Quantitative and structural characteristics of lipids in Chlorobium and Chloroflexus

Exposure of dark grown resting Euglena to light induced the synthesis of chloroplast valyl-tRNA synthetase. Ethanol, a specific inhibitor of Euglena chloroplast development had little effect on chloroplast valyl-tRNA synthetase induction during the first 12 h of light exposure. Ethanol, however, completely inhibited enzyme synthesis between 12–72 h of light exposure. Malate, an alternative carbon source, had little effect on the photoinduction of valyl-tRNA synthetase. When dark grown resting cells were exposed to 2 h of light and returned to the dark, chloroplast valyl-tRNA synthetase continued to accumulate for 8–12 h at a rate which was less than the rate in cells maintained continuously in the light. The mutant strain W₃BUL lacks detectable chloroplast DNA and phototransformable protochlorophyllide, but retains a plastid remnant. Exposure of strain W₃BUL to light induced the synthesis of chloroplast valyl-tRNA synthetase and enzyme induction was not inhibited by ethanol. After 72 h of light exposure in the presence or absence of ethanol, enzyme levels in strain W₃BUL were comparable to the levels found in the wildtype strain after 8–14 h of light exposure. These results suggest that the nonchloroplast photoreceptor regulates the initial phase of enzyme synthesis. Mutant strain W₁₀BSmL differs from strain W₃BUL in that the plastid remnant if present, is greatly reduced. Chloroplast valyl-tRNA synthetase was undetectable in the strain W₁₀BSmL suggesting that the levels of active, cytoplasmically synthesized, chloroplast localized enzymes may be related to the developmental status of the chloroplast through the extent to which the enzyme precursor can be accumulated and or posttranslationally processed into an active enzyme within the chloroplast or chloroplast remnant.This research was supported by National Institutes of Health Grant GM26994, Biomedical support grant RR-0755 and funds from the Research Council, University of Nebraska     

An unusual effect of the far-red absorbing form of phytochrome: Photoinhibition of seed germination inBromus sterilis L.

Seeds ofBromus sterilis L. germinated between 80–100% in darkness at 15° C but were inhibited by exposure to white or red light for 8 h per day. Exposure to far-red light resulted in germination similar to, or less than, that of seeds maintained in darkness. Germination is not permanently inhibited by light as seeds attain maximal germination when transferred back to darkness. Germination can be markedly delayed by exposure to a single pulse of red light following 4 h inhibition in darkness. The effect of the red light can be reversed by a single pulse of far-red light indicating that the photoreversible pigment phytochrome is involved in the response. The response ofB. sterilis seeds to light appears to be unique; the far-red-absorbing form of phytochrome (Pfr) actually inhibiting germination.Abbreviations Pr red absorbing form of phytochrome - Pfr far-red absorbing form of phytochrome     

Photo and Nutritional Regulation of the Light-Harvesting Chlorophyll a/b-Binding Protein of Photosystem II mRNA Levels in Euglena

In Euglena gracillis var bacillaris, light exposure increases the level of mRNA encoding the light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCPII) approximately twofold. LHCPII mRNA levels increased in the dark upon either malate or ethanol addition. LHCPII mRNA is present but LHCPII is not synthesized in the bleached mutants W₃BUL and W₁₀BSmL, which lack protochlorophyll(ide) and most if not all of the chloroplast genome. Light exposure increased LHCPII mRNA levels in W₃BUL but not in W₁₀BSmL. Carbon availability and light acting through a nonchloroplast photoreceptor appear to regulate LHCPII mRNA levels. A chloroplast photoreceptor and/or a product produced by the chloroplast appear to regulate LHCPII mRNA translation.     

Nutritional regulation of organelle biogenesis inEuglena: Photo- and metabolite induction of mitochondria

Exposure of dark-grown restingEuglena gracilis Klebs var.bacillaris Cori to light, ethanol, or malate produced an increase in the specific activity of fumarase (EC. 4.2.1.2) and succinate dehydrogenase (EC. 1.3.99.1) during the first 8–12 h of exposure to inducer, followed by a decrease in the specific activity of both mitochondrial enzymes between 12 and 72 h. The increased specific activity represented a net increase in the level of active enzyme, and it was dependent upon cytoplasmic protein synthesis. The photoinduction of fumarase required continuous illumination while the subsequent decrease in fumarase specific activity was independent of light. Light had little effect on the ethanol and malate induction of fumarase and succinate dehydrogenase. In the mutant W₃BUL, which has no detectable protochlorophyll(ide) and chloroplast DNA, light induced both mitochondrial enzymes and the kinetics of enzyme induction were similar to the induction kinetics in wild-type cells. The induction of mitochondrial enzymes appears to be controlled by a non-chloroplast photoreceptor. Dark-grown resting cells of the plastidless mutant W₁₀SmL have lost the ability to regulate fumarase levels. In this mutant, the specific activity of fumarase fluctuated and light had little effect on these fluctuations, indicating that fumarase synthesis was uncoupled from the nonchloroplast photoreceptor. Ethanol addition produced transient changes in fumarase specific activity in W₁₀SmL indicating that in this mutant, mitochondrial enzymes are still inductible by metabolites. Fumarase synthesis in wild-type cells was not induced in the dark by levulinic acid, a chemical inducer of the breakdown ofEuglena storage carbohydrates. Taken together, our results indicate that the photoinduction of mitochondrial enzyme synthesis is not a result of the photoinduction of carbohydrate breakdown. The mechanisms by which light and organic carbon induce the synthesis ofEuglena mitochondria may differ.     

Light-dependent PER-like proteins in the cephalic ganglia of an apterygote and a pterygote insect species

Antibodies targeted to a highly conserved tetradecapeptide region of the pivotal biological clock protein PER detect in the firebrat Thermobia domestica a 115-kDa protein and in the cockroach Periplaneta americana a 110-kDa protein that are present in the cytoplasm of a small set of brain cells. A similar cytoplasmic reaction occurs with antisera to the whole PER protein of Drosophila melanogaster, but these antisera also react with numerous cell nuclei. On western blots, they detect an 80-kDa antigen in T. domestica and 70- and 80-kDa antigens in P. americana. No indication of antigen translocation between cell nuclei and cytoplasm was found. Nuclear staining is maintained at a high constant level in T. domestica held at a 12:12 h light:dark photoperiod (LD) or in continuous light, but disappears rapidly in response to extended darkness. In P. americana under LD conditions, the number of immunoreactive nuclei and their staining intensity fluctuate in parallel, with maximal staining late in the day. The circadian changes are maintained in continuous light but all staining vanishes in continuous darkness. A 6-h light pulse in early night of an LD cycle induces maximal staining after about 10 h, suggesting that the effect of light on nuclear PER-like expression is indirect. The behaviour of nuclear antigens is opposite to that of the cytoplasmic PER-like proteins that persist in constant darkness and disappear in constant light. Under LD conditions, the cytoplasmic PER-like antigen cycles in T. domestica but remains at a steady level in P. americana. The sensitivity to photoregime suggests that both the nuclear and the cytoplasmic PER-like antigens are components of the biological clock.R. Závodská and H. Sehadová contributed equally to this work     

Long-term kinetics of the light-dependent regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in plants with and without 2-carboxyarabinitol 1-phosphate

The light-dependent modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was studied in two species: Phaseolus vulgaris L., which has high levels of the inhibitor of Rubisco activity, carboxyarabinitol 1-phosphate (CA1P), in the dark, and Chenopodium album L., which has little CA1P. In both species, the ratio of initial to fully-activated Rubisco activity declined by 40–50% within 60 min of a reduction in light from high a photosynthetic photon flux density (PPFD; >700 mol · m^–2 · s^–1) to a low PPFD (65 ± 15 mol · m^–2 · s^–1) or to darkness, indicating that decarbamylation of Rubisco is substantially involved in the initial regulatory response of Rubisco to a reduction in PPFD, even in species with potentially extensive CA1P inhibition. Total Rubisco activity was unaffected by PPFD in C. album, and prolonged exposure (2–6 h) to low light or darkness was accompanied by a slow decline in the activity ratio of this species. This indicates that the carbamylation state of Rubisco from C. album gradually declines for hours after the large initial drop in the first 60 min following light reduction. In P. vulgaris, the total activity of Rubisco declined by 10–30% within 1 h after a reduction in PPFD to below 100 mol · m^–2 · s^–1, indicating CA1P-binding contributes significantly to the reduction of Rubisco capacity during this period, but to a lesser extent than decarbamylation. With continued exposure of P. vulgaris leaves to very low PPFDs (< 30 mol · m^–2 · s^–1), the total activity of Rubisco declined steadily so that after 6–6.5 h of exposure to very low light or darkness, it was only 10–20% of the high-light value. These results indicate that while decarbamylation is more prominent in the initial regulatory response of Rubisco to a reduction in PPFD in P. vulgaris, binding of CA1P increases over time and after a few hours dominates the regulation of Rubisco activity in darkness and at very low PPFDs.Abbreviations CA1P 2-carboxyarabinitol 1-phosphate - CABP 2-carboxyarabinitol 1,5-bisphosphate - k_cat substrate-saturated turnover rate of fully carbamylated enzyme - PPFD photosynthetically active photon flux density (400–700 nm) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate     

The modulation of the conversion of l-aminocyclopropane-l-carboxylic acid to ethylene by light

Endogenous ethylene production of tobacco leaves was similar in light and in darkness. However, the rate of conversion of exogenously applied l-aminocyclopropane-l-carboxylic acid (ACC) to ethylene was reversibly inhibited by light. Virus-stimulated ethylene production, during the hypersensitive reaction of tobacco leaves to tobacco mosaic virus, was likewise inhibited by light. Under such circumstances ethylene production is limited at the level of the conversion of ACC to ethylene. Inhibition of the increase in ACC-stimulated ethylene production by cycloheximide and 2-(4-methyl-2,6-dinitroanilino)-N-methyl-propionamide after shifting leaf discs from light to darkness indicated that de novo protein synthsis was involved. Regulation of ACC-dependent ethylene production by reversible oxidation/reduction of essential SH groups, as suggested by Gepstein and Thimann (1980, Planta 149, 196–199) could be excluded. Instead, regulation of the ACC-converting enzyme at the level of both synthesis/degradation and activation/inactivation is suggested. Phytochrome was not involved in light inhibition, but low intensities of either red or blue light decreased the rate of ACC conversion. Dichlorophenyldimethylurea counteracted the inhibitory effect of light, indicating that (part of) the photosynthetic system is involved in the light inhibition. The ethylene production of Pharbitis cotyledons grown in darkness or light, either in the presence of absence of the inhibitor of carotenoid synthesis, SAN 9789 (norflurazon), supported this view.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - DCMU dichlorophenyldimethylurea - MDMP 2-(4-methyl-2,6-dinitroanilino)-N-methyl-propionamide - SAM S-adenosylmethionine - SH groups sulfhydryl groups - TCA trichloroacetic acid - TMV tobacco mosaic virus     

Visual pigment,dark adaptation and rhodopsin renewal in the eye of Pontoporeia affinis (Crustacea,Amphipoda)

The benthic amphipod Pontoporeia affinis lives in the Baltic sea and in northern European lakes in an environment where very little light is available for vision. The eyes, consisting of 40–50 ommatidia, are correspondingly modified. Microspectrophotometric recordings on isolated eyes show the presence of at least two kinds of screening pigments in the ommatidia with maxima at 540–580 nm and 460–500 nm. Difference spectra obtained from the rhabdoms after exposure to red and blue light, respectively, give evidence of a single rhodopsin with its maximum at 548 nm and a 500-nm metarhodopsin. In ERG recordings sensitivity in the dark-adapted state, after saturating exposures to blue and to red light, stabilizes at levels determined by the rhodopsin concentration. No change is observed during 10–14 h after the beginning of dark adaptation. However, using animals pre-exposed with a strong red light and then kept in darkness, it is found that after a delay of 20–40 h sensitivity of the dark-adapted eye begins to increase and finally, after 5–6 days reaches a level corresponding to 100% rhodopsin. Thus, a slow renewal of rhodopsin appears to occur in darkness, where a photoisomerization of metarhodopsin is excluded.Abbreviations ERG electroretinogram - IR infrared - MSP microspectrophotometry     

Biosynthesis of ribulose-1.5-bisphosphate carboxylase in greening cells of Euglena gracilis

Light induction of chloroplast development in Euglena leads to quantitative changes in the protein composition of the soluble cell part. One major part of these is the observed accumulation of ribulose-1.5-bisphosphate carboxylase/oxygenase (RuBPCase) enzyme (EC 4.1.1.39). As measured by immunoelectrophoresis, a small amount of RuBPCase (about 10^-6 pmol) is present in a dark-grown cell, whereas a greening cell (72h) contains 10–20 pmol enzyme. Both the cytoplasmic and chloroplastic translation inhibitors, cycloheximide and spectinomycin, have a strong inhibitory effect on the synthesis of the enzyme throughout the greening process of Euglena cells. Electrophoretic and immunological analyses of the soluble phase prepared from etiolated or greening cells do not show the presence of free subunits of the enzyme. For each antibiotic-treated greening cell, the syntheses of both subunits are blocked. Our data indicate that tight reciprocal control between the syntheses of the two classes of subunits occurs in Euglena. In particular, the RuBPCase small subunit synthesis in greening Euglena seems more dependent on the protein synthesis activity of the chloroplast than the syntheses of other stromal proteins from cytoplasmic origin.Abbreviations LSU large subunit of ribulose-1.5-bisphosphate carboxylase - RuBP ribulose-1.5-bisphosphate - RuBP-Case ribulose-1.5-bisphosphate carboxylase - SSU small subunit of ribulose-1.5-bisphosphate carboxylase     

Induction and decay of thermosensitivity in the flesh fly,Sarcophaga crassipalpis

When pharate adults of the flesh fly Sarcophaga crassipalpis are exposed to 40°C for 4 h they become more tolerant of high temperatures that are normally lethal (thermotolerance). In contrast, a 1-h exposure to 45°C decreases tolerance to a subsequent high temperature challenge (thermosensitivity). While control flies experience little mortality when held at 35°C for 24–48 h the thermosensitized flies die when exposed to 35°C. Sensitivity to a second thermal challenge slowly decays over a 72-h period. The acquisition of thermotolerance prevents the development of thermosensitivity. Brains from thermosensitized flies cultured at 43°C express the 72-kDa heat-shock protein and normal protein synthesis is inhibited. This implies that development of thermosensitivity is not associated with a loss in the capacity to express the 72-kDa heat-shock protein.Abbreviations ICN ICN Biomedicals, Inc. PO Box 19536, Irvine, CA 92713-9921 - LD light dark cycle - LT₅₀ time required to kill 50% of the test animals - SDS sodium dodecyl sulfate - TRIS Tris(hydroxymethyl)aminomethane     

Impact of light/dark regimen on growth rate,biomass formation and bacteriochlorophyll synthesis in Erythromicrobium hydrolyticum

The impact of illumination on specific growth rate, biomass formation, and synthesis of photopigment was studied in Erythromicrobium hydrolyticum, an obligately aerobic heterotrophic bacterium having the ability to synthesize bacteriochlorophyll a. In dark-grown continuous cultures the concentration of protein increased with increasing dilution rate, the concentration of bacteriochlorophyll a showed the opposite effect. At a dilution rate of 0.08 h^-1 (68% of _max in the dark) and S_R-acetate of 11.8 mM, the concentration of BChla of illuminated cultures in steady-state was 11–22 nM, compared to 230–241 nM in cultures incubated in darkness. No significant differences were observed in the concentration of protein. A shift from darkness to light conditions resulted in increased specific growth rates resulting in increased biomass formation, thus showing that light enhances growth by serving as an additional energy source. This phenomenon, however, was temporary because bacteriochlorophyll synthesis is inhibited by light. In contrast to incubation in continuous light or dark, incubation under light/dark regimen resulted in permanently enhanced biomass formation. In the dark periods, bacteriochlorophyll was synthesized at elevated rates (compared to constant darkness), thus compensating the inhibitory effect of light in the preceding period. It thus appears that the organism is well-adpated to life in environments with alternating light/dark conditions. The ecological relevance of the observations is discussed.Non-standard abbreviations BChla bacteriochlorophyll a - D dilution rate - spceific growth rate - K_s saturation constant - S_R concentration of limiting in inflowing medium of chemostat     

Control of the circadian rhythm of carbon dioxide assimilation in Bryophyllum leaves by exposure to darkness and high carbon dioxide concentrations

The circadian rhythm of CO₂ assimilation in detached leaves of Bryophyllum fedtschenkoi at 15° C in normal air and continuous illumination is inhibited both by exposure to darkness, and to an atmosphere enriched with 5% CO₂. During such exposures substantial fixation of CO₂ takes place, and the malate concentration in the cell sap increases from about 20 mM to a constant value of 40–50 mM after 16 h. On transferring the darkened leaves to light, and those exposed to 5% CO₂ to normal air, a circadian rhythm of CO₂ assimilation begins again. The phase of this rhythm is determined by the time the transfer is made since the first peak occurs about 24 h afterwards. This finding indicates that the circadian oscillator is driven to, and held at, an identical, fixed phase point in its cycle after 16 h exposure to darkness or to 5% CO₂, and it is from this phase point that oscillation begins after the inhibiting condition is removed. This fixed phase point is characterised by the leaves having acquired a high malate content. The rhythm therefore begins with a period of malate decarboxylation which lasts for about 8 h, during which time the malate content of the leaf cells must be reduced to a value that allows phosphoenolpyruvate carboxylase to become active. Inhibition of the rhythm in darkness, and on exposure to 5% CO₂ in continuous illumination, appears to be due to the presence of a high concentration of CO₂ within the leaf inhibiting malic enzyme which leads to the accumulation of high concentrations of malate in the leaf cells. The malate then allosterically inhibits phosphoenolpyruvate carboxylase upon which the rhythm depends. The results give support to the view that malate synthesis and breakdown form an integral part of the circadian oscillator in this tissue.Abbreviations B. Bryophyllum - PEPCase phosphoenolpyruvate carboxylase     

Organ specific analysis of the anaerobic primary metabolism in rice and wheat seedlings II: Light exposure reduces needs for fermentation and extends survival during anaerobiosis

Low oxygen stress in plants can occur during flooding and compromise the availability and utilization of carbohydrates in root and shoot tissues. Low-oxygen-tolerant rice and -sensitive wheat plants were analyzed under anaerobiosis in light to evaluate main factors of the primary metabolism that affect sensitivity against oxygen deprivation: activity of glycolysis and the rate of photosynthesis. Relatively stable ATP contents (93 and 58% of aerated control levels after 24 h anaerobiosis) in illuminated shoot tissues account for enhanced tolerance of rice and wheat seedlings to anaerobiosis upon light exposure in comparison to anoxia in darkness. Although the photosynthetic process was inhibited during low oxygen stress, which was partly due to CO₂ deficiency, more light-exposed than dark-incubated seedlings survived. Illuminated plants could tolerate a 70% lower anaerobic ethanol production in shoots in comparison to darkness, although still an 18-times higher ethanol production rate was determined in rice than in wheat leaves. In conclusion, light-exposed plants grown under anaerobiosis may recycle low amounts of generated oxygen between photosynthesis and dissimilation and generate additional energy not only from substrate phosphorylation during glycolysis but also from other sources like cyclic electron transport.     

Gene activation in suspension-cultured cells of Arabidopsis thaliana during blue-light-dependent plantlet regeneration

In cell-suspension cultures of Arabidopsis thaliana (L.) Heynh., transfer to auxin-free medium initiates regeneration leading to the formation of numerous rootlets around day 5. This process is promoted by continuous irradiation of the cell cultures with blue light (400–500 nm) while red light (600–700 nm) is ineffective in this respect. During the course of this process, two mRNA species, encoding, respectively, chalcone synthase and a plasmalemma channel protein, transiently accumulate. A second temporary increase in the steady-state level of these mRNAs is correlated with the onset of chloroplast development after 13–17 d of blue-light exposure of the cell cultures. During this cellular differentiation process a number of mRNAs start to accumulate which specify prominent plastid proteins: the small and the large subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (SSU and LSU), respectively the light-harvesting chlorophyll-a/b protein II (LHCPII). These findings are in accordance with those obtained with carrot suspension cultures where a clear sequence of development, i.e. the formation of somatic embryos followed by bluelight-dependent chloroplast differentiation, has also been observed.Abbreviations AthH2 intrinsic membrane protein of Arabidopsis thaliana (gene) - CHS chalcone-synthase - 2,4-D 2,4-dichlorophenoxyacetic acid - EFR energy fluence rate - LHCPII cab light harvesting chlorophyll-a/b protein of photosystem II (gene) - LSU rbcL large subunit of Rubisco - SSU rbcS small subunit of Rubisco - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase Dedicated to Prof. Wolfhart Rüdiger on the occasion of this 60th birthdayThe research was supported by the Deutsche Forschungsgemeinschaft. We thank Mrs. I. Liebscher for her competent assistance. For the generous gift of cloned gene sequences we thank Prof. Dr. G. Link (Pflanzliche Zellphysiologie, Bochum, Germany), Dr. A. Batschauer (Biologisches Institut II/Botanik, Freiburg, Germany) and Dr. B. Weißhaar (MPI für Züchtungsforschung, Köln, Germany).     

Induction of fumarase in resting Euglena

Exposure of dark-grown resting (carbon deficient) Euglena to light, ethanol or malate produced a transient increase in the specific activity of fumarase. Fumarase levels decreased 8–12 h after the start of induction and this decrease could not be prevented by additional inducer. During the period of fumarase accumulation, cycloheximide prevented further fumarase synthesis and enzyme levels decreased at a rate comparable to the rate of decline normally observed 8–12 h after the start of induction. Although the addition of ethanol to ethanol-induced cultures or malate to malate-induced cultures 12 or 24 h after the initial induction failed to maintain or induce additional fumarase synthesis, the addition of organic carbon to photoinduced cells 8 or 24 h after light exposure induced additional enzyme synthesis. Additional enzyme synthesis was not induced when ethanol- or malate-induced cells were exposed to light 12 or 24 h after organic carbon addition. Light exposure or ethanol addition failed to induce fumarase synthesis during balanced growth indicating that fumarse inducibility is a property of resting cells.     

Light inhibition of internode elongation in green plants

The clongation of the first internode of fully greenVigna sinensis L. is inhibited by white light (W). This inhibition is fluence-rate dependent between 0 and 70 Wm^–2. The kinetics of elongation rate in the light after darkness were investigated with linear displacement transducers. The internode elongation rate does not exhibit any endogenous rhythm. A rapid inhibition occurs during the first 2 or 3 h after the onset of light, and a second type of inhibition (slow reaction) increases from the beginning to the 8th hour of light. The rapid inhibition is not fluence-rate dependent between 20 and 70 Wm^–2, but the slow reaction is. There is no rapid inhibition in a low fluence rate white light to high fluence rate white light transition, only the slow reaction is observed. The responses to different wavebands, i.e., blue light (B), yellow and green light (YG), and red light (R), are the same for the two inhibition reactions. Each waveband used separately does not reproduce the full effect observed in W. Results show a stimulation with B, a greater inhibition activity with YG than with R, and a synergistic action of B and R which when given together lead to an inhibition similar to that obtained in W. Plants returned from the light to darkness progressively recover a high elongation rate without any latent period. The W light regulating internode elongation rate is mainly perceived by the growing internode itself.Abbreviations B blue light - D darkness - F far-red light - HW high fluence rate white light - LW low fluence rate white light - R red light - W white light - YG yellow and green light     

Cytokinin control of growth of Spirodela oligorrhiza in darkness

Summary Continuous heterotrophic growth of Spirodela oligorrhiza cultures following transfer to darkness requires cytokinins, or periodic brief treatment with red light. In the absence of cytokinins or red light growth ceases after 2–3 days. However, growth resumes spontaneously after 3–4 weeks in darkness to produce etiolated plants. The growth rate of these etiolated plants is not stimulated by kinetin.Although the kinetin concentration in treated plants reaches a plateau 30–60 min after adding kinetin to dormant plants in darkness new fronds do not appear for 24 h. Dormant colonies treated with kinetin in darkness for only 6–12 h subsequently grow in darkness at the same rate as plants treated with kinetin for 1, 2 or 3 days. Treatments which inhibit growth in the light, for example cold, chloramphenicol or actidione, eliminate the requirement for cytokinin and allow subsequent growth in darkness. The results suggest that a growth inhibitor may be present but ineffective in Spirodela growing in the light. The inhibitor is active in darkness but slowly decays. Kinetin appears to inactivate the inhibitor in darkness.     

Carbon-nitrogen relations during batch growth ofNannochloropsis oculata (Eustigmatophyceae) under alternating light and dark

Nannochloropsis oculata (strain CCAP 849/1) was sampled at least every 12 h over a 26-d period of batch culture growth in a 12 h/12 h light/dark illumination cycle. Exponential cell-specific growth rate was 0.5 d^–1. Cell division occurred during the dark phase, while ammonium uptake, pigment synthesis and cell volume increase occurred mainly during the light. Stationary phase cells were on average larger that the largest exponentially growing cells. The lag phase prior to cell division was short with the C/N ratio returning to 6.25 (from 28) within 2 d of refeeding with ammonium. Significant Chl.a synthesis commenced after this period; net synthesis of Chl.a ceased on exhaustion of the N-source with a 40% fall in levels by the end of the stationary phase. Levels of carotenoids per cell also declined during N-deprivation although per ml of culture levels remained constant. Ammonium-refeeding of N-deprived cells resulted in a very rapid rise in glutamine (Gln) and very high ratios of glutamine/glutamate (Gln/Glu peaking at 35 within 1 h); peak Gln/Glu was lower in cells refed in the dark or after a shorter period of N-deprivation. The major intracellular amino acids during exponential phase were Glu, Gln, alanine and arginine, but on exhaustion of the N-source, levels of Gln fell rapidly (Gln/Glu falling to below 0.1 from 0.5–0.9 in the light and 0.3 in darkness during exponential growth). During N-deprivation tyrosine accumulated within the cells. Comparisons are drawn with the growth ofIsochrysis galbana, another alga used in aquaculture, under identical conditions.Author for correspondence