Carbon and nitrogen content of Noctiluca scintillans in the Seto Inland Sea, Japan

The carbon and nitrogen content of Noctiluca scintillans cellsfrom the Seto Inland Sea, Japan was investigated in order toestimate its biomass in natural samples. The carbon contentof N.scintillans ranged from 123 to 627 ng C cell^–1 witha mean value of 353 ng C cell^–1, or 1.12 to 2.67 fg Cµm^–3 with a mean value of 1.98 fg C µm^–3.The nitrogen content ranged from 36.0 to 232 ng N cell^–1with a mean value of 131 ng N cell^–1, or 0.499 to 0.910fg N µm^–3 with a mean value of 0.694 fg N µm^–3.Total cell carbon and nitrogen increased but the carbon andnitrogen per cell volume decreased with increasing cell volume.The C/N ratio of the cells ranged from 2.3 to 4.4, which wasrelatively low compared with the Redfield ratio. The carbonand nitrogen content was extremely low (91.2 ng C cell^–1,41.8 ng N cell^–1) for starved cells, whereas it was extremelyhigh (528 ng C cell^–1, 205 ng N cell^–1) for cellswhich had ingested the large diatom, Coscinodiscus wailesii.Our results suggest that the carbon and nitrogen content ofN.scintillans varies depending on its physiological conditionand the type of food that it has recently consumed.     

Leaf Development in Lolium temulentum: Photosynthesis and Photosynthetic Proteins in Leaves Senescing under Different Irradiances

Changes in carbon fixation rate and the levels of photosyntheticproteins were measured in fourth leaves of Lolium temulentumgrown until full expansion at 360 µmol quanta m^–2s^–1 and subsequently at the same irradiance or shadedto 90 µmol m^–2 s^–1. Ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco), light-harvesting chlorophylla/b protein of photosystem II (LHCII), 65 kDa protein of photosystemI (PSI), cytochrome f (Cytf) and coupling factor 1 (CF₁) declinedsteadily in amount throughout senescence in unshaded leaves.In shaded leaves, however, the decrease in LHCII and the 65kDa protein was delayed until later in senescence whereas theamount of Cyt f protein decreased rapidly following transferto shade and was lower than that of unshaded leaves at the earlyand middle stages of senescence. Decreases in the Rubisco andCF₁ of shaded leaves occurred at slightly reduced rates comparedwith unshaded leaves. These results indicate that chloroplastproteins in fully-expanded leaves are controlled individually,in a direction appropriate to acclimate photosynthesis to agiven irradiance during senescence. (Received August 20, 1992; Accepted January 5, 1993)     

Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Autotrophic picoplankton populations in Lake Kinneret are composedof picocyanobacteria and picoeukaryotes. Overall, the ratesof photosynthetic carbon fixed by autotrophic picoplankton duringthis study were low (0.01–1.5 mg Cm^–3 h^–1).The highest chlorophyll photosynthetic activity of the <3µm cell-size fraction was found in spring, when picoeukaryotespredominated and in addition small nanoplankton passed throughthe filters. The maximum cell-specific photosynthetic rate ofcarbon fixation by picocyanobacteria and picoeukaryotes was2.5 and 63 fg C cell^–1 h^–1, respectively. The highestspecific carbon fixation rate of autotrophic picoplankton was11 µg C µg^–1 Chl h^–1 The proportionalcontribution of autotrophic picoplankton to total photosynthesisusually increased with depth. Picocyanobacteria collected fromthe dark, anaerobic hypolimnion were viable and capable of activephotosynthesis when incubated at water depths within the euphoticzone. Maximum rates of photosynthesis (P_max) for picocyanobacteriaranged from 5.4 to 31.4 fg C cell^–1 h^–1 with thehighest values in hypolimnetic samples exposed to irradiance.Photosynthetic efficiency (     

Simultaneous changes in cell quotas of microcystin, chlorophyll a, protein and carbohydrate during different growth phases of a batch culture experiment with Microcystis aeruginosa

The cell quotas of microcystin (Q_mcyst), protein (Q_prot), chlorophylla (Q_chloro) and carbohydrate (Q_carbo), as well as the net productionrates of these parameters, were determined during the exponentialand stationary phases in nine batch cultures of Microcystisaeruginosa (CYA 228) at light regimes from 33 to 53 µmolphotons m^–2 s^–1. The following results were obtained.(i) A parallel pattern was found in the changes of Q_mcyst, Q_prot,Q_chloro and Q_carbo during the entire growth cycle and significantcorrelations were recorded between Q_mcyst and Q_prot, Q_chloroand Q_carbo. (ii) The net microcystin production rate (µ_mcyst)was positively correlated with the specific cell division rate(µ_c), the chlorophyll production rate (µ_chloro)and the protein production rate. (iii) A significant inverselinear relationship was found between µ_c and Q_mcyst, i.e.cultures with a positive µ_c had a Q_mcyst between 110 and400 fg microcystin cell^–1, while declining cultures hadQ_mcyst values >400 fg microcystin cell^–1. Maximum variationin Q_mcyst within cultures was 3.5-fold. Collectively, the resultsshow that cells produced microcystin at rates approximatingthose needed to replace losses to daughter cells during divisionand that microcystin was produced in a similar way to proteinand chlorophyll, indicating a constitutive microcystin production.     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

Photoacclimation in the marine alga Nannochloropsis sp. (Eustigmatophyte): a kinetic study

Exponentially growing cultures of Nannochloropsis sp. were transferredfrom high light (650 µ.mol quanta m^–2 s^–1)to low light (30 (µrnol quanta m^–2 s^–1). Thekinetics of changes in the various parameters of the photoacclimationprocess were followed until steady state was reached. In thisorganism, the observed 4.25 increase in cellular chlorophylla in the low-light-acclimated cells was linearly correlatedwith an increase in the number of photosynthetic units (PSU),while the PSU size remained constant. The new steady state wasreached after a transition period of 87 h and following an initialovershoot in the chlorophyll a concentration. During the processof photoacclimation, harvested and utilized light was increaseddue to a combination of increased cellular chlorophyll and anincrease of the quantum yield. These improvements in efficiencywere coupled with the reduction of respiration. The overalleffect of photoacclimation, therefore, amounted to reducingthe impact of low light on the growth rate of the algae. Ourstudy demonstrates the importance of the photoacclimation processin the context of the planktonic way of life. An index of the'effectiveness of photoacclimation' is proposed.     

Changes in some Calvin Cycle Enzymes of the Tomato during Acclimation to Irradiance

The activities of three Calvin cycle enzymes, RuBPc (E.C. 4.1.1.39 [EC] ),3PGA phosphokinase (E.C. 2.7.2.3 [EC] ) and NADP-G3P dehydrogenase(E.C. 1.2.1.13 [EC] ), and the cytoplasmic enzyme PEPc (E.C. 4.1.1.31 [EC] )together with soluble protein and chlorophyll were measuredin extracts from young tomato leaves during acclimation to achange in irradiance. Leaf area and fresh weight were also measuredto show changes due to growth during treatments. Soluble proteinhad doubled on a unit leaf area basis 7 d after transfer from100 µmol quanta m^–2s^–1 PAR (low light) to400 µmol quanta m^–2s^–1 PAR (high light). Duringthis period the protein/chlorophyll ratio rose from 4•6to 10, RuBPc activity almost doubled and PEPc almost trebled.Following the reverse transfer from high to low light, solubleprotein decreased by 30% after 7 d and the protein/chlorophyllratio fell from 12 to 5•6. There was no change in RuBPcactivity 3 d after transfer from high to low light while PEPcactivity decreased by over 30%. There was no decrease in theactivity of 3PGA phosphokinase or NADP-G3P dehydrogenase 1 dafter transfer to low light, but decreases were apparent after3 d. The extracted kinase and dehydrogenase when fully activatedwere able to phosphorylate and reduce 3PGA at more than 2•5-foldits calculated rate of synthesis in the leaf. The data are discussedin relation to changes in the CO₂ exchange of the leaf. Key words: Photosynthetic acclimation, irradiance, tomato leaf, RuBP carboxylase     

Acclimation of Tomato Leaves to Changes in Light Intensity; Effects on the Function of the Thylakoid Membrane

When young tomato plants grown in high light (400 µmolquanta m^–2s^–1 PAR) were transferred to low light(100 µmol quanta m^–2s^–1 PAR), non-cyclic electrontransport capacity was decreased and the rate of dark re-oxidationof Q^–, the first quinone electron acceptor of photosystemII, was decreased within 1–2 d. In contrast, the amountof coupling factor CF₁, assayed by its ATPase activity, decreasedmore gradually over several days. The total chlorophyll contentper unit leaf area remained relatively constant, although thechlorophyll a/chlorophyll b ratio declined. When young tomato plants grown in low light were transferredto high light, the ATPase activity of isolated thylakoids increasedmarkedly within 1 d of transfer. This increase occurred morerapidly than changes in chlorophyll content per leaf area. Inaddition, in vivo chlorophyll fluorescence induction curvesindicate that forward electron transfer from Q^– occurredmore readily. The functional implications of these changes arediscussed. Key words: Tomato, leaves, light intensity, thylakoid membrane     

Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Ephyra larvae and small medusae (1.7–95 mm diameter, 0.01–350mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aureliaaurita were used in predation experiments with phytoplankton(the flagellate Isochrysis galbana, 4 µm diameter, {smalltilde}6 x 10^–6 µg AFDW cell^–1), ciliates (theoligotrich Strombidium sulcatum, 28 µm diameter, {smalltilde}2 x 10^–3 µg AFDW), rotifers (Synchaeta sp.,0.5 µg AFDW individual^–1) and mixed zooplankton(mainly copepods and cladocerans, 2.1–3.1 µg AFDWindividual^–1). Phytoplankton in natural concentrations(50–200 µg C I^–1) were not utilized by largemedusae (44–95 mm diameter). Ciliates in concentrationsfrom 0.5 to 50 individuals ml^"1 were consumed by ephyra larvaeand small medusae (3–14 mm diameter) at a maximum predationrate of 171 prey day^–1, corresponding to a daily rationof 0.42%. The rotifer Synchaeta sp., offered in concentrationsof 100–600 prey I^–1, resulted in daily rations ofephyra larvae (2–5 mm diameter) between 1 and 13%. Mixedzooplankton allowed the highest daily rations, usually in therange 5–40%. Large medusae (>45 mm diameter) consumedbetween 2000 and 3500 prey organisms day^"1 in prey concentrationsexceeding 100 I^–1. Predation rate and daily ration werepositively correlated with prey abundance. Seen over a broadsize spectrum, the daily ration decreased with increased medusasize. The daily rations observed in high abundance of mixedzooplankton suggest a potential ‘scope for growth’that exceeds the growth rate observed in field populations,and this, in turn, suggests that the natural populations areusually food limited. The predicted predation rate at averageprey concentrations that are characteristic of neritic environmentscannot explain the maximum growth rates observed in field populations.It is therefore suggested that exploitation of patches of preyin high abundance is an important component in the trophodynamicsof this species. ¹Present address: University of Bergen, Department of MarineBiology, N-5065 Blomsterdalen, Norway     

Physiological Responses of Phytoflagellates to Dissolved Organic Substrate Additions. 2. Dominant Role of Autotrophic Nutrition in Pyrenomonas salina (Cryptophyceae)

The enhancement of algal growth by organic substrate assimilationis a common laboratory observation, yet few studies have addressedthe interaction of dissolved organic compounds and environmentalfactors for controlling the relative contribution of heterotrophyand autotrophy to the nutrition of these algae. The effectsof light intensity and glycerol addition on the growth, cellvolume, pigmentation, and carbon uptake of the facultative heterotroph,Pyrenomonas salina Santore, were examined. Glycerol additionto cultures growing at a limiting light intensity increasedthe growth rate, increased the average cell volume and cellularstarch content, decreased the cellular phycoerythrin to chlorophyll ratio, and had no effect on the CO₂ fixation rate cell^–1.Glycerol addition to cultures growing at a moderate light intensitythat was saturating for photo-autotrophic growth increased theaverage cell volume and cellular starch content but had no effecton the CO₂ fixation rate cell^–1. The results indicatethat autotrophy was the major process for carbon acquisitionduring the growth of P. salina, but that carbon acquisitionfrom glycerol catabolism also was used to partially supportgrowth of the alga at the limiting light intensity. In addition,glycerol presumably was used to fulfill the energy and/or reductantrequirements of the alga, and to increase the reserve carbohydrate(starch). ¹ Current address and address for correspondences: Horn PointEnvironmental Laboratories, University of Maryland, PO Box 775,Cambridge, Maryland 21613, U.S.A. (Received October 29, 1990; Accepted May 31, 1991)     

Effects of chronic, low levels of UV radiation on carbon allocation in Cryptomonas erosa and competition between C.erosa and bacteria in continuous cultures

We conducted a long term (4 week) continuous culture studyto measure the chronic effects of UV radiation on the alga,Cryptomonas erosa, using three different fluence rates of UVradiation. We measured carbon allocation into carbohydrate,protein and lipid pools, as well as chlorophyll a concentrationsand algal and bacterial density. After 21 days, algal densityin the control and lowest UV treatment (treatment 1 = 3.4 Wm^–2 UVR unweighted) was significantly lower than in thetwo highest UV treatments (treatment 2 = 14.9 W m^–2 andtreatment 3 = 16.2 W m^–2 UVR unweighted), and did notrecover in the following week of no UV exposure. Chlorophylla and carbohydrate content (ng algal cell^–1) for the controland treatment 1 were clearly lower than treatments 2 and 3 byday 15, and did not recover by day 28. Percentage total lipidfor the control and treatment 1 also decreased compared withtreatments 2 and 3 by the end of the exposure period. However,by day 21, protein content for the control and treatment 1 wassignificantly higher than treatments 2 and 3, and demonstrateda further increase by day 28. The results were largely attributedto competition effects between C.erosa and bacteria in thesenon-axenic cultures. Bacterial density was significantly (x4)higher in the control and lowest UV treatment compared withthe two highest UV treatments. Our findings suggest a competitiveadvantage of phytoplankton over bacteria under these conditions.If UV radiation, in general, affects bacteria to a greater extentthan algae, there are likely to be changes in (i) bacterialutilization of dissolved organic matter produced by phytoplankton,(ii) competition between phytoplankton and bacteria for nutrientminerals and (iii) predation rates on bacteria by micro-flagellates.     

The Photocontrol of Spore Germination in the Fern Ceratopteris richardii

This paper describes how different wavelengths of light regulatespore germination in the fern Ceratopteris richardii. This speciesdoes not exhibit any dark germination. Maximum photosensitivityof the spores is reached 7 to 10 d after imbibition. An increasein the red light fluence above the threshold fluence of 10¹⁶quanta.m^–2 leads to a corresponding increase in germination.In sequential irradiation experiments, farred light can reversethis red light-mediated germination to the level observed withthe far-red light control. Blue light fluences above 10²⁰ quanta.m^–2can also block the germination response to red light. Moreover,this antagonistic effect of blue light is not reversed by subsequentirradiation with red light. It is therefore concluded that phytochromeand a distinct blue light photoreceptor control C. richardiispore germination. These interpretations are entirely consistentwith the published literature on other fern genera. (Received November 28, 1986; Accepted April 6, 1987)     

Distinct roles of PMCA isoforms in Ca2+ homeostasis of bladder smooth muscle: evidence from PMCA gene-ablated mice

We previously showed that plasma membrane Ca²⁺-ATPase (PMCA) activity accounted for 25–30% of relaxation in bladder smooth muscle (8). Among the four PMCA isoforms only PMCA1 and PMCA4 are expressed in smooth muscle. To address the role of these isoforms, we measured cytosolic Ca²⁺ ([Ca²⁺]_i) using fura-PE3 and simultaneously measured contractility in bladder smooth muscle from wild-type (WT), Pmca1^+/–, Pmca4^+/–, Pmca4^–/–, and Pmca1^+/–Pmca4^–/– mice. There were no differences in basal [Ca²⁺]_i values between bladder preparations. KCl (80 mM) elicited both larger forces (150–190%) and increases in [Ca²⁺]_i (130–180%) in smooth muscle from Pmca1^+/– and Pmca1^+/–Pmca4^–/– bladders than those in WT or Pmca4^–/–. The responses to carbachol (CCh: 10 µM) were also greater in Pmca1^+/– (120–150%) than in WT bladders. In contrast, the responses in Pmca4^–/– and Pmca1^+/–Pmca4^–/– bladders to CCh were significantly smaller (40–50%) than WT. The rise in half-times of force and [Ca²⁺]_i increases in response to KCl and CCh, and the concomitant half-times of their decrease upon washout of agonist were prolonged in Pmca4^–/– (130–190%) and Pmca1^+/–Pmca4^–/– (120–250%) bladders, but not in Pmca1^+/– bladders with respect to WT. Our evidence indicates distinct isoform functions with the PMCA1 isoform involved in overall Ca²⁺ clearance, while PMCA4 is essential for the [Ca²⁺]_i increase and contractile response to the CCh receptor-mediated signal transduction pathway. PMCA; bladder smooth muscle; gene-altered mice     

The allometry of algal respiration

For 30 years, study after study has shown that respiration ratesincrease as {small tilde}0.75 of body size for organisms rangingfrom protozoans to mammals. However, a number of studies suggestedthat the respiration-size relationship for algae may be a rareexception to this general rule. Algal respiration may be almostproportional to cell size, such that the slope of the respiration-sizerelationship is closer to unity. The present study examinedthe effect of cell size and taxon on phytoplankton respiration,using data collected from the literature. To this end, we collecteda data set of 178 observations of algal respiration and cellsize representing six divisions-chlorophytes. chrysophytes,cyanophytes, euglenophytes, pyrrophytes and rhodophytes. Therelationship between respiration (R, in p1 O₂ cell^–1 h^–1)and cell carbon content (C, in pg C cell^–1) is describedas R = 0.030C^{0 93} and the exponent is significantly >3/4.When we expressed cell size in terms of volume, the exponentdecreased to 0.88 but this is still significantly >3/4. Amongthe six divisions studied, chlorophytes, euglenophytes and rhodophytesseemed to differ significantly in their respiration-size relationshipfrom other taxa. However, euglenophytes and rhodophytes havesuch small size ranges that no meaningful relationships canbe developed for those groups alone. The chlorophyte respiration-sizerelationship has obvious patterns in its residuals which mayindicate that significant sources of error were not controlledin these heterogeneous data. Thus, for the present, the generalmodel seems most appropriate for the prediction of respirationrates of phytoplankton.     

Heterogeneity of cyanobacterial gas-vesicle volume and metabolic activity

The literature suggests that relative gas-vesicle volume andmetabolic activity differ between individuals within naturalpopulations of cyanobacteria. We demonstrate, using flow cytometryand fluorescein diacetate (FDA) conversion rates, that individualrelative gas-vesicle volumes differed by 6.7- and 4.6-fold,and individual cell photosynthetic rates ranged from 0.026 to0.127 and from 0.075 to 2.415 fmol O₂ cell^–1 min^–1within phosphorus-starved and phosphorus-replete Microcystisaeruginosa populations, respectively. The observation that thereis considerable heterogeneity in gas-vesicle volume and cellmetabolic activity in cultured populations suggests that thiscould also contribute to heterogeneity in buoyancy of fieldpopulations.     

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)     

A Comparison of the Growth, Photosynthesis, Stomatal Conductance and Water Use Efficiency of Moricandia and Brassica Species

When grown in pots and well-watered, the relative growth ratesof the above ground parts of two species of Moricandia (M. arvensis,an intermediate C₃–C₄ species, and M. moricandioides,a C₃ species) were inferior to those of two cultivated Brassicaspecies (B. campestris and B. napus). The Moricandia specieshad thicker leaves (greater d.wt per unit leaf area) with morechlorophyll than the Brassica species and had slightly greaterrates of photosynthesis per unit leaf area at an irradiance(400–700 nm) of 2000 µmol quanta m^–2 s ^–1.Leaves of M. arvensis, known to have a CO₂ compensation pointbetween that of C₃ and C₄ species, had a lower ratio of theintercellular to atmospheric partial pressure of CO₂ (C₁/C_a)and a greater instantaneous water use efficiency (WUE) thanthose of M. moricandioides and the Brassica species. Carbon isotope discrimination (     

High abundance of picoplankton-ingesting ciliates during late fall in Lake Kinneret, Israel

Small, aloricate ciliates dominated the biomass of heterotrophicprotists throughout the water column at the end of the periodof stratification in Lake Kinneret, Israel The integrated biomassof cilates was 5–20 times that of heterotrophic flagellatesDuring incubation experiments, ciliate growth rates in cpilimneticwater corresponded to population doubling times of 9.6–19.4h, while flagellate populations showed no growth. Most of thealiates were small forms (10–30 µm long), includingscuticocihates, choreotnchs, Coleps spp. and Colpoda spp., andappeared to be consuming bacteria, coccoid cyanobacteria, and<5 µm eukaryotic algae. Grazing rates of cihate assemblageson picoplankton in the epilimnion, as determined by the uptakeof fluorescently labeled bacteria and cyanobactena, ranged from62 to 86 nl cell^–1 h^–1 Colpoda steini, isolatedfrom lakewater, grew on a cultured freshwater Synechococcussp with a doubling time of 4.5 h, and a gross growth efficiencyof 48% The estimated daily requirements of ciliates for growthapproximately equalled total phytoplankton production. We calculatedthat ciliates in the epilimnion were clearing 4–10% ofthe bacterioplankton and cyanobactenal standing stocks per daySince this would not be sufficient food consumption to meetdaily carbon requirements of the aliates, it is likely thatthese organisms were also grazing a significant amount of autotrophicand heterotrophic eukaryotic cells in Lake Kinneret.     

Changes in Steady-State Photosynthetic 14C-Incorporation into cellular Metabolites of Chlorellax pyrenoidosa during Transitions from High to Low Irradiance

Changes in the levels of ¹⁴C-labelled metabolites were monitoredin Chlorella pyrenoidosa cells during a transition from highto low irradiance, i.e., from 700 to 430 µmol quanta (400–700nm) m^–2 s^–1. Chlorella cells assimilated ¹⁴CO₂ photosynthetically(steady-state ¹⁴C-labelling) for 12 min at the high irradianceand then 10 min at the low irradiance. With the transition tolow light, the level of ¹⁴C-labelled ribulose 1,5- bisphosphate(RuBP) did not decrease, even though the rate of total ¹⁴C-incorporationdecreased by 80%. The data suggest that RuBP carboxylase deactivatesrapidly (within 1 or 2 min) on exposure to low light, causingRuBP pool sizes to be maintained (or even increased) in spiteof a decreased rate of RuBP regeneration. There was also evidenceof light modulation of other enzymes, including some enzymesinvolved in sucrose synthesis. The rate of sucrose synthesisdecreased with decrease in light intensity while the level ofuridine diphosphoglucose increased, but within a few minutes,both returned to their former levels. ¹Present address: Chemical Biodynamics, Lawrence Berkeley Laboratory,Building 3, 1 Cyclotron Road, Berkeley, CA 94720, U.S.A. (Received March 8, 1986; Accepted June 25, 1986)