Planktonic bacterial DNA and RNA synthesis from algal extracellular products in a eutrophic lake

In order to better understand bacterial growth in the natural environment, a tracer experiment was carried out by adding [ 14 C]bicarbonate to samples from a eutrophic lake and enclosures which were constructed inside the lake to provide different substrate conditions. It was shown that extracellular dissolved organic carbon (EDOC) released from photosynthetic phytoplankton was one of the most important carbon sources for the growth of planktonic bacteria in the aquatic environment studied here. About 5% of photosynthetically fixed 14 C was transferred to the bacterial fraction between 4 and 8 h after the addtion of the [ 14 C]bicarbonate. This increased to 10% during the following 16 h in the dark. Between 50% and 60% of bacterial 14 C from EDOC was transformed to bacterial macromolecules. Analysis of cellular components showed that 36–39% of macromolecules produced were DNA during the light incubation, although the proportion of DNA relative to total macromolecules decreased in the dark. RNA synthesis increased with time, from 25% to 38% during the dark incubation. There was a correlation between bacterial growth rates and the ratio of RNA/DNA synthesized from EDOC (r = 0.81). However, the RNA/DNA ratio was very small, 1.8 for all samples examined, compared with previous data obtained for the laboratory bacteria. This is probably due to the very slow growth rate of the natural planktonic bacteria.