Growth of green sulphur bacteria in experimental benthic oxygen, sulphide, pH and light gradients

The green sulphur bacterium Prosthecochloris aestuarii (strain CE 2401) was cultured in a benthic gradient chamber to study its growth and photosynthetic activity in experimental gradients of oxygen, sulphide and light. An axenic biofilm was obtained within evenly inoculated artificial sediment after 5 weeks of incubation. The phototrophic biofilm was located 2.2-3.5 mm below the sediment surface, i.e. below the maximal penetration depth of oxygen, thus confirming that growth of P. aestuarii was restricted to strictly anoxic sediment layers. The activity was limited by the diffusive flux of sulphide, showing the role of molecular diffusion in growth of this benthic species. Scalar irradiance was attenuated strongly in the biofilm, with distinct attenuation maxima at 750 nm corresponding to bacteriochlorophyll c (Bchl c) absorption and at 800 nm corresponding to bacteriochlorophyll a (Bchl a) absorption. Using radiance attenuation data as a proxy for photopigment contents it was shown that the ratio Bchl a/Bchl c changed with depth. This indicates chromatic adaptation to changing light climates in the sediment. Total sulphide oxidation was estimated from the sulphide fluxes from below into the reaction zone. Measurements of sulphide oxidation as a function of scalar irradiance in the reaction zone showed that anoxygenic photosynthesis of the biofilm was saturated at a scalar irradiance (430-830 nm)>2 μmol photons m^-2 s^-1.