BIODEGRADATION OF AZOTOBACTER-VINELANDII EXOPOLYMER BY LAKE-SUPERIOR MICROBES

Exopolymer and cells from Azotobacter vinelandii, a soil bacterium, an d cells of Scenedesmus bijugatus, a eucaryotic alga, were radiolabeled to experimentally measure their mineralization by hypolimnetic microb es from Lake Superior during July and August 1992. Rates of mineraliza tion were calculated from measurements of microbially respired (CO2)-C -14 released in respiration bottles during 14-22-d incubations. Hypoli mnetic microbes mineralized the exopolymer from A. vinelandii (0.32-0. 36% d(-1)) about twice as fast as they mineralized S. bijugatus cells (0.16-0.20% d(-1)). A. vinelandii cells were mineralized 5 times faste r than S. bijugatus cells were mineralized. Bacterial growth efficienc ies for these substrates were estimated at the end of the experiments by dividing the cumulative amounts of bacterial biomass produced by th e mass of substrate that was utilized in the different respiration bot tles. Although the Azotobacter cells were rapidly mineralized, they we re a poor substrate for producing new bacterioplankton;biomass. Hypoli mnetic bacterioplankton were more efficient at converting S. bijugatus into new biomass (growth efficiency = 18-30%) than at converting eith er the A. vinelandii cells or exopolymer (2% and 8-10% growth efficien cies, respectively). Decaying bacterioplankton blooms may be rapidly m ineralized in the upper hypolimnion of Lake Superior, based on the res ults from the model bacterium (A. vinelandii). Conversely, exopolymers or phytoplankton from the epilimnion may support heterotrophic bacter ial production throughout the water column and limit bacterioplankton growth during periods when small amounts of these substrates are produ ced.