Photosynthesis and chlorophyll fluorescence in sunflower (Helianthus annuus L.) leaves as affected by phosphorus nutrition

The effects of phosphate concentration on plant growth and photosyntheticprocesses in primary leaves of young sunflower (Helianthus annuusL.) plants were examined. Plants were grown for 3 weeks on half-strengthHoagland's solution containing 0, 0.1, 0.5, 1.0, and 3.0 molm^–3 orthophosphate (Pi). It was shown that optimal photosynthesisand the highest light utilization capacity were achieved at0.5 mol m^–3 Pi in the growth medium, which was in goodagreement with the maximum content of organic phosphorus inthe leaves. Low phosphate in the medium inhibited plant growthrate. Phosphate deficiency appreciably decreased photosyntheticoxygen evolution by leaves, the efficiency of photosystem two(PSII) photochemistry and quantum efficiency of PSII electrontransport. High oxidation state of PSII primary electron acceptorQA, at 0.1 mol m^–3 Pi, however, indicates that photosyntheticelectron transport through PSII did not limit photosynthesisin Pi-deficient leaves. The results indicate that diminishedphotosynthesis under sub- and supra-optimal Pi was caused mainlyby a reduced efficiency of ribulose 1, 5-bisphosphate (RuBP)regeneration at high light intensities. These results suggestthat, under non-limiting C0₂ and irradiance, photosynthesisof the first pair of leaves could be diminished by both sub-and supra-optimal phosphorus nutrition of sunflower plants. Key words: Helianthus annuus L, phosphate nutrition, photosynthesis, photochemical efficiency     

Deriving reaction–diffusion models in ecology from interacting particle systems

We use a scaling procedure based on averaging Poisson distributed random variables to derive population level models from local models of interactions between individuals. The procedure is suggested by using the idea of hydrodynamic limits to derive reaction-diffusion models for population interactions from interacting particle systems. The scaling procedure is formal in the sense that we do not address the issue of proving that it converges; instead we focus on methods for computing the results of the scaling or deriving properties of rescaled systems. To that end we treat the scaling procedure as a transform, in analogy with the Laplace or Fourier transform, and derive operational formulas to aid in the computation of rescaled systems or the derivation of their properties. Since the limiting procedure is adapted from work by Durrett and Levin, we refer to the transform as the Durrett-Levin transform. We examine the effects of rescaling in various standard models, including Lotka-Volterra models, Holling type predator-prey models, and ratio-dependent models. The effects of scaling are mostly quantitative in models with smooth interaction terms, but ratio-dependent models are profoundly affected by the scaling. The scaling transforms ratio-dependent terms that are singular at the origin into smooth terms. Removing the singularity at the origin eliminates some of the unique dynamics that can arise in ratio-dependent models.Research partially supported by NSF grants DMS 99-73017 and DMS 02-11367     

Feeding, prey selection and prey encounter mechanisms in the heterotrophic dinoflagellate Noctiluca scintillans

The heterotrophic dinoflagellate Noctiluca scintillans has anegligible swimming ability and feeds predominantly on immobileprey. How, then, does it encounter prey? Noctiluca scintillansis positively buoyant and, therefore, we hypothesized that itintercepts prey particles during ascent and/or that microscaleshear brings it into contact with prey. Noctiluca scintillanshas a specific carbon content 1–2 orders of magnitudeless than that typical for protists and, thus, an inflated volume.It also has a density slightly less than that of the ambientwater and therefore ascends at high velocities (-1 m h^–1).In stagnant water, clearance rates of latex spheres (5–80µm) increased approximately with prey particle size squared.This scaling is consistent with N.scintillans being an interceptionfeeder. However, absolute clearance rates were substantiallylower than those predicted by modeling N.scintillans both asa spherical and as a cylindrical collector. The latter modelassumes that prey particles are collected on the string of mucusthat may form at the tip of the tentacle. Feeding, growth andprey selection experiments all demonstrated that diatoms arecleared at substantially higher rates than latex beads and otherphytoplankters, particularly dinoflagellates. We propose thatdiatoms stick more efficiently than latex beads to the mucusof N.scintillans and that dinoflagellates reduce fatal contactbehaviorally. We conclude that N.scintillans is an interceptionfeeder and that the high ascent velocity accounts for encounterswith prey. However, the flow field around the cell-mucus complexis too complicated to be described accurately by simple geometricmodels. Fluid shear (0.7–1.8 s^–1 had a negativeimpact on feeding rates, which were much less than predictedby models. Noctiluca scintillans can survive starvation forlong periods (>3 weeks), it can grow at low concentrationsof prey (-15 µg C l^–1), but growth saturates onlyat very high prey concentrations of 500–1000 µgC l^–1 or more. We demonstrate how the functional biologyof N.scintillans is consistent with its spatial and seasonaldistribution, which is characterized by persistence in the plankton,blooms in association with high concentrations of diatoms, andsurface accumulation during quiescent periods or exponentialdecline in abundance with depth during periods of turbulentmixing.     

Effect of Ammonium on Dark-CO2 Fixation and on Cytosolic and Vacuolar pH Values in Eremosphaera viridis de Bary (Chlorococcales)

At constant external [CO₂], rates of dark-CO₂ fixation of theunicellular green alga Eremosphaera viridis were drasticallyincreased (up to 40-fold) by addition of ammonium (NH₃+ NH₄⁺)at external pH values (pH₀) between 6.0 and 8.0. The cytosolicpH was monitored under identical conditions by micro-pH-electrodemeasurements, and cytosolic and vacuolar pH by the ³¹P-NMR technique.Addition of ammonium (5.0 mol m^– pH₀ 7.0) caused a rapidand transient acidification of the cytosol during the first4 min. Thereafter, the cytosolic pH remained constant at itsoriginal value. A rather constant cytosolic pH was also confirmedby ³¹P-NMR measurements, which, in addition, indicated a slowalkalization of the vacuole (about 0.5 units within 30 min afteraddition of ammonium). Since the dramatic stimulation of dark-CO₂ fixation by ammoniumis not mediated by an alkalization of the cytosol, nor by directammonium effects on phosphoenolpyruvate carboxylase (PEPC, E.C.4.1.1.31 [EC] ), the role of vacuolar alkalization as a possible triggerfor the stimulation of PEP-carboxylase is discussed. Key words: Cytosolic pH, dark-CO₂ fixation, pH-regulation, vacuolar pH     

Long-term acclimatization of Scenedesmus bicellularis to high-frequency intermittent lighting (100 Hz). I. Growth, photosynthesis and photosystem II activity

Responses of the green microalga, Scenedesmus bicellularis tohigh-frequency intermittent lighting (II., 100 Hz) were assessedafter a 4 week acclimatization. Effects of IL on growth, photosynthesisand photosystem II (PSII) activity were studied at limitingand saturating irradiances, and compared to those of continuouslight (CL) of the same instantaneous and daily irradiances.Even after a 4 week acclimatization period, the photosyntheticcapacity (P_max), the photosynthetic efficiency (}" BORDER="0">) and the photosyntheticactivity at growth irradiance, either expressed on a per cellor a chlorophyll a basis, showed little difference, neitherdid the index of light adaptation (I_k) or PSII activity. Incontrast, growth was lower under IL at saturating irradiance.Results are discussed considering the non-linearity of the relationshipbetween growth or photosynthesis and irradiance.     

Optical assessment of phytoplankton nutrient depletion

The ratio of light absorption at 480 and 665 nm by 90% acetoneextracts of marine phytoplankton pigments has been examinedas a potential indicator of phytoplankton nutritional statusin both laboratory and field studies. The laboratory studiesdemonstrated a clear relationship between nutritional status(carbon/nitrogen ratio) and the absorption ratio that was independentof light and temperature climate. The absorption ratio for nutrient-repletecells was shown to vary between taxonomic groups. However, theinter-specific variation was less than the differences observedbetween nutrient-replete and nutrient-depleted cells. The fielddata suggest that the absorption ratio may be a useful indicatorof nutritional status of natural phytoplankton populations,and can be used to augment the interpretation of other data.     

HCO-3 as an Exogenous Carbon Source for Aquatic Bryophytes Fontinalis antipyretica and Fissidens grandifrons

The aquatic bryophytes Fontinalis antipyretica Hedw. and Fissidensgrandifrons Brid. were investigated for their ability to utilizeHCO^–₃ and CO₂ as exogenous carbon sources for photosynthesis.In NaHCO₃ solutions Fontinalis increased the pH to a maximumof 9.6 corresponding to a CO₂ compensation point of 1.1 mmolm^–3 CO₂. Measured photosynthetic rates cannot be explainedonly by uptake of CO₂. Net photosynthesis decreased at highpH but did not decline to zero until pH 10.10 in Fissidens andpH 11.8–12.0 in Fontinalis. Furthermore, photosynthesiswas increased by higher HCO_–3 concentrations at constantCO₂ concentration. It is concluded that Fontinalis antipyreticahas the capability to utilize HCO₃. Key words: Carbon source, Photosynthesis, Aquatic bryophytes     

Short-term feeding rates of A carrier grani in natural conditions: diurnal variation

Short-term in situ diel grazing activity of adult male and femalesofAcania graniSars. were investigated at a coastal station inthe Alboran sea (SE of Spain). Gut pigment contents were measuredfluorometrically, and the ingestion rates, filtration ratesand daily ratios were calculated from gut fullness, evacuationrates and pigment concentrations in sea water. The results indicatethat there were size-related differences in the amount of foodingested by males and females. Gut contents of females wereon average >1.5 times that for males. Body-volume-specificfeedingestimates were, however, the same for both sexes. Gutpassagetimes, estimated from animals collected at the time of day whengut pigments were maximal, were shorter in females. Both sexesshowed diel changes in feeding activity and faecal pellet productionof about one order of magnitude;the maximum rates were observedat dusk, the minimum rates at midday. There was a directrelationshipbetween copepod grazing and pigment concentration in sea waterwhen a time lag was considered. Daytime and night-time in situincubations using natural sea water enriched withlaboratorycultured algae provided evidence of non-continuous feeding andof light intensity control of the diel patterns. The possibleadaptive advantages of differences in sexual size on feedingbehaviour and diurnal variability are discussed.     

Phytoplankton carbon incorporation patterns and biochemical composition of particulate matter in the eastern North Atlantic subtropical region

Photosynthetic carbon metabolism and the biochemical compositionof late-winter phytoplankton assemblages in the eastern NorthAtlantic Ocean were studied during an oceanographic cruise carriedout in March 1992. Enhanced levels of phytoplankton biomassand primary production were linked to the subtropical front-AzoresCurrent (STF/AC) region. High values of the relative incorporationof carbon into proteins indicated that the phytoplankton wasgrowing at a rate close to the maximum growth rate. A very highpercentage of carbon was incorporated into the lipid fractionat southern latitudes, whereas incorporation into polysaccharidespeaked at the AC. In general, the biochemical composition ofparticulate matter reflected the observed patterns of photosynthatepartitioning. Latitudinal changes in phytoplankton species compositionaccounted for the geographical variability in ¹⁴C labellingpatterns. Turnover times of particulate matter estimated forthe STF/AC region were relatively low (3–5 days) and suggestedbalancedgrowth of the microplankton community over daily timescales.