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 microorganisms to locales that offer optimal physical and chemical conditions for light-dependent growth provides obvious selective advantages. Among 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 of light, a process that is often described by the term "photophobic". We recently reported that "swarm colonies" of the purple photosynthetic bacterium Rhodospirillum centenum are capable of macroscopically visible phototactic behavior. In the present study we further characterize the phototactic behavior of R. centenum swarm colonies and provide an action spectrum that delineates regions of the spectrum that elicit positive and negative phototaxis.