Studies on the vegetative life cycle of Chlamydomonas reinhardi Dangeard in synchronous culture II. Effects of chloramphenicol and cycloheximide on the length of cell cycle

Cells of Chlamydomonas reinhardi Dangeard were synchronouslygrown under a 12 hr light— 12 hr dark regime. When thesecells were brought into contact with chloramphenicol for a shortperiod at early stages in the cell cycle, zoospore liberationwas delayed for a period which was nearly the same as that ofthe duration of contact with the antibiotic. When given at laterstages, the antibiotic caused no such effect. Cycloheximide,on the other hand, caused—when provided at some intermediatestage of the cell cycle— two different prolonging effectson the length of the cell cycle: one doubled the normal length(observed when the drug was administered at certain stages)and the other caused a delay similar to that caused by chloramphenicol.Interestingly, no prolonging effect was observed when cycloheximidewas given either at early stages or at later stages, such asduring the last 1/4 period of the cell cycle preceding zoosporeliberation. Based on these results, three phases were distinguishedin the algal cell cycle: "chloramphenicolsensitive", "cycloheximide-sensitive"and "insensitive" phases. Considering the known facts aboutthe modes of action of the two antibiotics inhibiting proteinsynthesis, discussions were made on the significance of proteinsynthesis in chloroplasts and in cytoplasm in determining thelength of the cell cycle. (Received October 12, 1970; )     

A short note on the effects of 6-methyl purine on the revolution of the cell cycle of Chlorella pyrenoidosa in synchronous culture

Short exposure of Chlorella pyrenoidosa cells to a high concentrationof 6-methyl purine atan early stage of the cell cycle causedan apparent "return-to-start" effectin which the cellsseemedto return to the starting point of a new cell cycle, with concurrentabortion of the cell cycle in process. (Received November 25, 1975; )     

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; )     

The Assimilation of Nitrate-nitrogen by Nitrogen-starved Cells of Platymonas (Tetraselmis) striata (Prasinophyceae)

Nitrate refeeding of nitrogen-starved cells of Platymonas striataresulted in approximately a doubling of average cellular nitrogenwithin 24 h. All the nitrate-nitrogen removed from the culturemedium could be accounted for as non-nitrate nitrogen withincells. Thus no significantly sized nitrate pool existed in Platymonasstriata and no assimilated nitrogen was lost from the cellsto the medium over the 48 h period studied. The slight fallin average cellular nitrogen which occurred from 24 to 36 hcould be attributed to cell division. The majority (70–80per cent) of the assimilated nitrate was recovered in the trichloroaceticacid (TCA)-insoluble fraction. There was some increase in thepercentage of nitrogen found in the TCA-soluble fraction duringthe period of most rapid nitrate assimilation (0–24 h).This presumably reflects an inability of the cells to assimilatelow-molecular-weight metabolic intermediates into macromoleculesat the same rate at which they were being formed. The majorityof the TCA-soluble fraction could be accounted for in termsof amino acids, purine and pyrimidine bases and ammonia. Cellswith adequate nitrogen nutrition seemed to maintain amino acidand purine + pyrimidine base nitrogen pools of about 0.8–0.9and 0.3–0.4 pg per average cell respectively. Algal amino acids, algal purine and pyrimidine bases, algal ammonia     

Studies on the vegetative life cycle of Chlamydomonas reinhardi Dangeard in synchronous culture V. The transition point for the effect of 6-methyl purine in the cell cycle

By following the nuclear division, chloroplast division, cytokinesisand zoospore liberation of Chlamydomonas reinhardi cells exposedto a high concentration of 6-methyl purine (6-MP), corroborativeevidence was obtained for our previous conclusion that 6-MP-exposedcells are brought to the starting point of a new round of thecell cycle with abortion of the cycle in process ("return-to-start"effect). This effect did not occur after cells had passed acritical stage (transition point) which seemed to be situatedshortly prior to the onset of nuclear division under the conditionsused. When 6-MP was applied to cells after the transition point,it caused an advancing effect on their zoospore liberation.A cycloheximide (CHI)-inhibition step existed shortly alterthe transition point for 6-MP. A model was proposed for theeffects of 6-MP and CHI. (Received August 8, 1977; )     

Environmental factors controlling the time of flower-opening in Pharbitis nil

Pharbitis nil, strain Violet, subjected to various photoperiods(24-hr cycle at 24?C) bloomed about 10 hr after light-off whenthe light period was 10 hr or longer, and about 20 hr afterlight-on when the light period was shorter. The higher the temperature(20–30?C) during the dark period, the later the time offlower-opening, with the temperature during the last half ofthe dark period having a stronger effect than that during thefirst half. In continuous dark or light, flower buds of Pharbitis openedabout every 24 hr at all temperatures tested between 20 and28?C, which suggests the participation of a circadian rhythmin determining the time of flower-opening. A light pulse given6–12 or 28–36 hr after the onset of the dark periodgreatly advanced the phase of this rhythm (8–10 hr). Phasedelay of this rhythm could not be obtained by light pulses givenat any time. (Received September 29, 1979; )     

An effect of zinc on M-phase and G1 of the plant cell cycle in the synchronous TBY-2 tobacco cell suspension

An analysis was undertaken of the effects of a toxic metal,zinc, on plant cell suspension cultures of the TBY–2 cellline of tobacco (Nicotiana tabacum cv. Bright Yellow 2) in orderto determine whether Zn acts in a cell cycle-specific manner.In the control treatment (0 Zn), following a 24 h synchronizationwith aphidicolin and 7 h after the release from the inhibitor,the mitotic index peaked at 45%. The inclusion of Zn in the24 h aphidicolin treatment (100, 200 or 300 µM Zn) resultedin a concentration–dependent decrease in the mitotic peakto 30%, 22% and 10%, respectively, but did not affect the timingof the peak. Hence, despite high concentrations of Zn, cellstraversed from S–phase to mitosis, albeit in smaller proportions,at the same rate as the controls. Cells treated with 0, 100or 200 µM Zn during synchronization and then releasedinto Zn–free media showed successive peaks in mitoticindex at 7 h and 21 h following release, i.e. Zn-treated cellsprogressed through a complete cell cycle at the same rate asthe controls. Synchronization and subsequent release into Zn–containingmedium (100 µM) examined the effect of the metal on predominantlylate G1 cells. In this treatment, the mitotic index peaked at7 h and 19 h, indicating a slightly faster cell cycle (12 h)compared with the control (14 h). Continuous exposure to 100µM Zn through both synchronization and release resultedin a cell cycle of 11 h and a differential effect on the componentphases: M–phase lengthened (1.5 h to 3.5 h) and G1 shortened(6 h to 1 h) compared with the control treatment. Vital staining (Evans Blue) revealed that cell mortality increasedfrom 2.7% (0 Zn) to 6.1% and 6.5% at 100 and 200 µM Zn,respectively. The Zn content of cells increased 40–lfoldfrom 0 to 100 µM Zn. The data are consistent with theeffects of Zn reducing the cycling cell population primarilythrough cell arrest rather than cell death, but also revealthat a substantial population of TBY–2 cells progressedthrough the cell cycle despite accumulating Zn. In particular,the duration of G2 and S-phase was remarkably invariant, clearlyindicating that once plant cells meet the requirements of lateG1 check-points, they are committed to divide, even in the presenceof toxic concentrations of Zn. The synchronous TBY–2 cell suspension, which lacks theheterogeneity and developmental constraints of plant meristems,is an excellent system to study the effects of known toxic metals,and indeed other environmental factors, on the plant cell cycle. Key words: Cell cycle, plant cell suspensions, Nicotiana tabacum, zinc, toxicity     

Growth and light absorption of some planktonic cyanobacteria, diatoms and Chlorophyceae under simulated natural light fluctuations

Specific growth rates of Limnozhrix redekei, Planktothrix agardhii(cyanobacteria), Synedraacus, Stephanodiscus minutulus (diatoms),Scenedesmus acuminatus and Scenedesmus armatus (Chlorophyceae)were compared under different time structures of illumination,but the same daily light exposure, at 20C. Fluctuating irradiancesimulating a uniform rapid transport of the algal cells acrossthe aquatic light field on a cloudless day with Z^eu/Z^mix=1 wascompared with constant irradiance throughout the same photoperiodof 12 h length as well as a photoperiod of 6 h length. Fluctuatinglight (30 min for a cycle) resulted in a decrease in specificgrowth rates as compared with constant irradiance at the samephotoperiod length. This decrease amounts to 15–20% fordiatoms, 20–25% for Chlorophyceae and 35–40% forcyanobacteria, respectively. The decrease is somewhat lowerif the fluctuations simulating mixing are slower (60 min fora cycle). The specific growth rate is also decreased by a shorterphotoperiod, but this effect is more species specific. Regardingthe in vivo absorption spectra, fluctuating light or a shorterphotoperiod has little or no effect on the Chlorophyceae anddiatoms studied, whereas cyanobacteria show an increase in lightabsorption by chlorophyll a and phycobilins.     

The action of skeleton photoperiods on flowering in Lemna perpusilla

The effects of 24 hr cycle skeleton photoperiodic schedulesinvolving two short light pulses on flowering in Lemna perpusillahave been studied. Simulation of complete photoperiods by correspondingskeletal ones is nearly perfect for all photoperiods up to 8hr and is unstable for periods of 9 to 13 hr. A jump in theresponse phase appears when skeleton photoperiods ranging from12 to 13hr are given. For all skeleton photoperiods longer than14 hr the phase is entrained so that it agrees with that givenby skeleton photoperiods of complemental lengths. That is, askeleton photoperiod of 18 hr is equivalent to that of 6 (=24–18) hr. Simulation is largely related to whether thesecond pulse is locked on to "dawn" or to "sunset" dependingon when it falls during the dark period following the firstpulse. The inductive action of skeleton photoperiods that gives unstableentrainment depends on the length of a preliminary dark periodgiven before the plant receives the first pulse, since in theseskeleton schedules the sensitive zone to the second pulse shiftswith the length of the preliminary darkness. Thus, we tentatively conclude that the circadian oscillationin L. perpusilla involves an entrainment mechanism and thatphotoperiodic induction is contingent on the coincidence oflight and a specific inductive phase in oscillation. (Received September 18, 1968; )     

Effect of Inorganic Phosphate on the Biosynthesis of Purine and Pyrimidine Nucleotides in Suspension-Cultured Cells of Catharanthus roseus

The levels of purine and pyrimidine nucleotides in suspensioncultures of Catharanthus roseus were determined 24 h after stationary-phasecells were transferred to fresh complete (‘+Pi’)or phosphate-deficient (‘–Pi’) Murashige-Skoogmedium. The levels of ATP, GTP, UTP and CTP were from approx.3 to 5-fold greater in the cells grown in ‘+Pi’medium than in the cells grown in ‘–Pi’ medium.The levels of almost all other nucleotides were slightly higherin the cells in ‘+Pi’ medium. The rates of de novoand salvage biosynthesis of purine and pyrimidine nucleotideswere estimated from the rates of incorporation of radioactivityfrom [¹⁴C]formate, [2–¹⁴C]glycine, NaH¹⁴CO₃, [6–¹⁴C]orotate,[8–¹⁴C]adenine, [8–¹⁴C]adenosine, [2–¹⁴C]uraciland [2–¹⁴C]uridine. The results indicated that the activityof both the de novo and the salvage pathway was higher in thecells in ‘+Pi’ medium than in the cells in ‘–Pi’medium. The rate of degradation estimated from the rate of releaseof ¹⁴CO₂ from labelled purines and pyrimidines indicated thatdegradation of uridine was significantly reduced in the cellsin ‘+Pi’ medium, but no significant difference wasfound in the degradation of adenine, adenosine and uracil. Thepossible role of Pi in the control of the biosynthesis of nucleotidesand in the degradation of uridine is discussed. Catharanthus roseus, Madagascar periwinkle, suspension culture, inorganic phosphate, nucleotides, purines, pyrimidines, biosynthesis, degradation     

The Effects of {delta}-Aminolevulinic Acid and Inhibitors of RNA and Protein Synthesis on Phytochrome Mediated Chlorophyll Accumulation in Sorghum bicolor

In 5-d-old etiolated seedlings of Sorghum bicolor, 12 h of darknessafter 5 min in red light eliminated a lag before the accumulationof chlorophylls in subsequent continuous white light. Increasingthe dark period to 24 h and 36 h, increased the rate of chlorophyllaccumulation in the later stages of greening. Exogenous -aminolevulinicacid neither completely removed the lag, nor increased the rateof chlorophyll accumulation. Cycloheximide (25 µg ml^–1)and 6-methyl purine (5.0 µg ml^–1), given continuouslyor only until the 12 h dark period following the red light irradiation,restored the lag and decreased the rate of chlorophyll accumulation.D-threo-chloramphenicol (400µg ml^–1) also decreasedthe rate of chlorophyll accumulation but did not restore thelag. Addition of these inhibitors even 12 h after red lightirradiation decreased the rate of chlorophyll accumulation.Rifampicin (Rifamycin SV, 400 µg ml^–1) did not havesuch effects. Key words: Chlorophylls, Phytochrome, -Aminolevulinic acid, Sorghum bicolor     

Effects of temperature, salinity and food level on the life history traits of the marine rotifer Synchaera cecilia valentina, n. subsp.

A strain of the marine rotifer Synchaeta cecilia valentina,n. subsp., isolated from the Hondo of Elche Spanish Mediterraneancoastal lagoon at 22 salinity, was cultured in the laboratoryin 20 ml test tubes and fed with the alga Tetrasemis suecica.The effect of two temperatures (20 and 24°C), four salinities(20,25,30 and 37) and two food levels (15 000 and 25000 cellsml^–1) on the life history traits of this rotifer werestudied in life tables performed with replicated individualcultures. Temperature and salinity had a significant negativeeffect (P < 0.001) on the average lifespan (LS) and on thenumber of offspring per female (R₀) The effect of food levelon LS is unclear, whereas R₀ is greater at 20°C with thelower concentration of algae and at 24°C with the higheralgal concentration. The maximum values of LS and R₀, 5.6 daysand 9.2 offspring per female, respectively, were recorded at20°C, 25o salinity and low food concentration. There isalso a clear negative effect on the intrinsic growth rate (r)due to salinity. The effect of temperature depends on the foodlevel and, as occurs with R₀ the maximum values of r occur withthe lower algal concentration at 20°C, whereas at 24°Cthey are obtained with the higher algal concentration. Theser values, from 1.04 to 1.10 day^–1, were reached at 24°C,salinities of 20–25 and with high food concentration.     

Studies on chloroplast development in Chlamydomonas reinhardtii I. Effect of brief illumination on chlorophyll synthesis

When dark grown cells of Chlamydomonas reinhardtii y-1 mutantwere exposed to continuous light, an immediate transformationof small amounts of protochlorophyll(ide), which had been presentin the dark grown cells, to chlorophyll was observed. Afterthis, there was a slow accumulation of chlorophyll lasting for2.5-3 hr before the start of exponential synthesis. Initialaccumulation of chlorophyll was distinctly slower at a highlight intensity (13,000 lux) than it was at moderate intensitiesof light (2,000–5,000 lux). However, the exponential synthesisof chlorophyll started after the same 2.5–3 hr of illumination. A brief pre-illumination of cells followed by incubation indarkness was effective in promoting chlorophyll synthesis undersubsequent continuous illumination at high, as well as moderatelight intensities. Pretreatment alleviated retardation of theinitial chlorophyll accumulation by light of high intensity.The promoting effect of preillumination on chlorophyll synthesiswas sufficient, even when a light impulse as short as 10 secwas given. However, the effect was dependent on length of thedark period after the short pre-illumination. The full extentof this effect was observed when the dark period was about 2.5–3hr long. Further dark incubation gradually decreased the effect. On the basis of these findings, it is assumed that a factor(s)responsible for promotion of chlorophyll (or chloroplast) synthesisin the process of greening of dark grown cells is produced duringthe dark period after a brief pre-illumination, and that thefactor is turned over at a relatively fast rate. The possiblenature of the presumed factor is discussed in relation to chloroplastdevelopment. ¹Present address: Department of Biology, Faculty of Science,Kobe University, Nada-ku, Kobe, Japan. (Received August 18, 1970; )     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. Part I. A comparative study of the growth-irradiance relationship of three marine phytoplankton species: Skeletonema costatum, Olisthodiscus luteus and Gonyaulax tamarensis

Three marine phytoplankton species (Skeletonema costatum, Olisthodiscusluteus andGonyaulax tamarensis) were grown in batch culturesat 15°C and a 14:10 L:D cycle at irradiance levels rangingfrom 5 to 450 µEinst m^–2 s^–1. At each irradiance,during exponential growth, concurrent measurements were madeof cell division, carbon-specific growth rate, photosyntheticperformance (both O₂ and POC production), dark respiration,and cellular composition in terms of C, N and chlorophyll a.The results indicate that the three species were similar withrespect to chemical composition, C:N (atomic) = 6.9 ±0.4, photo-synthetic quotient, 1.43 ± 0.09, and photosyntheticefficiency, 2.3 ±0.1 x 10^–3 µmol O₂ (µgChl a)^–1 h^–1 (µEinst m^–2 s^–1)^–1.Differences in maximum growth rate varied as the –0.24power of cell carbon. Differences in growth efficiency, werebest explained by a power function of Chl a:C at µ = 0.Compensation intensities, ranged from 1.1 µEinst m^–2s^–1 for S. costatum to 35 forG. tamarensis and were foundto be a linear function of the maintenance respiration rate.The results indicate that interspecific differences in the µ–Irelationship can be adequately explained in terms of just threeparameters: cell carbon at maximum growth rate, the C:Chl aratio (at the limit as growth approaches zero) and the respirationrate at zero growth rate. A light-limited algal growth modelbased on these results gave an excellent fit to the experimentalµ–I curves and explained 97% of the observed interspecificvariability. ¹Present address: Lamont-Doherty Geological Observatory Columbiaof University, Palisades, NY 10964, USA     

Salt Stress Causes Acceleration of Purine Catabolism and Inhibition of Pyrimidine Salvage in Zea mays Root Tips

Nucleotide metabolism was studied in apical 5.0 mm root tipsof corn plants (Zea mays L., cv. Pioneer 3906) hydroponicallycultured for 7 d and then salinized for 19 d at a rate calculatedto reduce the osmotic potential (_o) of the solutions by O.1MPad^–1 to a final _o = -0.4 MPa. Saline treatments withtwo different molar ratios of Ca₂₊/Na⁺ were employed, viz.,0–03 (2.5 mol m^–3 CaCl₂ + 86.5 mol m^–3 NaCl)for the NaCl treatment and 0.73 (31.5 mol m^–3 CaCl₂ +43.1 mol m^–3 NaCl) for the NaCl + CaCl₂ treatment. Bothsalt treatments reduced root growth by more than 30%. The capacityof roots to provide purine nucleotides either by de novo synthesisor by re-utilization of existing bases, e.g. salvage of hypoxanthineto adenine nucleotides, was not affected by either salt treatment.However, catabolism of hypoxanthine was accelerated more than3.5-fold by both salt treatments, demonstrating an increasedcapacity for purine catabolism which would shift the normal1: 1 ratio of synthesis: degradation of purine nucleotides observedfor the roots of healthy control plants to less than 0.2 duringsalt stress. The ratio of pyrimidine nucleotide synthesis: degradationwas also reduced. In this case, the unfavourable shift towardnucleotide degradation resulted because both salt treatmentsreduced salvage capacity by more than 25%, but had no compensatingeffect on de novo synthesis or catabolism of pyrimidines. Key words: Salinity, osmotic potential, nucleotide metabolism     

Assimilation numbers in cultures of marine phytoplankton

During exponential growth in batch culture, assimilation numbersof eleven algal species ranged from 1.6–20.8, with a meanvalue of 5.3 g C/g Chlorophyll a/hr. The highest assimilationnumber of 20.8 g C/g Chlorophyll a/hr was observed in Coccolithuspelagicus, due to the relatively low concentration of chlorophylla/cell. The assimilation number declined from exponential tostationary phase in batch cultures for ten algal species, butincreased with age in batch culture in Amphiprora paludasa (abenthic diatom). The assimilation number declined with decreasinggrowth rate in nitrate-limited chemostat cultures of Phaeodactylumtricornutum and in iron-limited chemostat cultures of Phaeodactylumtricornutum and Isochrysis galbana.     

The Effect of Adenine and Mevalonic acid on the Endogenous Cytokinins of a Cytokinin-dependent Strain of Soya Bean Callus

The combined application of 10^–6 M adenine and 10^–6M mevalonic acid to soya bean callus accelerated its growth.Two biologically active compounds that co-chromatographed withzeatin and isopentenyl adenine were extracted from this callus.Studies with labelled adenine and mevalonic acid indicated thatthe cytokinin-dependent soya bean callus incorporated only avery small amount of the radioactive precursors into the biologically-activecompounds, making it extremely difficult to determine whetherthese compounds were synthesized de novo or whether they aroseas by-products of tRNA turnover. As cytokinins do not accumulatein rapidly-growing cytokinin-dependent soya bean callus culturedon kinetin as a source of cytokinin it seems as if biosynthesisde novo occurs when the callus is supplied with adenine andmevalonic acid. Glycine max (L.) Merrill, soya bean, callus culture, adenine, mevalonic acid, endogenous cytokinins     

Cell Cycle Changes in the Shoot Apex of Silene coeli-rosa During the Second and Third Days of Floral Induction

The cell cycle in Silene coeli-rosa shoot apices was measuredto test whether or not early components of the floral stimulus,produced during the 2nd and 3rd long days (LD) of an inductiveLD treatment, resulted in an increase in the duration of G₂phase in constant 20–24 h cell cycles. Plants were grownat 20°C in short days (SD) of 8 h light and 16 h darknessfor 28 d (day 0). Starting on day 0, plants were given SD or3 LD each comprising an identical 8 h day and 16 h photo-extension,or 3 dark-interrupted (d.i.) non-inductive LD, interrupted at1700 h of each day with 1 h of darkness. The cell cycle (percentagelabelled mitoses method) and changes in cell number were determinedin the shoot apical meristem. During days 1–2 of the SDtreatment, the cell cycle and mean cell generation time (MCGT)was 18 and 32 h, respectively, giving a growth fraction of 56%.During days 2–3, the cell cycle and MCGT shortened to15 and 23 h, respectively (growth fraction = 65%). During days1–2 of the LD and d.i. LD treatments, cell cycles andMCGTs were 9–10 and 27–29 h, respectively, resultingin smaller growth fractions (about 33%). Thus, shortened cellcycles and altered growth fractions occurred regardless of whetheror not the treatment was inductive. The LD treatment resultedin a marked shortening of G₁ and, to a lesser extent, S-phase,whilst G₂ remained constant. These changes were consistent withincreases in the proportion of cells in G₂ during the photoextensionof each LD which were suppressed during the comparable periodsof the d.i. LD treatment. The latter treatment resulted in eachphase occupying virtually identical proportions of the cellcycle as in the SD treatment. Thus, the unique cell cycle responsesto the initial part of the inductive LD treatment were increasesin the proportion of cells in G₂ coupled with G₁ and G₂ beingof similar duration. Cell cycle, mean cell generation time, shoot apex, Silene coeli-rosa     

Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

Herbivorous nutrition of Cyclops vicinus: the effect of a pure algal diet on feeding, development, reproduction and life cycle

Nutritional requirements, functional response, development andreproduction of Cyclops vicinus were studied with exclusivelyalgal food. Phytoflagellates were found to be adequate foodresources for both juvenile development and egg production.Ingestion measurements were performed with Chlamydomonas reinhardii.The functional response data give evidence for low feeding efficiency,especially for the naupliar stages. A difference between naupliiand older instars was also found in their quantitative foodneeds. A higher threshold food concentration was observed fornaupliar development (0.4 mg C l^–1) than for copepoditedevelopment (0.2 mg C l^–1). The calculation of assimilationefficiencies suggests that the high food requirements are dueto low specific ingestion rates rather than poor assimilationefficiency. Development time decreased as algal density increasedand males developed more quickly than females at all food concentrations.Body size and carbon content increased with increasing foodconcentration. Continuous egg production was observed abovea food concentration of 0.5 mg C l^–1. The results haveimplications for the life cycle of C. vicinus. Summer diapauseis interpreted as a strategy to avoid starvation of the juvenilestages. The facultative herbivory of the adults might be anadvantage when competing against other more carnivorous cyclopoidcopepods. ¹Present address: Agricultural University of Wageningen, Departmentof Mathematics, Dreijenlaan 4, NL-6703 HA Wageningen, The Netherlands