Effect of light quality on sulfide photo-oxidation and growth in an artificial biofilm of the green sulfur bacterium Prosthecochloris aestuarii.

We have succeeded in culturing an axenic biofilm of the green sulfur bacterium Prosthecochloris aestuarii strain CE 2404 in an artificial sandy sediment under visible light (400-700 nm). This simulates the conditions of deep submerged sediments. A five-week incubation period, using a 16-hour light / 8-hour dark regime, was applied in the benthic gradient chamber (BGC). The biofilm was located below the oxygen penetration depth of 1.2 mm, namely between 1.5 and 2.5 mm and the biomass peak was at 2.1 mm depth. This is much shallower compared to previously described artificial mats of P. aestuarii, which were grown in the BGC under near infrared (NIR)-rich light. High resolution time courses of photosynthesis were measured as sulfide photo-oxidation rates and studied under visible light and visible light amended with NIR to assess the effect of light quality. Sulfide photo-oxidation rates were rather low under visible light and strongly stimulated at most depths under full light conditions. However, under the latter conditions the rates decelerated after a maximum rate was reached at 8-10 min, apparently due to diffusional limitation of sulfide supply. It was concluded that the top of the mat was not limited by the photon flux density, while the biomass peak and the bottom of the biofilm were severely light limited under the culture conditions. These results support the hypothesis that a biofilm of P. aestuarii can develop in deep submerged sediments, when the oxygen penetration depth is very shallow. Nevertheless, the addition of NIR light strongly enhances the potential of P. aestuarii to grow deeper in the sediment.