CARBON LIMITATION OF AMMONIUM UPTAKE BY HETEROTROPHIC BACTERIA IN THE SUB-ARCTIC PACIFIC

The purpose of this study was to test the hypothesis that NH4+ uptake by heterotrophic bacteria in the subarctic Pacific is C limited. Addition of glucose (0.5-1.0-mu-M) stimulated NH4+ uptake in unfractionated water but had no effect on bacterial abundance. Glucose stimulation of NH4+ uptake was even greater in the bacterial size fraction ( < 0.8-mu-m). Regeneration of 15NH4+ from added [N-15] amino acids was measurable in the bacterial size fraction, but glucose additions prevented net NH4+ excretion because of increased uptake of NH4+. In the subarctic Pacific, NH4+ concentrations range from undetectable (less-than-or-equal-to 0.05-mu-M) to 0.4 mu-M, whereas the maximal estimate of monosaccharide concentrations was 0.025-mu-M. These results indicate that the supply of C compounds like glucose can limit NH4+ uptake by heterotrophic bacteria.