To determine if dissolved organic matter (DOM) limits biomass production of heterotrophlc bacterioplankton in the subarctic Pacific, the effect of various DOM and ammonium additions on bacterial production ( 3~-t h y n~l d i n e and '4C-leucine incorporation) and bacterial abundance was examined. Addition of dissolved free amlno acids (DFAA) consistently stimulated 3~-t h y m i d i n e incorporation from 31 to 393 % con~pared 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 3~-t h y m i d i n e and 14C-leucine incorporahon to a lesser extent than the DFAA adhtion. Bacterial abundance did not vary significantly during incubatlons, nor were there any differences between treatments, indicating that DFAA add~tions stimulated the average growth rate of the bacterial assemblage. Bactenal growth appeared to b e Chmited, sometimes, since glucose alone stimulated 3H-thynlldine and '4C-leucine in 2 out of 7 expenments. The much greater stimulation of bactenal production by DFAA than by glucose plus ammonium indicated that heterotrophic bacteria in the subarctic Pacific were usually energy-limited.

Dynamics of heterotrophic bactena and phytoplankton m the Delaware Estuary (USA) were studied over 3 yr along a sampling transect that enconlpassed the enbre sahnity gradient (0 to 32%) Bactenal abundance and chlorophyll a concentration were only weakly correlated (r = 0 17) Area1 bactenal production (from 3H-thymidme and '4C-leucine incorporalon rates) covaned w t h phytoplankton production in all geographic regions of the estuary (r = 0 ?0), although phytoplankton production explained less of the variance in bactenal production upstream (12"C) Over the entire estuary, the ratio of annual bactenal production to phytoplankton production was 0 4 In contrast, in the upper estuary bactenal production exceeded phytoplankton production by 1 4-fold, suggesting that allochthonous sources of organic matter are more important than phytoplankton in supporting bactenal growth in t h s region Dunng 5 yr of study, the summer average (May to September) of bactenal abundance and production vaned 2-fold, whlch could largely be explained by the summer phytoplankton production Even though bacteria and phytoplankton are uncoupled during the change of seasons, year-to-year vanation in bactena is apparently controlled by pnmary production and the fraction of primary production processed by heterotrophlc bactena is relatively constant

We investigated the contribution of dissolved free amino acids (DFAA) and ammonium (NH;) to the nitrogen demand (N-demand) of bacterioplankton in the subarctic Pacific and the Delaware Estuary. Bacteria were isolated from other microorganisms by gravity-filtration through 0.8 pm filters, and then bacterial abundance and nutrient concentrations were measured over time. During exper~ments lasting between 36 and 130 h, DFAA and N H 2 contributed 51 + 45 % and 64 + 54 % (n = 14) to the estimated N-demand, respectively. In 9 of the 14 experiments, DFAA and NHZ contributed over 90 % of the estimated N-demand, implying that dissolved organic nitrogen aside from DFAA (e.g. dissolved combined amino acids) was not a significant source of nitrogen. Additions of glucose (0.1 to 1.0 pM) increased the contribution of NHZ and DON other than DFAA to the bacterial N-demand. In most cases, measurements of amino acid and NH; uptake are sufficient for estimating bacterial nitrogen use.

We examined whether or not the heterotrophic manne flagellate Paraphysomonas Imperforata releases macromolecular dissolved organic matter (DOM) 3H-bacterial prey (Vibrio spp ) were added to flagellate cultures and the accumulation of 3~-~~~ was measured Flagellates released a large amount of macromolecular DOM (6 to 57 % of 3~-~0~) that was nch in llpids (11pid macromolecular ratio = 0 20 0 92) One component of the macromolecular DOM was a digestive enzyme (acid phosphatase] of the flagellate The flagellate acld phosphatase was not degraded by proteases but it was suscephble to proteases after treatment with polymyxin B which destroys bacterial membranes These data suggest that bactenal membranes protected the released acid phosphatase from proteolyhc attack Ultracentnfugation showed that the sedimentation coefficient of the phosphatase was 15 to 42 S consistent with the hypothesis that the phosphatase IS part of a macromolecular complex We hypothesize that flagellates release thelr own digestive enzymes and incompletely digested membranes and probably other cellular components from bactenal prey and that these compounds form liposomehke organlc complexes These complexes may be important for understanding the chemical natuie and turnover of DOM in the oceans

The thymidine and leucine methods were examined and used to estimate bacterial production in the Delaware Estuary. During growth experiments that minimized grazing on bacteria, conversion factors for both thymidine and leucine were initially high and then rapidly decreased to values lower than commonly-used factors (2.0 X 10" cells mol-' for thymidine). The low thymidine conversion factors may have been due to l3~]thymidine incorporation into protein, which was 72 % of total incorporation in untreated samples from the estuary. Addition of glucose reduced the leucine conversion factor from 7.4 to 3.6 X 1016 cells mol-' and the thymidine conversion factor from 1.53 to 0.68 X 10'' cells mol-'. Ammonium additions had no effect. The thymidine and leucine approaches gave similar estimates of bacterial production in the Delaware Bay (20 to 70 X 106 cells I-' h-'). Both methods confirmed that bacterial production was highest at about 40 to 50 km upstream of the mouth, which coincides with the peak in primary production. Bacterial production was about 30 % of primary production in most regions of the estuary, but increased to over 100 O/ O in the turbidity maximum where primary production was low. Bacterial production in the Delaware Estuary is apparently controlled by phytoplankton production in spite of large allochthonous sources of dissolved organic matter.