Bacterioplankton RNA, DNA, protein content and relationships to rates of thymidine and leucine incorporation

Bacterial macromolecules have often been used successfully as estimates of bacterial biomass in environmental samples. Less is known about the relationship between macromolecular content and rates of growth and activity. The ratio of RNA to DNA in bacteria has long been thought to be proportional to levels of metabolic activity and growth. We have used the nucleic acid fluorochrome thiazole orange combined with specific nuclease digestions to determine RNA:DNA ratios and compared these data to more standard measures of bacterial growth and activity. More than 100 samples from a wide variety of estuarine and marine environments were examined and the average RNA, DNA, and protein content per bacterial cell was found to be 9.44 plus or minus 6.25 fg, 5.75 plus or minus 2.35 fg, and 29.29 plus or minus 11.57 fg, respectively. Initial experiments demonstrated a strong correlation of RNA:DNA ratios with growth rates for a laboratory organism. We then compared RNA:DNA ratios to cell specific rates of super(3)H-thymidine and super(14)C-leucine incorporation as estimates of growth rates for naturally occurring bacterioplankton communities in whole water and the