Dynamics of anoxygenic photosynthesis in an experimental green sulphur bacteria biofilm

The dynamics of sulphide oxidation in an experimental biofilm of the green sulphur bacterium, Prosthecochloris aestuarii, were studied using a newly d eveloped light-dark cycling procedure. The biofilm was grown for 6 weeks in a benthic gradient chamber, in which gradients of light, sulphide and oxyg en were imposed experimentally. The H2S concentrations and pH were measured with microsensors as a function of depth in the biofilm and of time after a change in illumination status. The sulphide oxidation rates were calculat ed as a function of time and depth in the biofilm using a numerical procedu re to salve the non-stationary general diffusion equation. A close agreemen t was found between the areal rates of anoxygenic photosynthesis during the cycling procedure and the steady state before the cycling experiment. For the different layers of the biofilm, the maximum activity was observed afte r 10-12 min of light exposure. After this maximum, sulphide oxidation decre ased concomitantly with sulphide concentration, indicating sulphide limitat ion of anoxygenic photosynthesis. This lag time limits the application of t he standard dark-light shift method with a brief light exposure of a few se conds and, therefore, the numerical procedure described in this study enabl es the depth distribution of anoxygenic photosynthesis rates in microbial m ats to be determined more accurately.