PLASMID TRANSFER TO INDIGENOUS MARINE BACTERIAL-POPULATIONS BY NATURAL TRANSFORMATION

Horizontal gene transfer among microbial populations has been assumed to occur in the environment, yet direct observations of this phenomeno n are rare or limited to observations where the mechanism(s) could not be explicitly determined. Here we demonstrate the transfer of exogeno us plasmid DNA to members of indigenous marine bacterial populations b y natural transformation, the first report of this process for any nat ural microbial community. Ten percent of marine bacterial isolates exa mined were transformed by plasmid DNA while 14% were transformed by ch romosomal DNA. Transformation of mixed marine microbial assemblages wa s observed in 5 of 14 experiments. In every case, acquisition of the p lasmid by members of the indigenous flora was accompanied by modificat ion (probably from genetic rearrangement or methylation) that altered its restriction enzyme digestion pattern. Estimation of transformation rates in estuarine environments based upon the distribution of compet ency and transformation frequencies in isolates and mixed populations ranged from 5 x 10(-4) to 1.5 transformants/1 day. Extrapolation of th ese rates to ecosystem scales suggests that natural transformation may be an important mechanism for plasmid transfer among marine bacterial communities.