Periodicity in cell division and physiological behavior of ditylum brightwellii,a marine planktonic diatom,during growth in light-dark cycles

Summary Cells of Ditylum brightwellii, a large marine centric diatom, were partially synchronized by employing an appropriate light-dark cycle. At 20°C this consisted of 8 hrs of illumination at an intensity of 0.05 cal/cm² min. A single 2.8 l culture was studied over a 20 day period by diluting the culture daily to a standard cell concentration. The sequence of events in cell development was as follows: daughter cells were formed late in the light period, in the dark they elongated and the numerous chromatophores began dividing. A minimum cell buoyancy was observed in the dark concurrent with cell elongation. Increase in cell phosphorus took place in the dark period. The photosynthetic rate of cells removed during the dark period decreased to a minimum. In the following light period photosynthetic rate increased to a maximum, photosynthetic pigments, cell carbon, nitrogen, and carbohydrate increased and cell division again took place. Cell silica content increased concomitant with cell division. Details of cell morphology during cell division, based upon light microscopy, are reported.Contribution of the Scripps Institution of Oceanography.     

Studies on the biochemistry and fine structure of silica shell formation in diatoms

Summary Cell division in Navicula pelliculosa (Bréb.) Hilse, strain 668 was synchronized with an alternating regime of 5 h light and 7 h dark. Cell volume and dry weight increased only during the light period. DNA synthesis, which began during the third h of light, was followed sequentially by mitosis, cytokinesis, silicic acid uptake, cell wall formation, and cell separation. Silicification and a small amount of net synthesis of DNA, RNA and protein occurred during the dark at the expense of carbohydrate reserves accumulated during the light period. Cells kept in continuous light, after synchronization with the light-dark regime, remained synchronized through a second division cycle; the sequence of morphological events was the same as that in the light-dark division cycle, but the biosynthesis of macromolecular components changed from a stepwise to a linear pattern. The silicon-starvation synchrony was improved by depriving light-dark synchronized cells of silicic acid at the beginning of their division cycle, then resupplying silicic acid to cells blocked at wall formation.Abbreviation L light - D dark Portions based on a thesis submitted by W.M.D. to the University of California, San Diego in partial fulfillment of the requirements for the PH.D degree     

Synchronous Growth and Plastid Replication in the Naturally Wall-less Alga Olisthodiscus luteus

Olisthodiscus luteus is a unicellular biflagellate alga which contains many small discoidal chloroplasts. This naturally wall-less organism can be axenically maintained on a defined nonprecipitating artificial seawater medium. Sufficient light, the presence of bicarbonate, minimum mechanical turbulence, and the addition of vitamin B₁₂ to the culture medium are important factors in the maintenance of a good growth response. Cells can be induced to divide synchronously when subject to a 12-hour light/12-hour dark cycle. The chronology of cell division, DNA synthesis, and plastid replication has been studied during this synchronous growth cycle. Cell division begins at hour 4 in the dark and terminates at hour 3 in the light, whereas DNA synthesis initiates 3 hours prior to cell division and terminates at hour 10 in the dark. Synchronous replication of the cell's numerous chloroplasts begins at hour 10 in the light and terminates almost 8 hours before cell division is completed. The average number of chloroplasts found in an exponentially growing synchronous culture is rather stringently maintained at 20 to 21 plastids per cell, although a large variability in plastid complement (4-50) is observed within individual cells of the population. A change in the physiological condition of an Olisthodiscus cell may cause an alteration of this chloroplast complement. For example, during the linear growth period, chloroplast number is reduced to 14 plastids per cell. In addition, when Olisthodiscus cells are grown in medium lacking vitamin B₁₂, plastid replication continues in the absence of cell division thereby increasing the cell's plastid complement significantly.     

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.     

Photoperiodic Regulation of Cell Division and Chloroplast Replication in Heterosigma akashiwo

Cell division and chloroplast replication in Heterosigma akashiwo(Hada) Hada occurred as separate synchronous events during thecell cycle when cells were subjected to light-dark regimes.Under three different photoperiodic cycles of 10L/14D (10 hlight/14 h dark), 12L/12D or 16L/8D, cell division began athour 19–20 and finished at hour 23–26 after theonset of the light period, while chloroplast replication beganat hour 20–22 after the onset of the dark period. Almostall the cells divided only once in the 12L/12D cycle. The rateof increase in chloroplast number during one light-anddark cyclewas always equal to that in cell number in every photoperiodexamined. Light was essential for both cell division and chloroplast replication,but the minimum light period necessary for each event differed.When the light period was shorter than 6 h, no cell divisionoccurred; when it was shorter than 3 h, no chloroplast replicationoccurred. (Received February 26, 1987; Accepted June 17, 1987)     

Synchronous division and the pattern of diel vertical migration of Heterosigma akashiwo (Hada) Hada (Raphidophyceae) in a laboratory culture tank

Heterosigma akashiwo (Hada) Hada (Raphidophyceae) causes red tides in Osaka Bay (Japan). A clonal culture of the alga was grown in a 2 m tall culture tank on a 12: 12 LD cycle to determine patterns of vertical migration and cell division. A specific growth rate of 0.43 ln unit · day⁻¹ was obtained during complete mixing conditions. Under weakly stratified conditions (≈ΔT = 3–4°C/1.5 m), H. akashiwo in the tank grew and showed a similar pattern of vertical migration to that observed in the field for at least 6 days. Cell concentration, mean cell volume, and photosynthetic capacity, estimated by DCMU-induced fluorescence increase of H. akashiwo, were monitored in the stratified tank at 2-h intervals over 24 h at three levels in the water column. Cell ascent began shortly before the light period and vertically swimming cells were smaller in size than those sampled near the bottom of the tank. The cell division cycle and the pattern of vertical migration were phased individually by the light regime and were well synchronized with each other. This synchrony must be due to the interrelation between these two processes or the existence of a clock which controlled endogenous rhythms of both processes and was entrained by a light: dark cycle. The relative increase of fluorescence with DCMU was higher for migrating cells than for non-migrating cells.     

Light regulation of the cell cycle in Euglena gracilis bacillaris

We have studied the light regulation of the cell division cycle in the photosynthetic alga Euglena gracilis bacillaris. Euglena grown under phototrophic conditions are easily synchronized to a 12 h light-12 h dark regime. By inoculating stationary phase, nondividing cells into fresh media and exposing the diluted cells to either light or darkness, we have determined that initiation of DNA synthesis for the cell division cycle is light dependent. By varying the length of time in light to which synchronized cells are exposed, we have shown that commitment to the cell cycle requires exposure to more than 6 h of light. We propose that this is to allow the accumulation, through photosynthetic electron transport, of an initiating factor that will enable DNA synthesis to begin. Flow cytometry analysis also shows that once cells are committed to the cell cycle, they complete the cycle in the dark, so mitosis is a light-independent step.     

Regulation of growth and photosynthesis by Oscillatoria agardhii grown with a light/dark cycle

Abstract The cyanobacterium Oscillatoria agardhii was grown in turbidostat cultures with the light energy supply in either the continuous mode or in the pulsed mode (8/16 h light/dark (L/D) cycle). The light irradiance value used was sufficient to allow the maximal growth rate to be attained, when supplied continuously. Adaptation of O. agardhii to the L/D cycle was characterized by an increase in pigment content and photosynthetic performance, accompanied by a decrease in growth rate. This mode of adaptation resembled the adaptation of O. agardhii to continuous low light intensities. It is suggested that in this case the L/D cycle provokes this adaptation in order to allow the cells to accumulate carbohydrate rapidly during the light period. This was attributed to the storage of polyglucose, which served as a carbon and energy source for growth in the dark. The utilization of polyglucose in the dark was able to sustain the synthesis of all other cell components at the same rate as when cells were growing in the light. The growth yield in the dark, whilst metabolizing internally stored polyglucose, was 0.52 g cell C/g polyglucose C, or 0.62 g cell dry weight/g polyglucose. Although in the pulsed mode there is a 66% loss in light irradiance per 24 h when compared with a continuous light regime, the growth rate of the cyanobacteria grown in the pulsed mode was only 35% lower than the growth rate of a culture grown in continuous light. This can be explained by a high growth yield in the dark and by increased CO₂ fixation rates in the light of cells grown in the pulsed mode.     

Suppression of circadian rhythms of pineal and testicular hormones following lithium treatment in normal and reversed light-dark cycles, constant light and constant dark in rats

The aim of the current investigation was to study the effect of lithium on circadian rhythms of pineal - testicular hormones by quantitations of pineal and serum serotonin, N-acetylserotonin and melatonin, and serum testosterone at four time points (06.00, 12.00, 18.00 and 24.00) of a 24-hr period under normal photoperiod (L:D), reversed photoperiod (D:L), constant light (L:L) and constant dark phase (D:D) in rats. Circadian rhythms were observed in pineal hormones in all the combinations of photoperiodic regimens, except in constant light, and in testosterone levels in all the photoperiodic combinations. Pineal and serum N-acetylserotonin and melatonin levels were higher than serotonin at night (24.00 hr), in natural L:D cycle, in reversed L:D cycle or similar to normal L:D cycle in constant dark phase, without any change in constant light. In contrast, testosterone level was higher in light phase (12.00 hr through 18.00 hr) than in the dark phase (24.00 hr through 06.00 hr) in normal L:D cycle, in reversed L:D cycle, similar to normal L:D cycle in constant dark (D:D), and reversed to that of the normal L:D cycle in constant light (L:L). Lithium treatment (2 mEq/kg body weight daily for 15 days) suppressed the magnitude of circadian rhythms of pineal and serum serotonin, N-acetylserotonin and melatonin, and testosterone levels by decreasing their levels at four time points of a 24-hr period in natural L:D or reversed D:L cycle and in constant dark (D:D). Pineal indoleamine levels were reduced after lithium treatment even in constant light (L:L). Moreover, lithium abolished the melatonin rhythms in rats exposed to normal (L:D) and reversed L:D (D:L) cycles, and sustained the rhythms in constant dark. But testosterone rhythm was abolished after lithium treatment in normal (L:D)/reversed L:D (D:L) cycle or even in constant light/dark. The findings indicate that the circadian rhythm exists in pineal hormones in alternate light - dark cycle (L:D/D:L) and in constant dark (D:D), but was absent in constant light phase (L:L) in rats. Lithium not only suppresses the circadian rhythms of pineal hormones, but abolishes the pineal melatonin rhythm only in alternate light - dark cycles, but sustains it in constant dark. The testosterone rhythm is abolished after lithium treatment in alternate light - dark cycle and constant light/dark. It is suggested that (a) normal circadian rhythms of pineal hormones are regulated by pulse dark phase in normal rats, (b) lithium abolishes pineal hormonal rhythm only in pulse light but sustains it in constant dark phase, and (c) circadian testosterone rhythm occurs in both pulse light or pulse dark phase in normal rats, and lithium abolishes the rhythm in all the combinations of the photoperiod. The differential responses of circadian rhythms of pineal and testicular hormones to pulse light or pulse dark in normal and lithium recipients are discussed.     

The Loss of the Circadian Rhythm in Photosynthesis in an Old Strain of Gonyaulax polyedra

Cultures of Gonyaulax polyedra Stein maintained in the laboratory for 15 to 20 years, including an axenic strain isolated in 1960, have gradually lost the ability to survive in darkness. G. polyedra (70A), isolated in 1970 and maintained in a 12:12 light:dark cycle, now tolerates continuous darkness for a much shorter time than a strain isolated in 1981. I have compared the properties of strain 70A with those of this newer strain (81N), to investigate changes in Gonyaulax with length of time in culture, which may account for poor survival in darkness. When grown in continuous light (13, 12, or 4.5 watts per square meter), strains 70A and 81N have similar growth rates, yields, cell diameters, protein contents, C/N ratios, respiration rates, pigment complements, and photosynthetic rates. When entrained by a light:dark cycle (12L:12D), 70A showed no photosynthesis rhythm, although such a rhythm was formerly present. However, the circadian rhythms in bioluminescence and cell division were normal in both strains. Thus, the circadian clock is apparently still intact in 70A as in 81N. The rate of photosynthesis in strain 70A was constant at a low level, the consequent smaller accumulation of photosynthetic products probably accounting for the limited survival in darkness. The defect in strain 70A may be the loss of a component either directly affecting P_max or necessary for transduction from the circadian clock to photosynthesis.     

Inductive and Inhibitory Effects of Light on Cell Division in Chattonella antiqua

The cell division of a red tide flagellate, Chattonella antiqua,was synchronously induced under light and dark regimes of 10L14D(a light period, L, for 10 h followed by a dark period, D, for14 h), 12L12D and l4L10D. In all regimes cell number began toincrease ca. 14 h after the onset of L and almost doubled duringone LD cycle. When the light-off timing of the last L was changedor the whole L was shifted, cells that had been synchronizedunder 12L12D invariably began to divide ca. 14 h after the onsetof L. This shows that the timing of cell division was determinedby the time of the onset of L. When cells were continuously exposed to light after a cell division,the subsequent cell division was inhibited. This effect waslimited to cells that had been synchronized under short-dayconditions. Thus it can be concluded that light has both inductive and inhibitoryeffects on cell division in this alga, the latter effect dependingupon the previously given light and dark regimes. (Received December 21, 1984; Accepted February 28, 1985)     

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.     

CHANGES IN ENDOGENOUS CYTOKININ CONCENTRATIONS IN CHLORELLA (CHLOROPHYCEAE) IN RELATION TO LIGHT AND THE CELL CYCLE1

Endogenous cytokinins were quantified in synchronized Chlorella minutissima Fott et Novákova (MACC 361) and Chlorella sp. (MACC 458) grown in a 14:10 light:dark (L:D) photoperiod. In 24 h experiments, cell division occurred during the dark period, and cells increased in size during the light period. Cytokinin profiles were similar in both strains, consisting of five cis‐zeatin (cZ) and three N⁶‐(2‐isopentenyl)adenine (iP) derivatives. Cytokinin concentrations were low during the dark period and increased during the light period. In 48 h experiments using synchronized C. minutissima (MACC 361), half the cultures were maintained in continuous dark conditions for the second photoperiod. Cell division occurred during both dark periods, and cells increased in size during the light periods. Cultures kept in continuous dark did not increase in size following cell division. DNA analysis confirmed these results, with cultures grown in light having increased DNA concentrations prior to cell division, while cultures maintained in continuous dark had less DNA. Cytokinins (cZ and iP derivatives) were detected in all samples with concentrations increasing over the first 24 h. This increase was followed by a large increase, especially during the second light period where cytokinin concentrations increased 4‐fold. Cytokinin concentrations did not increase in cultures maintained in continuous dark conditions. In vivo deuterium‐labeling technology was used to measure cytokinin biosynthetic rates during the dark and light periods in C. minutissima with highest biosynthetic rates measured during the light period. These results show that there is a relationship between light, cell division, and cytokinins.     

Photosynthetic Activity during the Division Cycle in Synchronized Euglena gracilis

Axenic populations of the photosynthetic protozoan Euglena gracilis, grown with autotrophic nutrition, were synchronized with respect to cell division by culture on an alternating light-dark cycle. No cell divisions occurred in the light periods; approximately 100% of the cells divided in the dark periods. In such cultures, the synthesis of photosynthetic pigments and accumulation of polysaccharide were confined to the light periods. The capacity for photosynthesis, however, increased continuously over the entire light-dark cycle, and is thus not directly correlated with pigment content. A correlation was seen between photosynthetic capacity and protein content, suggesting that enzymatic mechanisms of the photosynthetic apparatus might be the limiting factor. Estimates of total photosynthetic activity indicate that about 5 x 10(-6) calories are required for the synthesis of a new cell.     

Studies on the vegetative life cycle of Chlamydomonas reinhardi Dangeard in synchronous culture I. Some characteristics of the cell cycle

Cells of Chlamydomonas reinhardi Dangeard were grown synchronouslyunder a 12 hr light-12 hr dark regime. Time courses of nucleardivision, chloroplast division, "apparent cytokinesis" and zoosporeliberation were followed during the vegetative cell cycle inthe synchronous culture. Liberation of zoospores occurred atabout 23–24 hr after the beginning of the light periodat 25°C. Four zoospores were produced per mother cell underthe conditions used. At lower temperatures, the process of zoosporeliberation as well as length of the cell cycle was markedlyprolonged, but the number of zoospores produced per mother cellwas approximately the same. At different light intensities,lengths of the cell cycle were virtually the same, while thenumber of zoospores liberated was larger at higher rather thanat lower light intensities. During the dark period, nuclear division, chloroplast divisionand apparent cytokinesis took place, in diis order, and proceededless synchronously than did the process of zoospore liberation.When the 12 hr dark period was replaced with a 12 hr light periodduring one cycle, the time of initiation as well as the durationof zoospore liberation was litde affected in most cases, whereasnuclear division, chloroplast division and apparent cytokinesiswere considerably accelerated by extended illumination. Whenalgal cells which had been exposed to light for 24 hr were furtherincubated in the light, zoospore liberation started much earlierand proceeded far less synchronously, compared with that under12 hr light-12 hr dark alternation. (Received October 12, 1970; )     

Regulation of photosynthetic carbon metabolism in synchronously growing Chlorella pyrenoidosa

The physical properties and photosynthetic metabolism of synchronizedcells of Chlorella pyrenoidosa are described. Cells, synchronizedby successive periods of light and dark, photosynthesized understeady-state conditions for 30 min with ¹⁴CO₂. Pool sizes ofmetabolic intermediate compounds, and rates of flow of carbonthrough these pools, were determined. Cell properties and metabolismwere studied for cells just divided, at three periods duringthe growing stage, at the time of maximum DNA synthesis, justprior to division (after continuous light, and after 5 hr darkness),and following division after continuous light for 37 hr. Changes in pool sizes and flow rates are correlated with relativeshifts between amino acid and protein synthesis, which is greatestduring the growing period and DNA-synthesizing stage, and sucrosesynthesis, which is greatest in the divided cells (after darkness)and pre-division cells (after darkness). The effects of thestage of cell growth and of a prior period of darkness can beseparated to some extent by these studies, and in some respectsare additive. Specific sites of metabolic regulation discerned in these experimentsinclude the following: 1) Ribulose diphosphate carboxylase,2) fructose diphosphatase and sedoheptulose diphosphatase, 3)the synthesis of sucrose, probably at the reaction between fructose-6-phosphateand uridine diphosphoglucose to give sucrose phosphate and uridinediphosphate, 4) amino acid synthesis, at the level of nitratereduction, and 5) amino acid synthesis, at the level of carbonflow from the photosynthetic carbon reduction cycle to aminoacid carbon skeletons. (Received October 31, 1969; )     

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.     

DIURNAL CELL DIVISION REGULATED BY GATING THE G1/S TRANSITION IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)1

The cell‐cycle progression of Enteromorpha compressa (L.) Nees (=Ulva compressa L.) was diurnally regulated by gating the G₁/S transition. When the gate was open, the cells were able to divide if they had attained a sufficient size. However, the cells were not able to divide while the gate was closed, even if the cells had attained sufficient size. The diurnal rhythm of cell division immediately disappeared when the thalli were transferred to continuous light or darkness. When the thalli were transferred to a shifted photoperiod, the rhythm of cell division immediately and accurately synchronized with the shifted photoperiod. These data support a gating‐system model regulated by light:dark (L:D) cycles rather than an endogenous circadian clock. A dark phase of 6 h or longer was essential for gate closing, and a light phase of 14 h was required to renew cell division after a dark phase of >6 h.     

Division of chloroplast nucleoids and replication of chloroplast DNA during the cell cycle ofDunaliella salina grown under blue and red light

Summary Synchronous cultures of the algaDunaliella salina were grown in blue or red light. The relationships between replication of chloroplast DNA, cell size, cell age and the number of chloroplast nucleoids were studied. The replication of chloroplast DNA and the division of chloroplast nucleoids occurred in two separate periods of the chloroplast cycle. DNA replication was concomitant with that in the nucleocytoplasmic compartment but nucleoid division occurred several hours earlier than nuclear division. Red-light-grown cells were bigger and grew more rapidly than those grown in blue light. In newly formed daughter cells, the chloroplast nucleoids were small and spherical and they were localized around the pyrenoid. During the cell cycle they spread to other parts of the chloroplast. The number of DNA molecules per nucleoid doubled during DNA replication in the first third of the cell cycle but decreased several hours later when the nucleoids divided. Their number was fairly constant independent of the different light quality. Cells grown in red light replicated their chl-DNA and divided their nucleoids before those grown in blue light and their daughter cells possessed about 25 nucleoids as opposed to 15.Abbreviations DAPI 4,6-diamidino-2-phenylindole - chl-DNA chloroplast DNA - PAR photosynthetically active radiation     

INTERACTIONS OF LIGHT/DARK CYCLES AND NITROGEN PULSES ON THE TIMING OF CELL DIVISION IN THE NITROGEN-LIMITED MARINE DIATOM CYLINDROTHECA FUSIFORMIS (BACILLARIOPHYCEAE)1

Cell division in most eukaryotic algae grown on alternating periods of light and dark (LD) is synchronized or phased so that cell division occurs only during a restricted portion of the LD cycle. However, the phase angle of the cell division gate, the time of division relative to the beginning of the light period, is known to be affected by growth conditions such as nutrient status and temperature. In this study, it is shown that the phase angle of cell division in a diatom, Cylindrotheca fusiformis Reimann and Lewin, is affected by the N-limited growth rate; cell division occurred later in the dark period (12:12 h LD cycle) when the growth rate was infradian (D = 0.42 d^?1) than when it was ultradian (D = 1.0 d^?1). Nitrogen-pulses did not affect the phase angle of the division gate, but could shift the time of peak cell division activity within the division gate. The effects, if any, of N-pulses were dependent upon the growth rate and the time of day that the pulses were administered. These responses indicate that the timing of cell division in this diatom is not determined solely by the zeitgeber from the LD cycle, but rather that a LD cycle control mechanism and a N-mediated control mechanism are both involved and are somewhat interdependent. In addition, an increase in protein was observed immediately after administering a N-pulse to C. fusiformis in the ultradian growth mode indicating that the accumulation of protein can be uncoupled from the cell division cycle.