conditions, including aerobic heterotrophic, anaerobic phototrophic, anaerobic photoheterotrophic, and dark anaerobic conditions, using an alternative electron acceptor like dimethyl sulfoxide (DMSO) or trimethylamine-N-oxide. To grow and divide under these different conditions, R. sphaeroides can induce a range of alternative oxidases and terminal reductases, including one for DMSO and trimethylamine-N-oxide. Not all of the different carriers and terminal reductases are induced to high levels under all conditions, and there appears to be some redundancy. For example, during anaerobic growth in the dark using DMSO or trimethylamine-N-oxide as an electron acceptor, R. sphaeroides synthesizes a full complement of photosynthetic pigments, as well as the ''alternative'' cytochrome b 260 oxidase (38), and during anaerobic photoheterotrophic growth, both the b 260 and b 410 (KCN-sensitive) oxidases are induced (22,39). This redundancy allows R. sphaeroides populations to respond rapidly, in terms of electron transport, to changes in their environment.Previous work on the phenomenon of aerotaxis in Escherichia coli and Salmonella typhimurium has indicated that it is independent of the methyl-accepting chemotaxis proteins used as part of the primary sensing system in these organisms (26, 34), although methyl-accepting chemotaxis proteins have been implicated in aerotaxis in Bacillus subtilis (37). R. sphaeroides appears to have a chemotaxis system comprising both methylaccepting chemotaxis protein-dependent and -independent pathways (36). This, in conjunction with its flexible electron transport system, makes R. sphaeroides an interesting organism for the study of taxis to effectors linked to electron transport. Aerotaxis and phototaxis in the purple photosynthetic bacteria were first identifie...