BACTERIAL UPTAKE AND UTILIZATION OF DISSOLVED DNA

Concentrations and bacterial uptake of extracellular dissolved DNA (D- DNA) were followed during a 2 wk period in 5300 l mesocosms under natu ral and enriched nutrient conditions in an estuary. The nutrient addit ion (21 mg Cl-1, 6 mg Nl(-1) and 2.3 mg Pl(-1)) caused a 10- to 15-fol d increase of the natural bacterial populations, followed by a rapid d ecline. D-DNA in the mesocosms varied from 2 to 11 mu g l(-1). Maximum concentrations and uptake rates of D-DNA, determined from the uptake of [H-3]labelled lambda Hind DNA, coincided with the highest bacterial growth rates. Bacterial uptake of D-DNA ranged from 0.04 to 0.9 mu g l(-1) h(-1). Before the nutrient additions, D-DNA was estimated to acc ount for up to 6, 8 and 46% of the bacterial C, N and P requirements, respectively. Addition of the nutrients reduced these values to 0.8, 2 and 9%, respectively. Test of uptake preference by the bacterioplankt on for DNA at different sizes (100, 250 and 569 bp) demonstrated that the smallest DNA fragment was favoured over the larger fragments. Most of the assimilated DNA was incorporated into cell constituents and co uld not be extracted after treatment with trichloroacetic acid. Our ex periments suggest that D-DNA may be an important source of nutrients, especially phosphorus, to bacterioplankton.