Suspension feeding in Bithynia tentaculata (Prosobranchia,Bithyniidae), as affected by body size,food and temperature

The suspension feeding of Bithynia tentaculata was tested in laboratory experiments. The animals were fed in 1-1 aerated glass beakers, and filtration rates were calculated from changes in cell concentrations during the 6-h experiment. Temperature influenced the filtering rate, with minimum values of 5ml · ind^–1 · h^–1 at 5° C and maxima of 17.2 ml · ind^–1 · h^–1 at 18° C. Three food species of different size, motility and cell surface characteristics (Chlamydomonas reinhardii, Chlorella vulgaris and Chlorogonium elongatum) did not affect filtration rates. Suspension feeding increased with increasing food concentrations up to 12 nl · ml^–1, above which feeding rate was kept constant by lowering the filtering rates. Even the smallest animals tested (<4 mm body length) were found to be feeding on suspended food at a rate of 2.7 ml · ind^–1 · h^–1, and increasing rates up to 8.4 ml were found in the 6–7 mm size class. All size classes of Bithynia showed a circannual fluctuation of their filtration rates. The ecological consequences of Bithynia's ability to switch between two feeding modes, grazing and suspension feeding, are discussed.     

The photoprotective role of humus-DOC for Selenastrum and Daphnia

Cell numbers and fluorescence of the green algae Selenastrum capricornutuum and survival of Daphnia magna exposed to simulated sun-light was assessed along a gradient of DOC (0, 1, 5 10 and 50 mg C l^–1). When exposed to UV-doses and spectral distribution (295–750 nm) closely resembling surface solar radiation during mid summer, Selenastrum showed major losses of cell fluorescence. In the absence of DOC, fluorescence was severely depressed, with successively decreasing effects with increased DOC. Surviving cells also required an extensive recovery period (10–12 d) for regrowth after exposure, while an almost immediate recovery was observed at concentrations above 1 mg DOC l^–1. For Daphnia, survival was reduced to less than 10% after 4 h exposure, and almost zero after 8 h exposure in the absence of humus DOC, while no effects were observed in treatments with 10 and 50 mg C l^–1. Selenastrum and Daphnia that were not directly irradiated, but exposed to UV-irradiated water with the same concentrations of DOC did not reveal negative effects. This indicates negligible indirect effects mediated by long-lived free radicals or other toxic compounds. Irradiation of Daphnia under increased oxygen concentration (200% saturation) did not indicate acute effects, suggesting that effects of ambient radicals and oxidants would be of minor importance relative to intracellular photoproducts.     

Predatory — prey relationships in subtropical zooplankton: water mite against cladocerans in an Argentine lake

Summary In a small lake in northern Argentina pelagic water mite Piona sp. had the maximum of population density in January following with a five-day delay after the peak of zooplankton dominant — Daphnia laevis. The mite density was highly predicted by the previous variations of Daphnia density during 4 months of observation (December–March). Daphnia density was a negative delayed function of the predator density but only in December–January when Piona was abundant. During that period Daphnia death rate, d was also correlated with the mite density (r ²=0.80, P<0.005). In laboratory experiments water mites killed 1–7 Daphnia · predator^-1 · hour^-1 in a broad range of prey density. Another zooplankton component, Diaphanosoma birgei, was consumed at the same rates. The mite hardly consumed any copepods. In the pelagium during 24 hours the mite was more associated with Daphnia, than with Diaphanosoma, probably, because of the coincidence in photoreactions with Daphnia. Piona contribution to the death rates of its prey estimated by using the data on functional and numerical responses as well as by means of Edmondson-Paloheimo model, could reach 53% for Daphnia and 40% for Diaphanosoma. A computer experiment on the reconstruction of prey dynamics after subtraction of predator influence showed that the mite could have caused a depression in Daphnia numbers observed in the lake, but the declines in Diaphanosoma population were caused by other factors. After the removal of mite pressure model Daphnia population increased its average density 10-fold. Experiments on Piona feeding revealed a strong effect of interference among predators. This was eliminated by putting one mite per experimental vessel, which led to a 20-fold increase in predation rate. The effect explains the low feeding rates of Piona obtained by the previous authors who ignored the possibility of interference.     

Feeding behaviour and morphology of filtering combs of Daphnia galeata

The population of Daphnia galeata Sars from the fish pond Velký Pálenec (Blatná, Czechoslovakia) living in high food conditions (7 mgC l^–1) was characterized by a small size of the filtering comb on the thoracic limb 3, measured as seta length, length of the base of the comb and number of setae (population 1). One month cultivation of this population in low food conditions (1.5 mgC l^–1) in the laboratory resulted in twofold increase in size of the filtering comb (population 2). Filtering and ingestion rates of both populations were measured at eight concentrations of food (approximately 0.025–3.2 mgC l^–1) using ¹⁴C labeled Scenedesmus acutus. The results show that size of the filtering combs influences considerably feeding behavior of Daphnia. The comparison of animals with the same body length suggests that the population with a large comb feeds at concentration of food below 0.4 mgC l^–1 more intensively and reaches the maximum of the filtering rate at a lower concentration than the population with a small comb. The situation is opposite at concentration above 0.4 mgC l^–1. The higher values of theoretical flow in population with a small projection of filtering area suggest that this population has to compensate disadvantage of a small comb with the higher appendages beat frequency.     

Clearance rates of sessile rotifers: in vitro determinations

We measured laboratory clearance rates of 10 rotifer and one unidentified bryozoan species from 3 different lakes using ³²P labeled algae (Chlamydomonas) or yeast (Rhodotorula). Clearance rates for all rotifers fed yeast ranged from < 2.0 to > 260 µl · animal^–1 · h^–1 depending on species. The in vitro clearance rates of two sessile rotifers (Ptygura crystallina and P. pilula) were not significantly different from previously measured in situ rates (Wallace and Starkweather 1983). Clearance rates for 5 rotifers fed algae ranged from < 5.0 to > 90.0 µl · animal^–1 · h^–1. Ptygura beauchampi, P. crystallina, P. pilula, Floscularia conifera, and F. melicerta ingested both cell types but their clearance rates varied substantially among species and between cell types. There was a substantial time-dependent loss of 32P from formalin-fixed animals (Sinantherina socialis) awaiting processing. This loss stabilized at approximately 20 hours and was estimated to be about 40% of the initial ingested label. Clearance rates for the bryozoan fed yeast or algae were highly variable, ranging from < 1.0 to > 3 000 µl · animal^–1 · h^–1.     

Cardiac output and stroke volume in swimming harbor seals

Summary Cardiac output was measured by the thermodilution method in three young harbor seals, at rest and while swimming up to the maximum effort for which they could be trained. Stroke volume was determined by counting heart rate simultaneously with determination of cardiac output. Cardiac outputs varied widely between surface breathing (7.8 ml · kg^–1 · s^–1) and breath-holding while swimming under water (1.8 ml · kg^–1 · s^–1). Stroke volume while at the surface was almost twice the volume white submerged. Surface cardiac output was always near maximal despite work effort, whereas submerged cardiac output gradually increased at higher work efforts. The cardiovascular performance of seals at the maximum MO₂ we could induce from them is equivalent to that of the domestic goat.Abbreviations CO Cardiac output - HR Heart rate - SV Stroke volume - MO ₂ Metabolic rate - FS Forced sumersion - V Velocity - C _DF Frontal drag coefficient - CV Cardiovascular Present address: Institute of Marine Science, University of Alaska, Fairbanks, AK, USA     

The Polyarthra escape from response: Defense against interference from Daphnia

Direct observations show that almost all (97%) Polyarthra remata caught in the inhalant current of Daphnia pulex avoid inhalation by initiating escape responses. Microvideographic analysis shows that these responses are initiated, on average, 0.24 mm (about 2.5 body lengths) from the inhalant opening of the branchial chamber, when the Polyarthra are moving at a velocity of about 1.4 mm s^–1 (about 6 times their normal swimming speed — 0.24 mm s^–1). The stimulus for the escape response could be rapid acceleration or, more likely, shear. The average distance traveled during an escape response is at least 1.25 mm (about 12 body lengths).The Polyarthra escape response can provide a very effective defense against interference from Daphnia and may greatly increase the ability of Polyarthra spp. to coexist with Daphnia.     

Lateral hydraulic conductivity of early metaxylem vessels in Zea mays L. roots

The hydraulic conductivity of the lateral walls of early metaxylem vessels (Lp_x in m · s^–1 · MPa^–1) was measured in young, excised roots of maize using a root pressure probe. Values for this parameter were determined by comparing the root hydraulic conductivities before and after steam-ringing a short zone on each root. Killing of living tissue virtually canceled its hydraulic resistance. There were no suberin lamellae present in the endodermis of the roots used. The value of Lp_x ranged between 3 · 10^–7 and 35 · 10^–7 m · s^–1 · MPa^–1 and was larger than the hydraulic conductivity of the untreated root (Lp_r = 0.7 · 10^–7 to 4.0 · 10^–7 m · s^–1 · MPa^–1) by factor of 3 to 13. Assuming that all flow through the vessel walls was through the pit membranes, which occupied 14% of the total wall area, an upper limit of the hydraulic conductivity of this structure could be given(Lp_pm=21 · 10^–7 to 250 · 10^–7 m · s^–1 · MPa^–1). The specific hydraulic conductivity (Lp_cw) of the wall material of the pit membranes (again an upper limit) ranged from 0.3 · 10^–12 to 3.8 · 10^–12 m² · s^–1 · MPa^–1 and was lower than estimates given in the literature for plant cell walls. From the data, we conclude that the majority of the radial resistance to water movement in the root is contributed by living tissue. However, although the lateral walls of the vessels do not limit the rate of water flow in the intact system, they constitute 8–31% of the total resistance, a value which should not be ignored in a detailed analysis of water flow through roots.Abbreviatations and Symbols k_wr (T ₁ ^2/W ) rate constant (half-time) of water exchange across root (s^–1 or s, respectively) - Lp_cw specific hydraulic conductivity of wall material (m² · s^–1 · MPa^–1) - Lp_pm hydraulic conductivity of pit membranes (m · s ^–1 · MPa^–1) - Lp_r hydraulic conductivity of root (m · s^–1 · MPa^–1) - Lp_x lateralhydraulic conductivity of walls of root xylem (m · s ^–1 · MPa^–1) This research was supported by a grant from the Bilateral Exchange Program funded jointly by the Natural Sciences and Engineering Research Council of Canada and the Deutsche Forschungsgemeinschaft to C.A.P., and by a grant from the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 137, to E.S. The expert technical help of Mr. Burkhard Stumpf and the work of Ms. Martina Murrmann and Ms. Hilde Zimmermann in digitizing chart-recorder strips is gratefully acknowledged.     

Brachionus calyciflorus Pallas as agent for the removal of E. coli in sewage ponds

Brachionus calyciflorus (Pallas) is a common brachionid in sewage oxidation ponds. The uptake and assimilation of E. coli was optimal at concentrations of 2.7–6.9 × 10⁸ cells ml^–1 while assimilation coefficient per body weight of B. calyciflorus was found to be 10% · Ind.^–1 d^–1. More than two eggs per individual were produced during 24 hours when brachionids were fed with a mixutre of E. coli (10⁹ cells · ml^–1) and Chlorella spp. (10⁶ cells · ml^–1). The nutritional value of the mixture of E. coli and Chlorella spp. was found to be higher than that of bacteria alone.     

Feeding rate inhibition in crowded Daphnia pulex

Feeding rates of Daphnia pulex fed a range of levels of the alga Chlamydomonas reinhardi of 15 °C are strongly density-dependent. At lower densities, Daphnia (30 1^–1) fed at higher rates than crowded (270 1^–1) Daphnia which manifest a relatively depressed saturation feeding response. At 30 individuals/liter, Daphnia consumed 8.5 – 15.7 × 10⁴ cells d^–1h^–1 (on a volume basis, 12.1 – 22.2 × 10⁶ m³), at 270 L^–1 3.7 – 3.9 × 10⁴ (5.2 – 5.5 = 10⁶ m³ cells d^–1h^–1 when feeding on algae at 80 000 cells ml^–1 (11.3 × 10⁶ m³ ml^–1). The feeding rate data best fit an Ivlev feeding function. An autoallelopath might be causing the repression. Water preconditioned with crowded Daphnia completely repressed feeding in uncrowded Daphnia after six hours.     

Metabolism,thermogenesis and daily rhythm of body temperature in the wood lemming,Myopus schisticolor

Wood lemmings (Myopus schisticolor) were captured during their autumnal migration in September and October. The animals were maintained at 12°C and under 12L:12D photoperiod. Basal metabolic rate and thermogenic capacity of the wood lemming were studied. Basal metabolic rate was 3.54 ml O₂·g^-1·h^-1, which is 215–238% of the expected value. The high basal metabolic rate seems to be typical of rodents living in high latitudes. The body temperature of the wood lemming was high (38.0–38.8°C), and did not fluctuate much during the 24-h recording. The high basal metabolic rate and the high body temperature are discussed with regard to behavioural adaptation to a low-quality winter diet. Thermogenic capacity, thermal insulation and non-shivering thermogenesis of the wood lemming displayed higher values than expected: 53.0 mW·g^-1, 0.53 mW·g^-1·C^-1 and 53.2 mW·g^-1, respectively. Brown adipose tissue showed typical thermogenic properties, although its respiratory property was fairly low, but mitochondrial protein content was high compared to other small mammals. The 24-h recording of body temperature and motor activity did not reveal whether the wood lemming is a nocturnal animal. Possibly, the expression of a circadian rhythm was masked by peculiar feeding behaviour. It is concluded that the wood lemming is well adapted to living in cold-temperature climates.Abbreviations BAT brown adipose tissue; bm, body mass - BMR basal metabolic rate - C conductance - Cox cytochrome-c-oxidase - HP heat production - HP_max maximum heat production - M metabolism - NA noradrenaline - NST non-shivering thermogenesis - NST_max maximum non-shivering thermogenesis - RMR resting metabolic rate - RQ respiratory quotient - T _a anibient temperature - T _b body temperature - T _lc lower critical temperature - UCP uncoupling protein - vO₂ oxygen consumption - vO_{2 max} maximum oxygen consumption     

Water conservation and protein metabolism in northern elephant seal pups during the postweaning fast

Urine production and N output were monitored in northern elephant seal (Mirounga angustirostris) pups progressing through 10 weeks of a natural postweaning fast. Urine output declind by 84% (to 69±12 ml·day^–1) at 10 weeks (P<0.05). Glomerular filtration rate at 10 weeks was 51% of the 67±3 ml serum·min^–1 observed during week 1 (P<0.05). Urine N excretion fell by 69% to 1.2±0.17 g·day^–1, while urinary concentration increased (P<0.05). Serum urea declined from an initial 11 mmol·1^–1 to 5–7 mmol·1^–1 by 5 weeks. The fall in urinary N loss (and thus amino acid oxidation) was concomitant with depressed metabolic rate. Therefore, protein contributed little toward meeting energy demands (i.e., <4% of average metabolic rate) throughout fasting. These data indicate that fasting pups improve water conservation and minimize protein catabolism during prolonged natural fasts without an exogenous source of water.Abbreviations AA amino acid(s) - AMR average metabolic rate - ANOVA one-way analysis of variance - BMR basal metabolic rate - BUN blood urea nitrogen - EP end product - EWL evaporative water loss - [Gr]_s serum creatinine concentration - GFR glomerular filtration rate - LBM lean body mass - LML Long Marine Laboratory - MR metabolic rate - NEFA non-esterified fatty acids - RMR resting metabolic rate - TCA tricarboxylic acid - U:C ulinary urea: creatinine concentration ratio     

Combined effect of the insecticide carbaryl and the Chaoborus kairomone on helmet development in Daphnia ambigua

The cladoceran Daphnia ambigua was exposed to both the insecticide carbaryl and the kairomone released from the predator Chaoborus simultaneously, and its morphological changes were analyzed. Daphnia developed helmets in response to the kairomone, but not in response to carbaryl at low (sublethal) concentrations (1–3 µg 1^–1). However, the carbaryl enhanced the development of high helmets and prolonged the maintenance period of the helmets over instars in the presence of the kairomone. These results suggest that sublethal concentrations of the insecticide alter predator-prey interactions by inducing helmet formation in Daphnia, which may reduce vulnerability of the Daphnia to predation.     

Seasonal dynamics of metabolic activity of the Daphnia community in Lake Bled (Slovenia)

Seasonal changes of the community composition, oxygen consumption (R) and respiratory electron transport system (ETS)-activity of the Daphnia community living in Lake Bled (Slovenia) were studied between January and December 1998. The ETS activity of ovigerous Daphnia females at in situ temperature ranged from 3.27 l O₂ mg dw^–1 h^–1 in February to 20.91 l O₂ mg dw^–1 h^–1 in July. Respiration rates at in situ temperature varied from 4.04 l O₂ mg dw^–1h^–1 in December to 18.68 l O₂ mg dw^–1 h^–1in July. The influence of four factors (temperature, body size, fecundity, genetic differences) on the metabolism were investigated. Both ETS activity and respiration rate significantly correlated with temperature. The proportion of hybrid D. cucullata× galeata in Daphnia community correlated significantly with respiration rate at in situtemperature and ETS activity at standard temperature also. ETS activity and respiration rate showed no significant correlation with body size and the fecundity of Daphnia, whereas ETS activity in D. hyalina × galeata and D. cucullata× galeataseparately correlated with body size. ETS activity of D. hyalina × galeata also was correlated with fecundity. Hybrid D. hyalina× galeata had up to one third lower ETS activity than D. cucullata× galeata. The mean ETS/R ratio in the Daphnia community was 1.16±0.28 (N= 12). The ETS/R ratio did not correlate significantly with temperature, body size, fecundity or the proportion of D. cucullata× galeatain the Daphnia community. Laboratory experiments showed that both hybrids had similar ETS/R ratios.     

Toxicity of the new pyrethroid insecticide,deltamethrin, to Daphnia magna

The toxicity of deltamethrin, a synthetic pyrethroid insecticide, was determined under standardized conditions (ISO, 1982) in neonates and juveniles of Daphnia magna. Neonates (6 to 24 h old) were more sensitive than juveniles (48 to 72 h old). The 24- and 48-h EC₅₀s (immobilization) in neonates were 0.113 and 0.031 µg l^–1, respectively. The toxicity of deltamethrin was highly toxic. The 96-h EC₅₀ was in the ppt (µg l^–1) range. Toxicity tests with Daphnia may be used to detect toxic residues in water and sediment in areas treated with deltamethrin and other highly toxic pyrethroid pesticides.     

Chaff piles of harvester ant (Messor spp.) nests in a desert ecosystem

Summary The plant species composition of the chaff piles of three species of harvester ant (Messor spp.) and the contribution of the chaff to the organic pool were studied from August 1985 to July 1987. There were distinct differences in the plant species composition of the chaff of the three species. We attribute this to the different diets of the three species, which reflect the relative sizes of their individuals and their foraging strategies. The amount of chaff accumulated varies greatly between the three species (Messor rugossus: 127–196 g · ha^–1 · y^–1;Messor ebeninus: 2823–4437 g · ha^–1 · y^–1;Messor arenarius: 2165–2535 g · ha^–1 · y^–1), although the number of nests per hectare is virtually the same. We found that the amount of chaff is related to the rate of activity and the size of the individuals of each of the three ant species. The total chaff accumulated during the study period was 19.2 kg · ha^–1, which is an important contribution to the organic matter in the soil in the Negev desert ecosystem.     

Phase transitions and thermal expansion coefficients of plant cuticles

The temperature-induced volume expansion of enzymatically isolated cuticular membranes of twelve plant species was measured. All cuticular membranes exhibited distinct second-order phase transitions in the temperature range of about 40 to 50° C. Increases in the volumes of fruit cuticles (Lycopersicon, Cucumis, Capsicum, Solanum and Malus) were fully reversible, while leaf cuticular membranes (Ficus, Hedera, Nerium, Olea, Pyrus, Picea and Citrus) underwent irreversible structural changes. Below the phase-transition temperature, volumetric expansion coefficients ranged from 0.39·10^–6 m³·kg^–1·K^–1 to 0.62·10^–6 m³·kg^–1·K^–1, and above from 0.60·10⁶ m³·kg^–1·K^\-1 to 1.41· 10^–6 m³·kg^–1·K^–1. Densities of cuticles at 25° C ranged from 1020 kg·m^–3 to 1370 kg·m^–3. Expansion coefficients and phase transitions were characteristic properties of the polymer matrix as a composite material, rather than of cutin alone. It is argued that the sudden increase of water permeability above the transition temperature, is caused by an increase of disorder at the interface between the polymer matrix and the soluble cuticular lipids. Possible ecological and physiological consequences of these results for living plants are discussed.Abbreviations CM Cuticular membrane - CU cutin - MX polymer matrix - SCL soluble cuticular lipids (waxes) The authors greatfully acknowledge stimulating discussions with Drs. H. Gruler (Exp. Physik 3, Universität Ulm, FRG) and M. Riederer (Institut für Botanik und Mikrobiologie, Technische Universität München, München, FRG) and financial support by the Deutsche Forschungsgemeinschaft.     

The energetics of the common mole rat Cryptomyś, a subterranean eusocial rodent from Zambia

Body temperature, oxygen consumption, respiratory and cardiac activity and body mass loss were measured in six females and four males of the subterranean Zambian mole rat Cryptomys sp. (karyotype 2 n=68), at ambient temperatures between 10 and 35°C. Mean body temperature ranged between 36.1 and 33.2°C at ambient temperatures of 32.5–10°C and was lower in females (32.7°C) than in males (33.9°C) at ambient temperatures of 10°C but dit not differ at thermoneutrality (32.5°C). Except for body temperature, mean values of all other parameters were lowest at thermoneutrality. Mean basal oxygen consumption of 0.76 ml O₂·g^-1· h^-1 was significantly lower than expected according to allometric equations and was different in the two sexes (females: 0.82 ml O₂·g^-1·h^-1, males: 0.68 ml O₂·g¹·h^-1) but was not correlated with body mass within the sexes. Basal respiratory rate of 74·min^-1 (females: 66·min¹, males: 87·min^-1) and basal heart rate of 200·min^-1 (females: 190·min^-1, males: 216·min^-1) were almost 30% lower than predicted, and the calculated thermal conductance of 0.144 ml O₂·g^-1·h¹·°C^-1 (females; 0.153 ml O₂·g^-1·h^-1·°C^-1, males: 0.131 ml O₂·g^-1·h^-1·°C^-1) was significantly higher than expected. The body mass loss in resting mole rats of 8.6–14.1%·day^-1 was high and in percentages higher in females than in males. Oxygen consumption and body mass loss as well as respiratory and cardiac activity increased at higher and lower than thermoneutral temperatures. The regulatory increase in O₂ demand below thermoneutrality was mainly saturated by increasing tidal volume but at ambient temperatures <15°C, the additional oxygen consumption was regulated by increasing frequency with slightly decreasing tidal volume. Likewise, the additional blood transport capacity was mainly effected by an increasing stroke volume while there was only a slight increase of heart frequency. In an additional field study, temperatures and humidity in different burrow systems have been determined and compared to environmental conditions above ground. Constant temperatures in the nest area 70 cm below ground between 26 and 28°C facilitate low resting metabolic rates, and high relative humidity minimizes evaporative water loss but both cause thermoregulatory problems such as overheating while digging. In 13–16 cm deep foraging tunnels, temperature fluctuations were higher following the above ground fluctuations with a time lag. Dominant breeding females had remarkably low body temperatures of 31.5–32.3°C at ambient temperatures of 20°C and appeared to be torpid. This reversible hypothermy and particular social structure involving division of labour are discussed as a strategy reducing energy expenditure in these eusocial subterranean animals with high foraging costs.Abbreviations BMR basal metabolic rate - br breath - C thermal conductance - HR neart rate - LD light/dark - M _b body mass - MR metabolic rate - OP oxygen pulse - PCO₂ partial pressure of carbon dioxide - PO₂ partial pressure of oxygen - RMR resting metabolic rate - RR respiratory rate - T _a ambient temperature - T _b body temperature - TNZ thermal neural zone - O₂ oxygen consumption     

Aerobic nitrate and nitrite reduction in continuous cultures of Escherichia coli E4

Nitrate and nitrite was reduced by Escherichia coli E4 in a l-lactate (5 mM) limited culture in a chemostat operated at dissolved oxygen concentrations corresponding to 90–100% air saturation. Nitrate reductase and nitrite reductase activity was regulated by the growth rate, and oxygen and nitrate concentrations. At a low growth rate (0.11 h^–1) nitrate and nitrite reductase activities of 200 nmol · mg^–1 protein · min^–1 and 250 nmol · mg^–1 protein · min^–1 were measured, respectively. At a high growth rate (0.55 h^–1) both enzyme activities were considerably lower (25 and 12 nmol mg^–1 · protein · min^–1). The steady state nitrite concentration in the chemostat was controlled by the combined action of the nitrate and nitrite reductase. Both nitrate and nitrite reductase activity were inversely proportional to the growth rate. The nitrite reductase activity decreased faster with growth rate than the nitrate reductase. The chemostat biomass concentration of E. coli E4, with ammonium either solely or combined with nitrate as a source of nitrogen, remained constant throughout all growth rates and was not affected by nitrite concentrations. Contrary to batch, E. coli E4 was able to grow in continuous cultures on nitrate as the sole source of nitrogen. When cultivated with nitrate as the sole source of nitrogen the chemostat biomass concentration is related to the activity of nitrate and nitrite reductase and hence, inversely proportional to growth rate.     

Long-term kinetics of the light-dependent regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in plants with and without 2-carboxyarabinitol 1-phosphate

The light-dependent modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was studied in two species: Phaseolus vulgaris L., which has high levels of the inhibitor of Rubisco activity, carboxyarabinitol 1-phosphate (CA1P), in the dark, and Chenopodium album L., which has little CA1P. In both species, the ratio of initial to fully-activated Rubisco activity declined by 40–50% within 60 min of a reduction in light from high a photosynthetic photon flux density (PPFD; >700 mol · m^–2 · s^–1) to a low PPFD (65 ± 15 mol · m^–2 · s^–1) or to darkness, indicating that decarbamylation of Rubisco is substantially involved in the initial regulatory response of Rubisco to a reduction in PPFD, even in species with potentially extensive CA1P inhibition. Total Rubisco activity was unaffected by PPFD in C. album, and prolonged exposure (2–6 h) to low light or darkness was accompanied by a slow decline in the activity ratio of this species. This indicates that the carbamylation state of Rubisco from C. album gradually declines for hours after the large initial drop in the first 60 min following light reduction. In P. vulgaris, the total activity of Rubisco declined by 10–30% within 1 h after a reduction in PPFD to below 100 mol · m^–2 · s^–1, indicating CA1P-binding contributes significantly to the reduction of Rubisco capacity during this period, but to a lesser extent than decarbamylation. With continued exposure of P. vulgaris leaves to very low PPFDs (< 30 mol · m^–2 · s^–1), the total activity of Rubisco declined steadily so that after 6–6.5 h of exposure to very low light or darkness, it was only 10–20% of the high-light value. These results indicate that while decarbamylation is more prominent in the initial regulatory response of Rubisco to a reduction in PPFD in P. vulgaris, binding of CA1P increases over time and after a few hours dominates the regulation of Rubisco activity in darkness and at very low PPFDs.Abbreviations CA1P 2-carboxyarabinitol 1-phosphate - CABP 2-carboxyarabinitol 1,5-bisphosphate - k_cat substrate-saturated turnover rate of fully carbamylated enzyme - PPFD photosynthetically active photon flux density (400–700 nm) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate