Influence of the concentrations of d-xylose and yeast extract on ethanol production by Pachysolen tannophilus

To determine the most favorable conditions for the production of ethanol by Pachysolen tannophilus, this yeast was grown in batch cultures with various initial concentrations of two of the constituents of the culture medium: d-xylose (s_o), ranging from 1 g·l⁻¹ to 200 g·l⁻¹, and yeast extract (l_o), ranging from 0 g·l⁻¹ to 8 g·l⁻¹. The most favorable conditions proved to be initial concentrations of S_o=25 g·l⁻¹ and l_o=4 g·l⁻¹, which gave a maximum specific growth rate of 0.26 h⁻¹, biomass productivity of 0.023 g·l⁻¹·h⁻¹, overall biomass yield of 0.094 g·g⁻¹, specific xylose-uptake rate (q_s) of 0.3 g·g⁻¹·h⁻¹ (for t=50 h), specific ethanol-production rate (q_E) of 0.065 g·g⁻¹·h⁻¹ and overall ethanol yield of 0.34 g·g⁻¹; q_s values decreased after the exponential growth phase while q_E remained practically constant.     

COPPER‐UPTAKE KINETICS OF COASTAL AND OCEANIC DIATOMS1

We investigated copper (Cu) acquisition mechanisms and uptake kinetics of the marine diatoms Thalassiosira oceanica Hasle, an oceanic strain, and Thalassiosira pseudonana Hasle et Heimdal, a coastal strain, grown under replete and limiting iron (Fe) and Cu availabilities. The Cu‐uptake kinetics of these two diatoms followed classical Michaelis–Menten kinetics. Biphasic uptake kinetics as a function of Cu concentration were observed, suggesting the presence of both high‐ and low‐affinity Cu‐transport systems. The half‐saturation constants (K_m) and the maximum Cu‐uptake rates (V_max) of the high‐affinity Cu‐transport systems (～7–350 nM and 1.5–17 zmol · μm^?2 · h^?1, respectively) were significantly lower than those of the low‐affinity systems (>800 nM and 30–250 zmol · μm^?2 · h^?1, respectively). The two Cu‐transport systems were controlled differently by low Fe and/or Cu. The high‐affinity Cu‐transport system of both diatoms was down‐regulated under Fe limitation. Under optimal‐Fe and low‐Cu growth conditions, the K_m of the high‐affinity transport system of T. oceanica was lower (7.3 nM) than that of T. pseudonana (373 nM), indicating that T. oceanica had a better ability to acquire Cu at subsaturating concentrations. When Fe was sufficient, the low‐affinity Cu‐transport system of T. oceanica saturated at 2,000 nM Cu, while that of T. pseudonana did not saturate, indicating different Cu‐transport regulation by these two diatoms. Using CuEDTA as a model organic complex, our results also suggest that diatoms might be able to access Cu bound within organic Cu complexes.     

Competitive exclusion as a mode of action of a novel Bacillus cereus aquaculture biological agent

Aims: To determine the contribution of potential modes of action of a Bacillus cereus aquaculture biological control agent in inhibition of the fish pathogen, Aeromonas hydrophila. Methods and Results: When B. cereus was tested in plate well inhibition studies, no production of antimicrobial compounds was detected. Bacillus cereus had a high growth rate (0·96 h^?1), whereas Aer. hydrophila concentration decreased by c. 70% in co‐culture experiments. In nutrient limitation studies, B. cereus had a significantly higher growth rate when cultured under glucose (P < 0·05) and iron (P < 0·01) limitation in comparison with Aer. hydrophila. Bacillus cereus glucose (0·30 g l^?1 h^?1) and iron (0·60 mg l^?1 h^?1) uptake rates were also significantly higher (P < 0·01) than the Aer. hydrophila glucose (0·14 g l^?1 h^?1) and iron (0·43 mg l^?1 h^?1) uptake rates. Iron uptake was facilitated by siderophore production shown in time profile studies where relative siderophore production was c. 60% through the late exponential and sporulation phases. Conclusions: Competitive exclusion by higher growth rate, competition for organic carbon and iron, facilitated by siderophore production, could be identified as mechanisms of pathogen growth inhibition by B. cereus. Significance and Impact of the Study: This study is the first elucidation of the mechanism of action of our novel B. cereus biological agent in growth attenuation of pathogenic Aer. hydrophila. This study enhances the application knowledge and attractiveness for adoption of B. cereus NRRL 100132 for exploitation in aquaculture.     

Enhancement of docosahexaenoic acid production by Schizochytrium sp. using a two-stage oxygen supply control strategy based on oxygen transfer coefficient

Aims: To improve the yield and productivity of docosahexaenoic acid (DHA) by Schizochytrium sp. in terms of the analysis of microbial physiology. Methods and Results: A two‐stage oxygen supply control strategy, aimed at achieving high concentration and high productivity of DHA, was proposed. At the first 40 h, K_La was controlled at 150·1 h^?1 to obtain high μ for cell growth, subsequently K_La was controlled at 88·5 h^?1 to maintain high q_p for high DHA accumulation. Finally, the maximum lipid, DHA content and DHA productivity reached 46·6, 17·7 g l^?1 and 111 mg l^?1 h^?1, which were 43·83%, 63·88% and 32·14% over the best results controlled by constant K_La. Conclusions: This paper described a two‐stage oxygen supply control strategy based on the kinetic analysis for efficient DHA fermentation by Schizochytrium sp. Significance and Impact of the study: This study showed the advantage of two‐stage control strategy in terms of microbial physiology. As K_La is a scaling‐up parameter, the idea developed in this paper could be scaled‐up to industrial process and applied to other industrial biotechnological processes to achieve both high product concentration and high productivity.     

Regulation of cation transport pathways and glycolytic enzyme activity by alterations in red cell volume

In the presence of NH₄Cl and hypotonic solutions, Rana balcanica red cells respond by increasing their volume. The stimulation of cellular volume by hypotonicity is more rapid than that of NH₄Cl, while the maximum value is less than that observed in the presence of NH₄Cl. Depending on the cause of swelling, (net uptake of NH₄Cl or decrease in external osmolality) cells show specific responses. The NH₄Cl treatment causes a significant increase in intracellular Na⁺, from 5·14±0·78 to 29·84±0·47 mmoles l⁻¹ cell, while hypotonicity leads to a significant decrease of this cation, to 3·85±0·25 mmoles l⁻¹ cell in relation to the control, after 30 min of incubation of Rana balcanica erythrocytes. In addition, amiloride significantly reverses the NH₄Cl effect with respect to intracellular Na⁺. Both treatments cause a significant K⁺ loss in comparison with controls. Two glycolytic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH) and pyruvate kinase (PK) of Rana balcanica haemolysate were found to respond to the NH₄Cl effect by significantly decreasing their activity. Copyright © 1999 John Wiley & Sons, Ltd.     

Hardness does not affect the physiological responses of wild and domestic strains of diploid and triploid rainbow trout Oncorhynchus mykiss to short‐term exposure to pH 9.5

This study examined the effects of water hardness on the physiological responses associated with high pH exposure in multiple strains of diploid and triploid rainbow trout Oncorhynchus mykiss. To accomplish this, three wild strains and one domesticated strain of diploid and triploid O. mykiss were abruptly transferred from control soft water (City of Vancouver dechlorinated tap water; pH 6·7; [CaCO₃] < 17·9 mg l^?1) to control soft water (handling control), high pH soft water (pH 9·5; [CaCO₃] < 17·9 mg l^?1), or high pH hard water (pH 9·5; [CaCO₃] = 320 mg l^?1) followed by sampling at 24 h for physiological measurements. There was a significant effect of ploidy on loss of equilibrium (LOE) over the 24 h exposure, with only triploid O. mykiss losing equilibrium at high pH in both soft and hard water. Furthermore, exposure to pH 9·5 resulted in significant decreases in plasma sodium and chloride, and increases in plasma and brain ammonia with no differences between soft and hard water. There was no significant effect of strain on LOE, but there were significant differences between strains in brain ammonia and plasma cortisol. Overall, there were no clear protective effects of hardness on high pH exposure in these strains of O. mykiss.     

Catalytic Properties of Two Pyruvate Kinase Isoforms in Nordic Krill,Meganyctiphanes norvegica,With Respect to Seasonal Temperature Adaptation

Two isoforms of pyruvate kinase (PK I and PK II) were partly purified and characterized from the Nordic krill Meganyctiphanes norvegica. Both PK variants were present in summer and winter specimens with a tissue specificity in abdominal muscle (PK I) and cephalothorax (PK II). Obvious differences were found in chromatographic and kinetic characteristics. Enzymatic adaptations to low temperatures were found in PK I only, whereas PK II did not contribute to seasonal temperature adaptation. In winter specimens, the activation energy of PK I decreased significantly from 53.2 ± 1.5 to 50.2 ± 1.2 kJ·mol⁻¹. The affinity of PK I to phosphoenol-pyruvate was higher in winter (K_M = 0.024 ± 0.002 mmol·l⁻¹) compared to summer (K_M = 0.033 ± 0.003 mmol·l⁻¹). Both effects lead to an increased efficiency of this enzyme isoform in the cold. In contrast, K_M values of PK II showed no significant differences between summer (K_M = 0.181 ± 0.014 mmol·l⁻¹) and winter specimens (K_M = 0.193 ± 0.015 mmol·l⁻¹). The effects of cooperativity remained unchanged during the seasons with approximate values of n_Hill = 1.0 (PK I) and 1.5 (PK II). Fructose-1,6-bis-phosphate affected only PK II by shifting sigmoidal kinetics to hyperbolic curves resulting in a decrease of K_M to 0.027 mmol·l⁻¹. Further effectors were tested showing an inhibiting effect of oxalate on both isoforms with a reduction to 20% and 50% in PK I and PK II, respectively. Presumably, the ecophysiological effect of the capacity to regulate muscle PK is related to the necessity to increase motility during vertical migration and phases of feeding activity.     

The effects of copper, iron and zinc on digestive enzyme activity in the hybrid tilapia Oreochromis niloticus (L.) ×Oreochromis aureus (Steindachner)

The present experiment was conducted to study effects of Cu, Fe and Zn on activities of digestive enzymes of the hybrid tilapia Oreochromis niloticus×Oreochromis aureus. The acidic protease activities increased 65·5 and 55·1% by addition of homogenates of digesta‐containing stomach with copper (75 mg l⁻¹) and zinc (50 mg l⁻¹) respectively. Addition of Cu and Zn increased the activities of protease in the hepatopancreas homogenates by 132·7 and 38·1% respectively, and reduced the activity of protease in the digesta‐containing intestine homogenates by 11·0 and 13·8% respectively. Addition of Fe (50 mg l⁻¹) increased the acidic protease activity by 96·7% but did not alter the activities of protease in the intestine and hepatopancreas. Addition of Cu markedly inhibited activities of amylase in intestine and hepatopancreas homogenates, while Zn addition showed no effects. Addition of Fe reduced activities of amylase in the intestine homogenates by 47·9% but had no effect on amylase activities in the hepatopancreas. When Cu (75 mg kg⁻¹), Fe (50 mg kg⁻¹) and Zn (50 mg kg⁻¹) were supplemented to basal diet for 3 weeks, the activities of amylase in hepatopancreas homogenates increased 125·3, 215·6 and 70·0%, respectively, the activities of amylase in intestine increased 79·8, 74·6 and 48·5%, respectively, and the activities of lipase in intestine increased 90·5, 149·8 and 84·0%, respectively. Supplementation of Cu, Fe or Zn into diet had no effects on activity of protease in all digestive organs. Therefore, the results suggest that effects of Cu, Fe and Zn on activity of digestive enzymes in vitro were different from those seen in vivo, and that the positive effects of Cu, Fe and Zn supplemented to fish diet would be valuable information for formulating fish feed.     

Enzymatic production of trans-4-hydroxy-l-proline by proline 4-hydroxylase

Trans-4-hydroxy-l -proline (Hyp) is a useful chiral building block for production of many nutritional supplements and pharmaceuticals. However, it is still challenging for industrial production of Hyp due to heavy environmental pollution and low production efficiency. To establish a green and efficient process for Hyp production, the proline 4-hydroxylase (DsP4H) from Dactylosporangium sp. RH1 was overexpressed and functionally characterized in Escherichia coli BL21(DE3). The recombinant DsP4H with l -proline as a substrate exhibited K_m, k_cat and k_cat/K_m values up to 0.80 mM, 0.52 s⁻¹ and 0.65 s⁻¹·mM⁻¹ respectively. Furthermore, DsP4H showed the highest activity at 35°C and pH 6.5 towards l -proline. The highest enzyme activity of 175.6 U mg⁻¹ was achieved by optimizing culture parameters. Under the optimal transformation conditions in a 5-l fermenter, Hyp titre, conversion rate and productivity were up to 99.9 g l⁻¹, 99.9% and 2.77 g l⁻¹ h⁻¹ respectively. This strategy described here provides an efficient method for production of Hyp and thus has a great potential in industrial application.     

Acquisition of alanyl-alanine in an Agnathan: Characteristics of dipeptide transport across the hindgut of the Pacific hagfish Eptatretus stoutii

This study used ³H-_L-alanyl-_L-alanine to demonstrate dipeptide uptake using in vitro gut sacs prepared from the hindgut of the Pacific hagfish Eptatretus stoutii. Concentration-dependent kinetic analysis resulted in a sigmoidal distribution with a maximal (± SE) uptake rate (J_max-like) of 70 ± 3 nmol cm⁻² h⁻¹ and an affinity constant (K_m-like) of 1072 ± 81 μM. Addition of high alanine concentrations to transport assays did not change dipeptide transport rates, indicating that hydrolysis of the dipeptide in mucosal solutions and subsequent uptake via apical amino acid transporters was not occurring, which was further supported by a K_m distinct from that of amino acid transport. Transport occurred independent of mucosal pH, but uptake was reduced by 42% in low mucosal sodium. This may implicate cooperation between peptide transporters and sodium-proton exchangers, previously demonstrated in several mammalian and teleost species. Finally, apical _L-alanyl-_L-alanine uptake rates (i.e., mucosal disappearance) were significantly increased following a meal, demonstrating regulation of uptake. Overall, this examination of dipeptide acquisition in the earliest extant Agnathan suggests evolutionarily conserved mechanisms of transport between hagfish and later-diverging vertebrates such as teleosts and mammals.     

METHYLAMINE UPTAKE IN THE GREEN ALGA CHLORELLA PYRENOIDOSA1

Methylamine uptake in nitrogen-starved Chlorella pyrenoidosa Beij. follows Michaelis-Menten kinetics: maximum uptake is about 1.6 nmol μl^?1· cells · min^?1, half-saturation occurs at 4 μM methylamine, and the slope in the range where uptake is proportional to concentration is 0.4 nmol μl^?1· min^?1·μM^?1. In cells grown in the presence of a non-limiting nitrogen concentration, methylamine uptake is directly proportional to concentration up to at least 0.5 mM, and the slope is 1/500 that for starved cells. Similar uptake kinetics have been reported for Penicillium chrysogenum and attributed to an inducible “ammonium permease.” Apparently, a similar permease occurs in algae.     

Effects of dehydration and low temperatures on the oxidation of high-potential cytochrome c by photosynthetic reaction centers in Ectothiorhodospira shaposhnikovii

Effects of temperature and dehydration on the efficiency of electron transfer from membrane-bound high-potential cytochromes c_h to the reaction-center bacteriochlorophyll (P-890) in Ectothiorhodospira shaposhnikovii have been studied. A kinetic analysis of the cytochrome oxidation suggests that there are at least two conformational states of the c_h-P-890 complex, of which only one allows photoinduced electron transfer from cytochrome to P-890⁺. Lowering the temperature of dehydration leads to a change in the proportion of the populations in the two conformations. The observed 2-fold deceleration of cytochrome oxidation can be related only to the diminution of the amount of photoactive cytochromes per reaction center. The rate constant for the transfer of an electron from cytochrome c_h to bacteriochlorophyll is 2.8 · 10⁵ s⁻¹ and is independent of temperature and dehydration (as estimated within the accuracy of the experiments). The effects produced by low temperature and dehydration are completely reversible. The thermodynamic parameters of the transition of the cytochrome from the nontransfer to electron-transfer conformation were estimated. For room temperature (+ 20°C) in chromatophore preparations, ΔG = −5.4 kJ · M⁻¹, ΔH = 60 kJ · M⁻¹, ΔS = 0.22 kJ · M⁻¹ · K⁻¹. For Triton X-100 subchromatophore preparations, the absolute values of the above parameters are significantly lower: ΔG = −2.8 kJ · M⁻¹, ΔH = 18 kJ · M⁻¹, and ΔS = 0.075 kJ · M⁻¹ · K⁻¹. To a larger extent, the above parameters are diminished for chromatophore preparations in an 80% glycerol solution: ΔG = −1.7 kJ · M⁻¹, ΔH = 6 kJ · M⁻¹, ΔS = 0.025 kJ · M⁻¹ · K⁻¹. The data suggest the hydrophobic character of the forces that maintain the P-890-c_h complex in the electron-transfer conformation. The results obtained suggest that electron tunneling within the complex cannot occur until a specific conformational configuration of the complex is formed. The efficiency of cytochrome c_h oxidation is determined by the temperature, the degree of dehydration and the environmental conditions, whereas the transfer of an electron itself in the electron-transfer configuration is essentially independent of temperature and hydration.     

Cl− influx across posterior gills of the Chinese crab (Eriocheir sinensis): potential energization by a V-type H+-ATPase

Cl⁻ absorption across isolated, perfused gills of freshwater adapted Chinese crabs (Eriocheir sinensis) was analysed by measuring transepithelial potential differences (PD_te) and radioactive tracer fluxes across isolated, perfused posterior gills. Applying hemolymph-like NaCl salines on both sides of the epithelium PD_te amounted to −30±1 mV (n=14). Undirectional Cl⁻ influxes of 470±38 and effluxes of 245±27 μmol·hr⁻¹·g⁻¹ wet weight (ww) (n=14) resulted in a Cl⁻ net influx of 226±31 μmol·hr⁻¹·g⁻¹ ww. Symmetrical substitution of Na⁺ by choline resulted in a substantial hyperpolarisation of the gill. Cl⁻ influx was unchanged under these conditions. However, net influx of Cl⁻ decreased by 40%, due to an increase of the Cl⁻ efflux.Nevertheless, a significant Cl⁻ net influx remained which was independent of the presence of Na⁺. When 2 mmol/l ouabain were added to the internal perfusion medium, PD_te increased, although the fluxes remained unchanged. Following external application of 1μmol/l of the V-type H⁺-ATPase inhibitor bafilomycin, A_l PD_te and Cl⁻ effluxes were not significantly affected. However, Cl⁻ influxes decreased. These findings suggest that Cl⁻ can be taken up independently of Na⁺ and that active Na⁺ independent Cl⁻ uptake across the posterior gill of Eriocheir sinensis is probably driven by a V-type H⁺-ATPase localized in the apical membrane.     

Continuous production of an antibiotic polypeptide (nisin) by Lactococcus lactis using a bioreactor coupled to a microfiltration module

The continuous production of nisin, an antibiotic polypeptide, by Lactococcus lactis in a bioreactor system coupled to a microfiltration module is described. Nisin productivity with respect to both cultivation time (ND) and the quantity of glucose consumed (ND/S_f) in continuous production was enhanced by maintaining a low concentration of lactic acid in the broth. A maximum ND of 7.80 × 10⁴ U·l⁻¹·h⁻¹ and ND/S_f of 5.20 × 10³ U·g⁻¹·h⁻¹ were obtained when the glucose concentration in the feed medium was 15 g/l. These values represent about 4.1- and 4.5-fold increases, respectively, over those obtained in batch culture.     

Respiratory-cardiovascular physiology and chloroethane gill flux in the channel catfish, Ictalurus punctatus

A fish respirometer-metabolism chamber was used to obtain in vivo respiratory-cardiovascular and chloroethane gill flux data on transected channel catfish (Ictalurus punctatus). Methods used for spinal transection, attachment of an oral membrane (respiratory mast), placement and attachment of blood cannulas and urine catheters are described. Respiratory physiology, cardiac output and chemical extraction efficiencies for 1,1,2,2-tetrachloroethane (TCE), pentachloroethane (PCE), and hexachloroethane (HCE) were determined on 419–990 g catfish. The overall mean values (± s.d.) for ventilation volume (Q_v), effective respiratory volume (Q_w), oxygen consumption (Vo₂ and percentage utilization of oxygen (U) were 17-3 ±4–71 h^?1 kg^?1, 9·8±l·71 h^?1 kg^?1, 71·6±12·5mg h^?1 kg^?1, and 49± 10%, respectively, while cardiac output calculated via the Fick Method was 2·4±0·61 h^?1 kg^?1. Additional measurements were made on ventilation rate (V_r), total plasma protein, haematocrit (Hct), and urine volume; while both arterial and venous blood were analysed for pH, oxygen partial pressure (P02), carbon dioxide partial pressure (Pco₂), total oxygen (To₂), total carbon dioxide (Tco₂) and total ammonia (TAMM). Physiological measurements taken at 24 h were not significantly different from those taken at 48 h and indicated no deterioration of the in vivo preparation. All of these values agreed well with literature values on UTitransected channel catfish, except for Hct which was lower for cannulated animals used in this study. Overall, these data provide strong support for the use of transected channel catfish for in vivo collection of physiological and chemical gill flux data. The mean initial chemical extraction efficiencies for TCE, PCE and HCE were 41, 61 and 73%, respectively. Chemical clearances (Cl_X) for these same three chemicals were 5·9, 9·3 and 10·8 1 h^?1 kg^?1, respectively. The approximate 1: 1 relationship between effective respiratory volume (Q_w) and chemical clearance (Cl_x) indicated that branchial uptake of PCE and HCE was water flow-limited. Chemical gill flux observed for channel catfish and chloroethanes was similar to that observed for rainbow trout in previous studies and provided further support for the flow-limited model of chemical flux across fish gills.     

THE UPTAKE OF IODATE BY MARINE PHYTOPLANKTON1

Several studies have suggested that phytoplankton play a role in the iodine cycle. Using a short-term incubation technique for determining the uptake of iodate by phytoplankton, cultures of Thalassiosira oceanica Hasle, Skeletonema costatum (Greville) Cleve, Emiliania huxleyi (Lohmann) Hay and Mohler, and Dunaliella tertiolecta Butcher were found to be capable of assimilating iodate at rates ranging from 0.003 to 0.24 nmol IO₃^?·μg chlorophyll a^?1·h^?1. The kinetics for the uptake of iodate can be modeled, and the similarity between the model and experimental results suggests that there is a steady state between iodate uptake and release of dissolved iodine from the cells, presumably in the form of iodide. Two experiments were conducted in the Sand Shoal Inlet of the Cobb Bay estuary (37°15′N, 75°50′W). The uptake of iodate was 0.26 and 0.08 nmol IO₃^?·μg chlorophyll a^?1·h^?1 during high and low tide, respectively. Using field estimates based on measured levels of iodate in the estuary, we estimate that phytoplankton can take up as much as 3% of the ambient pool of iodate on a daily basis and the entire pool in about 1 month. Thus, phytoplankton can be a significant component of the global iodine cycle by mediating changes in the speciation of iodine in the marine environment.     

UPTAKE OF GUANINE BY THE DIATOM,PHAEODACTYLUM TRICORNUTUM1

Nitrate-cultured cells of Phaeodactylum tricornutum Bohlin lack the ability to take up guanine but can do so after a period of nitrogen deprivation, i.e. photosynthesis in nitrogen-free medium. Maximum rate of uptake occurred after 24 h of nitrogen deprivation. The development of ability to take up guanine required CO₂ fixation and was prevented by cycloheximide, ammonium or nitrate. The guanine taken up accummulated in the cells almost entirely as a compound which is probably methylated hypoxanthine. Guanine uptake was dependent upon metabolism and exhibited Michaelis-Menten like kinetics with a half-saturation value of 0.48 ± 0.05 μM guanine and a maximum uptake rate for guanine of ca. 200 nmol · 10^?8 cells · h^?1. Rate of uptake increased hyperbolically with Na⁺ concentration, with 8.25 mM Na⁺ supporting half-maximal rate, and it was inhibited by K⁺ ions.     

Iron-limited growth and kinetics of iron uptake in Magnetospirillum gryphiswaldense

Growth and magnetite formation in Magnetospirillum gryphiswaldense MSR-1 were found close to the maximum at an extracellular iron concentration of 15–20 μM. Ferrous iron was incorporated by a slow, diffusion-like process. Several iron chelators including various microbial siderophores were unable to promote transport of iron into the cells. In contrast, spent culture fluids stimulated the uptake of ferric iron in iron-depleted cells at a high rate, whereas fresh medium and transport buffer were unable to promote iron uptake. However, no siderophore-like compound could be detected in spent culture fluids by the Chrome Azurol S assay. Ferric iron uptake followed Michaelis-Menten kinetics with a K _m of 3 μM and a V _max of 0.86 nmol min^–1 (mg dry weight)^–1, suggesting a comparatively low-affinity, but high-velocity transport system. Iron incorporation was sensitive to 2,4-dinitrophenol and carbonylcyanide-m-chlorophenylhydrazone, indicating an energy-dependent transport process. Received: 21 May 1996 / Accepted: 7 August 1996     

Zooplankton and bacteria contribution to phosphorus and nitrogen internal cycling in a tropical and eutrophic reservoir: Pampulha Lake,Brazil

Nutrient regeneration and respiration rates of natural zooplankton from a tropical reservoir were experimentally measured. Excretion rates of ammonia (E_a), orthophosphate (E_p) and community respiration rates (R) were estimated considering the variations in the concentrations of ammonia, orthophosphate and dissolved oxygen between control and experimental units. The ranges obtained for these rates from the 2 h assays were E_a = 1.95–4.95 μg N-NH₄ · mg · DW⁻¹ · h⁻¹; E_p = 0.12–0.76 μg P-PO₄ mg DW⁻¹ · h⁻¹. Respiratory rates were quite constant (R = 0.01–0.02 mg O₂ · mg DW⁻¹ · h⁻¹). The uptake of nutrients due to bacteria can affect the experimental determination of excretion rates of zooplankton. Orthophosphate release increased from 0.28 to 0.82 μg P-PO₄ · mg DW⁻¹ · h⁻¹ when bacterial activity was depleted by antibiotic addition in experimental vessels (Exp IV). This demonstrates that free living bacteria are able to consume promptly most phosphorus excreted by zooplankton. Ammonia excretion rates were lower in experimental units containing antibiotics. Lower excretion rates were also obtained with longer exposure times and higher biomass levels in the experimental units. Finally, this study also showed that zooplankton excretion can affect significantly turn over rates of total phosphorus in Pampulha Reservoir. In some periods, specially during the dry season when zooplankton biomass was very high, phosphorus release by zooplankton, during one single day, can be as high as 40% of the total phosphorus content in lake water (Turn over time = 2.5 days).     

Inorganic carbon uptake by phytoplankton in Vineyard Sound,Massachusetts. II. Comparative primary productivity and nutritional status of winter and summer assemblages

Using time-course, natural-light incubations, we assessed the rate of carbon uptake at a range of light intensities, the effect of supplemental additions of nitrogen (as NH₄⁺ or urea) on light and dark carbon uptake, and the rates of uptake of NH₄⁺ and urea by phytoplankton from Vineyard Sound, Massachusetts from February through August 1982. During the winter, photoinhibition was severe, becoming manifested shortly after the start of an incubation, whereas during the summer, there was little to no evidence of photoinhibition during the first several hours after the start of an incubation. At light levels which were neither photoinhibiting nor light limiting, rates of carbon uptake normalized per liter were high and approximately equal during winter and summer (22–23 μg C·l^?1 · h^?1), and low during spring (<10 μgC·l^?1· h^?1). In contrast, on a chlorophyll a basis, rates of carbon fixation were as high during spring (15–20μg C·μg Chl a^?1·h^?1), when concentrations of chlorophyll a were at the yearly minimum (<0.5 μg · l^?1) as during the summer, when chlorophyll a concentrations were substantially higher (0.8–1.3 μg · l^?1). Highest rates of NH₄⁺ and urea uptake were observed during summer, and at no time of the year was there evidence for severe nitrogen deficiency, although moderate nitrogen nutritional stress was apparent during the summer months.