Pathway of dark inorganic carbon fixation in two species of diatoms: influence of light regime and regulator factors on diel variations

Planktonic algae submitted to vertical mixing with a short periodicitycommute many times a day from low to high irradiance levels.To study the influence of this light periodicity, two diatoms,Skeletonema coslatum and Nitzschia turgiduloides, were cultivatedunder alternating conditions of 2 h light/2 h dark (2 h/2 h),simulating vertical mixing in the natural environment. Two otherlight regimes were used: continuous light (CL) and alternatecycles of 12 h light/12 h dark (12 h/12 h). Products synthesizedin the dark by S.costmum during 60 s incubation for 2 h/2 hculture or during 5 min for 12 h/12 h culture were determined.They were essentially sugars, malate, aspartate and glyceratefor 2 h/2 h cells and 12 h/12 h cells taken at the beginningof the light period. In contrast, 12 h/12 h cells taken duringthe darkness or in the middle of the light period and set inthe dark synthesized only amino acids. Our results corroborateprevious reports on dark CO₂ fixation via phosphoenolpyruvatecarboxykinase (PEPCKase, enzyme allowing the fixation of CO₂on PEP and the synthesis of amino acids) with involvement ofa substrate synthesized during the light period, but demonstratethat incorporation also occurs by the C-3 pathway (pathway responsiblefor the major CO₂ fixation in the light) in the very early stagesof the dark period. Another important result highlighted bythis study is the appreciable rate of dark fixation: on average6.7, 8.3 and 12.7% of photosynthesis at saturating photon fluxdensity for N.turgiduloides cultivated under 2 h/2 h, CL and12 h/12 h regime respectively and nearly 12% for S.costatumin the 2 h/2 h light regime. Variation of dark fixation wasinvestigated as a function of hour in the two species. Skeletonemacostatum cells submitted to the 2 h/2 h cycle show a constantrate of light-independent assimilation throughout the day. Bycontrast, both N.turgiduloides grown under the 12 h/12 h or2 h/2 h regime and S.costatum cultured under the 12 h/12 h cycleundergo fluctuations in the rate of dark fixation over the light/darkcycle. The mean dark fixation rate is controlled by the lengthof the photoperiod or the frequency of light fluctuations, dependingon species. We argue that this phenomenon must be taken intoconsideration in primary production calculations. Dependingon whether they are synthesized at the beginning or at the endof the light period, products are somewhat different and therate of fixation varies. This leads us to suggest that the pathwayof dark fixation may be regulated by at least two factors: amountof available substrate and enzyme (RuBPCase and PEPCKase) activityand/or amount.     

Changes in Stoichiometry among PSI, PSII and Cyt b6-f Complexes in Response to Chromatic Light for Cell Growth Observed with the Red Alga Porphyra yezoensis

Stoichiometry among 3 thylakoid components, PSI and PSII andCyt b6-f complexes, was determined with the red alga Porphyrayezoensis with special reference to the regulation of PSI/PSIIstoichiometry in response to light regime. The ratio of PSIto PSII abundance was four times greater in thalli grown underorange light which excites mainly phycobilisome, thus PSII,than that under red light which excites preferentially Chl a,thus PSI. Cyt b₆-f abundance remained almost constant. The PSIand PSII content was regulated separately under the two growthlight conditions as was also observed with the red alga Porphyridiumcruentum by Cunningham et al. [(1990) Plant Physiol. 93: 888].This differs from the cyanophyte Synechocystis PCC 6714 whereadjustment occurs only in the PSI content [(1987) Plant CellPhysiol. 28: 1547]. However, results on the marine cyanophyteSynechococcus NIBB 1071 indicate that changes in the PSI/PSIIsoichiometry is similar to red algae. In this species, as inthe red algae, more than one PSII is associated with each phycobilisome.The light regime also induced changes in the phycobiliproteincomposition in Porphyra yezoensis. Under PSII light, phycoerythrinincreased, and phycocyanin decreased, while under PSI lightthe response was reversed. The change suggests an occurrenceof complementary chromatic adaptation. (Received April 8, 1994; Accepted June 1, 1994)     

An in situ pulse light fluorometer for chlorophyll determination as a monitor for vertical and horizontal phytoplankton distribution in lakes

Methodological problems with in vivo, fluorescence (IVF) measurementusing an in situ pulse light fluorometer were investigated inorder to validate this method for monitoring the vertical andhorizontal chlorophyll a (chl. a) distribution in lakes. Thecorrelation between chl. a and IVF was poor in the upper epilimnion(0–5 m) of a mesotrophic lake. The IVF of algal culturesand natural phytoplankton is very sensitive to changes in thelight environment. The response of the IVF to rapid light fluctuationsdepends on the amplitude of the light intensity and the lightconditions to which the algae were exposed before the onsetof light fluctuations. The variability of the ratio IVF:chl.a concentration makes a permanent calibration of the IVF againstchl. a necessary. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP), Second International Workshop heldat the National Oceanographic Institute, Haifa, Israel in April–May1984.     

Effect of Rate of Photosynthesis on the Pattern of Assimilate Distribution in the Graminaceous Plant

Infrared gas analysis and a quantitative radiocarbon tracertechnique were used to measure photosynthesis and the distributionof ¹⁴C-labelled assimilate in Lolium temulentum and a uniculmbarley exposed continuously or intermittently to contrastinglight intensities. Plants grown for 10–20 d in low light(<50 W m^–2 of visible light) exported a greater proportionof their assimilate to growing leaves at the terminal meristemand a smaller proportion to their roots and tillers than equivalentplants in high light (150 W m^–2). A single day's exposure(8.4 h photoperiod) to a contrasting light regime elicited achange in the pattern of assimilate distribution in the samediurnal period, but 2–3 d exposure was required for asubstantial shift in the pattern of supply of assimilate tomeristems. The data indicated that in terms of assimilate distributioncomplete adaption to a new light regime is attained in about7 d.     

Responses of respiration and photosynthesis of Scenedesmus protuberans Fritsch to gradual and steep salinity increases

The effect of an increase in salinity on the physiology of thehalotolerant chlorophyte Scenedesmus protuberans was studiedin light-limited continuous cultures. It was observed that agradual, as well as a steep increase in salinity resulted inlower biomass. However, the mechanisms by which this was achievedwere different. In the culture that was exposed to a gradualsalinity increase, respiration and the cellular protein contentof the culture were initially unaffected. However, this culturewas not able to maintain its cellular chlorophyll content and,consequently, gross and net photosynthesis decreased. The culturethat was exposed to a steep salinity increase rapidly reactedby increasing its respiration and cellular protein content,which is ascribed to an induction of osmoregulation. This culturewas able to maintain its gross photosynthesis rate. It is speculatedthat, in this species, a steep salinity increase induces a nearlyimmediate osmoregulatory response, allowing growth to continue.If the cells are exposed to a gradual salinity increase, inductionof osmoregulation lags behind and, consequently, photosynthesisand growth rate will be^* affected.     

Effect of UV-B Radiation on Leaf Optical Properties Measured with Fibre Optics

Changes in the internal light microenvironment in leaves ofplants of Brassica campestris L. cv. Emma, B. carinata L., andMedicago saliva L. cv. Armour in response to exposure to UV-B(UV-B, 280–320 nm) radiation were measured using a fibreopticmicroprobe. Plants were exposed for 2 weeks either to high visiblelight or to supplemental ultraviolet-B radiation. The spectral regime (400–700 nm; PAR) was measured eithermidway through the leaf palisade or the spongy mesophyll. Afterexposure to UV-B radiation leaves of Brassica campesiris attenuatedtransmitted light more than the controls. At the same time bothforward and back scattered light increased in the palisade andspongy mesophylls. In contrast, UV-treatment of Medicago salivaleaves increased light transmission into the palisade, whilethe back scattered component showed little change. Leaves ofcariiwla showed little change in response to UV. Other responsesto UV-B radiation included increases in leaf thickness, decreasedtotal chlorophyll content, and changes in UV-B screening pigmentsand chlorophyll fluorescence induction kinetics. Brassica campestriswas most sensitive to exposure to enhanced levels of UV-B radiation,whereas leaves of B. carinata were the least sensitive. Ourdata indicate that exposure to UV-B radiation altered the lightmicroenvironment within leaves of the species different ways.These changes appeared to be caused by alterations in pigmentcontent and leaf anatomy. In turn, the altered distributionof PAR within the leaf could influence photosynthesis. Key words: Brassica campestris, Brassica carinata, fibre optics, light scattering, Medicago saliva, optical properties, ozone depletion, photosynthesis, ultraviolet radiation     

Dynamics of Vertical Leaf Nitrogen Distribution in a Vegetative Wheat Canopy. Impact on Canopy Photosynthesis

The development of vertical canopy gradients of leaf N has beenregarded as an adaptation to the light gradient that helps tomaximize canopy photosynthesis. In this study we report thedynamics of vertical leaf N distribution during vegetative growthof wheat in response to changes in N availability and sowingdensity. The question of to what extent the observed verticalleaf N distribution maximized canopy photosynthesis was addressedwith a leaf layer model of canopy photosynthesis that integratesN-dependent leaf photosynthesis according to the canopy lightand leaf N distribution. Plants were grown hydroponically attwo amounts of N, supplied in proportion to calculated growthrates. Photosynthesis at light saturation correlated with leafN. The vertical leaf N distribution was associated with thegradient of absorbed light. The leaf N profile changed duringcrop development and was responsive to N availability. At highN supply, the leaf N profiles were constant during crop development.At low N supply, the leaf N profiles fluctuated between moreuniform and steep distributions. These changes were associatedwith reduced leaf area expansion and increasing N remobilizationfrom lower leaf layers. The distribution of leaf N with respectto the gradient of absorbed irradiance was close to the theoreticaloptimum maximizing canopy photosynthesis. Sensitivity analysisof the photosynthesis model suggested that plants maintain anoptimal vertical leaf N distribution by balancing the capacityfor photosynthesis at high and low light. Copyright 2000 Annalsof Botany Company Canopy photosynthesis, leaf nitrogen distribution, nitrogen, Triticum aestivum L, wheat     

The effects of UV radiation on photosynthesis in an Antarctic diatom (Thalassiosira sp.): Does vertical mixing matter?

The reduction of Antarctic stratospheric ozone results in significant increases in ultraviolet B radiation (UVB-R, 280-320 nm) reaching ocean's surface, potentially damaging phytoplankton. Several studies refer to the negative direct and indirect effects of UVB-R and ultraviolet A (UVA-R, 320-400 nm, which is not modified by ozone concentration) on different targets within algal cells. There are, however, internal and external processes, like vertical mixing, which can in part counteract such effects. The hypothesis that vertical mixing is a significant factor reducing the negative effects of ultraviolet radiation (UV-R, 280-400 nm) on planktonic algae photosynthesis was tested at Potter Cove (South Shetland Is., Antarctica). Three laboratory (solar simulator, SOLSI) and two field (natural Sun exposure) experiments were conducted. Vertical mixing was studied exposing cells of Thalassiosira sp., a typically bloom forming diatom in Antarctic waters, to variable light conditions simulating 6 h cycles (Mix treatment), whereas incubations at two fixed depths were used as controls (0.5 and 5 m, S_fix and D_fix treatments, respectively). Light effects were studied for each of the previous exposure conditions considering three treatments: PAR-T (exposure to PAR, photosynthetic active radiation, 400-700 nm), UVA-T (exposure to PAR and UVA-R) and UVB-T (exposure to PAR, UVA-R and UVB-R). During SOLSI experiments no significant differences were found between the different light treatments under simulated normal and medium ozone concentrations. Under low ozone conditions, 40% reduction in photosynthesis was observed in the UVB-T for surface incubations. In contrast, no significant differences were observed among the light treatments under mixing conditions. Field and laboratory experiments showed similar results. However, during one of the field experiments when ozone was low, not only S_fix but Mix incubations presented a significant reduction in photosynthesis, suggesting that vertical mixing under such conditions was not efficient enough to prevent harmful UVB-R effects. On the other hand, during a day with high insulation and normal ozone, but with elevated absorption of light in the water column, no significant effects of any of the studied factors were detected.In conclusion, vertical mixing was shown to play a significant role in protecting algae under low ozone concentrations, lessening photoinhibition by UVB radiation. The differences between laboratory and field experiments are discussed in terms of the relative significance of UVB-R dose and dose rate on both types of experiments.     

Photosynthetic responses of the diatom Phaeodactylum tricornutum to high frequency light fluctuations simulating those induced by sea surface waves

Under high-frequency light fluctuations (0.01–10 Hz),photosynthesis of exponentially growing Phaeodaciylum tricornutumwas higher than the theoretical maximum. In stationary growth,photosynthesis was near the average expected rate.     

Action of Near UV and Blue Light on the Photocontrol of Phycobiliprotein Formation; A Complementary Chromatic Adaptation

Action of near UV to blue light on photocontrol of phycoerythrin(PE) and phycocyanin (PC) formation was investigated with non-photobleachedTolypothrix tenuis and Fremyella diplosiphon; this study wasdone to evaluate the proposition of Haury and Bogorad [(1977)Plant Physiol., 60: 835] that near UV to blue light is as effectiveas green and red light for photocontrol of PE and PC formationin blue-green algae and that lack of the blue effect in previousexperiments was due to destruction of blue-absorbing pigment(s)by the photobleaching treatment involved in the experimentalmethod. In our present work, light effect was measured in heterotrophiccultures incubated in darkness following brief exposure to differentwavelengths of light. Results indicated that (1) near UV to blue light was not effectivefor induction of PE formation either in T. tenuis or in F. diplosiphon,and (2) PC formation was induced by near UV light at 360 nmbut not by blue light at 460 nm. These features are identicalwith those previously reported for photobleached cells but notwith those reported by Haury and Bogorad for non-photobleachedcells. We conclude that photobleaching treatment does not haveany influence on the action of near UV to blue light. Actionat 390 and 460 nm observed by Haury and Bogorad probably resultedfrom light effects other than photocontrol, e.g., the actionof photosynthesis. (Received December 18, 1981; Accepted April 8, 1982)     

Heat stress stimulates nitric oxide production in Symbiodinium microadriaticum: a possible linkage between nitric oxide and the coral bleaching phenomenon

Nitric oxide (NO) is a gas displaying multiple physiologicalfunctions in plants, animals and bacteria. The enzymes nitratereductase and NO synthase have been suggested to be involvedin the production of NO in plants and algae, but the implicationof those enzymes in NO production under physiological conditionsremains obscure. Symbiodinium microadriaticum, commonly referredto as zooxanthellae, is a marine microalga commonly found insymbiotic association with a cnidarian host including reef-buildingcorals. Here we demonstrate NO production in zooxanthellae uponsupplementation of either sodium nitrite or L-arginine as asubstrate. The nitrite-dependent NO production was detectedelectrochemically and confirmed by the application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide(cPTIO), a specific NO scavenger. Cells stained with the diaminofluorescein,DAF-2 DA, an NO fluorescent probe, showed an increase in fluorescenceintensity upon supplementation of both sodium nitrite and L-arginine.Microscopic observations of DAF-stained cells verified thatNO was produced inside the cells. NO production in S. microadriaticumwas found to increase upon exposure of cells to an acute heatstress which also caused a decline in the photosynthetic efficiencyof PSII (F_v/F_m). This study provides substantial evidence toconfirm that zooxanthellae can synthesize NO even when theyare not in a symbiotic association with a coral host. The increasein NO production at high temperatures suggests that heat stressstimulates the microalgal NO production in a temperature-dependentmanner. The implications of these findings are discussed inthe light of the coral bleaching phenomenon which is associatedwith elevated sea surface temperature due to global warming.     

Growth and light absorption of some planktonic cyanobacteria, diatoms and Chlorophyceae under simulated natural light fluctuations

Specific growth rates of Limnozhrix redekei, Planktothrix agardhii(cyanobacteria), Synedraacus, Stephanodiscus minutulus (diatoms),Scenedesmus acuminatus and Scenedesmus armatus (Chlorophyceae)were compared under different time structures of illumination,but the same daily light exposure, at 20C. Fluctuating irradiancesimulating a uniform rapid transport of the algal cells acrossthe aquatic light field on a cloudless day with Z^eu/Z^mix=1 wascompared with constant irradiance throughout the same photoperiodof 12 h length as well as a photoperiod of 6 h length. Fluctuatinglight (30 min for a cycle) resulted in a decrease in specificgrowth rates as compared with constant irradiance at the samephotoperiod length. This decrease amounts to 15–20% fordiatoms, 20–25% for Chlorophyceae and 35–40% forcyanobacteria, respectively. The decrease is somewhat lowerif the fluctuations simulating mixing are slower (60 min fora cycle). The specific growth rate is also decreased by a shorterphotoperiod, but this effect is more species specific. Regardingthe in vivo absorption spectra, fluctuating light or a shorterphotoperiod has little or no effect on the Chlorophyceae anddiatoms studied, whereas cyanobacteria show an increase in lightabsorption by chlorophyll a and phycobilins.     

Changes in the Cytochrome c Oxidase Activity in Response to Light Regime for Photosynthesis Observed with the Cyanophyte Synechocystis PCC 6714

Changes in the activity of cytochrome c oxidase (EC 1.9.3.1 [EC] ,Cyt-oxidase) in response to growth conditions were studied withthe cyanophyte Synechocystis PCC 6714 in relation to changesin PSI abundance induced by light regime for photosynthesis.The activity was determined with the V_max of mammalian cytochromec oxidation by isolated membranes. The activity of glucose-6-phosphate(G-6-P):NADP⁺ oxidoreductase (EC 1.1.1.49 [EC] ) was also determinedsupplementarily. Cyt-oxidase activity was enhanced by glucoseadded to the medium even when cell growth maintained mainlyby oxygenic photosynthesis. G-6-P:NADP⁺ oxidoreductase was alsoactivated by glucose. The enhanced level of Cyt-oxidase washigher under PSII light, which causes high PSI abundance, thanthat under PSI light, which causes low PSI abundance. The levelwas intermediate under hetetrotrophic conditions. Although theactivity level was low in cells grown under autotrophic conditions,the level was again lower in cells grown under PSI light thanunder PSII light. The change of Cyt-oxidase activity in responseto light regime occurred in the same direction as that for thevariation of PSI abundance. Results suggest that in SynechocystisPCC 6714, the capacity of electron turnover at the two terminalcomponents of thylakoid electron transport system, Cyt-oxidaseand PSI, changes in parallel with each other in response tothe state of thylakoid electron transport system. ¹Present address: Institute of Botany, Academia Sinica, Beijing100044, China ²Present address: Department of Botany, Utkal University, Bhubaneswar,India 751004     

An Evaluation of the Effects of Ethylene on Carbon Assimilation in Lycopersicon esculentum Mill

Leaf and whole plant gas exchange rates of Lycopersicon esculentumMill, were studied during several days of continuous exposureto ethylene. Steady-state photosynthesis and transpiration ratesof control and ethylene-treated individual leaves were equivalent.However, the photosynthesis and transpiration rates of treatedleaves required at least five times longer to reach 50% of thesteady-state rate. This induction lag was attributed to ethylene—inducedleaf epinasty and temporary acclimation to lower incident lightlevels immediately prior to measurement of gas exchange. Thewhole plant net carbon exchange rate (NCER) of a representativetreated plant was also reduced by 51% after 24 h exposure toethylene relative to both its pre-treatment rate and that ofthe control. Ethylene exposure reduced the growth rate of thetreated plant by 50% when expressed as carbon (C) gain. Theinhibition of NCER and growth rate associated with epinastywas completely reversed when the epinastic leaves were returnedto their original positions and light interception was re-established.The results demonstrate that the inhibition of whole plant CO₂assimilation is indirect and due to reduced light interceptionby epinastic leaves. Morphological changes caused by environmentalethylene are thus shown to reduce plant C accumulation withoutinhibiting leaf photosynthesis processes per se. Key words: Ethylene, carbon assimilation, growth     

Effects of Light and CO2 on Net Photosynthesis Rates of Stands of Aubergine and Amaranthus

Net photosynthetic rates per unit ground area for plant standsof Solanum melongena L. var. esculentum (aubergine) and Amaranthuscaudatus L. var. edulis (grain amaranth) were measured over10 min intervals in an airtight, glass, controlled-environmentcabinet for a range of light flux densities provided by thediurnal variation in daylight. Light response curves for photosynthesisof stands, grown at ambient CO₂ concentration, were definedat 400, 800 and 1200 vpm CO₂. Light compensation points for these stands were around 20-30J m^-2 s^-1 and decreased slightly at higher CO₂ concentrations.For aubergine, a C₃ species, the short-term effects of CO₂ enrichmentwere to increase the initial slope as well as the asymptoteof the light response curve, reducing light saturation at moderateto high light flux densities; but for amaranthus, a C₄ species,saturation was less apparent and CO₂ enrichment scarcely increasedphotosynthesis except at light flux densities above 150 J m^-2s^-1. The canopies intercepted 93-98% of incident light. The efficiencyof utilization of intercepted light in photosynthesis (µgCO₂ J^-1) increased from zero at the light compensation pointto a maximum at an optimum light flux density of about 100 Jm^-2 s^-1 (the optimum rose a little with CO₂ enrichment) anddecreased slightly with further increase in light. Maximum utilizationefficiencies at 400 vpm CO₂ were 8-9 µg CO₂ J^-1. Enrichmentto 1200 vpm did not affect the peak utilization efficiency ofthe C₄ amaranthus, but increased that aubergine to 12·2µg CO₂ J^-1 (equivalent to some 14% when using the heatof combustion of plant dry matter to convert to the dimensionlessform). This is among the highest recorded efficiencies of lightutilization for stands, and relates to the exceptionally favourableenvironment, with optimal control of CO₂ concentration, humidity,temperature, water supply and mineral nutrition.Copyright 1993,1999 Academic Press Amaranthus caudatus L. var. edulis, Solanum melongena L. var. esculentum, canopy photosynthesis, CO₂ enrichment, light interception, light utilization, photosynthetic efficiency     

Differential Sensitivity of Lichens to Heavy Metals

Zinc, Cd and Cu inhibited photosynthesis in lichens containingcyanobacterial phycobionts at substantially lower concentrationsthan those causing decreased photosynthesis in lichens containingchlorophycean phycobionts. This distinction was not relatedto differences in total thallus concentrations of Zn, Mg, Caor K or to the quantity of Zn taken up to intracellular sites.When incubated with concentrated Zn solutions the chlorophyceanlichen Cladonia rangiformis accumulated more Zn on extracellularexchangeable sites than did the cyanobacterial lichen Peltigerahorizontalis. algae, cyanobacteria, chlorophyceae, lichens, heavy metal, photosynthetic inhibition, zinc, cadmium, copper, cation uptake     

Effect of temperature and light intensity on the growth rate of Synura sphagnicola

The chrysophyte Synura sphagnicola Korsch. was isolated froma hypolimnion bloom in a Canadian Shield lake and its abilityto grow at low light and temperature was studied. Growth saturatinglight was much higher than in situ intensities and independentof temperature while compensation intensity decreased with decreasingtemperature. Optimum temperature decreased with decreasing lightintensity. While optimum temperature was lower than generallyseen among temperate water algae, compensating and saturatinglight were similar to those seen in other algae. At low lightand temperature, the growth rate of S. sphagnicola was lowerthan the growth rates under similar conditions of other algae,and appeared insufficient to account for the net rate of chlorophyllaccumulation observed in the bloom from which the alga was isolated.     

Convection in ice-covered lakes: effects on algal suspension

Convection occurs in ice-covered lakes if solar radiation warmsnear-surface water from the freezing point towards the temperatureof maximal density. One effect of convective mixing may be tosuspend non-motile phytoplankton in the upper water column,providing cells with enough light for growth during ice-coveredperiods. Observations of the diatom Aulacoseira baicalensisunder the ice cover of Lake Baikal, Siberia, support the hypothesisthat convective mixing causes net suspension of cells. Thispaper presents a theoretical examination of the conditions underwhich convective flow fields can suspend algae in the photiczone of the upper water column. It is shown that the efficiencyof algal suspension depends on the ratio of the still-wateralgal sinking rate, W_p, to convective updraft speed, W_u. Thesuspension efficiency is also shown to be affected by asymmetriesin the flow field and night-time cessation of convection, butonly if W_p and W_u are comparable in value. It is concluded thatconvection in Lake Baikal should be vigorous enough to increasethe mixed-layer residence time of A.baicalensis from a few daysto over a month, at least during years with thin snow cover.     

The Photosynthetic Apparatus of Phytoplankton from a Perennially Ice-Covered Antarctic Lake: Acclimation to an Extreme Shade Environment

Phytoplankton in perennially ice-covered Lake Bonney (Antarctica)are exposed to a limited range of light variation both in termsof intensity (<1–3% of incident) and spectral distribution(blue-green) during the austral spring and summer. This relativeconstancy is due to continuous sunlight, optical filtering throughthe 4.2 m ice cap and an absence of vertical mixing. The effectsof this unique light environment on the structure and functionof the photosynthetic apparatus were studied using measurementsof P₇₀₀ reaction center content and spectral variation in photosystemII (PSII) fluorescence kinetics. Light-induced absorbance changeat both 700 nm and 810 nm was used to measure P₇₀₀ concentration.The average ratio of total Chl/P₇₀₀ was 743 (mol mol^–1),with a range of 480 to 1,039. These ratios were low in comparisonto previous studies of phytoplankton growing in low-light culturesor algae growing beneath Arctic sea ice. A sample from the deep(17 m) layer dominated by Chlamydomonas subcaudata was grownin enriched culture media. PSII fluorescence kinetics were measuredon thylakoid preparations in the presence of DCMU under blue-green(481 nm) and red (660 nm) light. C. subcaudata utilized blue-greenlight for photosynthesis more efficiently than the photobiologicallywell characterized C. reinhardtii (strain CC-124). These results,together with pigment analyses, suggest that carotenoids inLake Bonney phytoplankton are more important in light harvestingas opposed to photoprotection. (Received March 23, 1994; Accepted December 5, 1994)     

Photosynthesis and Light Activation of Ribulose 1,5-bisphosphate Carboxylase in the Presence of Starch

Limitation of photosynthesis and light activation of ribulose,1,5-bisphosphate carboxylase (RuBPCO) were examined in the 5thleaf of seedlings of red clover (Trifolium pratense L. cv. Renova)for 5 d following an increase in photosynthetic photon fluxdensity (PPFD) from 200 to 550µmol quanta m^–2 s^–1.Net photosynthesis and its stimulation at 2.0 kPa O₂ initialactivity of rapidly extracted RuBPCO, standard activity of RuBPCOafter incubation of the extracts in the presence of CO₂, Mg²⁺,and inorganic phosphate and contents of soluble protein, starch,soluble sugars, and various photosynthetic metabolites weredetermined. Photosynthesis decreased and starch content increased.No decrease in photosynthesis was found if, when PPFD was increased,all leaves except the investigated 5th leaf were removed, suggestingthat the decrease in photosynthesis was due to accumulated carbohydrates.The stimulation of photosynthesis at 2.0 kPa O₂ did not decreaseand the ratio of the total foliar steady-state contents of triosephosphate to 3-phosphoglycerate increased suggesting that thedecrease in photosynthesis was not due to limiting inorganicphosphate in chloroplasts. Intercellular CO₂ partial pressureand RuBP content were not decreased. Nevertheless, the ratioof photosynthesis to initial RuBPCO activity decreased, suggestingthat the catalysis per active RuBPCO site was decreased. Theincrease in PPFD in the growth cabinet and the PPFD at whichleaves were preconditioned for 1 h, affected not only initialactivity but also the standard activity of RuBPCO. The resultssuggest that a varying proportion of RuBPCO was bound to membranesand was contained in the insoluble fraction of the extracts.A comparison of photosynthesis with extracted RuBPCO activitysuggested that membrane bound RuBPCO did not contribute to photosyntheticCO₂ fixation and that the binding and release to and from membranesmodulated actual RuBPCO activity in vivo. Key words: Photosynthesis, ribulose 1,5-bisphosphate carboxylase, starch