Tf. Thingstad et al., Trophic control of bacterial growth in microcosms containing a natural community from northwest Mediterranean surface waters, AQUAT MIC E, 18(2), 1999, pp. 145-156
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.