Variation in urea excretion in the gulf toadfishOpsanus beta

Under long-term (24 h) control measurements, significant urea was excreted (15 to 90% of excreted nitrogen) byOpsanus beta (Goode and Bean) collected in 1989 from Biscayne Bay, Florida, USA. Urea excretion rates and plasma urea concentrations were not affected by antibiotic treatments which decreased intestinal microbe populations. These results suggest that nitrogen recycling by gut microbe urease is probably not significant in this species. Urea excretion rates increased significantly following 8 h of air-exposure and in response to high levels of NH₄Cl. These results suggest that urea is synthesized and excreted by the toadfish primarily in situations that limit ammonia excretion. Thus, the ornithineurea cycle in the toadfish liver primarily maintains low concentrations of ammonia within the fish. High levels of variability in urea excretion rates and variation in response to air-exposure suggest that urea synthetic rates are affected by immediate past environmental conditions.     

Ammonia as confounding factor in toxicity tests with the sea urchin Paracentrotus lividus (Lmk)

The aim of this study was to investigate the toxicity of total ammonia towards sea urchin bioassays, in order to elucidate the role of ammonia as confounding factor in sediment quality assessment studies. New toxicity data, expressed as EC50, NOEC and LOEC are reported for Paracentrotus lividus based on experiments at differing pH values. Results confirmed that sperm cell toxicity test is not very sensitive to ammonia, while total ammonia embryotoxicity is strictly pH dependent. Accurate observations of embryotoxic effects at increasing total ammonia concentrations evidenced progressive shifts from malformed plutei to gastrula and blastula blockages. Toxicity data obtained for P. lividus are comparable with previous results with other echinoid species.     

Acid–base regulation in the Dungeness crab (Metacarcinus magister)

Homeostatic regulation allows organisms to secure basic physiological processes in a varying environment. To counteract fluctuations in ambient carbonate system speciation due to elevated seawater pCO₂ (hypercapnia), many aquatic crustaceans excrete/accumulate acid–base equivalents through their gills; however, not much is known about the role of ammonia in this response. The present study investigated the effects of hypercapnia on acid–base and ammonia regulation in the Dungeness crab, Metacarcinus magister on the whole animal and isolated gill levels. Hemolymph pCO₂ and [HCO₃ ^?] increased in M. magister acclimated to elevated pCO₂ (330 Pa), while pH remained stable. Additionally, hemolymph [Na⁺], [Ca²⁺], and [SO₄ ^2?] were significantly increased. When challenged with varying pH during gill perfusion, the pH of the artificial hemolymph remained relatively unchanged. Overall, ammonia production and excretion, as well as oxygen consumption, were reduced in crabs acclimated to elevated pCO₂, demonstrating that either (amino acid) oxidation is reduced in response to this particular stress, or nitrogenous wastes are excreted in an alternative form.     

Patterns of nitrogenous waste excretion and gill urea transporter mRNA expression in several species of marine fish

Many prior studies of nitrogenous waste excretion in marine fish have examined excretion patterns for short time periods, and with relatively coarse sampling schemes (e.g., an initial and a final sample point). Recent studies of a ureotelic marine fish (the gulf toadfish, Opsanus beta) have demonstrated that urea excretion in this species occurs in brief but massive bursts, lasting from 0.5 to 3 h, and often only once per day. The present study sought to determine if prior sampling protocols may have underestimated the amount of urea being excreted by marine fish. A survey of 16 marine species (the teleosts: Myoxocephalus octodecemspinosus, Scophthalamus aquosa, Cyclopterus lumpus, Lophius americanus, Aprodon cortezianus, Cymatogaster aggregatus, Parophrys vetulis, Microstomus pacificus, Hippoglossoides elassodon, Bathyagonus nigripinnus, Ophiodon elongatus, Hemilepidatus spinosus, Icelinus terrius; the elasmobranch: Raja rhina; and the hagfish: Eptatretus stoutii) was undertaken for ammonia-N and urea-N excretion using a long sampling period (48 h) and hourly sample collection. Apart from the obvious exception of an elasmobranch, ammonia excretion was confirmed to be predominant in marine fish, with urea excretion constituting between 1.4 and 23.8% of the total of ammonia plus urea excreted. Notably, no pulses of urea excretion were detected. Despite the relatively low level of urea excretion, expression of urea transporter-like mRNA (detected using the toadfish urea transporter, tUT, cDNA as a probe) was discovered in gills of many of the species surveyed for nitrogen excretion patterns, although no signal was detected in the hagfish. These results suggest that urea excretion takes place through a specific transport pathway. Finally, more detailed analysis of nitrogen excretion in one of the surveyed species, the plainfin midshipman (Porichthys notatus) demonstrates that "total" nitrogen excretion estimated by summing ammonia and urea excretion underestimates true total nitrogen excretion by 37-51%.     

Urea as a nitrogen source for the phytoplankton in the Oslofjord

Ambient concentrations of urea in the inner Oslofjord, Norway, showed a pronounced yearly cycle in 1980, with values in the range 0.1 to 10.0 μg-at N l^-1; this cycle resemble that of ammonia although urea concentrations were usually lower. The uptake of urea by phytoplankton was investigated using ¹⁵N. Urea was usually a less important N source than NH ₄ ⁺ , and accounted for 0 to 53% (mean 19%) of summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rates from April to October. Absolute as well as relative (specific) uptake rates of urea were higher in the summer (June–August) than at other times. Uptake of urea was inhibited by NH ₄ ⁺ concentrations higher than 1 to 2 μg-at N l^-1. The summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rate was exponentially related to temperature.     

Chemical deactivation of V2O5-WO3/TiO2 SCR catalyst by combined effect of potassium and chloride

V₂O₅-WO₃/TiO₂ catalyst was poisoned by impregnation with NH₄Cl, KOH and KCl solution, respectively. The catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma (ICP), N₂ physisorption, Raman, UV-vis, NH₃ adsorption, temperature-programmed reduction of hydrogen (H₂-TPR), temperature-programmed oxidation of ammonia (NH₃-TPO) and selective catalytic reduction of NO _x with ammonia (NH₃-SCR). The deactivation effects of poisoning agents follow the sequence of KCl>KOH?NH₄Cl. The addition of ammonia chloride enlarges the pore size of the titania support, and promotes the formation of highly dispersed V = O vanadyl which improves the oxidation of ammonia and the high-temperature SCR activity. K⁺ ions are suggested to interact with vanadium and tungsten species chemically, resulting in a poor redox property of catalyst. More importantly, potassium can reduce the Brønsted acidity of catalysts and decrease the stability of Brønsted acid sites significantly. The more severe deactivation of the KCl-treated catalyst can be mainly ascribed to the higher amount of potassium resided on catalyst.     

Effect of ammonia on survival and osmoregulation in different life stages of the lobsterHomarus americanus

Acute toxicity of ammonia was determined for cultured larval, postlarval, and wild adult lobsters (Homarus americanus) in 1988. Ammonia tolerance was found to increase with ontogenetic development. Based on 96-h LC₅₀ values of 58 mg l^–1 NH₄ ⁺ + NH₃ l^–1 seawater (0.72 mg NH₃ l^–1) for Stage I larvae, 87 mg NH₄ ⁺ + NH₃ l^–1 (1.7 mg NH₃ l^–1) for Stage II larvae, 125 mg NH₄ ⁺ + NH₃ l^–1 (2.13 mg NH₃) for Stage III larvae, 144 mg NH₄ ⁺ + NH₃ l^–1 (2.36 mg NH₃ l^–1) for Stage IV postlarvae, 377 mg NH₄ ⁺ + NH₃ l^–1 (5.12 mg NH₃ l^–1) for adult lobsters at 5°C and 219 mg NH₄ ⁺ + NH₃ l^–1 (3.25 mg NH₃ l^–1) for adult lobsters at 20°C, recommendations for safe levels of total ammonia and un-ionized ammonia were calculated using an application factor of 0.1. Effects of ammonia on osmoregulatory capacity were studied on postlarvae and adults. Ability of postlarvae and adults to hyper-regulate in low-salinity media decreased after exposure to ammonia. In postlarval lobsters, osmoregulatory capacity was significantly affected in ammonia concentrations exceeding 32 mg l^–1. Osmoregulatory capacity in adult lobsters (5 and 20°C) was affected at 150 mg l^–1. In postlarval lobsters, a minimum exposure time of 12 h was required to impair osmoregulatory capacity. The decrease in hemolymph osmotic pressure was caused by lower hemolymph sodium concentrations. The presence of ammonia in the external medium could markedly affect the Na⁺/NH₄ ⁺ transport mechanism by permanently, temporarily, or partially impairing the transport sites for sodium.     

Effect of pH on biologic degradation of Microcystis aeruginosa by alga-lysing bacteria in sequencing batch biofilm reactors

In this paper, the effect of pH on biological degradation of Microcystis aeruginosa by alga-lysing bacteria in laboratory-scale sequencing batch biofilm reactors (SBBRs) was investigated. After 10 d filming with waste activated sludge, the biological film could be formed, and the bioreactors in which laid polyolefin resin filler were used to treat algal culture. By comparing the removal efficiency of chlorophyll a at different aerobic time, the optimum time was determined as 5 h. Under pH 6.5, 7.5, and 8.5 conditions, the removal rates of Microcystis aeruginosa were respectively 75.9%, 83.6%, and 78.3% (in term of chlorophyll a), and that of Chemical Oxygen Demand (COD_Mn) were 30.6%, 35.8%, and 33.5%. While the removal efficiencies of ammonia nitrogen (NH ₄ ⁺ - N) were all 100%. It was observed that the sequence of the removal efficiencies of algae, NH ₄ ⁺ - N and organic matter were pH 7.5>pH 8.5>pH 6.5. The results showed that the dominant alga-lysing bacteria in the SBBRs was strain HM-01, which was identified as Bacillus sp. by Polymerase Chain Reaction (PCR) amplification of the 16S rRNA gene, Basic Local Alignment Search Tool (BLAST) analysis, and comparison with sequences in the GenBank nucleotide database. The algicidal activated substance which HM-01 strain excreted could withstand high temperature and pressure, also had better hydrophily and stronger polarity.     

Turnover and loss of nitrogenous compounds during composting of food wastes

Few people have so far explored into the research of the dynamics of various nitrogenous compounds (including water-soluble nitrogen) in composting of food wastes. This study aimed to investigate the solid-phase nitrogen, water-soluble nitrogen, nitrogen loss together with ammonia volatilization in the process of food wastes composting. A laboratory scale static aerobic reactor in the experiment was employed in the composting process of a synthetic food waste, in which sawdust was used as the litter amendment. In the experiment, oxygen was supplied by continuous forced ventilation for 15 days. The results have shown that the concentrations of total nitrogen and organic nitrogen decrease significantly in the composting process, whereas NH₄ ⁺-N concentration increases together with little fluctuation in NO₃ ^?-N. After composting, the total content of the water-soluble nitrogen compounds in the compost greatly increased, the total nitrogen loss amounted to 50% of the initial nitrogen, mainly attributed to ammonia volatilization. 56.7% of the total ammonia volatilization occurred in the middle and late composting of the thermophilic stage. This suggested that the control at the middle and late composting of thermophilic stage is the key to nitrogen loss in the food waste compost.     

The role of metabolic nitrogen in coral calcification

When pieces of the staghorn coral Acropora acuminata are incubated with ¹⁴C-urea, the label is incorporated into skeletal carbonate. Incorporation of this label differs from that of H¹⁴CO ₃ ^- , suggesting urea is not immediately hydrolysed to provide a further source of HCO ₃ ^- . The effects of certain organic substrates upon calcification suggest the ornithine cycle is involved. Citrulline, an ornithine cycle intermediate, is found in high concentrations in the tissues of hermatypic corals. Urea, allantoins, NH₃ and arginine are also present. These compounds are barely detectable in zooxanthellae or an ahermatypic coral. The allantoins may be present as calcium salts. It is suggested that allantoins are the medium by which Ca²⁺ and CO₂ are transported to sites of calcification. Hydrolysis of urea, formed by breakdown of allantoins, yields CO₂ and NH₃. The NH₃ may neutralise protons formed during precipitation of CaCO₃ and bring about their removal from sites of calcification. As well as providing urea, the ornithine cycle may also be involved in the removal of NH₃ from sites of calcification.     

Urea formation from carbon dioxide and ammonia at atmospheric pressure

Urea synthesis, currently the largest use of carbon dioxide in organic synthesis, is conventionally operated at high pressure and high temperature. Here, we report for the first time that urea forms at atmosphere and ambient temperatures by negative corona discharge in gas phase. The conversion of CO₂ and yields of a solid mixture of urea and ammonium carbamate, which was identified by the ¹³C NMR spectrum, rise with reducing temperatures and increasing molar ratios of NH₃/CO₂ and discharge frequencies. The conversion of carbon dioxide was found to be 82.16?% at 20?°C and 1?atm with a molar flow ratio of n(NH₃)/n(CO₂) of 2.5. High pressure and high temperature as energy inputs are not necessary.     

Nitrate reductase and urease enzyme activity in the marine diatom<Emphasis Type="Italic"> Thalassiosira weissflogii</Emphasis> (Bacillariophyceae): interactions among nitrogen substrates

The dissolved nitrogen pool in aquatic systems is comprised of many different nitrogen forms, both inorganic and organic. Interaction among these nitrogen forms at the level of uptake and enzyme activity is, with the exception of NH₄⁺ and NO₃^–, not completely understood. Nitrate reductase (NR) and urease (UA) activities in the marine diatom Thalassiosira weissflogii (Grunow) Fryxell et Hasle were measured in NO₃^–, NH₄⁺, and urea-sufficient cultures before and after challenge additions of NH₄⁺, NO₃^–, and urea in a factorial design. NR and UA were constitutively expressed during growth on NO₃^–, NH₄⁺, and urea. Growth on NH₄⁺ or urea resulted in NR activities that were <10% of the activity observed in the NO₃^–-grown culture, while growth on NO₃^– resulted in UA values that were ~35% of the activities during growth on either NH₄⁺ or urea. The addition of NH₄⁺ or urea to NO₃^–-grown cultures resulted in an immediate decrease in cellular NO₃^– uptake rate, which was not mirrored by an immediate repression of in vitro NR activity; however, the diel peak in NR was suppressed in these challenge experiments. The addition of NO₃^– or NH₄⁺ to urea-grown cultures resulted in non-significant decreases in the urea uptake rate. UA was not impacted by NO₃^– addition, but NH₄⁺ addition significantly decreased UA throughout the experiment. These studies demonstrate that the uptake and assimilation of NO₃^– and urea may not be subject to the same internal feedback mechanism when challenged with other nitrogen substrates.Communicated by J.P. Grassle, New Brunswick     

Effect of nitrogen source on short-term light and dark CO2 uptake by a marine diatom

Based on a series of short-term incubations involving the marine diatom Chaetoceros simplex (Bbsm), precultured in NH ₄ ⁺ -, NO ₃ ^- -and urea-limited continuous cultures at several dilution rates, we found that both the short-term specific rate of ¹⁴CO₂ uptake and the amount of CO₂ fixed after 8- and 16-min incubations were unaffected by enrichment with NH ₄ ⁺ , urea, or NO ₃ ^- when NH ₄ ⁺ or urea were the preconditioning forms of N, but were slightly suppressed when the cells were first grown on NO ₃ ^- . Similar enrichments in the dark, however, led to significant CO₂ uptake under all conditions of NH ₄ ⁺ enrichment and to similarly enhanced CO₂ uptake, but only at high growth rates, when urea was the source of enrichment nitrogen. Our light results are contrary to some contemporary findings, but there does seem to be agreement that photosynthetic rates of rapidly growing phytoplankton will not be affected by exposure to pulses of nitrogen. Enhanced dark uptake, in contrast, appears to be characteristic of phytoplankton under all degrees of N limitation, and, as such, may be useful as an “all or nothing” index of the nitrogen status of natural waters. There is some indication that the index may be useful in determining both the form of and the degree of N limitation as well.     

Maintenance of coelomic fluid pH in sea urchins exposed to elevated CO2: the role of body cavity epithelia and stereom dissolution

Experimental ocean acidification leads to a shift in resource allocation and to an increased [HCO₃ ^?] within the perivisceral coelomic fluid (PCF) in the Baltic green sea urchin Strongylocentrotus droebachiensis. We investigated putative mechanisms of this pH compensation reaction by evaluating epithelial barrier function and the magnitude of skeleton (stereom) dissolution. In addition, we measured ossicle growth and skeletal stability. Ussing chamber measurements revealed that the intestine formed a barrier for HCO₃ ^? and was selective for cation diffusion. In contrast, the peritoneal epithelium was leaky and only formed a barrier for macromolecules. The ossicles of 6 week high CO₂-acclimatised sea urchins revealed minor carbonate dissolution, reduced growth but unchanged stability. On the other hand, spines dissolved more severely and were more fragile following acclimatisation to high CO₂. Our results indicate that epithelia lining the PCF space contribute to its acid–base regulation. The intestine prevents HCO₃ ^? diffusion and thus buffer leakage. In contrast, the leaky peritoneal epithelium allows buffer generation via carbonate dissolution from the surrounding skeletal ossicles. Long-term extracellular acid–base balance must be mediated by active processes, as sea urchins can maintain relatively high extracellular [HCO₃ ^?]. The intestinal epithelia are good candidate tissues for this active net import of HCO₃ ^? into the PCF. Spines appear to be more vulnerable to ocean acidification which might significantly impact resistance to predation pressure and thus influence fitness of this keystone species.     

Habitat use,urea production and spawning in the gulf toadfish <Emphasis Type="Italic">Opsanus beta</Emphasis>

Field studies were conducted in Johnson Key Basin, Florida Bay, USA from September 2002 through September 2004 to examine physiological, ecological, and behavioral characteristics of the gulf toadfish, Opsanus beta (Goode and Bean in Proc US Natl Mes 3:333–345, 1880), in relation to nitrogen metabolism, habitat usage, and spawning. Fish collected 5 cm above sediments in experimental shelters (epibenthic) were compared with those collected by throw traps which were found on or burrowing within sediments. The relationship between microhabitat ammonia and urea excretion, as determined by the enzymatic activity of glutamine synthetase (GS), was examined. The hypothesis tested was that O. beta occupying epibenthic nests were less ureotelic with lower GS activities than non-nesting individuals on/in sediments, due to a decreased environmental ammonia burden. Porewater total ammonia (T _Amm) concentrations at a sediment depth of 5 cm, i.e., the approximate depth of burrowing toadfish, ranged from 0 to 106.5 μmol N l⁻¹ while the pH ranged from 7.48 to 9.14. There was a weak but significant correlation between environmental ammonia (NH₃) concentration and hepatic GS activity for epibenthic toadfish (P < 0.001, r ² = 0.10), but not for burrowing toadfish. Mean urea-N and T _Amm concentrations within shelters occupied by toadfish (n = 281) were 9.8 ± 0.83 μmol N l⁻¹ and 13.0 ± 0.7 μmol N l⁻¹, respectively. As predicted, hepatic GS activity was significantly lower in epibenthic toadfish captured in shelters (4.40 ± 0.24 μmol min⁻¹ g⁻¹; n = 281) as compared to individuals on/in sediments (6.61 ± 0.47 μmol min⁻¹ g⁻¹; n = 128). Glutamine synthetase activity generally peaked in March (spawning season) and was lowest in July. Gender differences in hepatic and branchial GS activity were also found during the spawning season, which is attributable to the fact that males brood and guard offspring in their epibenthic nests while females often rest on or burrow into the sediments. Finally, hepatic and branchial GS appeared to have different patterns of enzymatic activity suggesting functional differences in gene expression.     

Physiological and biochemical aspects of embryonic and larval development of the winter flounder Pseudopleuronectes americanus

Eggs and larvae of the winter flounder Pseudopleuronectes americanus Walbaum were hatched and raised in the laboratory under controlled conditions. Biochemical composition was measured during development and found to be similar to that of other species: 65 to 80 percent protein, 15 to 30 percent fat, and 0 to 5 percent carbohydrate. Ash content was 7 to 10 percent of dry weight. The chorion comprised more than half of the weight of an egg and the data suggested that it was possibly a source of nutrition to the developing embryo. The sequence of utilization appeared to be carbohydrate and then protein to hatching, lipid, mixed lipid and protein, the predominantly protein until feeding began. Carbohydrate was accumulated at first feeding and depleted when growth began. Protein and lipid were deposited in approximately constant proportions. Respiration rates of eggs were low, 0.002–0.015 l O₂ egg^-1 h^-1, but rose gradually from fertilization to hatching. Respiration rates of early larvae were from two to eight times that of eggs (0.033–0.131 l O₂ larva^-1 h^-1). Variation in larval respiration rates indicated a three-fold difference in rate according to level of activity. Eggs excreted ammonia at an increasing rate from fertilization to hatching. Larvae excreted ammonia, primary amines, and other unidentified organic nitrogenous substances. Rates of excretion and proportions of excretory products varied with stage of development. Primary amine excretion was variable and a major component in early stages. Ammonia-N excreted was two to 20 times primary amine N excreted. Unidentified substances were the predominant form of N excretion during early feeding. Ammonia accounted for most of the N excreted in older larvae. Early specific growth rates were 2.1 and 5.5%. Net caloric conversion and net and gross nitrogen efficiencies were low in first feeding larvae compared to adult fishes (32.2, 27.7, and 10.7% respectively).Contribution no. 5071 from the Woods Hole Oceanographic Institution     

A sensitive method for determining nitrogen-15 isotope in urea

A method is presented in which the ¹⁵N at.% of urea is determined with high precision on liquid samples containing as little as 10 nmol of urea. The method involves removal of interference from NH₄ ⁺ initially present in the sample by cation exchange. Urea in the sample is subsequently hydrolyzed to NH₄ ⁺ by the enzyme urease. Liberated NH₄ ⁺ is separated from the alkaline sample by diffusion as NH₃ through a helium gas phase where it is finally oxidized to N₂ by reaction with hypobromite iodine. The isotopically labeled N₂ thus formed is mixed with the N₂ initially present in the sample, and the ¹⁵N at.% of urea is calculated from the relative amounts of ¹⁴N¹⁵N and ¹⁵N¹⁵N by means of the isotope pairing principle. The presented method for ¹⁵N-urea analysis proved to be precise (SE < 0.4 at.%, n = 5), when applied on both marine and freshwater samples with an ¹⁵N enrichment >1 at.%, and interference was found only from volatile methyl amines. Received: 19 September 1996 / Accepted: 25 October 1996     

Effects of ammonia on survival and osmoregulation of various development stages of the shrimp Penaeus japonicus

In Penaeus japonicus, the tolerance to ammonia increased with the development from nauplius to late juvenile. The 48-h LC₅₀ of ammonia in nauplii (III–V), 96-h LC₅₀ in zoeae (I–III), mysis (I–III), post-larvae (PL1) and late juveniles (10.4±1.1 g) were respectively 5.0, 6.1 to 8.1, 9.4 to 10.9, 15.5 and 52.7 mg Nl^-1 (0.5, 0.6 to 0.7, 0.9, 1.3 and 3.1 mg NH₃–Nl^-1). In a chronic experiment (20 d), the LC₅₀ in post-larvae (PL1) was 19.1 (1.4) at 96 h and 16.2 mg Nl^-1 (1.3 mg NH₃–Nl^-1) at 480 h. Osmoregulatory capacity (OC) was calculated as the osmotic gradient between the hemolymph and the external medium at given salinities. The effects of ammonia on OC, Na⁺ and Cl^- regulation and gill Na⁺–K⁺ ATPase activity in late juveniles were examined in fullstrength seawater, SW (1050 mosm kg^-1, 36 S) and in dilute SW (450 mosm kg^-1, 15%.), after 48 or 96 h exposure to various concentrations of ammonia. Ambient ammonia disrupted both hypo- and hyper-osmoregulation; decreased OC resulted from impaired Na⁺ and Cl^- regulation. Gill Na⁺–K⁺ ATPase activity increased in SW and was not affected in dilute SW. The decrease of OC was ammonia-dose-dependent. The threshold ammonia concentrations affecting hypo-OC and hyper-OC were, respectively, 16 (1.3) and 32 mg Nl^-1 (2.3 NH₃–Nl^-1) for a 48 h exposure; these concentrations were lower than the 48-h LC₅₀ value, 65.3 mg Nl^-1 (3.5 NH₃–Nl^-1). The time course of exposure to sublethal ammonia (48 mg Nl^-1) demonstrated that the effect on osmoregulation was time-dependent. This effect was also temporary, and the exposed shrimps recovered control OC values after removal of excessive ambient ammonia. The possibility of using OC as an indicator of physiological condition in osmoregulating crustaceans and the acting mode of ammonia on osmotic and ionic regulation are discussed.     

Model of stomatal ammonia compensation point (STAMP) in relation to the plant nitrogen and carbon metabolisms and environmental conditions

Agricultural crops can be either a source or a sink of ammonia (NH₃). Most NH₃ exchange models developed so far do not account for the plants nitrogen (N) metabolism and use prescribed compensation points. We present here a leaf-scale simplified NH₃ stomatal compensation point model related to the plants N and carbon (C) metabolisms, for C₃ plants. Five compartments are considered: xylem, cytoplasm, apoplasm, vacuole and sub-stomatal cavity. The main processes accounted for are the transport of ammonium (NH₄⁺), NH₃ and nitrate (NO₃⁻) between the different compartments, NH₄⁺ production through photorespiration and NO₃⁻ reduction, NH₄⁺ assimilation, chemical and thermodynamic equilibriums in all the compartments, and stomatal transfer of NH₃.The simulated compensation point is sensitive to paramaters related to the apoplastic compartment: pH, volume and active transport rate. Determining factors are leaf temperature, stomatal conductance and NH₄⁺ flux to the leaf. Atmospheric NH₃ concentration seem to have very little effect on the compensation point in conditions of high N fertilization. Comparison of model outputs to experimental results show that the model underestimates the NH₃ compensation point for high N fertilization and that a better parametrisation of sensitive parameters especially active trasport rate of NH₄⁺ may be required.     

Smashing tests? Patterns and mechanisms of adult mortality in a declining echinoid population

Mass mortality of echinoids is well documented, and has potentially profound effects on benthic communities. However, no study to date has quantitatively investigated how regular, predictable events such as winter storms might lead to large mortality events in pivotal echinoid species. Hydrodynamic disturbances can be major drivers of crucial biological processes in benthic communities. For echinoid populations in particular, wave action in shallow waters generated by high winds (winter storms) can cause displacement, damage and even death to individuals. However, evidence for displacement-mediated mortality is scant in the literature, in part because it is so difficult to demonstrate in exposed environments where echinoids are frequently found. In this study, we examined mortality in a sheltered subtidal population of the European purple sea urchin Paracentrotus lividus over a 3-year period, and examined the role that dislodgement by wave action or predation might play in these mortality patterns. Because our study population has been in decline for the past three decades, we considered it important to evaluate its current status in addition to assessing the contribution that adult mortality makes to that decline. We sampled twice per month, using the density of freshly dead echinoid material to assess the extent of adult mortality. The Irish Meteorological Service provided our estimates of wind speed data. We compared historical and recent data on P. lividus size frequency data to investigate change the population structure (Poor recruitment would be caused by failure to spawn over a prolonged period). Our data suggest ongoing declines in this population, and support the theory that the decline of the P. lividus population of Lough Hyne is a result of persistent recruitment failure linked to repeated cool maximum sea surface temperatures. Although we found peaks of P. lividus mortality were coincident with spikes in wind speed, mortality was low, and seems unlikely to have contributed significantly to the dramatic decline in P. lividus in the past three decades.Communicated by J.P. Thorpe, Port Erin