Low Half-saturation Constants Research Articles

Daisyworld inhabited with daisies incorporating a seed size/number trade-off: the mechanism of negative feedback on selection from a standpoint of the competition theory

We reexamined a Daisyworld model from the traditional view of competition theory. Unlike the original model, white and black daisies in our model incorporate a seeding/germination trade-off against bare ground area without assuming the local temperature reward. As a result, the planetary temperature is automatically regulated by two species if the following conditions are met: (i) the species react equally to an environmental condition, but one can alter the environmental condition in the opposite direction to the other. (ii) that one of the two cannot have both a higher maximal growth rate ( μ max ) and lower half-saturation constant ( K) than those of the other. In other words, a pair of phenotypes incorporates a trade-off between quality and number of seeds. We found that the homeostatic regulation can also be reconciled with the adaptive evolution of optimal temperature. The results of simulation imply that biotic environmental feedback can also be maintained when the emergence of polymorphisms (black and white daisies) is closely linked to such a trade-off.

Growth kinetics of marine unicellular N2-fixing cyanobacterial isolates in continuous culture in relation to phosphorus and temperature

Unicellular N 2 -fixing cyanobacteria from tropical marine oligotrophic environments have been proposed to be major contributors to the global N cycle but still remain poorly characterized. These organisms are likely to be limited by phosphorus availability in situ. The aim of this study was to identify growth kinetics of isolates from the tropical North Atlantic and subtropical North Pacific in relation to phosphorus and temperature in continuous cultures. Cells from the Atlantic measured 2.5 μm in diameter (A-2.5). Genetically identical isolates from the Pacific showed 2 diameters depending on P-media concentrations (small: 3 μm, 1 pM PO 4 [P-3] and large: 7 pm, 4 pM PO 4 [P-7]). All 3 isolates were highly stenothermal, and optimal growth temperatures ranged between 26 and 30°C. Small cells (A-2.5 and P-3) had lower half-saturation constants (K s ) for PO 4 than large cells (P-7) (0.06 to 0.21 μM vs. 0.20 to 0.25 μM). Maximum growth rates and N:P ratios increased with temperature for all isolates; N:P ratios were close to Redfield ratios (N:P = 16) when isolates approached maximum growth rates. N 2 -fixation activity did not vary between growth rates, but did increase with temperature; rates were consistently lower than previously published rates for the same isolates under non-P-limiting conditions. From these studies, we conclude that both Atlantic and Pacific unicellular cyanobacteria that have the capacity to fix N 2 have a limited temperature range for growth and that smaller sized isolates could be better adapted for conditions of phosphorus limitation.

Uptake of urea and amino acids by the macroalgae Ulva lactuca (Chlorophyta) and Gracilaria vermiculophylla (Rhodophyta)

Dissolved organic nitrogen (DON) makes up a large fraction of the total dissolved nitro- gen pool in coastal waters, but is often ignored as a potential nitrogen source for primary producers. In laboratory experiments, we measured the uptake of small, labile DON compounds, urea and a variety of different amino acids, by the common estuarine macroalgae Ulva lactuca and Gracilaria vermiculophylla. Urea uptake was measured based on its disappearance from solution; amino acid uptake was measured using this method as well as by assimilation of 15 N- and 13 C-labeled amino acids. The Michaelis-Menten uptake parameters (Km, Vmax, Vmax/Km) were calculated for all com- pounds. Whereas both species were capable of assimilating urea and amino acids, U. lactuca consis- tently exhibited significantly higher uptake rates than G. vermiculophylla. There were 2 distinct phases of uptake for urea, an initially rapid 'surge' uptake phase and a slower, 'sustained' phase. This suggests that both species can take advantage of pulsed urea availability. Vmax rates for urea for U. lactuca were lower than published values for ammonium uptake, but were still high enough to be a significant factor at natural urea concentrations. We did not observe surge uptake of amino acids by either species and the uptake rates varied substantially among the 6 amino acids studied. The differ- ential uptake of 15 N and 13 C by U. lactuca suggested that both alanine and glycine are decarboxy- lated prior to uptake. However, following decarboxylation, the residual of the glycine molecule is assimilated, while the amine group on alanine is likely removed prior to assimilation. Glycine uptake rates by N-starved or NH4 + -fertilized macroalgae were higher than uptake rates by NO3 - -fertilized macroalgae, which suggests that the induction of glycine and ammonium uptake may be similar. The low half-saturation constant (Km) and the high affinity at low concentrations (Vmax/Km) that we measured for amino acids suggests that macroalgae can take advantage of the low concentration of amino acids found in estuarine waters. This study shows that macroalgae are capable of utilizing DON and that under conditions of low inorganic nitrogen availability, organic nitrogen may provide a significant portion of the total N demand.

Bacterial Metabolism of n-Alkanes and Ammonia under Oxic, Suboxic and Anoxic Conditions

n-Alkanes are widespread in the biosphere. Due to the lack of functional groups, these alkanes exhibit low chemical reactivity. However, many microorganisms have evolved pathways to utilise n-alkanes as a growth substrate, and moreover, fortuitous alkane oxidation may play an important role in alkane degradation. This review discusses the ecology of n-alkane-degrading and ammonia-oxidising bacteria with a focus on alkane metabolism in the transition from oxic to anoxic conditions, the pathways of n-alkane and ammonium oxidation, and the enzymes catalysing n-alkane and ammonia activation. n-Alkane degrading bacteria occur in oxic as well as strictly anoxic environments, and they live in very diverse habitats, including marine or fresh water, soils, sediments or aquifers. Aerobic ammonium-oxidising as well as methanotrophic bacteria are often found in stratified habitats such as biofilms and sediments. Aerobic pathways involving oxygenases that catalyse the initial activation of n-alkanes and ammonium are well known. However, anaerobic ammonium oxidation as well as anaerobic utilisation of hydrocarbons have been demonstrated only in the past decade and are the subject of current research efforts. Enzyme systems that catalyse aerobic alkane oxidation involve a number of well-characterised monooxygenases such as cytochrome P450 monooxygenases, multi-component alkane monooxygenases (also known as ω-hydroxylase systems), methane monooxygenases, and ammonia monooxygenase. Alternative enzymes, for example an n-alkyl hydroperoxide-forming dioxygenase, have also been postulated, but contrary to the monooxygenases, an n-alkane oxidising dioxygenasehas not yet been biochemically characterised. The oxygenase components of soluble methane monooxygenase and alkane monooxygenase contain binuclear iron centres that mediate dioxygen activation, whereas particulate methane monooxygenase, ammonia monooxygenase, and presumably distinct butane monooxygenases are copper-containing enzymes. Little is known about the impact of the oxygen concentration on bacterial alkane degradation, and it has not yet been investigated which pathways and enzymes are active in bacteria which utilise alkanes at suboxic or even quasi-anoxic conditions. Methane monooxygenase as well as ammonia monooxygenase have low half-saturation constants for oxygen and, in addition, both have an ample substrate spectrum. Activation of n-alkanes by cooxidation has been demonstrated for both types of enzymes. In suboxic to quasi-anoxic habitats, in which alkane, ammonium and methane oxidising bacteria as well as other organotrophic microorganisms live in close vicinity, a cooperative effect with respect to n-alkane degradation may occur.

Growth rate and carbon affinity ofUlva lactuca under controlled levels of carbon, pH and oxygen

We examined the growth rate (µ) ofUlva lactuca L. (collected from Roskilde Fjord, Denmark in 1987) at different levels of dissolved inorganic carbon (DIC), pH and oxygen in two culture facilities. Growth was faster in Facility A (µ max ca 0.3 d−1) than in B (µ max ca 0.2 d−1), probably because of more efficient stirring and higher light intensity. The growth-DIC response curve exhibited low half-saturation constant (K 1/2) values (0.35 mM DIC in A, 0.55 mM in B) and growth rates close toµ max at natural seawater concentration of 2 mM DIC. Growth rate showed a low sensitivity to oxygen over a wide range of DIC and oxygen concentrations. Collectively, the results demonstrated an efficient mechanism for DIC use, unaffected by acclimatization to DIC concentrations between 0.2 and 3 mM. The growth rate decreased little between pH 7.5 and 9 at 2 mM DIC, but steeply above pH 9 approaching zero just above pH 10. The decline of growth at high pH may result from direct pH effects on cell pH, reduced HCO 3 - availability and impaired operation of the carbon uptake process. The growth responses ofU. lactuca to DIC, pH and oxygen resembled those observed in previous short-term photosynthetic experiments. This similarity is probably due to the fast growth ofU. lactuca which means that photosynthetic products are rapidly converted into cell growth. Based on the culture experiments we argue that field plants ofU. lactuca not exposed to stagnant water and DIC depletion are likely to be limited in growth by environmental factors other than DIC (e.g. light and nutrients). Dense mats ofU. lactuca, however, may show reduced growth as a result of DIC depletion, high pH and self-shading.

Nutritional strategy of a benthic filamentous bacterium

Filibacter limicola is a filamentous gliding bacterium isolated from the profundal sediment of a eutrophic lake. It is an obligate amino acid utilizer. The kinetic parameters for the metabolism of four amino acids byF. limicola, Vitreoscilla spp. and the bacterial populations of water and sediment samples were compared.F. limicola exhibited low half-saturation constants (K) which were of the same order as those obtained with water samples. The K values for theVitreoscilla spp. and the sediment were an order of magnitude higher. It would appear that the bacterium is a specialist, inhabiting a niche which is sufficiently nutrient rich to support an organism with a limited substrate range. It also possesses a high affinity uptake system for some amino acids which may permit it to compete effectively during periods of nutrient depletion.

Monosaccharide transport into hemocytes of a sipunculan worm Themiste dyscrita.

The hemerythrin-containing blood cells, or hemocytes, of the sipunculan worm Themiste dyscrita were found to have a stereospecific and nonconcentrative monosaccharide transport system. The transport system transferred both D-glucose and 3-O-methyl-D-glucose (3-OMG), and transport into cells by this system was rapid, reaching 50% equilibrium in approximately 20 s at 10 degrees C with an initial concentration gradient of 0.1 mM; the contribution to total uptake by simple diffusion was very small. 3-OMG uptake showed saturation kinetics with a low half-saturation constant (Km less than or equal to 0.1 mM). The uptake of labeled 3-OMG by the hemocytes was strongly inhibited by unlabeled 3-OMG, 2-deoxy-D-glucose, alpha- and beta-D-glucose, D-galactose, and D-mannose. It was moderately inhibited by D-xylose, only slightly by alpha-methyl-D-glucoside and D-fructose, and uninhibited by sucrose, L-glucose, or D-sorbitol. Phloretin was more potent than phloridzin in blocking entry of 3-OMG. Cytochalasin B did not bind tightly to the T. dyscrita transporter and was not a potent inhibitor of transport; it half-maximally inhibited 3-OMG transport at 0.1 mM. Therefore, despite some differences the data suggest functional similarities in the mechanism of monosaccharide transport into blood cells of mammals and this invertebrate.

Kinetics of sulfate uptake by freshwater and marine species ofDesulfovibrio

Apparent half-saturation constants (Km) and maximum uptake rates (Vmax) for sulfate were determined in four species ofDesulfovibrio of freshwater and marine origin using a35S-sulfate tracer technique. The lowerstKm (5 μM) was found in the freshwater speciesDesulfovibrio vulgaris (Marburg) and the highestKm (77 μM) in the marine speciesDesulfovibrio salexigens. Maximum specific rates of sulfate uptake (i.e.,Vmax) were proportional to the growth rates observed in batch cultures. The halophilicDesulfovibrio salexigens did not change itsKm andVmax between 1 and 6,000 μM SO42-, and apparently did not induce a low-affinity uptake system at high sulfate concentrations. The low half-saturation constants measured for sulfate uptake explain why high rates of bacterial sulfate reduction occur in surface sediments of freshwater lakes, and why sulfate reduction can be a quantitatively important process in anaerobic carbon mineralization in low-sulfate environments. The results shows that extremely low sulfate concentrations must occur before sulfate reduction is completely outcompeted by methanogenesis.

Summer growth of Chordaria flagelliformis (O.F. Muell.) C. Ag.: Physiological strategies in a nutrient stressed environment

The objective of this study was to determine the relative importance of mechanisms by which Chordaria flagelliformis (O.F. Muell.) C. Ag. maintains rapid summer growth in sea water of low nitrogen content. Growth and nutrient uptake kinetic measurements showed that a steady-state uptake of ammonium, urea, and nitrate at ambient concentrations could more than satisfy the demand for growth. This finding is exemplified by the low half-saturation constants for growth ( K s ) on the three nitrogen sources estimated in culture under natural irradiance. Wind and tidally related nutrient inputs were found to be insignificant. The contribution of ammonia and urea regenerated from animal excretion, though potentially significant was most likely overshadowed by the steady-state uptake of nitrogen. It was also shown that the specific growth rate of plants is as dependent on the internal thallus nitrogen quota as it is on the external sea-water concentration. Field measurements showed a high tissue polymeric nitrogen concentration early in the growing season. With the proposed uncoupling between growth and uptake, this reserve was shown to be sufficient to account for growth throughout the month of May.

Nitrogen uptake characteristics of Chordaria flagelliformis (Phaeophyta) in batch mode and continuous mode experiments

The uptake of NH 4 + and urea by Chordaria flagelliformis was measured in a perturbed system (batch mode) and in an apparent steady-state system (continuous mode). The maximum uptake rates (Vmax) measured in short-term depletion experiments greatly exceeded those determined under a steady nutrient supply. C. flagelliformis briefly exposed to high nutrient concentrations has an impressive capacity for rapid uptake (high Vmax)_which is uncoupled from growth. In contrast, under continous, homogenous low N concentrations the alga appears well equipped (low half-saturation constant, Km) to take up the available N. The results show that previous work based on batch mode uptake experiments probably overestimated the N requirements for the growth of many seaweeds.

Kinetic Analysis of Competition Between Sulfate Reducers and Methanogens for Hydrogen in Sediments

The competition between sulfate-reducing and methanogenic bacteria for hydrogen was investigated in eutrophic lake sediments that contained low in situ sulfate concentrations and in sulfate-amended sediments. Sulfate reduction and methane production coexisted in situ in lake surface sediments (0 to 2 cm), but methane production was the dominant terminal process. Addition of 10 to 20 mM sulfate to sediments resulted in a decrease in the hydrogen partial pressure and a concomitant inhibition of methane production over time. Molybdate inhibition of sulfate reduction in sulfate-amended sediments was followed by an increase in the hydrogen partial pressure and the methane production rate to values comparable to those in sediments not amended with sulfate. The sulfate reducer population had a half-saturation constant for hydrogen uptake of 141 pascals versus 597 pascals for the methanogen population. Thus, when sulfate was not limiting, the lower half-saturation constant of sulfate reducers enabled them to inhibit methane production by lowering the hydrogen partial pressure below levels that methanogens could effectively utilize. However, methanogens coexisted with sulfate reducers in the presence of sulfate, and the outcome of competition at any time was a function of the rate of hydrogen production, the relative population sizes, and sulfate availability.

Structural effects in the action of antibiotics on the ion permeability of lipid bilayers. II. Kinetics of tyrocidine B

1. 1. The kinetics of tyrocidine B action on phospholipid and phospholipid-cholesterol bilayer shows autocatalytic behavior described by an hyperbolic tangent function 1 2 [1 + Tanh k 1 + (t−t 0)] characterized by two constants: a rate k 1 + and a latency parameter μ = exp[− k 1 + t 0]. 2. 2. These constants are studied as functions of transmembrane potential and concentration differences, as well as the usual kinetic parameters. 3. 3. The results are summarized in a reaction scheme for which a minimal interpretation is that tyrocidine acts through bimolecular transmembrane structures, the formation of which is autocatalytic and rate limiting. 4. 4. Prior to this a fast adsorption process is required which has a low (< 1 · 10 −9 M) half-saturation constant. If it is assumed that the latter is ion dependent, being much smaller for K + than for Na +, then anomalous results found previously for Na +-K + bionic potential as function of antibiotic concentration are explained. 5. 5. There is also a fast bimolecular reaction, which may take place in the aqueous phase. Conductance development requires that its product reacts with the transmembrane structures above, to give a “carrier complex”. 6. 6. These results are markedly dependent on the polarity of lipid used, indicating that both mechanisms under (3) and (5) above are influenced by the charge profile of the bilayer. The latter can be changed by adsorption of Ca 2+ on the one hand or acidic glycolipid (ganglioside) on the other, added to the aqueous phase. 7. 7. The dimers, referred to under (3) and (5) above, may be interpreted as non-polar and polar associations respectively. 8. 8. In all the above respects tyrocidine occupies a position intermediate between simple carriers and highly associative pore forming antibiotics.