Use of 13N as tracer for bacterial and algal uptake of ammonium from sea-water

Abstract

The relative importance of phytoplankton and heterotrophlc bacteria in the ut~lization of ammonium in a temperate coastal pelaglc environment (Long Island Sound. New York, USA) was examined uslng the short-lived radioisotope, I3N Uptake of ' 3~~: Into different slze fractions under simulated in situ temperature and light condihons was compared to size fractionations of bacterial abundance, chlorophyll, and uptake of tritlated thymidme and 14C-b~carbonate In January and Apnl, little ' 3~ entered the bactenal slze fract~on, but when bacteria were more metabolically a c t~v e in May and July, about '/3 of the total I3N uptake appeared in the size fraction conta~ning most of the bactena but httle of the chlorophyll (0 2 to 0 6 ,pm) Exposure to up to 8 mC1 I-' of I3N radioactivity d~d not ~n h~b~t bacterial or phytoplankton activity, and hlled controls showed virtually no uptake Ammonium uptake rates measured by I3N were comparable to those measured by the stable isotope 15N in 2 of 3 experiments The size fract~onation results were consistent wlth estimates of bactenal and phytoplankton demand tor nitrogen for growth Turnover rates for the dissolved a m m o n~u m pool ranged from 0 6 % h-' in April to 116 O/ O h-' in July In separate experiments .r\~th mlxed seawater cultures of bactena, the half-saturation constant (K,) for ammonium uptake was 1 0 1 uM, and ammonium uptake was reduced more by additions (100 to 1000 nM) of glutamine than by glutamate or othel ainlno a c~d s suggesting that glutamlne is directly ~nvolved in a m m o n~u m uptake In these bacteria

Dissolved free amino acids in the Sargasso Sea: uptake and respiration rates, turnover times, and concentrations

“…If bacteria in the Sargasso Sea were growing on C : N substrates > 10: 1 then they may not have been significant regenerators of inorganic nitrogen. Thls is consistent with observations from several marine systems showing that bacteria take up ammonium (Wheeler & Kirchman 1986, Fuhrman et al 1988, Suttle et al 1990, and provides further evidence that much of the regeneration of inorganic nitrogen results from consumption of phytoplankton and bacteria by predators (e.g. Sherr et al 1983, Goldman et al 1985.…”

Rapid ammonium cycling and concentration‐dependent partitioning of ammonium and phosphate: Implications for carbon transfer in planktonic communities

“…Incubations were carried out under simulated in situ conditions of light and temperature in l-liter polycarbonate flasks. As in other studies with i3NH,+ in seawater (Fuhrman et al 1988), we found that Formalin-killed controls did not adsorb appreciable isotope-presumably because of the many competing cations in seawater. At closely spaced time intervals, 15-ml subsamples were filtered through 0.2-, l.O-, and 3.0-pm polycarbonate filters (Nuclepore).…”

“…2C). Other observations have shown that this uptake is attributable to bacteria (Fuhrman et al 1988). The partitioning of this flux into different size classes was very sensitive to small changes in NH,' concentration.…”

Contribution of dissolved free amino acids and ammonium to the nitrogen requirements of heterotrophic bacterioplankton

“…Billen & Fontigny (1987) consistently observed DFAA uptake rates that exceeded N-demand by as much as 600 % . Fuhrman et al (1988) and Fuhrman (1990) also observed DFAA uptake that occasionally exceeded N-demand. Our approach was quite different from that of these previous investigators.…”

“…Another potentially important nitrogen source for bacteria is ammonium. Bacteria can account for a substantial fraction (ca 50 %) of total NHZ uptake in seawater (Wheeler & Kirchman 1986, Fuhrman et al 1988, Kirchman et al 1989. Tupas & Koike (1990) showed that NH; could support as much as 50 to 88 % of bacterial N-demand, even when large amounts of dissolved organic nitrogen (DON) were utilized.…”