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The significance of organic carbon substrates for the chemotaxis and physiology of phototrophic consortia was investigated in a dense chemocline community of Pelochromatium roseum. For the first time, the monopolar monotrichous flagellation of the central bacterium could be visualized. In situ, intact motile P. roseum consortia were strongly attracted by sulphide and 2-oxoglutarate, which indicated a potential role of these compounds in the metabolism of P. roseum. In chemocline water samples, 2-[14C(U)]-oxoglutarate was utilized at nanomolar concentrations (half saturation constant of uptake Kt < or = 10-40 nM), and at a maximum uptake rate of Vmax approximately 6 nM h-1. The calculated turnover of 2-oxoglutarate at in situ concentrations was approximately 6 h. Microautoradiography of chemocline water samples revealed that 87.5% of the P. roseum consortia incorporated 2-oxoglutarate when both light and sulphide were present, whereas uptake was detected in less than 1.4% of the consortia if either light or sulphide were absent. Because the green sulphur bacterial epibionts in P. roseum have been shown to grow autotrophically, 2-oxoglutarate most likely is taken up and utilized by the central bacterium. Thus, our results indicate that incorporation of 2-oxoglutarate by the central bacterium is regulated by the metabolic state of the green sulphur bacterial epibionts.