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

Abstract

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 ± 6.25 fg, 5.75 ± 2.35 fg, and 29.29 ± 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 3H-thymidine and 14C-leucine incorporation as estimates of growth rates for naturally occurring bacterioplankton communities in whole water and the <0.8 μm size fraction in estuarine and coastal waters in the Northern Gulf of Mexico. Nucleic acid ratios were generally very low in the natural communities, usually below those determined for late stationary phase pure cultures. No significant or consistent relationship was observed between RNA:DNA ratios and rates of 3H-thymidine and 14C-leucine incorporation in naturally occurring bacterioplankton communities. No relationship could be ascertained whether compared against location or water temperature. Thymidine incorporation rates were found to be completely independent of cellular protein content while leucine incorporation rates were more related to protein content Our data suggest that while RNA, DNA, and protein content are strongly correlated with bacterial numbers in our environmental samples, the low metabolic activity and the heterogeneous composition of bacterioplankton communities may preclude the use of these parameters as biochemical indicators of activity in the environment.