Double-stranded DNA measurement in lakes with the fluorescent stain PicoGreen and the application to bacterial bioassays

We used the double-stranded DNA (dsDNA) stain PicoGreen with a microplate f luorometer to measure bacterial abundance, biomass, and growth rates in lak e water. PicoGreen fluorescence units (PFU) correlated closely with bacteri al abundance measured with acridine orange direct counts (R-2 = 0.95 to 0.9 8) as well as bacterial biomass inferred from image analysis (R-2 = 0.95 to 0.98) in eutrophic waters. PicoGreen fluorescence increased proportionally with bacterial size, indicating that it was a good indicator of biomass as well as abundance. In oligotrophic Lake Superior, there was a weaker, but significant (p < 0.05) correlation between PFU and abundance (R-2 = 0.52) a s well as PFU and biomass (R-2 = 0.54). Growth rate measurements in bottle cultures showed a similar relationship, with PFU, abundance, and biomass be ing more tightly correlated in productive waters than in oligotrophic water s. Parallel dilution cultures were performed in microplate wells (nanocosms ) and 11 bottles, The slope of nanocosm fluorescence to bottle fluorescence was ca 1, indicating that nanocosms mimicked bacterial abundance and growt h in bottle cultures. Preservation of Escherichia coli with formaldehyde sh owed that there was an initial loss of dsDNA of about 10 to 15% with little subsequent loss for 2 wk, indicating that bacteria from growth experiments conducted in the field could be preserved for subsequent analysis in the l aboratory, Bacterial cellular dsDNA content in 3 Minnesota lakes varied bet ween 0.6 and 6.2 fg cell(-1) and was highest in the most eutrophic and most rapidly growing bacterial community, i.e., the more eutrophic lakes. These results suggest that the PicoGreen method is effective for growth bioassay s in systems with moderate to high levels of bacterial productivity. PicoGr een coupled with a microplate fluorescence reader is a promising method for determining bacterial biomass and growth rates, especially in meso- to eut rophic systems.