Utilization and turnover of labile dissolved organic matter by bacterial heterotrophs in eastern north Pacific surface waters

Seawater incubation experiments were conducted in June and October 1992 to examine bacterial utilization of labile dissolved organic matter (DOM) in open ocean surface waters of the eastern North Pacific. Natural plankton extract-DOM (PE-DOM) and selected model compounds were added to seawater samples to evaluate bacterial utilization and respiration rates relative to bacterial carbon production rates for the various amendments. PE-DOM always stimulated bacterial production and DOM utilization, and the primary nitrogen source supporting this bacterial production was dissolved organic nitrogen (DON). Utilization of DON during exponential growth was balanced by the production of ammonium for samples amended with PE-DOM. Bacterial growth efficiencies for samples amended with PE-DOM ranged between 3.4 and 8.8 % and generally were slightly higher in June than in October. Of the model compounds tested, net bacterial biomass production was observed only in samples amended with glucose, glucose plus ammonium (glucose+NH4+), and dissolved free amino acids (DFAA). Bacterial growth efficiencies for these amendments were 0.8, 1.9, and and 9.3.%, respectively. Bacterial production at in situ DOM concentrations was observed in June but not in October. Using the bacterial dissolved organic carbon (DOC) utilization rates observed in this study together with other detailed information pertaining to bulk DOC at our study site, we estimate that the turnover time for labile DOC in these surface waters ranges from approximately 2 to 6 d depending on the labiliity of the standing stock of DOC. On the basis of (a) the exclusive use of DON as a nitrogen source in PE-DOM amendments, (b) the stimulation of ammonium utilization in the glucose+NH4+ amendment, and (c) the higher growth efficiencies observed for samples amended with either PE-DOM or DFAA, we suggest that bacterioplankton biomass production in eastern North Pacific surface waters is primarily energy limited. As a result of this energy limitation, bacterial production appears to be additionally constrained by the quality of the nutrients available for assimilation. Thus, the quality of the DOM substrate, specifically the DOC:DON ratio, can be a major determinant of bacterial production in pelagic marine systems.