Resource competition and community structure in aquatic microorganisms: experimental studies of algae and bacteria along a gradient of organic carbonto inorganic phosphorus supply

Two microbial communities were grown in chemostats receiving a low supply o f inorganic Phosphorus (P) (10 mu M) and different supplies of organic carb on (OC), ranging from 0 to 600 mu M, either as glucose or a mixture of orga nic substrates. One community was a natural assemblage of lake plankton and the other was a model community composed of cultured organisms. As the sup ply ratio of OC to inorganic P increased, concentrations of dissolved OC in creased, concentrations of dissolved P decreased and abundances of phototro phic algae decreased. Abundances of bacteria and phagotrophic organisms did nut consistently change with the OC:P supply ratio. The model community wa s first established with a phototroph (Scenedesmus quadricauda) and bacteri a; the steady states of this community were invasible by the mixotroph Ochr omonas danica under all OC:P supply ratios used. When OC supply was high, b oth microbial communities persisted with higher concentrations of dissolved OC when mixed substrates, rather than glucose, were supplied. Otherwise, t he effects of organic substrate composition appeared to be secondary to tho se of the OC:P supply ratio. These experiments confirm some elements of pub lished theory on resource-based interactions among heterotrophic bacteria a nd phototrophic algae.