Ammonium uptake and incorporation by Chesapeake Bay phytoplankton: Short term uptake kinetics1

Abstract

Using 15N‐labeled NH4+, we assessed the time‐course of NH4+ uptake, the parameters of saturation kinetics, and the rates of incorporation of NH4+ into trichloroacetic acid‐insoluble material by Chesapeake Bay phytoplankton. The amount of NH4+ taken up by the phytoplankton increased linearly with time for 2 h for all samples growing on or exposed to high concentrations (>1 µg‐atom N · liter−1) of NH4+, but extrapolations to time zero did not intercept the origin. These results could be explained by rapid isotopic equilibration of internal and external NH4+ pools. For stations for which ambient NH4+ bordered on detection limits, rapid but nonsaturable uptake was observed in 1‐ and 5‐min incubations, whereas saturation was attained in 15–60‐min incubations. The experiments on rates of NH4+ incorporation into TCA‐insoluble material demonstrated that most (>85%) of the 15N label was incorporated into the macromolecular fraction by ≈15 min. These data thus demonstrate that rates of NH4+ uptake may become limited within minutes by rates of assimilation and incorporation into macromolecules and that reported kinetic parameters reflect these metabolic rates rather than transport rates.

Measuring gross uptake of 15N‐labeled nitrogen by marine phytoplankton without particulate matter collection: Evidence of low 15N losses to the dissolved organic nitrogen pool

“…Although some cases of large and long-lasting internal DIN pools have been reported for diatom cultures (Dortch 1982), the accumulation of DIN is generally a transient phenomenon that results from an imbalance between uptake and assimilation during the first 1 or 2 h after nutrient addition (Dortch et al 1984;Raimbault 1984;Slawyk and Rodier 1986). Short-term NH,' and NO; uptake experiments with natural planktonic assemblages have shown that most (>85%) of the 15N-labeled DIN is incorporated into the macromolecular fraction within I 1 h (Wheeler et al 1982;Glibert and McCarthy 1984). For longer incubation periods, uptake and assimilation are generally balanced, and internal DIN does not account for more than 0.3-2.1% of PON (Col10s and Slawyk 1976).…”

“…However, we suggest that in conventional 15N incubation experiments lasting 22 h (Dugdale and Wilkerson 1986), values of 15N abundance of the intracellular DON pool are close to those Of the PON pool (RDONi G &N) for at least two reasons. First, it has been shown that most of the DI15N taken up is rapidly (5 1 h) shunted into structural high-molecular-weight DON (Wheeler et al 1982;Glibert and McCarthy 1984). Second, we know from experience that the 15N abundance of the total PON pool is not significantly different from the TCA precipitable proteinaceous fraction for incubation periods lasting from 2 to 21 h (unpubl.…”

Dynamics of the 1990 winter/spring bloom in Chesapeake Bay

“…5), well above levels normally taken to saturate uptake by phytoplankton (Wheeler et al 1982). While light limitation of carbon fixation likely prevented the bloom from becoming established In North Bay as early in the year as at the other stations, once the bloom was established other factors likely contributed to its demise.…”

The time‐course of uptake of inorganic and organic nitrogen compounds by phytoplankton from the Eastern Canadian Arctic: A comparison with temperate and tropical populations

“…PO:-: Nalewajko and Lean 1980). This, however, has not been a satisfactory explanation for short term enhanced uptake of NH,+ observed in the field and in cultures; intracellular NH,+ pools are generally too small in marine phytoplankton to accommodate the large isotope fluxes measured (Wheeler et al 1982;Goldman and Glibert 1982). Glibert et al (1982b) have shown that isotope dilution, resulting primarily from microplankton excretion of unlabeled NHJ+, may contribute substantially to the nonlinear NH,' uptake pattern observed over longer incubations.…”

“…A comparison of uptake rates computed from 20-min and 24-h incubations from several locations in the Eastern Arctic showed that short term uptake rates were substantially greater than long term estimates; uptake ratios ranged from I.4 to 12.9 with a median value of 3.1 (Table 2). However, these enhanced rates were not sustained; '"N accumulation by the particulate matter during the Glibert and Goldman 1981;Wheeler et al 1982). In contrast to my findings for the arctic, however, these other data showed that a substantial portion (often >50%) of the 15N accumulation occurred in the first lo-20 min of incubation.…”