Ammonium regeneration: its contribution to phytoplankton nitrogen requirements in a eutrophic environment

In both frontal and stratified water of the Strait of Georgia, changes in dissolved nitrogen concentrations provided evidence for the simultaneous uptake of ammonium, nitrate and urea by a summer phytoplankton community. Chlorophyll a specific uptake rates of ammonium and urea were ca 2 and 2.4 times greater in stratified water than in frontal water, whereas chlorophyll a specific nitrate uptake rates were ca 1.6 times greater in frontal water. Ammonium and urea regeneration rates, calculated using a mass balance approach, were similar in frontal water, but urea regeneration rates were 2 to 5 times greater in the stratified water during the first 12 h of the experiment. Increases in particulate nitrogen could not be accounted for by corresponding decreases in total concentration of dissolved inorganic nitrogen and urea, or by 15N accumulation in the particulates. In frontal water the change in total dissolved inorganic nitrogen and urea consistently overestimated the change in particulate nitrogen, and in stratified water the change in total dissolved inorganic nitrogen and urea consistently underestimated the change in particulate nitrogen. These data suggest that the plankton community In frontal water was losing nitrogen in the form of dissolved organic nitrogen. By contrast, the plankton community in stratified water took up nitrogen compounds which were not measured as part of the total dissolved inorganic and urea nitrogen, but were most likely dissolved organic nitrogen compounds. Results stress the importance of determining uptake rates of all 3 nitrogen substrates (NHfi, N O and urea) using 15N isotopes and by simultaneously measuring the change in concentration of these compounds throughout the incubation period.

Section: Nh: and Urea Regenerationsupporting

confidence: 88%

Section: Methodsmentioning

confidence: 99%

Time course of uptake of inorganic and organic nitrogen by phytoplankton in the Strait of Georgia: comparison of frontal and stratified communities

Price¹,

Cochlan²,

Harrison³

1985

“…It is noteworthy that the fastest regeneration rates reported in the literature (>0.2 pmole 1-' h-') were associated with estuarine environments such as Kaneohe Bay (Caperon et al 1979). Chesapeake Bay (Glibert 1982) and Bedford Basin (La Roche 1983). The present range of regeneration rates for the shelf region of the southern Benguela are intermediate between those reported for coastal and oligotrophic waters.…”

Section: Ammonium Assimilationcontrasting

confidence: 49%

Ammonium regeneration by microplankton in an upwelling environment

Probyn¹

1987

Size-frachonated measurements of ammonium flux In the southern Benguela upwelllng system provided evldence that regenerahon by microplankton (<200um) was concentrated In the upper water column and generally supported the majonty of phytoplankton demands Based on the present study and literature data lt was demonstrated that ammonium reminerahzabon was dependent on the concentrahon of particulate nitrogen present when viewed on a broad scale The < 15 pm size class, including bactena and nanoflagellates proved to be the predominant mineralizers accounting for 95 % of total microplankton regeneration on average Although ammomf~cahon by heterotrophic p~coplankton (presumably mainly bactenal) was Important in some instances bacte~nvorous nanoflagellates (1 to 15 urn) appeared to supply the bulk of mlcroplankton excretion, i e a mean of 87 % in surface waters and 55 % near the base of the euphot~c zone Bactena were relatively more active at depth than at the surface although in the majonty of cases absolute rates were fastest In surface watersThe posslbdlty e x s t s that bactenal reminerallzation was underesbmated in the < 1 um fracbon due to the absence of nanoflagellates dunng the ~ncubahon Nevertheless, nanoflagellate feeding compnses a n essenhal component of the reminerahzat~on process Ammonlum uptake by the dlfferent size classes was dlrectly proportional to chlorophyll a concentrations Results of thls study are cons~stent wlth the proposed cruclal role for small protozoans as intermediaries between bacterla and hlgher trophic levels and as m~nerallzers of nutrients for pnmary producers

“…Nitrate uptake rates were measured using the stable isotope 15 N. Duplicate samples from each culture were inoculated with 10 µM 15 NO 3 -and incubated at 8, 17 or 25°C alongside the original cultures in 500 ml polycarbonate bottles for 3 to 4 h. After incubation, samples for 15 N analysis were collected on precombusted GF/F filters and analyzed using the micro-Dumas dry-combustion technique, as described by La Roche (1983) and Harrison (1983), and a Model N-150 (Jasco International, Tokyo, Japan) emission spectrometer (Fiedler & Proksch 1975). Nitrate uptake rates were calculated following Dugdale & Wilkerson (1986).…”

Section: Methodsmentioning

confidence: 99%

Effects of temperature on growth rate, cell composition and nitrogen metabolism in the marine diatom Thalassiosira pseudonana (Bacillariophyceae)

Berges¹,

Varela²,

Harrison³

2002

141

Although temperature effects on phytoplankton growth and photosynthesis can be clearly demonstrated in the laboratory, their relevance in the field is much harder to establish. Recently, however, it has been recognized that temperature has a significant influence on nitrogen uptake. In particular, temperate marine diatom species may be limited by their ability to acquire nitrate at temperatures above approximately 16°C. In order to explore this idea, we grew the diatom Thalassiosira pseudonana at 8, 17 and 25°C, and compared cell composition, and rates of growth (µ), 15 N incorporation, calculated nitrate incorporation (the product of µ and cell N content), and the activity of nitrate reductase (NR), a key enzyme involved in nitrate incorporation. Cell N content, protein and volume remained relatively constant across different temperatures, but cell C, chlorophyll a (chl a), and C:N ratio increased with increasing temperature, suggesting that C metabolism was affected more strongly than N metabolism. Classical temperature models suggested that growth and various indices of nitrate metabolism all responded to temperature, with Q 10 values of close to 2 over the whole temperature range. However, Q 10 values over the interval from 8 to 17°C were higher than 2 and much lower than 2 between 17 and 25°C. Limitations to the Q 10 concept are considered. Temperature effects on different measures of nitrate metabolism were very similar, supporting the hypothesis that the effects of temperature on diatom nitrate metabolism are mediated at the level of NR activity. Recent biochemical data for NR also supports this idea.