Growth of bacterioplankton and consumption of dissolved organic carbon in the Sargasso Sea

Lability of the bulk dissolved organic carbon (DOG) pool and the amount available to bacterioplankton on short time scales (hours to days) were examined in oligotrophic Sargasso Sea water (near Bermuda). We examined bacterial growth and DOC utilization using seawater culture methodology in combination with measurements of bacterial abundance, cell volume, and DOC. Bulk DOC concentrations were determined by high temperature combustion (HTC) analysis, which proved to be a sensitive method for detecting small changes in natural concentration of DOG. Measurable bacterial growth and DOC utilization only occurred in unamended cultures when initial DDC concentrations were greater than observed in the mixed layer at the Bermuda Atlantic Time Series station. In unamended cultures exhibiting growth, approximately 6 to 7% of the bulk pool was available and considered a labile component. This material was utilized with an average bacterial growth efficiency (BGE) of 14 +/- 6%. Nutrient enrichment experiments were also conducted with NH4, PO4, glucose, dissolved free amino acid (DFAA) and algal lysate additions. In all experiments bacterial growth rates, bacterial carbon production, and BGE increased with the addition of organic carbon supplements. There were no enhancements of bacterial production or DOC utilization above the control when inorganic nutrients were added, indicating that at the lime these experiments were conducted bacterial growth was limited by available carbon.