Abstract:
The motility of the purple sulfur bacterium Marichromatium gracile was investigated under different light regimes in a gradient capillary setup with opposing oxygen and sulfide gradients. The gradients were quantified with microsensors, while the behavior of swimming cells was studied by video microscopy in combination with a computerized cell tracking system. M. gracile exhibited photokinesis, photophobic responses, and phobic responses toward oxygen and sulfide. The observed migration patterns could be explained solely by the various phobic responses. In the dark, M. gracile formed an ~500-µm-thick band at the oxic-anoxic interface, with a sharp border toward the oxic zone always positioned at ~10 µM O2. Flux calculations yielded a molar conversion ratio Stot/O2 of 2.03:1 (Stot = [H2S] + [HS-] + [S2-]) for the sulfide oxidation within the band, indicating that in darkness the bacteria oxidized sulfide incompletely to sulfur stored in intracellular sulfur globules.
