GROWTH OF GREEN SULFUR BACTERIA IN EXPERIMENTAL BENTHIC OXYGEN, SULFIDE, 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 p hotosynthetic activity in experimental gradients of oxygen, sulphide a nd light. An axenic biofilm was obtained within evenly inoculated arti ficial sediment after 5 weeks of incubation. The phototrophic biofilm was located 2.2-3.5 mm below the sediment surface, i.e. below the maxi mal penetration depth of oxygen, thus confirming that growth of P. aes tuarii was restricted to strictly anoxic sediment layers. The activity was limited by the diffusive flux of sulphide, showing the role of mo lecular diffusion in growth of this benthic species. Scalar irradiance was attenuated strongly in the biofilm, with distinct attenuation max ima at 750 nm corresponding to bacteriochlorophyll c (Bchl c) absorpti on and at 800 nm corresponding to bacteriochlorophyll a (Bchl a) absor ption. Using radiance attenuation data as a proxy for photopigment con tents it was shown that the ratio Bchl a/Bchl c changed with depth. Th is indicates chromatic adaptation to changing light climates in the se diment. Total sulphide oxidation was estimated from the sulphide fluxe s from below into the reaction zone. Measurements of sulphide oxidatio n as a function of scalar irradiance in the reaction zone showed that anoxygenic photosynthesis of the biofilm was saturated at a scalar irr adiance (430-830 nm) >2 mu mol photons m(-2) s(-1).