Fluorometric Determination of DNA in Aquatic Microorganisms by Use of Hoechst 33258

A method for the determination of microbial DNA in aquatic environments by the use of Hoechst 33258 has been developed. With unsophisticated instrumentation and simple extraction procedures, it is possible to detect from 0.05 to 10 μg of DNA in bacterial cultures or natural water samples. The method is specific for DNA; DNase I treatment of extracts of natural microbial populations removed 95 to 100% of the observed fluorescence. DNA content ranged from 165 ng ml−1 for relatively eutrophic Potomac River water to 27 ng ml−1 for coastal Atlantic Ocean water and was correlated to an acridine orange direct count (r = 0.90).