Resource competition and community structure in aquatic micro-organisms: experimental studies of algae and bacteria along a gradient of organic carbon to inorganic phosphorus supply

Two microbial communities were grown in chemostats receiving a low supply of inorganic Phosphorus (P) (10 μM) and different supplies of organic carbon (OC), ranging from 0 to 600 μM, either as glucose or a mixture of organic substrates. One community was a natural assemblage of lake plankton and the other was a model community composed of cultured organisms. As the supply ratio of OC to inorganic P increased, concentrations of dissolved OC increased, concentrations of dissolved P decreased and abundances of phototrophic algae decreased. Abundances of bacteria and phagotrophic organisms did not consistently change with the OC:P supply ratio. The model community was first established with a phototroph (Scenedesmus quadricauda) and bacteria; the steady states of this community were invasible by the mixotroph Ochromonas danica under all OC:P supply ratios used. When OC supply was high, both microbial communities persisted with higher concentrations of dissolved OC when mixed substrates, rather than glucose, were supplied. Otherwise, the effects of organic substrate composition appeared to be secondary to those of the OC:P supply ratio. These experiments confirm some elements of published theory on resource-based interactions among heterotrophic bacteria and phototrophic algae.