Sulphate uptake and metabolism in the chrysomonad,Monochrysis lutheri

The intracellular concentration of inorganic ³⁵SO₄ in Monochrysis lutheri cells exposed to 0.513 mM Na₂ ³⁵SO₄ for up to 6-hr remained constant at about 0.038 mM. The exchange rate of this ³⁵SO₄ with the external unlabelled sulphate was negligible compared to the rate of influx across the plasmalemma (0.032 μmoles/g cells/hr). The flux of free ³⁵SO₄ to organic ³⁵S was 0.029 μmoles/g cells/hr. Assuming an internal electrical potential in the cells of-70 mV, this intracellular concentration of inorganic ³⁵SO₄ was well in excess of that obtainable by passive diffusion as calculated from the Nernst equation. These results indicate that sulphate is accumulated by an active mechanism rather than by facilitated diffusion. Sulphate uptake appears to occur via a carrier-mediated membrane transport system which conforms to Michaelis-Menten type saturation kinetics with a K _m of 3.2×10^-5 M and a V _max of 7.9×10^-5 μmoles sulphate/hr/10⁵ cells. Uptake was dependent on a source of energy since the metabolic inhibitor CCCP almost completely inhibited uptake under both light and dark conditions and DCMU caused a 50% decrease in uptake under light conditions. Under dark conditions, uptake remained at about 80% of that observed under light conditions and was little affected by DCMU, indicating that the energy for uptake could be supplied by either photosynthesis or respiration. A charge and size recognition site in the cell is implied by the finding that sulphate uptake was inhibited by chromate and selenate but not by tungstate, molybdate, nitrate or phosphate. Chromate did not inhibit photosynthesis. Cysteine and methionine added to the culture medium were apparently capable of exerting inhibition of sulphate uptake in both unstarved and sulphate-starved cells. Cycloheximide slightly inhibited sulphate uptake over an 8-hr period indicating, either a slow rate of entry of the inhibitor into the cells or a slow turnover of the proteins(s) associated with sulphate transport.     

Interactions between Mitochondria and Chloroplasts in Cells: I. Action of Cyanide and of 3-(3,4-Dichlorophenyl)-1,1-dimethylurea on the Spore of Funaria hygrometrica

The effects of cyanide and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on photosynthesis and respiration of intact chlorophyllic moss (Funaria hygrometrica) spore was investigated. Thirty micromolar cyanide strongly inhibited dark respiration, was without effect on photosynthesis at high light intensities (above the saturation plateau values), and stimulated photosynthesis at low light intensities (below the saturation plateau values). Three hundred nanomolar DCMU inhibited the photosynthesis and was without effect, even under light conditions, on the dark respiration. It seems likely, therefore, that in the chlorophyllic moss spore the cytochrome oxidase pathway is not functioning under high light intensities unless the photosynthesis is inhibited by DCMU.     

Polarographische Messung der Nitritreduktion von Chlorella im monochromatischen Licht

Summary In green plant cells nitrite is reduced by two systems, one dependent on photosynthesis and the other upon respiration. Using a polarographic method for continuous measurement of nitrite uptake, the relationship between light driven and respiration linked nitrite reduction of Chlorella cells was studied.Photosynthetic nitrite reduction is characterized by a pronounced increase in the velocity of nitrite uptake upon illumination. After the light is turned off the velocity immediately returns to the preillumination value. Photosynthetic nitrite reduction of Chlorella is separated from respiration linked nitrite reduction by illumination with red light under anaerobic conditions; it is stimulated by CO₂ and is inhibited by DCMU, findings which confirm earlier observations.In white light a special blue light stimulation of nitrite uptake is overlapped by photosynthetic nitrite reduction. In contrast to photosynthetic nitrite reduction this type of light stimulation is characterized by a lag period of about I min from the onset of illumination; it continues about 10 min when the light is turned off. It is separated from photosynthetic nitrite reduction by irradiation of the algae with low intensities of short wavelength light (<500 nm). Blue light stimulation of nitrite uptake of Chlorella is strongly dependent on the developmental stage of the cells. It is observed with young cells (autospores) of synchronized algae only.There is no evidence for any connection between blue light stimulation of nitrite uptake and photosynthesis. From the sensitivity of this process towards anaerobic conditions and antimycin A it is concluded to be a stimulation of respiration linked nitrite reduction.Under conditions of low exogenous nitrite concentration a temporary inhibition of steady state dark nitrite reduction appears immediately after the light is turned off. From several observations it is concluded that the inhibition already exists during the preceding illumination and decreases the rate of total nitrite uptake in the light. This process is suppressed by inhibition of respiration as well as by the inhibitor of photosynthesis, DCMU.If nitrate is the source of nitrogen an excretion of nitrite is found following illumination. The kinetics of this process agree with those observed for the light induced inhibition of steady state dark nitrite reduction immediately after illumination.     

Photoassimilation of Glycolate, Glycine and Serine by Euglena gracilis

SYNOPSIS. Glycolate was readily utilized for growth by Euglena gracilis , strain Z, in the light at pH 3.8 under a variety of atmospheric conditions, including CO₂-free air and nitrogen. Glycolate did not support growth in the dark as sole carbon source; no significant uptake of glycolate was observed under these conditions. However, cells grown in the light with glycolate as sole carbon source were still capable of glycolate uptake for up to 3 hr after transfer to darkness, and glycolate was taken up by cells utilizing glucose in the dark. The energy requirement for glycolate utilization could thus be met either by light, or by the aerobic metabolism of glucose in the dark. DCMU, an inhibitor of photosystem II, inhibited photoassimilation of glycolate. In the light, but again not in the dark, glycine and serine also served as sole source of carbon under CO₂-free air, but not under nitrogen. Net release of ammonia to the medium accompanied the photoassimilation of glycine and serine. Of the several metabolicallyrelated compounds tested, only glycolate was utilized as sole carbon source in the light under "anaerobic" conditions. A lag in net chlorophyll synthesis occurred during the photoassimilation of glycolate glycine or serine. Determination of rates of photosynthetic ¹⁴CO₂ fixation confirmed that some inhibition of photosynthetic capacity had occurred in response to utilization of glycolate and related compounds.     

Effect of a light pulse during the dark on photoperiodic regulation of the rate of thyroxine-induced, spontaneous, and prolactin-inhibited metamorphosis in Rana pipiens tadpoles

Since Rana pipiens tadpoles injected with thyroxine (T4) early in the dark develop more slowly than those injected in the light, we studied the effect of giving a light pulse of 1 hr early in the dark. Tadpoles injected under a 7.5-W red light bulb in a darkened room with 0.2 microgram T4 daily at 2200 hr went through metamorphosis faster on a 12L:3D:1L:8D cycle with a light pulse after injection than on a 12L:12D cycle without a light pulse, and even faster on a 12L:1.5D:1L:9.5D cycle with a light pulse before the injection. Thus a 1-hr light pulse counteracted the metamorphic delay resulting from administration of T4 in the dark, and set in motion the conditions that resulted in a more rapid response to an injection of T4. However, a 1-hr light pulse in the early dark had no effect on growth and development of older or younger untreated tadpoles or those constantly immersed in 30 micrograms/liter T4. Larvae on 21L:3D with T4 injection in the dark and on 12L:3D:1L:8D with T4 injection at 0700 hr just before the start of the main light phase progressed faster than 12L:3D:1L:8D with injection at 2200 hr in the dark before only a 1-hr light pulse. Thus the length of the light phase immediately after T4 injection was significant. There was no difference on 12L:12D and 12L:3D:1L:8D cycles in the effectiveness of daily injections of 10 micrograms prolactin (PRL) in the early dark at 2200 hr in promoting tail growth or antagonizing tail resorption induced by T4 immersion. Under these conditions, PRL utilization did not appear to be inhibited by the light pulse.     

Sulphate Influx in Characean Cells: II. LINKS WITH LIGHT AND METABOLISM IN CHARA AUSTRALIS

Light filters and metabolic inhibitors have been used to investigatefurther the active transport of sulphate into Chara australis.Two states of influx, light (basal) and dark (transiently stimulated),have been described. The stimulated state noted on transferto dark has been found when the incident intensity of monochromaticlight is reduced, and when photosystem 2 in photosynthesis isinhibited, either by use of cut-ofT filters or by DCMU. Thelight influx is insensitive to CCCP when photosynthetic ¹⁴CO₂fixation is totally inhibited, and is less sensitive to DNPthan the dark influx. Dark influx is inhibited by CCCP, DNP,and NaCN but is insensitive to DCMU. It is proposed that a respiratoryATP source may be sufficient energy supply for sulphate influxand that the state of influx is under separate control. It issuggested that a ‘triggering’ mechanism may bringabout the change from the light- to the dark-influx state.     

The germination characteristics of phytochrome-deficient aurea mutant tomato seeds

The role of light reactions in anthocyanin synthesis was studied in both attached and detached corollas of Petunia hybrida (cv. Hit Parade Rosa), the latter grown in vitro in media containing 150 m M sucrose and 50 μ M gibberellic acid (GA). Light was essential for the synthesis of anthocyanin in detached corollas, whereas in intact corollas its effect was only to enhance anthocyanin synthesis. Continuous white light at a fluence rate of at least 20 μmol m⁻² s⁻¹ was needed for anthocyanin synthesis in detached corollas. Blue light was more effective than red or green, and far-red was ineffective. Pigmentation of detached corollas exposed to light was inhibited by the photosynthetic inhibitor 3-(4-dichlorophenyl)-1,1-dimethylurea (DCMU). The chloroplast uncoupler NH₄Cl did not affect anthocyanin synthesis, which was, however, inhibited by the blocking of ATP synthesis in both the chloroplast and the mitochondria by dicyclohexylcarbodiimide (DCCD). Sucrose uptake in vitro was inhibited by DCMU and by darkness, and was promoted equally by blue and red light. The activity of phenylalanine ammonialyase (EC 4.3.1.5) was inhibited in detached corollas grown in the dark or in the light in the presence of DCMU. The activity of chalcone isomerase (EC 5.5.1.6) was not affected by light. These findings suggest that at least two different light reactions are involved in the regulation of anthocyanin synthesis in petunia corollas, namely the high irradiance reaction (HIR) and photosynthesis.     

Daily changes in concentrations of prolactin in serum of prepubertal bulls exposed to short- or long-day photoperiods

Early temporal changes in concentrations of prolactin (PRL) in serum after a sudden change in photoperiod and daily responsiveness to PRL-releasing and inhibiting factors were investigated in prepubertal Holstein bull calves exposed to different photoperiods. In calves switched from 8-hr light: 16-hr dark to 16-hr light:8-hr dark, there was no observable change in the daily pattern of serum concentrations of PRL after 1, 2, or 4 days. On the other hand, in animals switched from 16-hr light:8-hr dark to 8-hr light:16-hr dark, there was a consistent increase in serum PRL from 33.4 ng/ml on Day 0 to maximum values of 57.3, 62.7, and 78.9 ng/ml between 14 and 18 hr after onset of light on Days 1, 2, and 4, respectively. Thus, absence of light allowed expression of a daily rhythm in serum concentrations of PRL that persisted for at least 4 days after the photoperiod switch. There were no differences in L-dopa inhibition of PRL release in animals exposed to 16-hr light:8-hr dark at 3 or 15 hr after onset of light. However, thyrotropin-releasing hormone-induced release of PRL was greater 3 hr after onset of light (11 hr after onset of dark) compared with release at 9, 15, and 21 hr after onset of light in animals exposed to 16-hr light:8-hr dark, but not in bulls exposed to 8-hr light:16-hr dark. The results provide evidence that the cue for the putative photosensitive period of PRL secretion in cattle may be more closely associated with onset of dark, not onset of light.     

Some effects of light on algal respiration and the validity of the light and dark hottle technique for measuring primary productivity

SUMMARY. Measurements of the rate of oxygen uptake in a number of blue-green algae and diatoms were carried out under both field and laboratory conditions to determine the effects of light on such rates. The light history of algal cells was an important controlling factor of oxygen uptake. When measured in the light, with dichlorophenyl-dimethylurea (DCMU), oxygen uptake was sometimes different from uptake measured in the dark. The results cast some doubt on the validity of the light and dark bottle method for determining primary productivity. It is suggested that oxygen uptake measurements should be made in the presence of DCMU.     

Mineralization in rat metaphyseal bone exhibits a circadian stage dependency

Density gradient fractionation analysis of rat metaphyseal bone was used to delineate the biorhythmic changes in bone matrix mineralization. Seventy-two 4-week-old rats were entrained to 12-hr light, 12-hr dark cycles (light, 0800-2000 hr; darkness, 2000-0800 hr) for 4 weeks. All animals were fed ad lib. on Purina laboratory rat chow and tap water. Groups of 10-12 rats were killed by cervical dislocation at 4-hr intervals during a 24-hr period, and the tibias were then biopsied and frozen in liquid N2. Metaphyseal bone was fractionated via bromoformtoluene density gradients into specific gravity fractions ranging from 1.7 to 2.8. Density gradient fractions were analyzed for concentrations of calcium and inorganic phosphorus. Chronograms indicated that the accumulation of both calcium and inorganic phosphorus into the newly forming/least-dense mineral moieties of bone (1.3-1.7 sp grav) showed a single peak in the biorhythm of the rat. A statistically significant circadian rhythm of mineralization was detected for calcium (P less than 0.001) and inorganic phosphorus (P less than 0.039), with peaks during the environmental dark span. These results suggest that the physiological phasing of bone mineralization in the light-dark synchronized rat, is similar to that previously noted for cartilage mineralization and is antiphasal to the midday peak in bone collagen synthesis.     

CHLORIDE UPTAKE BY ANACYSTIS NIDULANS (CYANOPHYCEAE)1

Anacystis nidulans (Richt.) Drouet & Daily (UTEX 625), grown in batch culture with 0.5% CO₂ in air, was supplied with chloride labelled with ³⁶Cl in light and dark. Uptake in light was stimulated relative to uptake in darkness. A single transport system for Cl^? with an apparent K_m for Cl^? of 0.14 mM was identified. Chloride in the cells reached a maximum value after 30–50 min at 25 C. At this point the internal Cl^? concentration was calculated to be 60-fold the external (0.1 mM) in light and 37-fold in darkness. DCMU (3-[3,4-dichlorophenyl]–1, 1-dime-thylurea), at concentrations which abolished photosynthetic O₂ evolution did not inhibit Cl^? uptake in light. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP), at uncoupling concentrations for photosynthesis and dark respiration, strongly inhibited Cl^? uptake in light and darkness. N,N'-dicyclohexyl carbodiimide (DCCD), an energy transfer inhibitor, inhibited light Cl^? uptake more slowly than photosynthesis but had no effect on dark Cl^? uptake. It is concluded that Cl^? uptake in A. nidulans was active in light and darkness, and that ATP was the probable energy source for transport.     

Uptake and Utilization of Fructose by Anabaena variabilis ATCC 29413. Effect on Respiration and Photosynthesis

Anabaena variabilis ATCC 29413 showed a constitutive mechanismfor fructose uptake which was further enhanced by growing thecells with fructose. The uptake process was energydependentas indicated the inhibitory effect of the uncoupler carbonylcyanidem-chlorophenylhydrazone (CCCP) and the reduction induced byincubating the cells in the dark or in the light with DCMU.Cells adapted to growth on fructose showed increased rates ofrespiration both in the dark and in the light. The rate of ¹⁴CO₂evolved from radiolabelled fructose was lower in the light thanin the dark or in the presence of DCMU. This fact can be partiallyexplained by the photosynthetic reutilization of respiratory¹⁴CO₂. Modifications in photosynthesis were observed in fructose-growncells. PS I and PS II activity measured in spheroplasts obtainedby lysozyme treatment were enhanced by fructose probably asa way to compensate the lower concentration of chorophyll showedby fructose-grown cells. The photosynthetic affinity for externalCO₂ and the rate of photosynthesis dependent on external inorganiccarbon were reduced by fructose. (Received September 24, 1991; Accepted February 14, 1992)     

Increase in Nitrate Reductase Activity in Bean Leaves by Light Involves a Regulator Protein

Nitrate reductase activity, assayed either in vivo or in vitro was considerably higher in bean (Phaseolus vulgaris L.) leaves from 7-day-old light grown seedlings than those from dark grown, both in the absence as well as presence of nitrate. Cytochrome c reductase activity was however similar in both regimes, while peroxidase was lower in light than in dark. The light stimulated increase in nitrate reductase activity in leaf segments from dark grown seedlings was inhibited by cycloheximide, DNP, chloramphenicol, and sodium tungstate and was unaffected by lincomycin and DCMU. Under similar conditions, the increase in total chlorophyll was inhibited completely by cycloheximide and DNP, partially by chloramphenicol and lincomycin, and was unaffected by tungstate and DCMU. A supply of 1~5 mm reduced glutathione increased enzyme activity in the dark and also to some extent in light. The substrate induction of enzyme activity started after a lag of one hr in light or dark and continued for either 5 hr in the dark or 8 hr in light. Two proteinaceous inhibitors (Factors I and II) of nitrate reductase were isolated by ammonium sulfate precipitation and Sephadex gel filtration. The amount of Factor I was higher in the dark than in light. The amount and activity of Factor II was however, almost equal in light and dark. The inhibition of enzyme activity by these inhibitors increased with their concentration. It is proposed that light increases nitrate reductase activity by decreasing the amount of a nitrate reductase inhibitor.     

Chondroitin sulfate proteoglycan synthesis and reutilization of beta-D-xyloside-initiated chondroitin/dermatan sulfate glycosaminoglycans in fetal kidney branching morphogenesis

Branching morphogenesis and chondroitin sulfate proteoglycan synthesis by explanted fetal mouse kidneys were previously shown to be inhibited by p-nitrophenyl beta-D-xylopyranoside (beta-D-xyloside) while glomerular development and heparan sulfate proteoglycan synthesis were unaffected. The metabolic fate of fetal kidney explant proteoglycans was investigated to determine whether or not recovery of proteoglycan synthesis and morphogenesis occur after exposure to beta-D-xyloside. Chondroitin sulfate proteoglycan synthesis resumed within 4 hr of removal of beta-D-xyloside and was enhanced once beta-D-xyloside-initiated chondroitin/dermatan-35SO4 glycosaminoglycans (GAGs) were released from the tissue. Radioactivity incorporated into beta-D-xyloside-initiated chondroitin/dermatan-35SO4 GAGs during labeling in the presence of beta-D-xyloside was reutilized in the synthesis of chondroitin-35SO4 proteoglycan during a 24-hr chase in nonradioactive medium without beta-D-xyloside. Further, highly purified beta-D-xyloside-initiated chondroitin/dermatan-35SO4 GAGs were taken up by kidneys more avidly than was free [35S]sulfate. These 35S-GAGs were degraded and reutilized in the synthesis of chondroitin-35SO4 proteoglycan. Ureteric bud branching resumed 48 hr after beta-D-xyloside was removed from the incubation medium. These findings support the idea that both chondroitin sulfate proteoglycan synthesis and proteoglycan processing may be involved in branching morphogenesis.     

Glycogen content and nitrogenase activity in Anabaena variabilis

Nitrogenase (=acetylene-reducing activity) was followed during photoautotrophic growth of Anabaena variabilis (ATCC 29413). When cell density increased during growth, (1) inhibition of light-dependent activity by DCMU, an inhibitor of photosynthesis, increased, and (2) nitrogenase activity in the dark decreased. Addition of fructose stabilized dark activity and alleviated the DCMU effect in cultures of high cell density.The resistance of nitrogenase towards oxygen inactivation decreased after transfer of autotrophically grown cells into the dark at subsequent stages of increasing culture density. The inactivation was prevented by addition of fructose. Recovery of acetylene-reducing activity in the light, and in the dark with fructose present, was suppressed by ammonia or chloramphenicol. In the light, also DCMU abolished recovery.To prove whether the observed effects were related to a lack of photosynthetic storage products, glycogen of filaments was extracted and assayed enzymatically. The glycogen content of cells was highest 10 h after inoculation, while light-dependent nitrogenase activity was at its maximum about 24 h after inoculation. Glycogen decreased markedly as growth proceeded and dropped sharply when the cells were transferred to darkness. Thus, when C-supply (by photosynthesis or added fructose) was not effective, the glycogen content of filaments determined the activity of nitrogenase and its stability against oxygen. In cells lacking glycogen, nitrogenase activity recovered only when carbohydrates were supplied by exogenously added fructose or by photosynthesis.Abbreviations Chl chlorophyll a - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea     

GLUCOSE UPTAKE BY CYCLOTELLA CRYPTICA: DARK INDUCTION AND LIGHT INACTIVATION OF TRANSPORT SYSTEM1,2

Glucose transport capacity of C. cryptica increases in an exponential manner over 24 hr after transfer of the cells from light to complete darkness with little simultaneous increase in cell number. The transport system is rapidly inactivated when cells are transferred back to continuous light. Most of the inactivation takes place while there is still little changes in cell number. When grown on a continuous light regime, the capacity for glucose transport per cell depends on the light intensity. At intensities sufficient to saturate photosynthesis the glucose transport system is only about 5% that of dark-grown cells, while cells grown at intensities close to the light compensation point have about 30% of the capacity of dark-grown cells. The action spectrum for inactivation of glucose transport is identical to that for photosynthesis. Cells, whether grown under continuous light, in the dark in the presence of glucose, or kept in the dark without glucose, contain high levels of glucokinase and phosphofructokinase. The glucose transport system is highly specific for glucose; only galactose inhibits the uptake of glucose by about 50% when present at 10 times the concentration of glucose. The glucokinase is even more specific for glucose and is not inhibited by galactose. The phosphofructokinase is inhibited by high concentrations of ATP in cells grown under all conditions. cycloheximide inhibits the induction of glucose transport in the dark, but not the inactivation of the system in the light.     

Einfluss verschiedener Lichtwellenlängen auf die Zusammensetzung von Chlorella in Glucosekultur bei gehemmter Photosynthese

Summary Increasing blue light intensity inhibits the growth of Chlorella pyrenoidosa in glucose culture in which photosynthesis is blocked by DCMU, whereas red light supports growth which is the same as or better than that in dark controls.The action spectrum of light induced protein synthesis from exogenous glucose (photosynthesis inhibited, blue light addition resulting in growth >90% of the dark control) shows only one broad maximum at 450–490 nm which resembles the absorption spectrum of carotenoids.     

Influence of photosynthesis and chlorophyll synthesis on polypeptide accumulation in greening euglena

Two-dimensional gel electrophoresis resolves total cellular protein from Euglena gracilis klebs var bacillaris Cori into 640 polypeptides, 79 of which are induced by light exposure. The inhibition of chloroplast translation by streptomycin, the direct inhibition of photosynthesis as well as the indirect inhibition of chlorophyll synthesis by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and the specific inhibition of photosynthesis but not chlorophyll synthesis by DCMU in the presence of 17 millimolar ethanol failed to inhibit the accumulation of 40 polypeptides. These polypeptides appear to be synthesized on cytoplasmic ribosomes and their accumulation is independent of the developmental status of the chloroplast. Streptomycin but not DCMU completely inhibited the accumulation of six polypeptides which are undetectable in mutants lacking chloroplast DNA suggesting that these polypeptides are translated on chloroplast ribosomes. The accumulation of seven polypeptides which are detectable in mutants lacking chloroplast DNA was also inhibited by streptomycin but not by DCMU suggesting that the accumulation of these polypeptides is dependent upon stabilization by a chloroplast translation product. The accumulation of 12 polypeptides was inhibited by streptomycin and by DCMU under conditions in which chlorophyll synthesis was inhibited, but not under conditions in which chlorophyll synthesis was unaffected by DCMU. The inhibition by DCMU of the accumulation of these polypeptides appears to be due to the inhibition of chlorophyll synthesis suggesting that they are components of pigment protein complexes. The accumulation of six polypeptides was inhibited under all conditions in which photosynthesis was inhibited suggesting that the accumulation of these polypeptides is dependent upon a product of photosynthesis.     

Evidence for the occurrence of photorespiration in synurophyte algae

The fluxes of CO(2) and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO(2,) only a slow drawdown to compensation concentrations of 26 μM for T. volvocina and 18 μM for M. papillosa, when O(2) evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO(2), indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO(2) was observed similar to the post-illumination burst characteristic of C(3) higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO(2) burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O(2). This was investigated in four synurophyte species T. volvocina, M. papillosa, Synura petersenii, and Synura uvella: photosynthetic O(2) evolution rates in all four algae, measured by O(2) electrode, were significantly higher (40-50%) in media at low O(2) (4%) than in air-equilibrated (21% O(2)) media, indicating an O(2) inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species.     

Effects of 3-(3,4-Dichlorophenyl)-1,1-Dimethylurea on the Cell Cycle in Euglena gracilis

The cell cycle of the photosynthetic unicellular alga Euglena gracilis growing in phototrophic medium is regulated by light. To investigate the relationship of this cell cycle response to light stimulated photosynthesis, we have tested the effect of the photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on Euglena cell cycle transit. While DCMU does not block light stimulated cells from entering the S phase of the cell cycle, it does inhibit the transit through G₂/M. The specificity of this response and its relationship to photosynthesis was studied by looking at the effect of DCMU on dark grown wild-type cells, and on two bleached variants of Euglena (W₃BUL and W₁₀BSmL) that lack chloroplasts. The drug does block G₂/M in these cells, but not entrance into the cell cycle. Our studies show that entrance of cells into the cell cycle from a quiescent state does not require active photosynthesis, and that DCMU has effects on G₂/M transit that are independent of the photosynthetic capacity of the cells.