Velocity Changes, Long Runs, and Reversals in the Chromatium minus Swimming Response

The velocity, run time, path curvature, and reorientation angle of Chromatium minus were measured as a function of light intensity, temperature, viscosity, osmotic pressure, and hydrogen sulfide concentration. C. minus changed both velocity and run time. Velocity decreased with increasing light intensity in sulfide-depleted cultures and increased in sulfide-replete cultures. The addition of sulfide to cultures grown at low light intensity (10 microeinsteins m^-2 s^{- 1}) caused mean run times to increase from 10.5 to 20.6 s. The addition of sulfide to cultures grown at high light intensity (100 microeinsteins m^-2 s^{- 1}) caused mean run times to decrease from 15.3 to 7.7 s. These changes were maintained for up to an hour and indicate that at least some members of the family Chromatiaceae simultaneously modulate velocity and turning frequency for extended periods as part of normal taxis.