Limitation of bacterial growth by dissolved organic matter in the subarctic Pacific

Abstract

To determine if dissolved organic matter (DOM) limits biomass production of heterotrophic bacterioplankton in the subarctic Pacific, the effect of various DOM and ammonium additions on bacterial production (3H-thymidine and 14C-leucine incorporation) and bacterial abundance was examined. Addition of dissolved free amino acids (DFAA) consistently stimulated 3H-thymidine incorporation from 31 to 393% compared with unamended controls. Addition of glucose or glucose plus ammonium sometimes stimulated bacterial production, but the effect was always less than that due to DFAA additions. A mixture of alkylamines either had no effect or stimulated 3H-thymidine and 14C-leucine incorporation to a lesser extent than the DFAA addition. Bacterial abundance did not vary significantly during incubations, nor were there any differences between treatments, indicating that DFAA additions stimulated the average growth rate of the bacterial assemblage. Bacterial growth appeared to be C-limited, sometimes, since glucose alone stimulated 3H-thymidine and 14C-leucine in 2 out of 7 experiments. The much greater stimulation of bacterial production by DFAA than by glucose plus ammonium indicated that heterotrophic bacteria in the subarctic Pacific were usually energy-limited.