Trophic control of bacterial growth in microcosms containing a natural community from northwest Mediterranean surface waters

A simple steady-state model based on P-Limited bacterial growth rate and pr edator controlled bacterial biomass predicts bacterial production (BP) to b e proportional to the square of ciliate biomass (C). Changes in ciliate bio mass will then drive changes in bacterial production and carbon demand. Thi s model was compared to experimental microcosms where natural mixed microbi al communities from Villefranche Bay (northwest Mediterranean) were given d aily additions of phosphate and glucose in a factorial design. In accordanc e with the proposed model, we found no effect of glucose additions on the i ncorporation of C-14-leucine, except when combined with phosphate. Phosphat e enrichment had a stimulatory effect on bacterial production, even when no glucose was added, but after an initial phase of ca 2 d, leucine incorpora tion increased more in carboys receiving glucose and phosphate in combinati on than in carboys enriched with phosphate alone. After 3 to 4 d, leucine i ncorporation culminated. These observations are consistent with a scenario where, initially, bacterial growth rate was P-limited. Stimulation of bacte rial carbon demand resulting from transfer of added phosphorus into ciliate biomass was, however, apparently larger than the increase in the system's production of labile organic C. The explanation suggested for the observed culmination in bacterial production is thus a switch from P- to C-limited b acterial growth rate. In such a scenario, glucose addition allows a larger increase in bacterial consumption before the pool of labile DOC is depleted . DOC accumulation was found only in carboys to which glucose was added wit hout phosphate.