Bacterial uptake and utilization of dissolved DNA

Abstract

Concentrations and bacterial uptake of extracellular dissolved DNA (D-DNA) were followed during a 2 wk period in 5300 l mesocosms under natural and enriched nutrient conditions in an estuary. The nutrient addition (21 mg Cl-1, 6 mg Nl-1 and 2.3 mg Pl-1) caused a 10- to 15-fold increase of the natural bacterial populations, followed by a rapid decline. D-DNA in the mesocosms varied from 2 to 11 μg l-1. Maximum concentrations and uptake rates of D-DNA, determined from the uptake of [3H]labelled λHind DNA, coincided with the highest bacterial growth rates. Bacterial uptake of D-DNA ranged from 0.04 to 0.9 μg l-1 h-1. Before the nutrient additions, D-DNA was estimated to account 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 bacterioplankton 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 could not be extracted after treatment with trichloroacetic acid. Our experiments suggest that D-DNA may be an important source of nutrients, especially phosphorus, to bacterioplankton.