The turnover of extracellular DNA was investigated in oligotrophic springs of the Crystal River and the eutrophic Medard Reservoir of southwest Florida. The Medard Reservoir possessed large populations of bacterioplankton and phytoplankton (6.8 x 109 cells per liter and 28.6 ,ug of chlorophyll a per liter, respectively), while the Crystal River springs only contained a fraction of the microbial biomass found in the Medard Reservoir. Although dissolved DNA values were greater in the Medard Reservoir, higher rates of DNA removal resulted in similar extracellular DNA turnover times in both environments (9.62 + 3.6 h in the Crystal River and 10.5 + 2.1 h in the Medard Reservoir). These results indicate that regardless of trophic status or microbial standing stock, extracellular DNA turns over rapidly in subtropical planktonic freshwater environments. Therefore, recombinant DNA sequences from released genetically engineered microorganisms might not be ekpected to survive for long periods of time in freshwater planktonic environments. The processes governing the distribution and dynamics of high-molecular-weight dissolved organic matter in lakes and rivers are complex (7, 17). Biological inputs (3) and abiotic transformations affect rates of formation and configuration of the high-molecular-weight materials (8, 13). Less is known concerning the turnover of biologically important dissolved macromolecules such as DNA in freshwater environments (18). We have recently completed a seasonal study of the distribution of dissolved DNA and microbial biomass and activity in an oligotrophic and a eutrophic river in southwest
The factors which affect the production of extracellular DNA by genetically altered strains of Escherichia coli, Pseudomonas aeruginosa, Pseudomonas cepacia, and Bradyrhizobium japonicum in aquatic environments were investigated. Cellular nucleic acids were labeled in vivo by incubation with [3H]thymidine or [3H]adenine, and production of extracellular DNA in marine waters, artificial seawater, or minimal salts media was determined by detecting radiolabeled macromolecules in incubation filtrates. The presence or absence of the ambient microbial community had little effect on the production of extracellular DNA. Three of four organisms produced the greatest amounts of extracellular nucleic acids when incubated in low-salinity media (2%c artificial seawater) rather than high-salinity media (10 to 50%c artificial seawater). The greatest production of extracellular nucleic acids by P. cepacia occurred at pH 7 and 37°C, suggesting that extracellular-DNA production may be a normal physiologic function of the cell. Incubation of labeled P. cepacia cells in water from Bimini Harbor, Bahamas, resulted in labeling of macromolecules of the ambient microbial population. Collectively these results indicate that (i) extracellular-DNA production by genetically altered bacteria released into aquatic environments is more strongly influenced by physiochemical factors than biotic factors, (ii) extracellular-DNA production rates are usually greater for organisms released in freshwater than marine environments, and (iii) ambient microbial populations can readily utilize materials released by these organisms.
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