MACROSCOPIC PHOTOTACTIC BEHAVIOR OF THE PURPLE PHOTOSYNTHETIC BACTERIUM RHODOSPIRILLUM-CENTENUM

Most photosynthetic microorganisms have the capability of photosensing light quality and intensity. Movement of motile photosynthetic microo rganisms to locales that offer optimal physical and chemical condition s for light-dependent growth provides obvious selective advantages. Am ong phototrophs, many cyanobacteria and algae migrate towards or away from the direction of light, a process termed phototaxis. In contrast, anoxygenic photosynthetic bacteria are believed to respond to changes in light intensity in a manner that is not related to the direction o f light, a process that is often described by the term ''photophobic'' . We recently reported that ''swarm colonies'' of the purple photosynt hetic bacterium Rhodospirillum centenum are capable of macroscopically visible phototactic behavior. In the present study we further charact erize the phototactic behavior of R. centenum swarm colonies and provi de an action spectrum that delineates re ions of the spectrum that eli cit positive and negative phototaxis.