Effets des fluctuations rapides de la lumiere sur la photosynthese du phytoplancton

[¹⁴C]-assimilation rates were measured on cultures of two unicellulargreen algae (Chlamydomonas sp. and Oocystis sp.) as a functionof light intensities (saturation curves), under steady lightand also under rapidly alternating high and low light intensities.Assimilation rates vary according to the frequency of the intermittentlight regime and it falls under two categories: (1) at 0.1 and0.2 Hz, the assimilation rate is equal to the average of therates observed at high and low light intensities under steadylight, and (2) under 1.0, 1.6 and 10 Hz the assimilation rateis equal to the rate observed under a mean steady irradiance.Moreover, the range of assimilation rates at a given frequencydepends on the difference between the high and the low intensities.Batch cultures of Oocystis sp. have been grown under intermittentlight of 0.1, 1.0 and 10.0 Hz (same mean intensity). Growthrate under intermittent light of 0.1 Hz is –40% lowerthan the control under steady light. Photosynthetic potential(P^B_max)and efficiency () change with the growth stages of thecultures. At the end of the logarithmic growth phase, both photosyntheticparameters are maximum at 1.0 Hz and minimum at 0.1 Hz. Averagecell concentrations of chlorophyll a increase as the frequencyof the light regime decreases. During the log phase, concentrationof carotenoids relative to chlorophyll a increases at 1.0 Hz,decreases at 0.1 Hz, and remains constant at 10.0 Hz. Underclear sky conditions, wave-induced light fluctuations in thephotic layer may therefore enhance primary production, especially(1) in the lower part of the photic layer, where low frequencylight changes might cause cell chlorophyll a to increase, and(2) at a depth of 1–4 m, where the main frequencies (ofthe order of 1.0 Hz), might cause a significant increase ofboth the photosynthetic potential (P^B_max)and efficiency (). ¹Contribution au programme du GIROQ (Groupe interuniversitairede recherches océanographiques du Québec) ²Adresse actuelle: Centre de recherches en nutrition, UniversitéLaval, Québec, Qué. G1K 7P4, Canada     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. II. A general review

The causes of interspecific differences in the µ-l relationshipare examined in the context of a mechanistic model which relatesµ to irradiance in terms of six factors:, k_c photosyntheticquotient (PQ), Chl a:C, respiration and excretion. The effectof cell size on the light saturated growth rate is also considered.It is shown that photosynthetic efficiency and PQ exhibit remarkablylittle interspecific variability, and average 0.024 ±0.005 µg C(µg Chl a)^–1 h^–1 (µEm^–2 s^–1)^–1 and 1.5 ± 0.2 mol 02 molC^–1 (when NO₃^– is the nitrogen source) respectively.Two useful relationships were derived: (i) between growth efficiency,_g and Chl a:C at µ. = 0; (ii) between the compensationintensity, I_c and the Chl a-specific maintenance respirationrate. Both relationships were independent of temperature anddaylength. Species best adapted to growth at low light werefound to exhibit high Chl a:C ratios and low maintenance respirationrates. As a group, diatoms were consistently the best adaptedfor growth at low irradiance. Chiorophytes, haptophytes, chrysophytesand cryptophytes were intermediate in their performance at lowirradiance. Dinoflagellates exhibited extreme diversity, withspecies spanning the spectrum from very good performance atlow irradiance to very poor. A new µ_max-cell carbon relationshipis given based on growth rates normalized to 15°C. Evidenceis presented to show that noise in this relationship can besignificantly reduced by using only carbon-specific growth ratesand using only data for species grown at the same daylength.     

Planktonic primary production in the German Wadden Sea

By combining weekly data of irradiance, attenuation and chlorophylla concentrations with photosynthesis (P) versus light intensity(E) curve characteristics, the annual cycle of planktonic primaryproduction in the estuarine part of the Northfrisian WaddenSea was computed for a 2 year period. Daily water column particulategross production ranged from 5 to 2200 mg C m^–2 day^–1and showed a seasonal pattern similar to chlorophyll a. Budgetcalculation yielded annual gross particulate primary productionsof 124 and 176 g C m^–2 year^–1 in 1995 and 1996,respectively. Annual amounts of phytoplankton respiration, calculatedaccording to a two-compartment model of Langdon [in Li,W.K.W.and Maestrini,S.Y. (eds), Measurement of Primary Productionfrom the Molecular to the Global Scale. International Councilfor the Exploration of the Sea, Copenhagen, 1993, pp. 20–36],and dissolved production in 1996, were both in the range of24–39 g C m^–2 year^–1. Annual total net productionwas thus very similar to particulate gross production (127 and177 g C m^–2 year^–1 in 1995 and 1996, respectively).Phytoplankton growth was low or even negative in winter. Inspring and summer, production/biomass (Pr/B) ratios varied from0.2 up to 1.7. Phytoplankton growth during the growth seasonalways surpassed average flushing time in the area, thus underliningthe potential of local phytoplankton bloom development in thispart of the Wadden Sea. The chlorophyll-specific maximum photosyntheticrate (P^B_max) ranged from 0.8 to 9.9 mg C mg^–1 Chl h^–1and was strongly correlated with water temperature (r² = 0.67).By contrast, there was no clear seasonal cycle in ^B, which rangedfrom 0.007 to 0.039 mg C mg^–1 Chl h^–1 (µmolphotons m^–2 s^–1)^–1. Its variability was muchless than P^B_max and independent of temperature. The magnitudeand part of the variability of P^B_max and ^B are presumably causedby changes in species composition, as evidenced from the rangeof these parameters found among 10 predominant diatom speciesisolated from the Wadden Sea. The ratio of average light conditionsin the water column (E_av) to the light saturation parameterE_k indicates that primary production in the Wadden Sea regionunder study is predominantly controlled by light limitationand that nutrient limitation was likely to occur for a few hoursper day only during 5 (dissolved inorganic nitrogen) to 10 (PO₄,Si) weeks in the 2 year period investigated.     

Photoinhibition and the diurnal variation of phytoplankton photosynthesis--I. Development of a photosynthesis--irradiance model from studies of in situ responses

Diurnal series of fluorescence and photosynthesis assays wereconducted in high altitude (3803 m), tropical (16°), LakeTiticaca (Peru/Bolivia). Near-surface diurnal thermoclines formedon typical days of high photon flux density (PFD, {small tilde}2000 µE m^–2 s^–1). In the depth range of diurnalstratification profiles of in vivo fluorescence, both without(F_a and with (F_b DCMU, exhibited a mean decrease of 64% frommorning to mid-day, but little change (mean increase of 1.5%)through the afternoon. Three times during the day surface, mid-depth(3–5 m) and deep (15–20 m) phytoplankton sampleswere incubated with H¹⁴CO₃^– under short (<2 h) exposuresto a range of in situ PFDs. Comparison of phytoplankton in differentsamples (ANOVA) showed identical photosynthetic response insunrise (isothermal) samples but a significant drop in surfaceand mid-depth photosynthesis at all PFDs during times of diurnalstratification. Similarly, both low-light () and light-saturated(P² _max photosynthetic parameters were lower in mid-day surfacesamples compared to deep samples. In addition, previously photoinhibitedsamples had a higher threshold intensity for photoinhibition,I_T. These results, together with diurnal time series of fluorescencefrom in situ incubations, demonstrate that recovery from extendedepisodes of photoinhibition during diurnal stratification isslower than suggested by previous observations in vitro. Photosynthesisby near-surface phytoplankton is different in light increasingup to I_T than light decreasing from I_T. This effect can be modeledby reducing and P_max as a function of the maximum photoinhibitingPFD in the diurnal light history. ¹Present address: Division of Molecular Plant Biology, Universityof California, Berkeley, Berkeley, CA 94720, USA     

Taurine Transport in N-deprived Chlorella fusca

The development of taurine uptake into the unicellular greenalga Chlorella fusca 211-8b was characterized as a specificresponse to either nitrate or sulphate limitation. Taurine transportunder nitrogen starvation was stimulated by low pH and showeda biphasic kinetics with K_m-values of 1.1 x 10^–3 mol dm^–3and 1.0 x 10^–2 mol dm^–3. Uptake was substantiallyinhibited by all - and ß-amino acids tested, whereassulphonate analogues failed to diminish taurine accumulation.Thus, uptake seemed to be mediated by a ‘general aminoacid permease’, unable to discriminate between carboxyland sulphonyl groups. However, Chlorella fusca could not catabolizethis unusual ß-amino acid and mobilize the amino-boundnitrogen for growth. Only a small group of -amino acids supportedthe growth of Chlorella fusca as an efficient nitrogen source. Key words: Taurine uptake, nitrogen starvation, amino acid uptake, Chlorella fusca.     

Nitrate and ammonium uptake by plankton in an Amazon River floodplain lake

Uptake of ammonium and nitrate by plankton was measured in tropicalLake Calado, Brazil. Nitrate uptake was strongly influencedby light and was light saturated at {small tilde}340 µEm^–2 s^–1. In contrast, uptake of ammonium was lessinfluenced by light, and saturated at {small tilde}250 µEm^–2 s^–1. Uptake rates of both forms of nitrogenwere inhibited by up to 80% at light intensities higher thanthose required for saturation. Concentrations of ammonium andnitrate also had a strong influence on uptake rates. Half-saturationconstants (0.3–5 µM) were usually greater than ambientconcentrations (0.1–0.6 µM), indicating that uptakerates at ambient concentrations were less than one-half of thesaturated rates. Ammonium is the more important type of inorganicnitrogen for plankton of Lake Calado because nitrate concentrationsremain low to undetectable except during periodic inputs ofnitrate-rich water from the Amazon River. Using the observeddependence of uptake on concentration and light, maximum uptakerates per unit chlorophyll were computed to be in reasonableagreement with rates derived from P^B_m values for carbon uptake. ¹ Present address: Florida Department of Natural Resources,Marine Research Laboratory, St Petersburg, FL 33701, USA     

A comparison of net and gross rates of oxygen production as a function of light intensity in some natural plankton populations and in a Synechococcus culture

In vitrorates of gross and net oxygen production were measuredas a function of light intensity in some plankton communitiescollected from Bedford Basin, Nova Scotia, and in a monoclonalculture of Synechococcus. The rate of gross oxygen productionwas measured by a technique in which the stable oxygen isotope,¹⁸O, serves as a photosynthetic tracer Net oxygen productionwas measured by automated Winkler technique. The rate of communityrespiration in the light was then determined by the differencebetween gross and net rates of oxygen production. In the naturalpopulations examined, neither gross nor net oxygen productionrates were significantly inhibited at the highest light intensitymeasured (500–800 µE m^–2 s^–1) In a samplein which the dark respiration rate was small relative to themaximal rate of production [P_max;sensu Platt et al (1980) JMar. Res., 38, 687–701] the rates of ‘light’respiration were 3 times greater. In two other communities,with high rates of dark respiration relative to P_maxthe ratesof ‘light’ respiration were closer to rates of darkrespiration. In the Synechococcus clone, both gross and netoxygen production rates were inhibited at high light intensities.Rates of ‘light’ respiration were found to varyas a function of light intensity. The greatest rates of respirationwere measured in samples incubated at light intensities thatwere just saturating (100 µE m^–2 s^–1). Therates of ¹⁴C production were also measured as a function oflight intensity The photosynthetic quotients, based on ¹⁴C productionrates and gross oxygen production rates, average 1 9     

The optical properties of a turbid reservoir and its phytoplankton in relation to photosynthesis and growth (Mount Bold Reservoir, South Australia)

In situ light measurements were used to obtain information oninherent and apparent optical properties. The average verticalattenuation coefficient K_d(ave) varied from 1.1 to 4.6 In unitsm^–1 During three periods the variation in K_d(ave) correlatedwith changes in chlorophyll a concentration and specific attenuationcoefficients K_s, of 0.013, 0.014 and 0.022 m² mg Chl a^–1were calculated. Chlorophyll-specific diffuse absorption coefficients(A,) for these periods were 0.012. 0.013 and 0.017 m² mg Chla^–1 and only varied significantly from estimates of K_sin the period when scattering was intense. Absorption coefficientsa(z^mid) and scattering coefficients b(z^mid) calculated for themid-point of the euphotic zone ranged between 0.45 and 2.9 mand 3.5–52.0 m respectively. Chlorophyll-specific absorptioncoefficients K_a, of 0.005, 0.006 and 0.007 m² mg Chl a^–1and scattering coefficients K_b of 0.05. 0.09 and 0.191 m² mgChl a^–1 were measured during the three periods. The highK_b value occurred when gas-vacuolate cyanobactena were dominant.Algal photosynthesis and light absorption were related throughthe maximum quantum yield _m which varied between 0.019 and 0.11mol C Einstein^–1 while average quantum yields _a, variedbetween 0.006 and 0.024 with a mean of 0.013 mol C Einstein^–1A comparison of changes in the mean irradiance of the mixedzone and chlorophyll concentration indicated that growth waslight limited below 0.04–0.05 Einsteins absorbed mg Chla^–1 day^–1.     

Photoadaptation in Antarctic phytopfankton: variations in growth rate, chemical composition and P versus I curves

The response of phytoplankton to variations in the light regimewas studied during the VULCAN and ACDA cruises in the Antarctic.Unenriched batch cultures of 12–19 days' duration reachedchl concentrations of 10–50 µg^–1 and exhibitedexponential growth rates, with the maximal rate being 0.41 doubl,day^–1. Ice edge algae exhibited maximum growth rates atphoton flux densities (PFD) of 30–100 µE m^–2S^–1and the growth rate was reduced by about 30% at 500–1000µE m^–2S^–1 The chl/C ratio ranged between 0.004and 0.018, with the lowest ratios at PFDs above 500 µEm^–2S^–1 chl/C ratios were also below maximum at PFDsbelow 40–50 µE m^–2S^–1 The C:N:P ratioswere close to the Redfield ratios; the Si/C ratio averaged 0.16(atoms), and the ATP/C ratio averaged from 0.0024 to 0.0050in different culture senes. When thawed after having been frozenfor 10 days, shade-adapted cultures were in a much better conditionthan sun-adapted ones. P versus I data showed that the maximumassimilation number varied from 0.75 to 4.4 µg C (µgchl)^–1h^–1. It varied inversely with the chl/C ratio;therefore the maximum carbon turnover rate varied little betweensamples (0.024/0.035 h^–1). Low biomass communities exhibitedrelatively high values for (the initial slope of P versus Icurves), low values for 1_sat (160–330 µE m^–2S^–1),and they were susceptible to photoinhibition. In contrast, communitiesdominated by Odontella weissflogii exhibited low values for, a high value for I_sat (560 µE m^–2S^–1 andthey tolerated high PFDs. The photo-adaptational status of thephytoplankton in natural water samples is discussed relativeto the profile of water column stability and mixing processes.     

Ecological studies on the phytoplankton of Boknafjorden, western Norway. II. Environmental control of photosynthesis

Both predicted (incubator) and measured (in situ) ¹⁴C-assimilationrates were studied from February to November 1981 at three stationsin Boknafjorden, a deep silled fjord of western Norway. Sampleswere taken from different light depths within the euphotic zone.A high degree of conformity was found between the two approaches.Daily values of carbon assimilation integrated over the euphoticzone varied between 0.05 and 1.4 g C m^–2. Yearly primaryproduction varied between stations from 82 to 112 g C m^–2(120–148 g C m^–2 when based on average light conditions).The light-saturation curve parameters ^B and P^B_max ranged from0.0056 to 0.0537 mg C mg Chla^–1 h^–1 µE^–1m² and from 0.7 to 8.5 mg C mg Chla^–1 h^–2 (in situassimilation numbers ranged from 0.9 to 9.3 mg C mg Chla^–1h^–1) respectively, which compare well with those publishedfrom the northwestern side of the Atlantic. The overall importanceof light in controlling photosynthesis throughout the year wasrevealed by the light utilization index , estimated to be 0.43mg C mg Chla^–1 E^–1 m². The maximum quantum yieldwas encountered on August 17, with 0.089 mol CE^–1. Chla/Cratios above and below 0.010 were found to be typical for shade-and light-adapted cells respectively. Assimilation numbers andgrowth rates were linearly related only when considering light-adaptedcells. Consistent with the findings of this study, the applicabilityof I_K, ^B and P^B_max as indicators of light-shade adaptation propertiesshould be questioned. Maximum growth rates were encounteredduring an autumn bloom of the dinoflagellate Gyrodinium aureolum(1.0 doublings day^–1), while 0.7–0.8 doublings day^–1were found for a winter bloom (water temperature of 2°C)of the diatom Skeletonema costatum. No unambiguous temperatureeffect on assimilation number and growth of phytoplankton couldbe recognized in Boknafjorden. A tendency towards increasedassimilation numbers coinciding with increased water columnstability was revealed. The highest P^B_max values were oftenencountered at almost undetectable nutrient concentrations.At least during summer this could be attributed to recyclingof nutrients by macro- and/or microzooplankton, responsiblefor a greater part of the primary production now being grazeddown. This study supports the convention that the depth of theeuphotic zone may extend considerably below the 1% light depth.     

Frond and flower production in Lemna gibba G 3 in presence of respiratory inhibitors

Effects of respiratory inhibitors on frond and flower productionin light culture of a long-day duckweed, Lemna gibba G 3, wereinvestigated. The inhibitors examined could be divided into3 groups based on their specific actions: (A) 2,4-Dinitrophenol(10^–6M), arsenate (10^–4M), malonate (10^–2M),o-phenanthroline (10^–6M), ,'-dipyridyl (10^–5M) andazide (10^–6M) inhibited flower production by suppressingthe rate of flower production without affecting the inductionperiod. Frond production, however, was promoted by these reagents.Effective time of application came one day after the end ofthe induction period. (B) Iodoacetate (10^–6M) and fluoride(10^–4M) inhibited both flower production and, less significantly,frond production. Reduced rate of flower production was responsiblefor the inhibition of flowering. Effective time of applicationpreceded by one day that of A group inhibitors. (C) Salicylaldoxime(10^–6M), diethyldithiocarbamate (10^–6M) and 8-hydroxyquinoline(10^–7M) enhanced flower production by reducing the lengthof the induction period, and simultaneously slightly inhibitedfrond production. Effective time of application was the latterhalf of the induction period. The implications of these findingsare discussed with special reference to the component processesinvolved in photoperiodic induction of flowering in duckweed. (Received March 27, 1969; )     

In Vitro Morphogenesis in Nardostachys jatamansi DC.: Shoot Regeneration from Callus Derived Roots

Callus cultures of Nardostachys jatamansi DC. maintained onMurashige and Skoog's medium containing 3.0 mg 1^–1 of-naphthaleneacetic acid and 0.25 mg 1^–1 of kinetin whenshifted to medium containing 0.25–1.0 mg 1^–1 ofindole-3-acetic acid or indole-3-butync acid showed profuserhizogenesis. The callus-regenerated roots when transferredto medium containing 2.0–6.0 mg 1^–1 of kinetin producedshoot buds. The de novo shoot bud regeneration took place eitherdirectly from cortical cells or from the inner stelar region.In addition, direct, concomitant root-shoot development wasalso observed. Nardostachys jatamansi, organogenesis, root-buds     

Propagation of Date Palm (Phoenix dactylifera L.) in vitro

Adventitious plantlets were obtained from lateral buds, shoottips, embryos, and pieces of stem and rachilla tissue of Phoenixdactylifera L. cultured on a modified Murashige and Skoog mediumcontaining 3 mg l^–1 N-(²-isopenty)adenine 0?1–100mg l^–1 -naphthaleneacetic acid or 2,4-dichlorophenoxyaceticacid, and 3 g l^–1 activated charcoal. Additions of auxinswere necessary to induce explants to produce callus, adventitiousplantlets, and roots. Plantlets were obtained from explantscultured 3–4 months in vitro. No difference in growthresponses between male and female explants was observed duringculture. Complex addenda of activated charcoal and polyvinylpyrrolidonewere tested in the nutrient media at various concentrationsto prevent explant browning. Activated charcoal fostered satisfactorygrowth by reducing the browning and inhibition of growth ofexplants.     

Photosynthetic Units and Carbon Assimilation in Leaves of Grain Sorghum under Different Light Intensities

The Cyt f and P700 contents in leaves of three Sorghum, varietieswere measured, in relation to their carbon assimilation, underdifferent light intensities during growth. At the maximum irradiationused (1,800 µE m^–2 s^–1) the ratio of P700to Cyt f was close to unity, whereas under low irradiation (450µE m^–2 s^–1) the ratio of P700 to Cyt f rangedfrom two to three. A strikingly positive correlation existedbetween the P700 contents of the leaves and their rates of carbondioxide fixation, dry matter production and Cyt f contents,only when the plants were grown under high light intensities.The P700 content of the leaves in plants grown under low irradiationwas unrelated to the contents of Cyt f. Thus, at a high lightintensity there is a close relationship between the Cyt f andP700 levels, but at low intensities the amounts of electroncarriers and the reaction centre are independent. (Received March 7, 1983; Accepted August 24, 1983)     

Identification and oligosaccharide structure analysis of rhodopsin glycoforms containing galactose and sialic acid

The N-linked oligosaccharides of frog (Rana pipiens) rhodopsinwere analysed by sequential exoglycosidase digestion and gelfiltration chromatography, following reductive tritiation. Inaddition, selected tryptic glycopeptides obtained from frogretinal rod outer segment membranes were examined by electrospraymass spectrometry (ES-MS), fast atom bombardment mass spectrometry(FAB-MS), amino acid sequence and composition analysis, andcarbohydrate composition analysis. The amino acid sequence datademonstrated that the glycopeptides were derived from rhodopsinand confirmed the presence of twoN-glycosylation sites, at residuesAsn² and Asn¹⁵. The predominant glycan (60% of total) had thestructure GlcNAcß1–2Man1–3(Man1–6)Manß1–4GlcNAcß1–4GlcNAc-(Asn),with the remaining structures containing 1–3 additionalhexose residues, as reported previously for bovine rhodopsin.Unlike bovine rhodopsin, however, a sizable fraction of thetotal giycans of frog rhodopsin also contained sialic acid (NeuAc),with the sialylated oligosaccharides being present exclusivelyat the Asn² site. FAB-MS analysis of oligosaccharides releasedfrom the Asn² site gave, among other signals, an abundant quasimolecularion corresponding to a glycan of composition NeuAc₁Hex₆HexNAc₃(where Hex is hexose and HexNAc is N-acetylhexosamine), consistentwith a hybrid structure. The potential biological implicationsof these results are discussed in the context of rod outer segmentmembrane renewal. glycoforms oligosaccharide structure rhodopsin     

The influence of temperature and light on the upper limit of Microcystis aeruginosa production in a hypertrophic reservoir

Primary production was measured for 7 years, using the in situ¹⁴C-method in hypertrophic Hartbeespoort Dam, South Africa,to examine the influence of light and water temperature on theupper limit of Microcystis aeruginosa production. Water temperaturesvaried from 11 to >25°C and chlorophyll concentrationsreached 6500 mg m^–3. The maximum volumetric rate of production(A_max) was 12->8800 mg C m^–3 h^–1 with areal productions(A) of 69->3300 mg C m^–2 h^–1 for euphotic zonedepths of <0.5–8.4 m. The intrinsic parameters of phytoplanktonproduction (, A_max/B, I_k) indicated that the phytoplankton populationwas adapted to high light levels. Both A_max/B and I_k were correlatedwith temperature. Under optimal conditions, , the theoreticalupper limit of A, was calculated to be 2.8 g Cm^–2 h^–1,while the measured rate was 2.5 g Cm^–2 h^–1. Measuredareal rates exceeding were overestimated due to methodologicalproblems when working with Microcystis scums. Light and watertemperature interacted to yield high production rates: watertemperature through its direct effect on photosynthetic ratesand indirectly in the formation of diurnal mixed layers; lightindirectly through water temperature and directly through itsattenuation and induction of light-adapted physiology in Microcystis.     

Reversible Structural Changes Associated With Callus Formation and Plantlet Development from Aseptically Cultured Shoots of Taro (Colocasia esculenta var antiquorum)

Taro callus maintained on Knop's medium with 2, 0·2 or0·02 mg l^–1 2,4,5-trichiorophenoxyacetic acid (2,4,5-T)or Linsmaier-Skoog (LS) containing 1 mgl^–1 of the cytokininadenine-N-benzyl-9-tetrahydro-2H- pyran-2-yl (SD8339) or 6 dimethylaininopurineand 0·1 mgl^–1 -naphthaleneacetic acid underwenta transition to a stable organized growth form which is referredto as a calloid. On transfer to LS medium th 0·2 mgl^–12,4,5-T in the absence of cytokinin the calloid reverts backto callus. Colocasia esculenta(L.)Schott, taro, callus, calloid, in vitro selection, histology, micropropagation, tissue culture, cytokinin     

Photosynthetic parameters, pigment composition and respiration rates of the marine diatom Skeletonema costatum grown in continuous light and a 12:12 h light-dark cycle

Nutrient-sufficient cultures of a Trondheimsfjord (Norway) cloneof the marine centric diatom Skeleionema costatum (Grev.) Clevewere grown at 75 µmol m^–2 s^–1 and 15C at24 and 12 h daylength to study diurnal variations and the effectof daylength on pigment and chemical composition, photosyntheticparameters, dark respiration rates and scaled fluorescence excitationspectra (F), the latter used as estimates for the absorptionof energy available to Photosystem II. Specific growth rateswere 1.06 and 0.56 day^– in 24 and 12 h daylength, respectively,while dark respiration rates were generally 85% of the net growthrate. The Chla-normalized photosynthetic coefficients P^B_m anda^B were {small tilde}20–25% higher in continuous lightthan at 12 h daylength, while the Chla:C ratio was {small tilde}15%lower (0.051 versus 0.061 w:w). Thus, the carbon-normalizedcoefficients P^c_m and a^c were <11% lower at 24 h than at 12h daylength. The maximum quantum yield _max, the Chla:C ratioand F differed negligibly, as did the light saturation indexl_k, the N:C ratio and the ratios Chlc:Chla and Fucoxanthin:Chla. P^B_m and l_k did not exhibit diurnal variations at 24 hdaylength, and varied within 23% of the daily mean at 12 h daylength.Predictions of the daily gross photosynthetic rate based ondata for a given time of the day should thus not be >10%in error relative to an integrated value based on several datasets collected through 24 h. _max was 0.084–0.117 mol O₂(mol photons)^– for gross oxygen evolution. However, ifused in mathematical models for predicting the gross and netgrowth rates (i.e. the gross and net carbon turnover rates),‘practical’ values of 0.076 and 0.040 g-at C (molphotons)^–, respectively, should be employed. Correspondingly,values for a^B and P^B_m should be adjusted pro rata. ¹Present address: College of Marine Studies, Sjmannsveien 27,N-6008 lesund, Norway     

Changes in the Levels of Ribulose-l,5-bisphosphate Carboxylase/Oxygenase (Rubisco) in Tetraedron minimum (Chlorophyta) during Light and Shade Adaptation

Exponentially growing cultures of the chlorophyta Tetraedronminimum were allowed to photoadapt to low (50µmole quantam^–2s^–1) and high (500µmole quanta m^–2–1)irradiance levels. In these cultures, various aspects of theorganization of the photosynthetic apparatus and related differencesin its performance were studied. In this organism, the observed five-fold increase in pigmentationof low-light adapted cells was due to increases in the numbersof PSU's, while their sizes remained constant. Using radioimmunoassay technique, we found that high-light adaptedalgae had over five times more Rubisco per PSU than their low-lightadapted counterparts. The high-light adapted algae also exhibited far higher (x2.3)light saturated photosynthetic rates per chl a. This increasewas the result of a reduction of tau, , the turnover time ofPS II reaction centers. We propose that the increase in Rubisco per PSU in high-lightadapted algae explains the reduction in , which results in thehigher P_max rates per chl a in these algae. The relationship is non linear, since the increase in Rubiscoper PSU was x5.3 whereas that in P_mM per chl a was only x2.3. (Received July 30, 1988; Accepted December 2, 1988)     

Modulation of Sucrose Content by Fructose 2,6-bisphosphate during Photosynthesis in Rice Leaves Growing at Different Light Intensities

Reddy, A. R. and Das, V. S. R. 1987. Modulation of sucrose contentby fructose 2,6-bisphosphate during photosynthesis in rice leavesgrowing at different light intensities.—J. exp. Bot. 38:828–833. The relationship between the rate of CO₂ fixation and sucroseconcentration in the leaves of rice (Oryza sativa L.) grownat different light intensities was investigated. Maximum sucrosecontent coincided with maximum rates of CO₂ fixation, achievedat a photon flux density of 1600 µmol m^–2 s^–1.The levels of sucrose and fructose 2,6-bisphosphate were alsocompared in the leaves under different light intensities. Fructose2,6-Msphosphate accumulated during growth at low light. Theactivity of fructose-6-phosphate 2-kinase was high in the leavesgrown at low light while that of fructose-2,6-bisphosphatasewas low. The activities of phosphoglucose isomerase and phospho-glucomutasewere slightly increased by growth at low light The activitiesof UDP glucose pyrophosphorylase were adversely affected invitro with increased concentrations of fructose 2,6-bisphosphatewhile those of sucrose phosphate synthase were moderately affected.Phosphoglucose isomerase and phosphoglucomutase were activatedby fructose 2,6-bisphosphate (8-0 mmol m^–3) by 12-15%.The results suggested that low light intensities during growthresult in an accumulation of fructose 2,6-bisphosphate whichmodulates the key enzymes of sucrose biosynthesis thus regulatingcarbon flow under conditions of limited photosynthesis. Key words: Oryza sativa, photosynthesis, sucrose synthesis, fructose 2,6-bisphosphate, light