GROWTH OF BACTERIOPLANKTON AND CONSUMPTION OF DISSOLVED ORGANIC-CARBON IN THE SARGASSO-SEA

Lability of the bulk dissolved organic carbon (DOG) pool and the amoun t available to bacterioplankton on short time scales (hours to days) w ere examined in oligotrophic Sargasso Sea water (near Bermuda). We exa mined bacterial growth and DOC utilization using seawater culture meth odology in combination with measurements of bacterial abundance, cell volume, and DOC. Bulk DOC concentrations were determined by high tempe rature combustion (HTC) analysis, which proved to be a sensitive metho d for detecting small changes in natural concentration of DOG. Measura ble bacterial growth and DOC utilization only occurred in unamended cu ltures when initial DDC concentrations were greater than observed in t he mixed layer at the Bermuda Atlantic Time Series station. In unamend ed cultures exhibiting growth, approximately 6 to 7% of the bulk pool was available and considered a labile component. This material was uti lized with an average bacterial growth efficiency (BGE) of 14 +/- 6%. Nutrient enrichment experiments were also conducted with NH4, PO4, glu cose, dissolved free amino acid (DFAA) and algal lysate additions. In all experiments bacterial growth rates, bacterial carbon production, a nd BGE increased with the addition of organic carbon supplements. Ther e were no enhancements of bacterial production or DOC utilization abov e the control when inorganic nutrients were added, indicating that at the lime these experiments were conducted bacterial growth was limited by available carbon.