INFLUENCE OF ENVIRONMENTAL FACTORS AND POPULATION COMPOSITION ON THE TIMING OF CELL DIVISION IN THALASSIOSIRA FLUVIATILIS (BACILLARIOPHYCEAE) GROWN ON LIGHT/DARK CYCLES1

Cell division patterns in Thalassiosira fluviatilis grown in a cyclostat were analyzed as a function of temperature, photoperiod, nutrient limitation and average cell size of the population. Typical cell division patterns in populations doubling more than once per day had multiple peaks in division rate each day, with the lowest rates always being greater than zero. Division bursts occurred in both light and dark periods with relative intensities depending on growth conditions. Multiple peaks in division rate were also found, when population growth rates were reduced to less than one doubling per day by lowering temperature, nutrients, or photoperiod and the degree of division phasing was not enhanced. Temperature and nutrient limitation shifted the timing of the major division burst relative to the light/dark cycle. Average cell volume of the inoculum was found to be a significant determinant of the average population growth rate and the timing and magnitude of the peaks in division rate. The results are interpreted in the context of a cell cycle model in which generation times are “quantized” into values separated by a constant time interval.     

Phasing effects of light on cell division in exponentially increasing cultures of Tetrahymena grown at low temperatures

Cultures of Tetrahymena pyriformis (W) grown at 10 ° ± 0.05 °C on proteose peptone-liver extract either in darkness or in relatively low light intensities exhibit marked differences in generation times during the exponential growth phase: about 20 h and 30 h, respectively. If a diurnal LD: 12, 12 or LD: 6, 18 light cycle was imposed on cultures that had been growing in the dark, cell division was phased so that the division bursts (each resulting in an approximate doubling of cell number) occurred once every 24 h and were confined primarily to the dark periods. Microscopic determination of the division index demonstrated that divisions virtually ceased during at least a portion of the light periods; indeed, a decrease in the index often anticipated the actual onset of light. Long trains of 24 h oscillations in apparent cell number could also be obtained in semicontinuous culture in LD: 6, 18. Furthermore, these entrained rhythmic division bursts persisted for at least 6 days with a circadian period if the culture was placed in constant darkness. These results are consistent with the hypothesis that an endogenous, self-sustaining circadian clock mechanism may underlie the observed persisting division rhythmicity as it does in many other microorganisms.     

Influence of light intensity on diel variations in rates of growth, respiration and organic release of a marine diatom: comparison of diurnally constant and fluctuating light

Effects of variations in light intensity on diel patterns ofgrowth, respiration and organic release of Skeletonema costatum(Grev.) Cleve in a cyclostat were evaluated. Light intensitywas either constant during the tight period at levels from 1500to 15 µEm^–2 s^–1 or fluctuated throughout thelight period from 500 to 10 µEm^–2 s^–1 at ratesof either 1 or 12 cycles day^–1. Periodicity in cell divisionwas observed only at light intensities of 130 Em^–2 s^–1and was decreased under diurnally varying tight. Under all lightconditions carbon and pigment growth were maximal during thelight period but well coupled throughout the 24 h period. Carbonassimilation during the dark period varied from 19 to 34% oftotal daily production and was a linear function of growth rate.Respiratory activity during the light period increased relativeto total daily respiration as growth rate increased. The increasein night-time carbon assimilation with growth rate interactedwith night-time respiration through the refixation of respiredcarbon, thus, influencing the pattern of respiratory loss ofcarbon. Rates of organic release (E^c) were maximal during thelight period and did variations consistently increased withtight intensity. Fluctuating light increased E^c relative toconstant light. Net growth efficiency was maximal at 130 µEm^–2s^–1 when cell division periodicity was greatest. Underother light conditions relatively higher rates of cell divisionoccorred at night and cell division periodicity was reducedas well as net growth efficiency. Cellular chemical fractionationindicated that under high or variable light conditions fixedcarbon was stored during the tight period for subsequent synthesisof protein and pigments, and division at night. Such an uncouplingof photosynthesis and other growth parameters resulted in greatermetabolic costs to the cell. ¹Present address: Marine Biology, Lamont Doherty GeologicalObservatory, Palisades, NY 10964, USA     

Models of coupled population oscillators using 1-D maps

 Coupled population oscillators are investigated with the use of coupled logistic maps. Two forms of coupling are employed, reproductive and density. Three biologically distinct situations are investigated: populations independently oscillating in a two point cycle, populations independently chaotic, and populations independently approach a stable point. Both entrained and phase reversed patterns are observed along with complicated forms of chaos as the coupling parameters are varied.     

X-Ray and Ultraviolet Sensitivity of Synchronously Dividing Escherichia coli

A paper pile filtration technique was used to obtain synchronously dividing populations of E. coli strains B and B/r from cultures in the exponential growth phase. Three generations of highly phased cell division were obtained by rapid pressure filtration which selected approximately 1 per cent of the exponentially growing culture. The sensitivity of E. coli strain B to x-ray and UV inactivation as a function of the cell division cycle was determined on synchronous populations. E. coli strain B showed a sharp decrease in sensitivity to inactivation by both radiations in the middle of the division cycle, and a further decrease near the end of the cycle. The sensitivity of E. coli strain B/r to x-irradiation was also investigated. Only the mid-cycle decrease in sensitivity was found during the division cycle of this strain. It was concluded that the repetition of the observed sensitivity patterns in both strains through the first three cycles after synchronization indicates that the same basic sensitivity patterns are probably also present in the individual cells of an exponential phase culture.     

Reoccurring neural stem cell divisions in the adult zebrafish telencephalon are sufficient for the emergence of aggregated spatiotemporal patterns

Regulation of quiescence and cell cycle entry is pivotal for the maintenance of stem cell populations. Regulatory mechanisms, however, are poorly understood. In particular, it is unclear how the activity of single stem cells is coordinated within the population or if cells divide in a purely random fashion. We addressed this issue by analyzing division events in an adult neural stem cell (NSC) population of the zebrafish telencephalon. Spatial statistics and mathematical modeling of over 80,000 NSCs in 36 brain hemispheres revealed weakly aggregated, nonrandom division patterns in space and time. Analyzing divisions at 2 time points allowed us to infer cell cycle and S-phase lengths computationally. Interestingly, we observed rapid cell cycle reentries in roughly 15% of newly born NSCs. In agent-based simulations of NSC populations, this redividing activity sufficed to induce aggregated spatiotemporal division patterns that matched the ones observed experimentally. In contrast, omitting redivisions leads to a random spatiotemporal distribution of dividing cells. Spatiotemporal aggregation of dividing stem cells can thus emerge solely from the cells’ history.

An interdisciplinary study of the rules governing cell divisions in a population of neural stem cells in the zebrafish brain reveals the existence of aggregated spatio-temporal division patterns of rapid cell cycles in stem cells, and shows that these patterns can be explained by a simple agent-based model relying solely on the cells‘ division history.     

A "Point of No Return" in the Cell Cycle of Chlorella

In a Chlorella culture growing synchronously at pH 6.3 undera 12 hr light/12 hr dark regime, DNA replication occurs betweenthe 8th and the 12th hour of the cycle, the main period of proteinand chlorophyll synthesis occurring between the 4th and 12thhour of the cycle. When the culture is transferred to alkalinepH at any time up to the 8 hr of the cycle, autospore releaseis prevented, and the pattern of synthesis of DNA, protein andchlorophyll is altered. However, when the culture is transferredto alkaline conditions after the 8th hour of the cycle, thepattern follows that of a culture growing at pH 6.3 with respectto cell number and volume, as well as protein, chlorophyll andDNA contents. Thus, a transition point seems to occur afterthe 8 hr of the cycle. The existence of such a point was alsodemonstrated by reciprocal experiments in which Chlorella wascultured at an alkaline pH and transferred to pH 6.3 at varioustimes in the cell cycle. ¹ Present address: Applied Research Institute, Ben-Gurion Universityof the Negev, P.O. Box 1025, Beer-Sheva 84110, Israel. (Received October 2, 1981; Accepted January 20, 1982)     

Wall structure of a bitunicate ascus

Summary Ultraviolet light-induced, bleached Euglena clones exhibit synchronous steps of cell division in response to daily cycles of light and dark. The cyclic division activity, in the bleached cells, will persist in constant lighting conditions with a period, independent of temperature, of about 24 h. This persisting rhythm of cell division supports the hypothesis that this phase of the cell cycle may be coupled to the fluctuations of the endogenous circadian clock of the cell.Newly isolated bleached clones are sensitive to light in their growth rates and metabolic characteristics, showing light induced difference in substrate-stimulated respiration, and production of the polyglucan, paramylon. After repeated subculturing of a bleached clone the photosensitivity of the metabolic characteristics and of the growth rate are diminished along with the ability to photo-entrain division synchrony. Division control and the induction of cell synchrony in this organism apparently involve both the temporal influence of the endogenous cell clock and one or more other photosensitive reactions in the metabolism of the cell.A unique culture mixing technique utilizing the bleached Euglena, failed to support the hypothesis of the involvement of intercellular communication in the maintenance of cell synchrony in constant lighting conditions.     

MEASUREMENT OF MICROALGAL CELL VOLUME BY FLOW CYTOMETRY

Single cell analysis by flow cytometry is a powerful tool that has been employed to identify many different characteristics of phytoplankton populations. Cell volume is an important physiological component of many cellular processes. We have used a Coulter EPICS XL flow cytometer to measure cell volume in the spheroid dinoflagellate Amphidinium operculatum as a function of forward scatter. Cell volume measurements of this alga were quantified as equivalent spherical diameters from a standard curve obtained with latex beads of known diameter. This parameter was used to monitor cell diameter throughout the cell division cycle. In log phase cultures, A. operculatum showed increasing cell volumes throughout the light phase and a maximum cell volume concurrent with the onset of cell division late in the light phase. The maximum equivalent spherical diameter measured 14 μm, while the minimum equivalent spherical diameter was 10 μm that occurred late in the dark phase. Stationary phase cultures of A. operculatum did not exhibit oscillating cell volumes throughout the diel cycle. Chemical inhibition of the cell cycle using 100 μM olomoucine diminished cell volume changes during the light phase. These results suggest a coupling of size control to the cell division cycle.     

The cell cycle of<Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>: the role of the commitment point

Chlamydomonas reinhardtii cells can double their size several times during the light period before they enter the division phase. To explain the role of the commitment point (defined as the moment in the cell cycle after which cells can complete the cell cycle independently of light) and the moment of initiation of cell division we investigated whether the timing of commitment to cell division and cell division itself are dependent upon cell size or if they are under control of a timer mechanism that measures a period of constant duration. The time point at which cells attain commitment to cell division was dependent on the growth rate and coincided with the moment at which cells have approximately doubled in size. The timing of cell division was temperature-dependent and took place after a period of constant duration from the onset of the light period, irrespective of the light intensity and timing of the commitment point. We concluded that at the commitment point all the prerequisites are checked, which is required for progression through the cell cycle; the commitment point is not the moment at which cell division is initiated but it functions as a checkpoint, which ensures that cells have passed the minimum cell size required for the cell division.     

The "Point of no Return" Concept in Cell Division. The Effects of Some Metabolic Inhibitors on Synchronized Chlorella pyrenoidosa

The coarse of growth and cell division in synchronized cultures of Chlorella pyrenoidosa was studied after the addition of metabolic inhibitors at differing times during the cell cycle (14 h light - 10 h darkness with nitrate as nitrogen source. 12 h light: 12 h darkness with urea as nitrogen source). Dinitrophenol (DNP) added to a final concentration of 0.3 mM at any time in the synchronization cycle, the compound remaining in the suspension from the time of addition to the end of the dark period, inhibited spore formation completely. Growth measured as increase in cell volume was less sensitive to the action of the inhibitor. Chloramphenicol (CAP) added dining the 0–5 h interval to a final concentration of 0.1 mM resulted in 80 per cent inhibition of cell division. Similar treatment started at successive times thereafter resulted in a gradual decrease of the inhibition. Treatment at the 14th hour and during the dark period did not affect the sporulation. Similar experiments with 0.9 mM puromycin added at various times during the illumination period gave almost complete inhibition of cell division, while the growth was reduced by only 25 per cent. para-Fluorophenylalanine (p-FPhe) at 3.3 × 10^?2 mM stopped cell division nearly completely irrespective of addition time in the light period. Addition during the dark period also prevented an increase in the number of tree cells. In this case about half of the cells produced spores which were not released. It is concluded that DNP inhibits all stages of preparation for cell division, as well as the division process itself. With CAP a genuine transition point of preparation for cell division was observed, although its interpretation as related to protein synthesis is somewhat uncertain. With puromycin and p-FPhe no transitions were observed.     

Commitment to Division in Ciliate Cell Cycles

Near the end of the cell cycle, ciliates commit irreversibly to cell division. The point of commitment occurs at the time of oral polykinetid assembly and micronuclear anaphase. The commitment is a checkpoint which requisites a threshold cell mass/ DNA ratio and stomatogenesis. It is also a physiological transition point, involving cdk protein kinases similar to those of other eukaryotes. Both P34 kD and P36 kD kinases, similar to the S. pombe cdc2 kinases, have been described to have activity as monomers. Subsequent to commitment to division, dramatic cytoskeletal modifications occur for separation of organelles, cortex morphogenesis and cytokinesis. Numerous mutants affecting cytoskeletal function associated with the division process have been obtained in several species. Of these, only the ccl mutant in Paramecium affects cell cycle progression prior to commitment to division. The material reviewed is used to speculate about the mechanisms of regulation of pre-fission morphogenesis and cell division related processes in ciliates.     

Application of the frequency of dividing cells technique to estimate the in situ growth rate of Microcystis (Cyanobacteria)

1. Diel patterns of the frequency of dividing cells (FDC) of the bloom‐forming cyanobacteria Microcystis were investigated using both culture strains and natural populations. 2. In laboratory experiments, diel division cycles were examined twice in a 24‐h light/dark cycle during time‐course batch incubations of six culture conditions using two strains (morphospecies) of Microcystis (M. aeruginosa and M. wesenbergii). While both strains clearly showed phased cell division in the light period during the logarithmic growth phase, the peaks of FDC became unclear towards the stationary phases. Some dividing cells were always found in the dark period regardless of whether or not division had paused at the same time. 3. This result implied the inadequacy of applying the model of McDuff & Chisholm [Limnology and Oceanography (1982) vol. 27 , pp. 783–787] directly to calculate the duration of cell division. Modified equations are proposed to calculate the duration of cytokinesis as a terminal event, in which the FDC values at night are regarded as 0% and all FDC values are subtracted by the minimum FDC value. 4. The diel FDC in natural populations of M. aeruginosa and M. wesenbergii were examined at five sites from a harbour to several distances offshore in Lake Biwa. While both species showed phased cell division patterns in the daytime at the harbour, no peaks in FDC were discernible in the samples taken from the offshore sites. These results strongly suggested that Microcystis growth was higher inshore than offshore. The in situ growth rates were estimated using the new equations.     

Timing of micronuclear mitosis and its relation to commitment to division inParamecium tetraurelia

This study examines the timing of micronuclear mitosis during the vegetative cell cycle and shows that mitosis begins early in the division process and coincides approximately with the earliest stages of oral morphogenesis (about 0.6 in the cell cycle in synchronous cell samples). The cc1 mutation blocks cell cycle progression prior to the point of commitment to division. Although the cc1 mutation blocks macronuclear DNA synthesis under restrictive conditions, it does not block micronuclear DNA synthesis. However, absence of functional cc1 gene product leads to blockage of micronuclear mitosis prior to completion of anaphase. This point coincides with commitment to division and is also the point at which oral morphogenesis is blocked in cc1 cells. The tim-ings of the transition points for micronuclear mitosis and oral morphogenesis in cc1 cells are closely associated in both synchronous cell samples and in asynchronous cultures. © 1992 Wiley-Liss, Inc.     

Turnover of Cell Wall Polysaccharides during the Cell Cycle in a Synchronous Culture of Catharanthus roseus

Pulse-chase experiments were done using a synchronous cultureof Catharanthus roseus in order to study cell wall turnoverduring the cell cycle. [¹⁴C]Glucose was fed for 1 h to cells35 and 49 h after the re-start of the cell cycle. Radioactivitywas then diluted with a large amount of cold glucose and chasedduring the early G1 phase after the first cell division, thetime at which an increase in the amount of cell walls mainlytook place. A pulse-chase with [¹⁴C]glucose was also made duringthe S phase when cell walls had not increased so much. Radioactivity of the EDTA-soluble (pectin) fraction decreasedduring the chase in the early G1 phase; whereas, the radioactivitiesof the other cell wall fractions, as well as extracellular polysaccharide(ECP) increased during the chase, both in the early G1 and inthe S phases. The radioactivity of uronic acid in ECP was higherin the early G1 phase than in the S phase. These results indicatethat an active turnover of pectin may take place in the earlyG1 phase after the first cell division. ¹ Present address and reprint requests: Biological Institute,Tohoku University, Sendai 980, Japan. (Received November 5, 1984; Accepted April 2, 1985)     

Effect of cell flotation on growth of Anabaena circinalis under diurnally stratified conditions

We tested the hypothesis that the growth rate of Anabaena circinalis,under diurnally stratified conditions, would increase as flotationvelocity increased owing to higher light availability. An insitu experiment compared the growth of diurnally stratifiedpopulations of A. circinalis with flotation velocities of 0.5and 1.0 m h^–1, with neutrally buoyant populations thatwere exposed to either mixed or persistently stratified conditions.The experiment was conducted in the turbid lower Murray Riverin South Australia (vertical attenuation coefficient = 4.52± 0.36 m^–1). To represent the mixing patterns,A. circinalis was contained in diffusion chambers that weremoved to different positions in the water column throughoutthe day. Diurnal populations with flotation velocities of 1.0and 0.5 m h^–1 grew at 0.23 ± 0.01 and 0.15 ±0.01 day^–1, respectively. Mixed populations grew at 0.19± 0.01 day^–1, whereas persistently stratified populationsgrew at 0.43 ± 0.01 day^–1. Results were used toextend a model that predicts growth of A. circinalis when exposedto the different mixing patterns. The model showed that bloomsare unlikely to be formed when the period of diurnal stratificationis <1 week, regardless of flotation velocity. When the diurnallystratified period is >1 week, flotation velocity is importantand a bloom may form depending on values assigned to the growthperiod and maximum mixed depth (Z_m).     

DIEL PERIODICITY OF PHOTOSYNTHESIS AND CELL DIVISION COMPARED IN THALASSIOSIRA WEISSFLOGII (BACILLARIOPHYCEAE)1

Diel patterns of photosynthesis and cell division were examined in Thalassiosira weissflogii Grun. (clone Actin) grown in nitrogen-limited cyclostat culture. Ammonia (NH₄+) was either supplied continuously or as a daily pulse to cultures grown in constant light or in a light: dark cycle. When either nitrogen or light was supplied periodically, both cell division and photosynthetic capacity were periodic. When both nitrogen and light were supplied periodically, cell division was coupled to the N-pulse whereas periodicities of photosynthetic capacity were modified but remained coupled to the light-dark cycle. Diel oscillations in photosynthesis were i) largely independent of cellular pigmentation and ii) similar for light-limiting and saturating irradiances. Periodicity in photosynthetic capacity also persisted following transfer of non-dividing batch cultures to constant light. Results suggest that photosynthesis but not cell division was coupled to a circadian clock in T. weissflogii. A circadian rhythm of photosynthesis may optimize carbon assimilation in phytoplankton exposed to intermittent nutrient supply by ensuring that maximum photosynthetic capacity occurs during the day.     

Light dependency of the mating process in Closterium acerosum

Sexual cell division and activation of gametangial cells forconjugation in Closterium acerosum were induced by light. L₂₀₀cells conjugated at maximum level under the following conditions;(i) a light intensity higher than 1,000 lux in a 16-hr lightand 8-hr dark regime and (ii) an illumination time longer than12 hr at 3,000 lux. L₂₀₀ cells also conjugated under continuousillumination at 3,000 lux. The action spectrum for the activation of gametangial cellshad peaks around 450, 611 and 665 nm. 3-(4'-Chlorophenyl)-l,l-dimethylurea (CMU) inhibited the accumulationof carbohydrates and sexual cell division at 10^–5 M andthe activation of gametangial cells for conjugation at 10^–4M. (Received August 15, 1977; )     

Excretion of glycolate during synchronous culture of Ankistrodesmus braunii in the presence of disalicylidenepropanediamine or hydroxypyridinemethanesulfonate

The rate of excretion of glycolate by the unicellular greenalga Ankistrodesmus braunii changes during its life cycle. Itis high in the main growth phase during the light period witha maximum 6 hr after the start of illumination, and low duringthe period of cell division in the dark. The glycolate excretion is stimulated by DSPD and HPMS, whilethe total ¹⁴CO₂-fixation is inhibited by DSPD and enhanced byHPMS. Changes in the effects of DSPD and HPMS on glycolate excretionas well as on photosynthetic ¹⁴CO₂-fixation during the courseof the algal life cycle were followed using the technique ofsynchronous culture. How far the change of glycolate excretion is due to a changeof glycolate oxidase activity during the life cycle and to achange of C₂-supply from the carbon reduction cycle is discussed.The effect of DSPD on glycolate excretion suggests a participationof ferredoxin in the glycolate pathway. (Received August 10, 1968; )     

DNA Synthesis and Cell Division During Spore Germination in Lygodium japonicum

This paper describes the ontogenetic sequence of cell divisionsand associated DNA synthetic patterns observed in sectionedspores of Lygodium japonicum (Thunb.) Sw., collected at differentstages of germination. Following exposure to a saturating doseof red light, the spore undergoes an asymmetric division toform a basal cell, which retains nearly all of the storage inclusions,and an apical cell which expands and protrudes from the rupturedsporoderm. Division of the apical cell results in formationof a protonemal cell and an intermediate cell. Subsequently,the latter cell divides to form the primary rhizoid and a wedgecell adjacent to the protonemal cell. Secondary rhizoids mayarise from later divisions of either the basal cell or the wedgecell. In addition, the wedge cell appears to have the capacityto form a secondary prothal-lial filament. Histochemical localizationof cell constituents indicates an increasing concentration ofcytoplasmic RNA and protein in the presumptive protonemal regionof the spore cell prior to division. Autoradiography of ³H–thymidineincorporation has shown that synthesis of nuclear DNA precedeseach cell division. Although strictly nuclear DNA synthesisoccurs during early stages of germination, extra-nuclear DNAsynthesis increases greatly following division of the sporecell. The results are discussed in relation to earlier studieson cell division patterns seen in whole mount preparations ofgerminating spores of different species of Lygodium. Lygodium japonicum, spore germination, cell division, DNA synthesis