Motility of Marichromatium gracile in response to light, oxygen, and sulfide

The motility of the purple sulfur bacterium Marichromatium gracile was inve stigated under different light regimes in a gradient capillary setup with o pposing oxygen and sulfide gradients. The gradients were quantified with mi crosensors, while the behavior of swimming cells was studied by video micro scopy in combination with a computerized cell tracking system. M. gracile e xhibited photokinesis, photophobic responses, and phobic responses toward o xygen and sulfide. The observed migration patterns could be explained solel y by the various phobic responses. In the dark, M. gracile formed an simila r to 500-mum-thick band at the oxic-anoxic interface, with a sharp border t oward the oxic zone always positioned at similar to 10 muM O-2. Flux calcul ations yielded a molar conversion ratio S-tot/O-2 of 2.03:1 (S-tot = [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. In the light, M. gracile spread into the an oxic zone while still avoiding regions with > 10 muM O-2. The cells also pr eferred low sulfide concentrations if the oxygen was replaced by nitrogen. A light-dark transition experiment demonstrated a dynamic interaction betwe en the chemical gradients and the cell's metabolism. In darkness and anoxia , M. gracile lost its motility after ca. 1 h. In contrast, at oxygen concen trations of > 100 muM with no sulfide present the cells remained viable and motile for ca. 3 days both in light and darkness. Oxygen was respired also in the light, but respiration rates were lower than in the dark. Observed aggregation patterns are interpreted as effective protection strategies aga inst high oxygen concentrations and might represent first stages of biofilm formation.