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Ammonium Uptake (ammonium + uptake)
Selected AbstractsCharacterizing nitrogen dynamics, retention and transport in a tropical rainforest stream using an in situ15N additionFRESHWATER BIOLOGY, Issue 1 2002Jeffrey L. Merriam 1.,This study was part of the Lotic Intersite Nitrogen eXperiment (LINX); a series of identical 15NH4 tracer additions to streams throughout North America. 15NH4Cl was added at tracer levels to a Puerto Rican stream for 42 days. Throughout the addition, and for several weeks afterwards, samples were collected to determine the uptake, retention and transformation pathways of nitrogen in the stream. 2.,Ammonium uptake was very rapid. Nitrification was immediate, and was a very significant transformation pathway, accounting for over 50% of total NH4 uptake. The large fraction of NH4 uptake accounted for by nitrification (a process that provides energy to the microbes involved) suggests that energy limitation of net primary production, rather than N limitation, drives N dynamics in this stream. 3.,There was a slightly increased 15N label in dissolved organic nitrogen (DON) the day after the 15NH4 addition was stopped. This DO15N was < 0.02% of DON concentration in the stream water at the time, suggesting that nearly all of the DON found in-stream is allochthonous, or that in-stream DON production is very slow. 4.,Leptophlebiidae and Atya appear to be selectively feeding or selectively assimilating a very highly labelled fraction of the epilithon, as the label found in the consumers became much higher than the label found in the food source. 5.,A large spate (>20-fold increase in discharge) surprisingly removed only 37% of in-stream fine benthic organic matter (FBOM), leaves and epilithon. The fraction that was washed out travelled downstream a long distance (>220 m) or was washed onto the stream banks. 6.,While uptake of 15NH4 was very rapid, retention was low. Quebrada Bisley retained only 17.9% of the added 15N after 42 days of 15N addition. Most of this was in FBOM and epilithon. Turnover rates for these pools were about 3 weeks. The short turnover times of the primary retention pools suggest that long-term retention (>1 month) is minimal, and is probably the result of N incorporation into shrimp biomass, which accounted for < 1% of the added 15N. [source] Forest age, wood and nutrient dynamics in headwater streams of the Hubbard Brook Experimental Forest, NHEARTH SURFACE PROCESSES AND LANDFORMS, Issue 8 2007Dana R. Warren Abstract Instream processing may substantially alter nutrient export from forested watersheds. This study tested how instream uptake of N and P were affected by successional differences in the accumulation of large wood and debris dams in a 66-year chronosequence formed by five watersheds within the Hubbard Brook Experimental Forest (HBEF), NH. Nutrient enrichment releases in summer 1998 were used to measure the uptake velocities of phosphate, nitrate and ammonium for five streams within HBEF, and results indicated that uptake of PO43, was closely associated with forest age. In 2004, we quantified volume and abundance of large wood in each stream to test whether large wood abundance could be linked to nitrate uptake as well as phosphate. The volume of instream wood increased with forest age, at an apparent rate of 0·03 m3 (100 m),1 per year for these early to mid-successional forests (r2 = 0.95); however, debris dam frequency did not. Instead, debris dam frequency, when controlled for stream size, followed a U-shaped distribution, with high dam frequency in very young forests, low frequency in forests around 20,30 years of age and increasing dam frequency again as forests matured. Phosphate uptake velocity increased strongly with both forest age and large wood volume (r2 = 0·99; p < 0·001 in both cases); however, nitrate and ammonium uptake were not related to either factor. We attribute the positive relationship between phosphate uptake velocity and forest age/large wood volume to increased abiotic adsorption of phosphate by the inorganic sediments retained by wood. Nitrogen uptake in these streams is primarily biologically driven and did not vary predictably with these structural features of channels. We expect wood abundance to increase in HBEF streams as the forest matures, with a subsequent increase in stream phosphate uptake capacity. Copyright © 2007 John Wiley & Sons, Ltd. [source] CONTRASTING EFFECTS OF METHIONINE SULFOXIMINE ON UPTAKE AND ASSIMILATION OF AMMONIUM IN ULVA INTESTINALIS (CHLOROPHYCEAE),JOURNAL OF PHYCOLOGY, Issue 4 2004Neill G. Barr Ammonium is assimilated in algae by the glutamine synthetase (GS),glutamine:2-oxoglutarate aminotransferase pathway. In addition to the assimilation of external ammonium taken up across the cell membrane, an alga may have to reassimilate ammonium derived from endogenous sources (i.e. nitrate reduction, photorespiration, and amino acid degradation). Methionine sulfoximine (MSX), an irreversible inhibitor of GS, completely inhibited GS activity in Ulva intestinalis L. after 12 h. However, assimilation of externally derived ammonium was completely inhibited after only 1,2 h in the presence of MSX and was followed by production of endogenous ammonium. However, endogenous ammonium production in U. intestinalis represented only a mean of 4% of total assimilation attributable to GS. The internally controlled rate of ammonium uptake (Vi) was almost completely inhibited in the presence of MSX, suggesting that Vi is a measure of the maximum rate of ammonium assimilation. After complete inhibition of ammonium assimilation in the presence of MSX, the initial or surge (Vs) rate of ammonium uptake in the presence of 400 ,M ammonium chloride decreased by only 17%. However, the amount that the rate of ammonium uptake decreased by was very similar to the uninhibited rate of ammonium assimilation. In addition, the decrease in the rate of ammonium uptake in darkness (in the absence of MSX) in the presence of 400 ,M ammonium chloride matched the decrease in the rate of ammonium assimilation. However, in the presence of 10 ,M ammonium chloride, MSX completely inhibited ammonium assimilation but had no effect on the rate of uptake. [source] Do nutrient additions alter carbon sink strength of ectomycorrhizal fungi?NEW PHYTOLOGIST, Issue 2 2001M. I. Bidartondo Summary ,,Carbon sink strength differences are examined here between ectomycorrhizal fungi in interaction with additions of ammonium and apatite (a phosphorus- and calcium-containing mineral). ,,Pinus muricata associated with Paxillus involutus and four suilloid isolates (Suillus pungens and members of three Rhizopogon section Amylopogon species groups) were used in microcosm nutrient addition experiments. ,,The associations differed in ectomycorrhizal biomass, mycelial growth rate, biomass and respiration. P. involutus produced the lowest biomass of ectomycorrhizal connections to P. muricata, but it consumed proportionally more carbon per connection and transferred more than twice as much ammonium to the host per unit mycorrhizal biomass. Paxillus also colonized the soil more rapidly and intensely than the other fungi, but its mycelial respiration was lowest. Ammonium and apatite addition resulted in a marked increase in respiration and mycelial biomass, respectively, by the suilloid fungi. ,,The high carbon cost of ammonium uptake is suggested as one explanation for reduced sporocarp production and mycelial growth by ectomycorrhizal fungi commonly found after high levels of nitrogen addition. [source] Elevated carbon dioxide increases nitrate uptake and nitrate reductase activity when tobacco is growing on nitrate, but increases ammonium uptake and inhibits nitrate reductase activity when tobacco is growing on ammonium nitratePLANT CELL & ENVIRONMENT, Issue 11 2001P. Matt Abstract The influence of elevated [CO2] on the uptake and assimilation of nitrate and ammonium was investigated by growing tobacco plants in hydroponic culture with 2 mm nitrate or 1 mm ammonium nitrate and ambient or 800 p.p.m. [CO2]. Leaves and roots were harvested at several times during the diurnal cycle to investigate the levels of the transcripts for a high-affinity nitrate transporter (NRT2), nitrate reductase (NIA), cytosolic and plastidic glutamine synthetase (GLN1, GLN2), the activity of NIA and glutamine synthetase, the rate of 15N-nitrate and 15N-ammonium uptake, and the levels of nitrate, ammonium, amino acids, 2-oxoglutarate and carbohydrates. (i) In source leaves of plants growing on 2 mm nitrate in ambient [CO2], NIA transcript is high at the end of the night and NIA activity increases three-fold after illumination. The rate of nitrate reduction during the first part of the light period is two-fold higher than the rate of nitrate uptake and exceeds the rate of ammonium metabolism in the glutamate: oxoglutarate aminotransferase (GOGAT) pathway, resulting in a rapid decrease of nitrate and the accumulation of ammonium, glutamine and the photorespiratory intermediates glycine and serine. This imbalance is reversed later in the diurnal cycle. The level of the NIA transcript falls dramatically after illumination, and NIA activity and the rate of nitrate reduction decline during the second part of the light period and are low at night. NRT2 transcript increases during the day and remains high for the first part of the night and nitrate uptake remains high in the second part of the light period and decreases by only 30% at night. The nitrate absorbed at night is used to replenish the leaf nitrate pool. GLN2 transcript and glutamine synthetase activity rise to a maximum at the end of the day and decline only gradually after darkening, and ammonium and amino acids decrease during the night. (ii) In plants growing on ammonium nitrate, about 30% of the nitrogen is derived from ammonium. More ammonium accumulates in leaves during the day, and glutamine synthetase activity and glutamine levels remain high through the night. There is a corresponding 30% inhibition of nitrate uptake, a decrease of the absolute nitrate level, and a 15,30% decrease of NIA activity in the leaves and roots. The diurnal changes of leaf nitrate and the absolute level and diurnal changes of the NIA transcript are, however, similar to those in nitrate-grown plants. (iii) Plants growing on nitrate adjust to elevated [CO2] by a coordinate change in the diurnal regulation of NRT2 and NIA, which allows maximum rates of nitrate uptake and maximum NIA activity to be maintained for a larger part of the 24 h diurnal cycle. In contrast, tobacco growing on ammonium nitrate adjusts by selectively increasing the rate of ammonium uptake, and decreasing the expression of NRT2 and NIA and the rate of nitrate assimilation. In both conditions, the overall rate of inorganic nitrogen utilization is increased in elevated [CO2] due to higher rates of uptake and assimilation at the end of the day and during the night, and amino acids are maintained at levels that are comparable to or even higher than in ambient [CO2]. (iv) Comparison of the diurnal changes of transcripts, enzyme activities and metabolite pools across the four growth conditions reveals that these complex diurnal changes are due to transcriptional and post-transcriptional mechanisms, which act several steps and are triggered by various signals depending on the condition and organ. The results indicate that nitrate and ammonium uptake and root NIA activity may be regulated by the sugar supply, that ammonium uptake and assimilation inhibit nitrate uptake and root NIA activity, that the balance between the influx and utilization of nitrate plays a key role in the diurnal changes of the NIA transcript in leaves, that changes of glutamine do not play a key role in transcriptional regulation of NIA in leaves but instead inhibit NIA activity via uncharacterized post-transcriptional or post-translational mechanisms, and that high ammonium acts via uncharacterized post-transcriptional or post-translational mechanisms to stabilize glutamine synthetase activity during the night. [source] | ||||||||||