Nutritional relationships among microorganisms in an epilithic biofilm community

Previous studies of an epilithic algal-bacterial community in a pristine mountain stream suggested that heterotrophic bacteria were responding to the metabolic activities of the phototrophic population. Subsequent studies were performed to follow the flow of labeled carbon, from its initial inorganic form, through the trophic levels of the mat community. A majority of primary production metabolites were excreted by the algal population during active growth; this shifted to an incorporation into cellular material as phototrophic activity declined. Results suggest that there was a direct flux of soluble algal products to the bacterial population, with little heterotrophic utilization of dissolved organics from the overlying stream water. Both phototrophic productivity and bacterial utilization of algal products peaked at approximately the same time of year. Activity of the diatom-dominated algal population declined as silica concentrations in the stream water dropped, leading to a situation in which the sessile bacteria were substrate limited. These events resulted in an almost complete disappearance of the community in early September.