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

Abstract

A simple steady-state model based on P-limited bacterial growth rate and predator controlled bacterial biomass predicts bacterial production (BP) to be proportional to the square of ciliate biomass (C). Changes in ciliate biomass will then drive changes in bacterial production and carbon demand. This model was compared to experimental microcosms where natural mixed microbial communities from Villefranche Bay (northwest Mediterranean) were given daily additions of phosphate and glucose in a factorial design. In accordance with the proposed model, we found no effect of glucose additions on the incorporation of 14C-leucine, except when combined with phosphate. Phosphate enrichment had a stimulatory effect on bacterial production, even when no glucose was added, but after an initial phase of ca 2 d, leucine incorporation increased more in carboys receiving glucose and phosphate in combination than in carboys enriched with phosphate alone. After 3 to 4 d, leucine incorporation culminated. These observations are consistent with a scenario where, initially, bacterial growth rate was P-limited. Stimulation of bacterial 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 bacterial 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 without phosphate.