Variation in cell-specific rates of leucine and thymidine incorporation by marine bacteria with high and with low nucleic acid content off the Oregon coast

Abstract

The balance between rates of protein synthesis (measured by incorporation of radioactively labeled leucine) and rates of DNA synthesis (measured by incorporation of radioactively labeled thymidine) has been used to evaluate growth state in marine bacterioplankton. Our objective was to determine if variability in leucine and thymidine incorporation could further elucidate ecological differences in the role of high nucleic acid (HNA) and low nucleic acid (LNA) cells in marine ecosystems. We report here the first data set in which cell-specific rates of leucine and thymidine incorporation have been compared for HNA and LNA bacterial cells in the open ocean. In general, HNA cells had higher cell-specific incorporation rates of both leucine and thymidine, and had higher leucine:thymidine (Leu:TdR) incorporation ratios than LNA cells. The higher Leu:TdR ratios for HNA cells suggest that increases in the ratio may be associated with increased metabolic activity by marine bacterioplankton. Leu:TdR incorporation ratios were significantly positively correlated to temperature, but not to chlorophyll a concentrations or phytoplankton biomass calculated from specific carbon:chlorophyll a ratios, indicating that in this region temperature had a greater effect on bacterial growth state than did substrate supply. The proportion of total heterotrophic bacterial activity attributable to LNA cells was significantly greater at slope and basin stations compared to the mesotrophic shelf station. Finally, the difference in incorporation rates between HNA and LNA cells was greater for protein synthesis than for DNA synthesis. © Inter-Research 2006.