Monitoring of oxygenic and anoxygenic photosynthesis in a unicyanobacterial biofilm, grown in benthic gradient chamber

In order to assess the role of cyanobacteria in the formation and dynamics of microenvironments in microbial mats, we studied an experimental biofilm of a benthic, halotolerant strain, belonging to the Halothece cluster of cy anobacteria. The 12-week-old biofilm developed in a sand core incubated in a benthic gradient chamber under opposing oxygen and sulfide vertical conce ntration gradients. At the biofilm surface, and as a response to high light irradiances, specific accumulation of myxoxanthophyll was detected in the cells, consistent with the typical vertical distribution of sun versus shad e species in nature. The oxygen turn-over in terms of gross photosynthesis and net productivity rates was comparable to oxygen dynamics in natural mic robial mats. Sulfide blocked O-2 production at low irradiances in deep biof ilm layers but the dynamics of H2S and pH demonstrated that sulfide removal by anoxygenic photosynthesis was taking place. At higher irradiances, as s oon as H2S was depleted, the cells switched to oxygenic photosynthesis as h as been postulated for natural communities. The similarities between this e xperimental biofilm and natural benthic microbial mats demonstrate the cent ral role of cyanobacteria in shaping microenvironmental gradients and proce sses in other complex microbial communities. (C) 2000 Federation of Europea n Microbiological Societies. published by Elsevier Science B.V. All rights reserved.