Nitrogen,phosphorus and Daphnia grazing in controlling phytoplankton biomass and composition – an experimental study

The role of nitrogen as a factor controllingphytoplankton biomass was studied in nutrientenrichment incubations in the laboratory using waterfrom pelagic region of two mesotrophic lakes ineastern Finland, Lake Kallavesi (in year 1994) andLake Juurusvesi (in year 1995). We used differentcombinations of phosphorus and nitrogen additions ina total of eight experiments. Furthermore, we includedDaphnia grazing treatment to the experimentaldesign in Lake Juurusvesi experiments. The nitrogentreatments did not increase chlorophyll aconcentration in any of the experiments compared withthe controls. Chlorophyll a content was highestin those nutrient treatments where phosphorus wasadded with or without nitrogen. Daphnia grazingdecreased chlorophyll a concentration comparedwith non-grazed treatments. In some cases grazing alsocaused higher ammonium concentrations. Theseexperiments, as well as the nutrient ratio of the lakewater used, suggest that phosphorus is likely tocontrol the amount of phytoplankton biomass. This revised version was published online in July 2006 with corrections to the Cover Date.     

In vitro microsome-mediated aflatoxin B1-DNA binding and its inhibition by cytosol of various organs of the hamster and quail

We studied thein vitro activation of aflatoxin B1 (B1) by microsomes and its inactivation by the cytosol of various quail and hamster organs, using B1-DNA binding as an index. The microsomal activity of the liver to bind B1 to DNA was not largely different between the two species and was higher than that of the other organs examined in either species. The microsomal activity of the kidney and lung was very low in the quail compared with the hamster, indicating the very small contribution of the lung and kidney microsomes to the activation of B1 in birds. Only the hamster liver cytosol showed strong inhibition of microsome-mediated B1-DNA binding.     

Thresholds in development

The interpretation of gradients in positional information is considered in terms of thresholds in cell responses, giving rise to cell states which are discrete and persistent. Equilibrium models based on co-operative binding of control molecules do not show true thresholds of discontinuity, though with a very high degree of co-operativity they could mimic them; in any case they do not provide the cells with any memory of a transient signal. A simple kinetic model based upon positive feedback can account both for memory and for discontinuities in the pattern of cell states. The model is an example of a bistable control circuit, and transitions from one state to another may be brought about not only by morphogenetic signals, but also by disturbances in the parameters determining the kinetics of the system. This might explain some aspects of transdetermination in insects.An attempt is made to analyse the precision with which a spatial gradient of a diffusible morphogen could be interpreted by a kinetic threshold mechanism, in terms of the length of the field, the steepness of the concentration gradient, and the intrinsic random variability of cells. It is concluded that it would be possible to specify as many as 30 distinct cell states in a positional field 1 mm long with a concentration span of 10³. Mechanisms for reducing the positional error are considered.     

Labelling studies in vivo on the metabolism of the acyl and glycerol moieties of the glycerolipids in the developing maize leaf.

1. When [2-3H]glycerol was supplied to developing maize-leaf laminae, label entered 3-sn-phosphatidycholine at a linear rate essentially from zero time, whereas other lipids were labelled at accelerating rates. On transfer of laminae from [3H]glycerol to unlabelled glycerol, radioactivity was rapidly lost from 3-sn-phosphatidylcholine and accumulated in other lipids, principally monogalactosyl diacyglycerol. 2. Degradation of these lipids showed that 3H was present only in the glycerol moiety of the lipids. 3. In double-labelling pulse-chase experiments with [14C]acetate, which labelled essentially only fatty acids and [3H]glycerol similar amounts of 14C and 3H radioactivity were lost from 3-sn-phosphatidylcholine and accumulated by monogalactosyl diacylglycerol. 4. The different molecular species of both lipids isolated from laminae during a double-labelled pulse-chase study were separated by argentation t.l.c., and the changes in the amount of radioactivity and the 14C/3H ratio in different species were compared. The greatest loss of radioactivity during the period in unlabelled substrates occurred from the 3-sn-phosphatidylcholine species containing oleate and from the dilinoleate species, and radioactivity accumulated by monogalactosyl diacyglycerol was mainly in the dilinolenate species. However, despite the considerable change in the radioactivity in these species during the chase, the 14C/3H ratio in each of them remained relatively unchanged. 5. It is proposed that 3-sn-phosphatidylcholine in the developing leaf may serve as a donor or linoleate-containing diacyl-glycerols which are incorporated into other lipids, principally monogalactosyl diacylglycerol.     

Production of diacylglycerol by exogenous phospholipase C stimulates CTP:phosphocholine cytidylyltransferase activity and phosphatidylcholine synthesis in human neuroblastoma cells.

The involvement of endogenous diacylglycerol production in the stimulation of phosphatidylcholine synthesis by exogenous phospholipase C was examined using a neuroblastoma (LA-N-2) cell line. Phospholipase C treatment (0.1 unit/ml) of intact cells stimulated CTP:phosphocholine cytidylyltransferase activity significantly more effectively than did maximally effective concentrations of the synthetic diacylglycerol sn-1,2-dioctanoylglycerol (1 mM). When added to cells together with phospholipase C, oleic acid, but not dioctanoylglycerol, further increased cytidylyltransferase activity with respect to phospholipase C treatment alone, indicating that the enzyme was not maximally activated by the lipase. This suggests that the lack of additivity of diacylglycerol and phospholipase C reflects a common mechanism of action. The time course of activation of cytidylyltransferase by phospholipase C paralleled that of [3H]diacylglycerol production in cells prelabeled for 24 h with [3H]oleic acid. Diacylglycerol mass was similarly increased. Significant elevations of [3H]oleic acid and total fatty acids occurred later than did the increases in cytidylyltransferase activity and diacylglycerol levels. No significant reduction in total or [3H]phosphatidylcholine was elicited by this concentration of phospholipase C, but higher concentrations (0.5 unit/ml) significantly reduced phosphatidylcholine content. The stimulation of cytidylyltransferase activity by phospholipase C or dioctanoylglycerol was also associated with enhanced incorporation of [methyl-14C]choline into phosphatidylcholine. Dioctanoylglycerol was more effective than phospholipase C at stimulating the formation of [14C]phosphatidylcholine, and the effects of the two treatments were additive. However, further analysis revealed that dioctanoylglycerol served as a precursor for [14C]dioctanoylphosphatidylcholine as well as an activator of cytidylyltransferase; and when corrections were made for this effect, the apparent additivity disappeared. The results indicate that the generation of diacylglycerol by exogenous phospholipase C (and possibly the subsequent production of fatty acids via diacylglycerol metabolism) activates cytidylyltransferase activity in neuronal cells under conditions in which membrane phosphatidylcholine content is not measurably reduced.     

Phosphorus Distribution in Red Pine Roots and the Ectomycorrhizal Fungus Hebeloma arenosa

Red pines (Pinus resinosa Ait.) were grown in a pasteurized sandy loam either unamended with phosphate or fertilized with one of two levels of phosphate (34 or 136 mg/kg) as superphosphate, and with and without addition of Hebeloma arenosa inoculum. Shoot and total dry weights of mycorrhizal seedlings grown in soil unamended with P were greater than those for nonmycorrhizal seedlings grown in the same soil, but less than the dry weights of seedlings grown in soil amended with middle to high levels of P. Mycorrhizal infection was inhibited at the highest level of P amendment. ³¹P nuclear magnetic resonance spectra of intact mycorrhizal roots showed the presence of two dominant peaks, orthophosphate (Pi) and polyphosphate (polyP). The polyP peak was absent in spectra of nonmycorrhizal roots. The ratio for areas under the two peaks, Pi/polyP, was 1.8 for mycorrhizal roots grown in both unamended soil and soil that had received middle levels of superphosphate. Apparently, the fungus strongly mediates the supply of phosphate to the tree through the production of polyP, even at growth-limiting levels of soil P, and regulates compartmentalization of P in the mycorrhizal roots.     

1H NMR studies on ferricytochromec 3 fromDesulfovibrio vulgaris Miyazaki F and its interaction with ferredoxin I

Summary The¹H NMR signals of the heme methyl, propionate and related chemical groups of cytochromec ₃ fromDesulfovibrio vulgaris Miyazaki F (D.v. MF) were site-specifically assigned by means of ID NOE, 2D DQFCOSY and 2D TOCSY spectra. They were consistent with the site-specific assignments of the hemes with the highest and second-lowest redox potentials reported by Fan et al. (Biochemistry,29 (1990) 2257–2263). The site-specific heme assignments were also supported by NOE between the methyl groups of these hemes and the side chain of Val¹⁸. All the results contradicted the heme assignments forD.v. MF cytochromec ₃ made on the basis of electron spin resonance (Gayda et al. (1987)FEBS Lett.,217 57–61). Based on these assignments, the interaction of cytochromec ₃ withD.v. MF ferredoxin I was investigated by NMR. The major interaction site of cytochromec ₃ was identified as the heme with the highest redox potential, which is surrounded by the highest density of positive charges. The stoichiometry and association constant were two cytochromec ₃ molecules per monomer of ferredoxin I and 10⁸ M^–2 (at 53 mM ionic strength and 25°C), respectively.     

Single cell analysis of mesoderm formation in the Xenopus embryo

We have examined the developmental specification of individual cells in the Xenopus blastula using a new in vitro culture system. Regional differences are apparent at the mid-blastula stage when animal hemisphere cells form only ectodermal cell types, while many clones from below the pigment boundary contain mesodermal cell types. A number of clones give rise to more than one differentiated cell type indicating that the initial steps of mesoderm induction are potentially reversible. Animal hemisphere cells can be induced to form mesoderm by fibroblast growth factor (FGF). Different cell types predominate at different FGF concentrations and the neighbours in this sequence are also the pairs of cell types most usually associated in mixed clones derived from the marginal zone. We propose that the specification of individual cells depends upon both the concentration of inducing factor and on stochastic intracellular events.     

To lek or not to lek: mating strategies of male fallow deer

We studied the mating system of fallow deer (Dama dama) for6 years in central Italy. Males in this population could defendterritories that were either single, clumped in leks, or satelliteto leks. The most highly successful males in our study werein leks. When we considered all males, there were no significantdifferences in average copulatory success according to territorytype because many lek males did not achieve any copulations,which were seen in only a few lek territories. The variancein copulatory success, however, was much greater for leks thanelsewhere. Single territories were occupied for shorter timesduring the rut than lek territories. Fighting among males wasmore frequent in the lek, even when we excluded highly successfullek males from the analysis. Chases of nonterritorial malesand harem size were correlated with the number of copulationsachieved by individual males, but did not vary according toterritory type. Copulatory success of some individuals increasedwith age, but there were no age differences among males holdingdifferent types of territories. Satellite males switched tolek territoriality in the course of one rut, but switches fromsingle territory to lek territory were rare. We suggest thatmales in single territories are inferior competitors that selecta low-risk, lowbenefit strategy, whereas those in lek territorieswhere no copulations were seen may be attempting to establishthemselves on the lek to increase their copulatory success infuture years.