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.     

Diurnal Rb+ Transport from Roots to the Laminar Pulvinus in Phaseolus vulgaris L.

The primary leaves of kidney bean (Phaseolus vulgaris L.) openunder light and close in the dark by the deformation of thepulvinus resulting from diurnal distribution changes of K⁺,Cl^–, organic acid (or H⁺) and NO₃^–. When Rb⁺ was added as a tracer of K⁺ to the seedlings throughtheir roots, it was transported to the pulvinus cells duringthe light period but not during the dark period. Transpirationoccurred vigorously in the light but almost stopped in the dark.We concluded that Rb⁺ absorbed by the roots was carried to thepulvinus by the transpiration stream. Phaseolus vulgaris L., pulvinus, Rb⁺, diurnal transport transpiration stream     

Effects of dark preincubation and chloramphenicol on blue light-induced CO2 incorporation in Chlorella cells

Chlorella cells incubated in the dark longer than 12 hr showedpronounced blue light-induced ¹⁴CO₂ fixation into aspartate,glutamate, malate and fumarate (blue light effect), whereasthose kept under continuous light showed only a slight bluelight effect, if any. 2) During dark incubation of Chlorellacells, phosphoenolpyruvate carboxylase activity and the capacityfor dark ¹⁴CO₂ fixation decreased significantly, whereas ribulose-1,5-diphosphatecarboxylase activity and the capacity for photosynthetic ¹⁴CO₂fixation (measured under illumination of white light at a highlight intensity) did not decrease. 3) In cells preincubatedin the dark, intracellular levels of phosphoenolpyruvate and3-phosphoglycerate determined during illumination with bluelight were practically equal to levels determined during illuminationwith red light. 4) The blue light effect was not observed incells incubated widi chloramphenicol, indicating that blue light-inducedprotein synthesis is involved in the mechanism of the effect. (Received April 9, 1971; )     

SOME OBSERVATIONS ON THE VERTICAL MIGRATION OFDINOFLAGELLATES 12

The technique of measuring chlorophyll concentration in vivo by fluorometric analysis has been adapted to studying the diurnal migration of dino-flagellates in the sea and also in a deep tank (3 m in diameter by 10 m deep). The downward migration of Ceratium furca was followed during a bloom off the California coast. The main band of cells migrated from the upper 2 m to a depth of 5 m about 2 hr after sunset, and was dispersed between 5 and 16 m 4.5 hr after sunset. Cultures of Gonyaulax polyedra and Cachonina niei both migrated to the surface of the deep lank during illumination and migrated downward during darkness at a rate of 1-2 mjhr. The downward migration was observed to begin before the light was turned off, indicating that migration is correlated with a cellular periodicity which is to some extent independent of the light regime. Further evidence for such a periodicity was afforded by observations that C. niei start to migrate up in the water column before start of the light period. Nitrogen-limited cells of G. polyedra showed no diurnal migration, but within 1 day after addition of a nitrogen source they recovered their full migratory ability. Cells of C. niei, however, continued to migrate during 5 days of N-starvation, although they did not concentrate in the upper 1/2 m as did the control cells.     

TWO LIGHT REACTIONS IN THE PHOTOPERIODIC CONTROL OF FLOWERING OF LEMNA PERPUSILLA AND L. GIBBA

The effects of light quality on the photoperiodic control inthe flowering of a SD duckweed, Lemna perpusilla strain 6746,and a LD duckweed, L. gibba strain G3, were investigated withspecial reference to the interaction between R and B or FR lights. In the diurnal alternation of R or G light and dark periods,L. perpusilla responded as a SDP, but in that of B or FR lightit was almost daylength-indifferent. On the other hand, L. gibbaresponded as a LDP under B, R or FR light, although the criticallight length was altered by the light quality. In the diurnal alternation of R and B or FR light periods containingno dark period, L. perpusilla flowered with the shortening ofthe optimal and critical R light lengths, compared with theplant exposed to that of R light and dark period. The floweringresponse of L. gibba to the R light length showed double peaks,that is, the first peak at the R duration less than 9 hours,and the second at the R duration longer than 9 hours. The firstpeak corresponds to the optimal R light length in L. perpusilla. Under the CL with a mixture of R and B or FR lights, the floweringand frond production were influenced by the intensity ratioof two light given. In both plants, the optimal ratio of B toR or FR to R for the flowering was always greater than thatfor the frond production. It is suggested that the B or FR light interacts with the Rlight in the photoperiodic process in the plants and this interactionbetween the R and B or FR lights should be of importance forobtaining a better understanding of photoperiodism. (Received August 28, 1965; )     

Effect of far-red light pulse on induction and production of flowers in Lemna paucicostata 6746 in darkness

A short-day duckweed, Lemna paucicostata 6746, was exposed tocontinuous darkness at 26^?C, and the changes in the floral parameters(3) due to far-red and/or red light pulse given at various timesof the dark period were studied. Parameters a (vegetative growth rate) and (flowering ratio)were respectively decreased and increased with a far-red lightpulse given at the outset of the dark period. The decreaseda and the increased remained almost unchanged until the 7thhour, but returned to their initial levels thereafter. The far-redlight actions on a and were reversed by subsequent exposureto red light. Parameter P₁ (pre-flower induction period) wasextended by 1 day when far-red and/or red pulse was given atabout the 7th hour of the dark period. A far-jed pulse givenat the outset of the dark period only affected parameter P₂(flower induction period). Although the sensitivity of P₂ tored light increased with time, its sensitivity to far-red lightremained constant and at about the 7th hour was equally sensitiveto far-red and red lights. Both red and far-red pulses givenlater than the 7th hour were increasingly ineffective on P₂.The red/far-red reversibility occurred only for the action onP₂ of the far-red pulse applied during the early dark period.Parameter P₄ (flower production period) varied rhythmicallyin length with a far-red puke, the maximum shortening and extensionbeing induced by the pulse given at about the 7th and 19th hours,respectively. The sensitivity of P₄ to red light also changedrhythmically with an inverse phase angle to the rhythmic responseto farred light, and the far-red and red light actions werereversed respectively by subsequent red and far-red lights. These findings suggested that multiple timing devices includingan hourglass-type clock and a circadian clock are involved induckweed flowering. (Received October 25, 1978; )     

Nocturnal Accumulation of Acid in Leaves of Wall Pennywort (Umbilicus rupestris) Following Exposure to Water Stress

Physiological responses to water stress (drought) have beeninvestigated in Umbilicus rupestris (wall pennywort) by comparingcontrol (well-watered) and draughted plants with respect to(i) diurnal fluctuations in the acid content of the leaves,(ii) CO₂ exchange patterns and (iii) stomatal conductance. Controlplants show no diurnal fluctuations in acid content, whereasafter 6 d of drought a clear CAM-type pattern (nocturnal acidificationfollowed by deacidification in the light) is observed. In controlplants, the CO₂ exchange pattern over a 24 h period is of atypical C-3 ‘square-wave’ type, with extensive CO₂uptake in the light and CO₂ output in the dark. In droughtedplants the day-time CO₂ uptake is confined to a morning ‘burst’,whilst night-time CO₂ output is markedly reduced. There is howeverno net noctural uptake of CO₂. In control plants, stomatal conductanceis high during the day (especially in the first half of theday) falling to a low level at the onset of darkness, and thenrising slowly through the remainder of the night. In droughtedplants, stomatal conductance is very low, except that thereis morning ‘burst’ of high conductance and a periodduring the night when conductance is higher than in controlplants. These results are discussed in relation to the response of U.rupestris to drought both in laboratory and in field conditions. Umbilicus rupestris, wall pennywort, CO₂ exchange, Crassulacean acid metabolism, drought, stomatal conductance, water stress     

Effect of red light pulse on induction and production of flowers in a short-day duckweed, Lemna paucicostata 6746, in darkness

Flowering (number of flowers) of a short-day duckweed, Lemnapaucicostata 6746, in continuous darkness at 26^?C was affectedby a red light pulse in various ways depending on the time ofapplication. A conspicuous inhibition and a slight promotionwere respectively caused by the pulse given at the 7th and 19thhours of the dark period. Of the recently introduced floral parameters (4), a (vegetativegrowth rate) and (flowering ratio) were almost unchanged bythe pulse given at any time. P₁ (pre-flower induction period)was extended by one day when the pulse was given at about the7th hour of the dark period. The pulse greatly extended P₂ (flowerinduction period) when given at about the 7th hour of the darkperiod. A pulse given earlier or later was increasingly ineffectiveon P₂. P₄ (flower production period) changed rhythmically (i.e.,was extended or shortened) with the time of the red light pulse,the maximum extension and shortening being induced by the pulsegiven at about the 7th and 19th hours, respectively. Differenttiming mechanisms were suggested as controlling the sensitivitiesto the red light pulse of P₁ and P₂ or P₄. The floral response (number of flowers) vs. the red light pulseapplication time curve was explained in terms of the sum ofthe responses of P₂ and P₄ to the pulse. Floral parameters P₁and P₂ were defined more clearly. (Received September 4, 1978; )     

Diurnal changes in malate-decarboxylation activity in the leaves of a CAM plant, Bryophyllum daigremontianum

Bryophyllum diagremontianum plants grown under light-dark regimeswere exposed to one more cycle of the regime or to continuousdarkness for 24 hr. Photosynthetic O₂ evolution by leaf segmentsfrom these plants was investigated in the presence of 15 mMNaHCO₃ (CO₂-dependent O₂ evolution) or in the absence of CO₂(malate-dependent O₂ evolution). The malate-dependent O₂ evolutionserved as an index of the activity of malate decarboxylation.Malate content was respectively 67, 64 and 85 µmoles/g.fwin leaves measured at 7 hr 30 min in light and 6 hr 26 min inthe dark from plants under the light-dark regime (light 12 hr/dark12 hr) and those measured at 6 hr 26 min in the dark from plantsunder the continuous dark regime. The malate- and CO₂-dependentphotosynthetic O₂ evolutions in the same leaves were 9.7 and22, 0.2 and 17, and 16 and 26 µmoles/g.fw.hr, respectively.Thus, the diurnal change in capacity for malate-dependent O₂evolution was relieved by continuous dark treatment. These results suggest that the diurnal change in malate decarboxylationin this crassulacean acid metabolism plant does not occur byan endogenous rhythm. This further indicates lack of an endogenousrhythm for the influx-efflux of malate across the vacuole andin malate decarboxylation enzyme activity. (Received August 1, 1979; )     

Diurnal Geotropic responses of the Primary Leaves of Phaseolus vulgaris

When seedlings of Phaseolus vulgaris with leaves in the daytimeposition (almost horizontal to the ground) were turned upside-downduring the light period, their leaves moved upward away fromthe ground after about 20 min and ceased moving after about1.5 h. But when seedlings with leaves in the night time position(directed downward) were turned upside-down, their leaves moveddownward toward the ground after about 30 min and stopped movingabout 2 h later. Thus, Phaseolus primary leaves showed positiveor negative geotropic responses that correspohded to the darkor light period. This geotropic response of primary leaves was accompanied bythe redistribution of K⁺, Cl^– and NO₃- in the laminarpulvinus. These facts suggest that the circadian endogenousclock that is assumed to exist in Phaseolus vulgaris has atleast two regulation echanisms; one which measures time andanother which determines leaf postition in relation to gravityby changing the ion distribution in the pulvinus (Received February 12, 1983; Accepted May 17, 1983)     

14CO2 Pulse-Chase Labelling in Clusia minor L.

Conditions and maintenance of growth were chosen so that plantsof Clusia minor L. were obtained which showed the C₃- and CAM-modes of CO₂-exchange, respectively. C. minor is known to accumulateconsiderable amounts of citric acid in addition to malic acidduring the dark-phase of CAM. ¹⁴CO₂-pulse-chase experiments were performed with these plants.Patterns of labelling during the pulse and redistribution oflabel during the chase in the C₃-mode were as expected for C₃-photosynthesis.Pulse-labelling in the CAM-mode during the last hour of thelight period, during the first part of the dark period and duringthe last hour of the dark period always led to an almost exclusiveincorporation of label into malate. Redistribution of labelfrom malate after the pulse at the end of the dark period duringthe chase in the subsequent light period followed the patternexpected for light-dependent reassimilation of CO₂ remobilizedfrom malate in CAM during the light period. During the chasesin the dark period, label was transferred from ^l4C-malate tocitrate. This suggests that during accumulation of citric acidin the dark period of CAM in C. minor, citrate is synthesizedin the mitochondria from malate or oxaloacetate after formationof malate via phosphoenolpyruvate carboxylase. The experiment also showed that no labelled compounds are exportedfrom leaves in the CAM-mode during the dark period. In plantsof the C₃-mode the roots proved to be strong sinks. Key words: Clusia minor, labelling, pulse-chase, ¹⁴CO₂     

The Effect of Salinity on Light and Dark CO2-Fixation of Salt-Adapted and Unadapted Cell Cultures of Atriplex and Tomato

The effect of salinity on light and dark CO₂,-fixation was determinedin cells of A triplex portulacoides and tomato (Lycopersiconesculenturn Mill.) grown in culture. CO₂,-fixation of tomatocells was also determined in cultures adapted to mannitol andpolyethylene glycol (PEG). Salinity up to 400 mM NaCI in thecase of A triplex and up to 50 mM in the case of tomato enhancedthe rate of light-induced CO₂,-flxation in unadapted cells.Higher salt concentrations led to a marked decline in CO₂-flxationin both species. In salt-adapted A triplex cells no declinein the rate of light CO²,-flxation was seen even at 500 mM NaCl.Dark CO₂,-fixation was approximately 40% and 80% of the lightfixation in control cell cultures of A triplex and tomato, respectively.No enhancement in dark CO₂,-flxation was seen as salinity wasincreased, but a decline was found at similar salt concentrationsthat decreased fixation in the light. Mannitol-and PEG-adaptedtomato cells fixed CO₂, at somewhat lower rates than the controlcells in the light but not in the dark. Key words: Salinity, CO₂-fixation, cell cultures, Atriplex, tomato     

Effects of n-Alkylamines on the Motility and Viability of Heterosigma akashiwo Cells

The cytotoxic effects of positively charged liposomes and theircomponents on Heterosigma akashiwo cells were investigated.Positively charged liposomes reduced cell motility and eventuallycaused cytolysis. Negatively charged and non-charged liposomeshad little effect on cell motility and/or cell viability. Damagewas also induced by a single application of some n-alkyl-amines.Among n-alkylamines tested, laurylamine (C₁₂) was most effectivein reducing motility and causing cytolysis of cells. The extentof the deleterious effects increased with increasing concentrationsof laurylamine and with the duration of treatment. The extentof damage to cells by laurylamine changed periodically duringthe cell cycle. The effects of laurylamine began to increaseat the forth hour of the light period and began to decreaseat the first hour of the dark period, under condition of 12hours of light and 12 hours of darkness. Laurie and myristicacids, which each have a carboxyl group in place of the aminogroup of the corresponding alkylamines, laurylamine and myristylamine,had little effect on the cells. (Received December 21, 1988; Accepted March 29, 1989)     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

Scavenging of Hydrogen Peroxide in Prokaryotic and Eukaryotic Algae: Acquisition of Ascorbate Peroxidase during the Evolution of Cyanobacteria

Ascorbate (AsA) peroxidase was found in six species of cyanobacteriaamong ten species tested. Upon the addition of H₂¹⁸O₂ to thecells of AsA peroxidase-containing cyanobacteria, ¹⁶O₂ derivedfrom water and ¹⁸O₂ derived from H₂^I8O₂ were evolved in thelight. The evolution of ¹⁶O₂ was inhibited by DCMU and did notoccur in the dark, but ^I8O₂ was evolved even in the dark orin the presence of DCMU. Similar light-dependent evolution of¹⁶O₂ was observed in the cells of AsA peroxidase-containingEuglena and Chlamydomonas. However, the cells of AsA perox-idase-lackingcyanobacteria evolved only ¹⁸O₂ in either the light or dark.Furthermore, the quenching of chlorophyll fluorescence inducedby hydrogen peroxide was observed only in the cells of the AsAperoxidase-containing Synechocystis 6803, and not in the cellsof Anacystis nidulans which lacks AsA peroxidase. Thus, cyanobacteriacan be divided into two groups, those that has and those thatlacks AsA peroxidase. The first group scavenges hydrogen peroxidewith the peroxidase using a photoreductant as the electron donor,and the second group only scavenges hydrogen peroxide with catalase. (Received July 23, 1990; Accepted October 18, 1990)     

An Investigation into the Stomatal Behaviour of a Wilty Mutant of Pisum sativum

The water relations and stomatal behaviour of a wilty line ofpea (JI 1069) were investigated and compared with those of severalnon-wilty lines (JI 1180, JI 1194, and JI 74). The leaves ofthe wilty line were found to have a lower percent water content,water potential and diffusive resistance and the dimensionsof the stomatal cells were larger than those of the non-wiltytypes. The aperture of stomata on epidermal samples taken from plantsin the light or dark period of a diurnal rhythm was consistentlylarger for the wilty pea than for the non-wilty lines, however,their stomatal responses on detached epidermis to light, CO₂and KC1 concentration were similar. There was no differencein response to ABA of stomata on detached epidermis of wiltyor non-wilty types of pea. Key words: Pisum sativum, Wilty mutant, Water relations, Stomatal behaviour     

Diurnal regulation of NO3- uptake in soybean plants III. Implication of the Dijkshoorn-Ben Zioni model in relation with the diurnal changes in NO3- assimilation

According to the Dijkshoorn-Ben Zioni model, NO₃^– uptakein the roots is stimulated by NO₃^– assimilation in theshoots, through downward phloem transport of malate synthesizedin response to reduction of NO₂^– to NH³. In this paper,one hypothesis resulting from this model was tested, i.e. thatthe diurnal changes in NO₃^– uptake are due to the lightdependence of NO₃^– reduction in the leaves. This dependencewas studied in detached leaves transferred to deionized wateror supplied via the transpiration stream with similar amountsof ¹⁵NO₃^– in light or darkness. In the dark, the reductionof previously stored NO₃^– or xylem-borne ¹⁵NO₃^–was generally about 40–50% of that measured in the light.Glucose supply to the detached leaves stimulated NO₃^–reduction in the dark, but not enough to increase it up to thesame rate as in the light. Nitrite reduction in detached leaveswas much less affected by darkness, and could be maintainedat a high level by exogenous supply of substrate. Advantagewas taken from this last observation to sustain NO₂^–reductionin attached darkened shoots at the same rate as in the light,by ensuring an appropriate delivery of NO₂^– from the xylem.Although this was assumed to restore the light level of theassociated synthesis of malate, it led to a marked inhibitionof NO₃^– uptake. In addition, the direct supply of malateto the shoots or to the roots failed to prevent the decreaseof NO₃^– uptake in darkness. Thus, our conclusion is thatthe mechanisms evoked in the Dijkshoorn-Ben Zioni model do notplay an important role in the diurnal variations of NO₃^–uptake in soybean plants. Key words: Glycine max, light/dark cycle, malate synthesis, NO₃^– reduction, NO₃^– uptake     

A Quantitative Study of Daily Variation in the Cellular Ultrastructure of Palisade Chlorenchyma from Sunflower Leaves

Stereology was used to describe cytological changes which occurin palisade cells of fully expanded leaves as part of theirnormal daily activity. These changes were evaluated by describingthe relationship between organelle volume and cell volume asratio values (i.e. percentage volumes, V_v; surface-to-volume,S_v). These ratios describe an average cell in terms of its volumecommitment to each organelle compartment. Cells were also describedin terms of actual volume (µm³) or surface area (µm²)of membrane present in an average cell. ANOVA-LSD and Mann-Whitneystatistics indicate significant changes occur in the ratio valuesof the vacuole, chloroplast, oil, starch and microbody compartmentsover the 24 h period. This indicates a re-allocation of cellspace to these compartments during this period. The S_v ratioof internal membranes of the chloroplast and mitochondria showedno significant change over 24 h indicating that there is a constantrelationship between volume and membrane surface area in theseorganelles. Significant changes occurred in average cell volumeover 24 h with maximum volume during the dark period. Sincechanges in cell volume affected the actual volume expressionof all of the organelle compartments there were diurnal variationin the actual size of these compartments, including the internalmembranes of chloroplasts and mitochondria which more than doubledin surface area. Helianthus annuus L, sunflower, cytology, stereology, quantitative microscopy, diurnal, morphometrics, ultrastructure, chlorenchyma, chloroplast, mitochondria, microbodies     

Diurnal mixed layers and the long-term dominance of Microcystis aeruginosa

The population dynamics of the cyanobacterium Microcystis aeruginosain a hypertrophic, subtropical lake (Hartbeespoort Dam, SouthAfrica) were followed over 4.5 years. We examined the hypothesisthat M.aeruginosa dominated (>80% by volume) the phytoplanktonpopulation up to 10 months of each year because it maintaineditself within shallow diurnal mixed layers and was thus ensuredaccess to light, the major limiting resource. Wind speeds overHartbeespoort Dam were strong enough to mix the entire epilimnionthrough Langmuir circulations only 12% of the time. At othertimes solar heating led to the formation of diurnal mixed layers(z₁) that were shallower (<2 m) than the euphotic zone (z_cu;mean = 3.5 m, range: 0.45–8.4 m) while the seasonal mixedlayer (z_m) was always deeper than z_cu (range: 7–18 m).By means of its buoyancy mechanism M.aeruginosa maintained thebulk of its population within z₁, while non-buoyant speciessank into dark layers. Adaptation to strong light intensitywas implicated from low cellular chlorophyll a content (0.132µg/106 cells) and high I_k (up to 1230 µE m^–2s^–1). Ensured access to light, the post-maximum summerpopulations persisted throughout autumn and winter, despitesuboptimal temperatures, by sustaining low losses. Increasedsedimentation losses caused a sharp decline of the populationat the end of winter each year, and a short (2–3 months)successional episode followed, but by late spring M.aeruginosawas again dominant. The data from Hartbeespoort Dam point outthe importance of distinguishing between z_m and z₁, and showthe profound effect that the daily pattern of z₁ as opposedto the seasonal pattern of z_m, can have on phytoplankton populationspecies composition.     

Measurement of the Carbon Dioxide Compensation Point and the Rate of Loss of 14CO2 in the Light and Dark in Some Bryophytes

The CO₂ compensation point at 25 °C and 250 µEinsteinsm^–2 s^–1 wasmeasured for 27 bryo-phyte species, andwas found to be in the range of 45–160 µl CO₂ I^–1air. Under the same conditions Zea mays gave a value of 11 µlI^–1 and Horde um vulgare 76 µI^–1. The rate of loss of photosyntheticallyfixed ¹⁴CO₂ in the light and dark in six bryophytes (three mosses,two leafy liverworts, one thalloid liverwort) was determinedin CO₂-free air and 100% O₂. The rate of ¹⁴CO₂ evolution inthe light was less than that in the dark in CL₂-free air, butin 100% O₂ the rate in the light increased, so that in all butthe leafy liverworts it was greater than that in the dark. Raisingthe temperature tended to increase the rate of 14CO₂ evolutioninto CO₂-free air both in the light and dark, so that the light/dark(L/D) ratio did not greatly vary. The lower rate of loss of¹⁴CO₂ in the light compared tothe dark could be due to partialinhibition of ‘dark respiration’ reactions in thelight, a low rate of glycolate synthesis and oxidation, or partialreassimilation of the ¹⁴CO₂ produced, or a combination of someor all of these factors.