Respiration and nitrogen fixation in nodulated roots of soya bean and pea

Summary Oxygen uptake, carbon dioxide evolution and nitrogenase activity, measured either as hydrogen evolution (under argon 80%, oxygen 20%) or as the reduction of acetylene to ethylene, were assayed over the same time period by a direct mass-spectrometric method. When carbon dioxide evolution was used to estimate carbohydrate consumption, the results agreed with other work on whole plants. The RQ values obtained in these experiments were always less than 1.0 and thus the carbohydrate consumption calculated from oxygen uptake suggests that previous estimates, using carbon dioxide evolution as a measure of the cost of nitrogen fixation may be underestimates. Lag periods observed in the reduction of acetylene to ethylene suggest that there is a resistance to diffusion of gases in the root nodules.     

A re-examination of the problem of photochemical nitrogen reduction by blue-green algae

Summary Although it was possible in the light in the absence of carbon dioxide to obtain a ratio of nitrogen fixed to oxygen evolved in nitrogen-starved cells of A. cylindrica near to 1:1.5, that quoted by other workers, ratios varying between 1:0.9 and 1:3.0 were also obtained. The amount of oxygen evolved under the same conditions by normal cells in the presence of pyruvate was increased considerably. Since the addition of pyruvate also resulted in increased carbon dioxide output in the dark with the same algal material, oxygen output in the light was attributed to the production of factors necessary for carbon assimilation.Addition of pyruvate to nitrogen-starved and normal cells in the light resulted in similar rates of oxygen evolution after an initially higher rate in the starved cells. The ratio of overall nitrogen fixed to oxygen evolved, was 1:6.6 for the starved cells and 1:6.4 for the normal cells, showing that the presence of an added substrate increased oxygen output relative to nitrogen uptake. ¹⁴CO₂ was recovered from sodium pyruvate-1-¹⁴C in flasks incubated in the dark, showing that, at least in the dark, pyruvate was decarboxylated.The interpretation of these results is that endogenous and exogenous substrates available to cells of A. cylindrica become decarboxylated and that, in the light, carbon dioxide produced may be assimilated photochemically with accompanying oxygen evolution. This interpretation has been discussed in relation to reports of photochemical nitrogen reduction in blue-green algae.     

The effect of nitrogen refeeding on the carbohydrate content into nitrogen-starved cells of Platymonas striata Butcher

Studies on the mean cellular carbohydrate contents of Platymonas striata Butcher under conditions of nitrogen-starvation, and after refeeding these starved cultures with either nitrate or ammonium ions (growing under continuous illumination or with an alternating light/dark regime) have shown that nitrogen-starved cells accumulated abnormal amounts of cellular carbohydrate and that nitrogen refeeding produced a marked drop in the cellular carbohydrate. Cells grown in a light/dark regime accumulated less carbohydrates than those grown in continuous light. The mean cellular carbohydrate levels 16 h after nitrogen refeeding were still much in excess of those of cells grown with normal nutrition. It was therefore suggested that the differences in nitrogen uptakes in this period — when comparing either the uptake of cells grown in continuous light with that of cells grown in a light/dark regime; or when comparing the uptakes of cells presented with either nitrate or ammonium ions and grown in a light/dark regime —cannot be directly due to shortages of carbohydrate for the provision of carbon skeletons for nitrogen assimilation.     

Physiological studies on nitrogen fixation in the blue-green alga anabaena cylindrica

Summary Short-term manometric experiments with bacteria-free cultures of Anabaena cylindrica showed that the close dependency of nitrogen fixation upon photosynthesis could be temporarily eliminated in nitrogen-starved cells. Initial rates of nitrogen uptake by these cells in the absence of carbon dioxide were equally rapid in the light and dark, decreasing and finally ceasing after two hours. Continued steady nitrogen uptake was only maintained for long periods in the presence of carbon dioxide in the light. In the dark, nitrogen uptake was accompanied by carbon dioxide evolution.More oxygen was evolved in the light by cells fixing nitrogen than by those incubated under argon. This additional oxygen evolution could be accounted for by extra carbon dioxide fixation in the presence of nitrogen.Of a number of organic compounds tested, only sodium pyruvate stimulated nitrogen fixation. This stimulation was achieved both in the light and dark and in the presence and absence of carbon dioxide, showing that the role of pyruvate was other than acting as a carbon skeleton.Three metabolic inhibitors, cyanide and chlorpromazine (chiefly respiratory) and phenylurethane (photosynthetic) differentially inhibited photosynthesis and nitrogen fixation. The latter inhibitor had a more marked effect on photosynthesis while the two chiefly respiratory inhibitors had a stronger effect on nitrogen fixation.     

Oxygen and carbon dioxide fluxes from barley shoots depend on nitrate assimilation

A custom oxygen analyzer in conjunction with an infrared carbon dioxide analyzer and humidity sensors permitted simultaneous measurements of oxygen, carbon dioxide, and water vapor fluxes from the shoots of intact barley plants (Hordeum vulgare L. cv Steptoe). The oxygen analyzer is based on a calciazirconium sensor and can resolve concentration differences to within 2 microliters per liter against the normal background of 210,000 microliters per liter. In wild-type plants receiving ammonium as their sole nitrogen source or in nitrate reductase-deficient mutants, photosynthetic and respiratory fluxes of oxygen equaled those of carbon dioxide. By contrast, wild-type plants exposed to nitrate had unequal oxygen and carbon dioxide fluxes: oxygen evolution at high light exceeded carbon dioxide consumption by 26% and carbon dioxide evolution in the dark exceeded oxygen consumption by 25%. These results indicate that a substantial portion of photosynthetic electron transport or respiration generates reductant for nitrate assimilation rather than for carbon fixation or mitochondrial electron transport.     

IMPACT OF IRON LIMITATION ON THE PHOTOSYNTHETIC APPARATUS OF THE DIATOM CHAETOCEROS MUELLERI (BACILLARIOPHYCEAE)

Iron starvation induced marked increases in flavodoxin abundance and decreases in light-saturated and light-limited photosynthesis rates in the diatom Chaetoceros muelleri. Consistent with the substitution of flavodoxin for ferredoxin as an early response to iron starvation, increases of flavodoxin abundance were observed before declines of cell division rate or chl a specific photosynthesis rates. Changes in the abundance of flavodoxin after the addition of iron to iron-starved cells indicated that flavodoxin was not actively degraded under iron-replete conditions. Greater declines in light-saturated oxygen evolution rates than dark oxygen consumption rates indicated that the mitochondrial electron transfer chain was not affected as greatly by iron starvation as the photosynthetic electron transfer chain. The carbon:nitrogen ratio was unaffected by iron starvation, suggesting that photosynthetic electron transfer was a primary target of iron starvation and that reductions in nitrate assimilation were due to energy limitation (the C:N ratio would be expected to rise under nitrogen-limited but energy-replete conditions). Parallel changes were observed in the maximum light-saturated photosynthesis rate and the light-limited initial slope of the photosynthesis-light curve during iron starvation and recovery. The lowest photosynthesis rates were observed in iron-starved cells and the highest values in iron-replete cells. The light saturation parameter, I_k, was not affected by iron starvation, nor was the chl-to-C ratio markedly reduced. These observations were consistent with iron starvation having a similar or greater effect on photochemical charge separation in PSII than on downstream electron transfer steps. Declines of the ratio of variable to maximum fluorescence in iron-starved cells were consistent with PSII being a primary target of iron starvation. The functional cross-section of PSII was affected only marginally (<20%) by iron starvation, with the largest values observed in iron-starved cells. The rate constant for electron transfer calculated from fast repetition rate fluorescence was found to covary with the light-saturated photosynthesis rate; it was lowest in the most severely starved cells.     

The effect of nitrogen refeeding on starved cells of Platymonas striata Butcher

A rapid uptake of nitrogen was observed in nitrogen-starved cells of Platymonas striata after refeeding with ammonium or nitrate ions. This was followed by a net loss of nitrogen per cell. Cells initially grown in and then starved in a regime of continuous light showed greater increases in average cell nitrogen on refeeding with ammonium or nitrate ions than did cells initially grown in and then starved in a regime of alternating light and darkness. A particulate subcellular location was observed for nitrate reductase (EC 1.6.6.1) in broken cell suspensions prepared by sonication. Nitrite reductase (EC 1.6.6.4) was located in the soluble fraction of these cell suspensions. Broken cell preparations displayed a lowered nitrate reductase activity as compared with the particulate component of these preparations. This was shown not to be due to heat-stable inhibitors present in the soluble phase of the cell. It appeared to be an artefact produced by the high nitrite reductase activity of the broken cell preparations, which removed much of the nitrite as it was formed. Nitrogen starvation of nitrate-grown cultures produced cellular increases in nitrate reductase and nitrite reductase activities which were further increased after the addition of nitrate. The results are discussed.Abbreviations ASP2 complete culture medium - ASP2 INF medium lacking in inorganic nitrogen - ASP2 NF medium lacking all nitrogen - NAR nitrate reductase - NIR nitrite reductase - EDTA Ethylenediaminetetracetic acid - PVP Polyvinylpyrollidone, M.W. 44,000     

Dynamic aspects of phycobilisome structure

In exponentially growing cells of Synechococcus sp. 6301, over 95% of the phycobiliproteins are located in phycobilisomes, and the remainder is present in the form of low molecular weight aggregates. In addition to the subunits of the phycobiliproteins (C-phycocyanin, allophycocyanin, allophycocyanin B), the phycobilisomes of this unicellular cyanobacterium contain five non-pigmented polypeptides. During the initial phase of starvation (24 h after removal of combined nitrogen from the growth medium), the phycobiliproteins in the low molecular weight fraction largely disappeared. Phycocyanin was lost more rapidly from this fraction than allophycocyanin. Simultaneous changes in the phycobilisome were (1) a decrease in sedimentation coefficient, (2) a decrease in phycocyanin: allophycocyanin ratio, (3) a shift in the fluorescence emission maximum from 673 to 676 nm, and (4) a selective complete loss of a 30,000 dalton non-pigmented polypeptide. Upon extensive nitrogen starvation (72 h), the intracellular level of phycocyanin decreased by over 30-fold. These results indicate that in the early stage of nitrogen starvation, the free phycobiliproteins of the cell are degraded, as well as a significant proportion of the phycocyanin from the periphery of the phycobilisome. However, the structures partially depleted of phycocyanin still function efficiently in energy transfer. On extended starvation, total degradation of residual phycobilisomes takes place, possibly in conjunction with the detachment of these structures from the thylakoids.None of the effects of the absence of combined nitrogen were seen when cells were starved in the presence of chloramphenicol, or in a methionine auxotroph starved for methionine.Abbreviations Used NaK-PO₄ NaH₂PO₄ titrated with K₂HPO₄ to a given pH - SDS sodium dodecyl sulfate - Tris Tris(hydroxymethyl)aminomethane     

Nitrate levels affect the development of the black locust-Rhizobium symbiosis

Summary Experiments with black locust (Robinia pseudoacacia L.) seedlings grown under strictly controlled laboratory conditions indicated that the availability of nitrate has a marked impact on nitrogen fixation. When nitrate concentrations were very low, both nodulation and seedling growth were impaired, whereas nitrate concentrations high enough to promote plant growth strongly inhibited symbiotic nitrogen fixation. When nitrate was added to the growth medium after infection, nodulation and nitrogen fixation of the seedlings decreased. This effect was even more marked when nitrate was applied before infection with rhizobia. Higher nitrogen concentrations also reduced nodule number and nodule mass when applied simultaneously with the infecting bacteria. The contribution of symbiotic nitrogen fixation to black locust shoot mass by far exceeded its effects on shoot length and root mass. When nitrate availability was very low, specific nitrogen fixation (i. e. nitrogenase activity per nodule wet weight) was improved with increasing nitrogen supply, but rapidly decreased with higher nitrogen concentrations.     

Photosystem II Independent Carbon Dioxide Fixation in Plectonema boryanum During Photoautotrophic Growth Under Nitrogen Fixation Conditions

The non-heterocystous filamentous cyanobacterium Plectonema boryanum UTEX 594 grew rapidly microaerobically under nitrogen-starvation conditions in continuous high light intensity by conducting oxygenic photosynthesis and oxygen sensitive nitrogen-fixation in alternating cycles. During diazotrophic phase, the light harvesting pigment phycocyanin declined with a concomitant depression in light dependent oxygen evolution by the cyanobacterium. A substantial component of light dependent carbon dioxide fixation during diazotrophic phase was not inhibited by DCMU in spite of complete cessation of photosynthetic oxygen evolution. Endogenous-reductant dependent electron transfer to photosystem I during diazotrophic phase is postulated even during photoautotrophic growth.     

The development of nitrate reductase in Chlorella and its repression by ammonium

Summary Chlorella vulgaris, grown with ammonium sulphate as nitrogen source, contains very little nitrate reductase activity in contrast to cells grown with potassium nitrate. When ammonium-grown cells are transferred to a nitrate medium, nitrate reductase activity increases rapidly and the increase is partially prevented by chloramphenicol and by p-fluorophenylalanine, suggesting that protein synthesis is involved. The increase in nitrate reductase activity is prevented by small quantities of ammonium; this inhibition is overcome, in part, by raising the concentration of nitrate. Although nitrate stimulates the development of nitrate reductase activity, its presence is not essential for the formation of the enzyme since this is formed when ammonium-grown cells are starved of nitrogen and when cells are grown with urea or glycine as nitrogen source. It is concluded that the formation of the enzyme is stimulated (induced) by nitrate and inhibited (repressed) by ammonium.     

Effect of nutrients and aeration on O2 evolution and photosynthetic pigments ofAnabœna torulosa during akinete differentiation

The addition of a nitrogen (nitrate) and carbon sources (acetate, citrate and fructose) and phosphate deficiency (nitrate medium deficient in phosphate) under unaerated conditions induced akinete differentiation inAnabœna torulosa. Aerated cultures of this organism in these nutrients did not differentiate akinetes. Oxygen evolution by aerated cultures was higher when compared to unaerated cultures, which concurred with high chlorophyll content of aerated cultures. Nitrate nitrogen supported high phycocyanin content in unaerated cultures, phycocyanin and allophycocyanin contents were low under aerated conditions. The contents of phycocyanin, allophycocyanin, phycoerythrin and carotenoids gradually decreased at the mature akinete phase. Under aerated conditions, chlorophyll content rose and the content of all the pigments increased with the growth rate of the organism.     

The photosynthetic response of nutrient-depleted dilute cultures of Skeletonema costatum to pulses of ammonium and nitrate; the importance of phosphate

Experimental data on changes in carbon fixation rate causedby nutrient pulses in dilute cultures of nutrient-depleted Skeletonemacostatum are reported. Pulses contained single nutrients ormixtures of a nitrogen source and phosphate, concentrationsranging from 0 to 5 µM for ammonium and nitrate and from0 to 1 µM for phosphate. The cultures were incubated overnightafter pulsing to allow the rapid stage of nutrient uptake tocome to completion before measurement of carbon fixation thenext day. Increments in N:C ratios due to nitrogenous nutrientuptake depended upon the P:C ratio in the cells as well as theconcentration of the pulse. When P:C ratios were low (<0.005),increases in nitrogen repressed photosynthesis. Phosphate uptakewas independent of the absence or presence of a nitrogen sourceand had only a small stimulatory effect upon carbon fixation.When added jointly with ammonium or nitrate, however, largeincreases in the rate of photosynthesis resulted. These weredue mainly to rises in chlorophyll a concentrations resultingfrom higher N:C ratios in the cells. Chlorophyll-specific carbonfixation rates were hyperbolic functions of P:C ratios but exhibiteda C-shaped relationship to N:C ratios. The stimulatory and repressiveeffects of the nutrient pulses are shown to be consistent withthe view that the rate of photosynthesis at constant illuminationand carbon dioxide partial pressure is mainly controlled bythe chlorophyll a concentration and phosphate availability inthe phytoplankton.     

Heat-induced formation of nitrogen oxides in water

It was found by the fluorimetric method using 2,3-diaminonaphthalene that moderate heating of water (60–80°C, for up to 4 h) leads to the fixation of atmospheric nitrogen with the formation of nitrite. The kinetic parameters of this process were determined. The energy of activation of ${\rm NO}_{2}^{-}$ formation was estimated to be 139 kJ/mol. It was found that the amount of nitrite formed depends on the concentration of dissolved oxygen and nitrogen. It was shown by two independent methods (Griess reagent/VCl₃ and 2,3-diaminonaphthalene/nitrate reductase) that heating of water (80°C, 1 h) results in the formation of nitrate; with the use of the fluorescent probe dihydrorhodamine 123, the generation of nitrogen dioxide (peroxynitrite) was revealed. Nitrite, nitrate, and nitrogen dioxide are formed in water upon heating in approximately equal amounts. A scheme of reactions proceeding with bidistilled water by the action of heat with the formation of nitrogen oxides is proposed.     

Physiological and Biochemical Characteristics of Sugar Beet Plants Grown at an Increased Carbon Dioxide Concentration and at Various Nitrate Doses

Three-week-old sugar beet (Beta vulgaris L.) seedlings were grown for an additional four weeks under controlled conditions: in river sand watered with a modified Knop mixture containing one half-fold (0.5N), standard (1N), and or threefold (3N) nitrate amount, at the irradiance of 90 W/m² PAR, and at the carbon dioxide concentrations of 0.035% (1C treatment) or 0.07% (2C treatment). The increase in the carbon dioxide concentration and in the nitrogen dose resulted in an increase in the leaf area and the leaf and root dry weight per plant. With the increase in the nitrogen dose, morphological indices characterizing leaf growth increased more noticeably in 1C plants than in 2C plants. And vice versa, the effects of increased CO₂ concentration were reduced with the increase in the nitrogen dose. Roots responded to the changes in the CO₂ and nitrate concentrations otherwise than leaves. At a standard nitrate dose (1N), the contents of proteins and nonstructural carbohydrates (sucrose and starch) in leaves depended little on the CO₂ concentration. At a double CO₂ concentration, the content of chlorophyll somewhat decreased, and the net photosynthesis rate (P _n) calculated per leaf area unit increased. An increase in the nitrogen dose did not affect the leaf carbohydrate content of the 1C and 2C plants except the leaves of the 2C-3N plants, where the carbohydrate content decreased. In 1C and 2C plants, an increase in the nitrogen dose caused an increase in the protein and chlorophyll content. Specific P _n values somewhat decreased in 1C-0.5N plants and had hardly any dependence on the nitrate dose in the 2C plants. The carbohydrate content in roots did not depend on the CO₂ concentration, and the content was the highest at 0.5N. Characteristic nitrogen dose-independent acclimation of photosynthesis to an increased carbon dioxide concentration, which was postulated previously [1], was not observed in our experiments with sugar beet grown at doubled carbon dioxide concentration.     

Cell wall glycanases and their activity against the hemicelluloses from pine hypocotyls

O₂ evolution and chlorophyll a fluorescence emission have been monitored in intact cells of the cyanobacterium Anacystic nidulans 1402–1 to stdy the influence of carbon and nitrogen assimilation on the operation of the photosynthetic apparatus. The pattern of fluorescence induction in dark-adapted cyanobacterial cells was different from that of higher plants. Cyanobacteria undergo large, rapid state transitions upon illumination, which lead to marked changes in the fluorescence yield, complicating the estimation of quenching coefficients. The Kautsky effect was not evident, although it could be masked by a state II–state I transition, upon illumination with actinic light. The use of inhibitors of carbon assimilation such as D,L-glyceraldehyde or iodoacetamide allowed us to relate changes in variable fluorescence to active CO₂ fixation. Ammonium, but not nitrate, induced non-photochemical fluorescence quenching, in agreement with a previous report on green algae, indicative of an ammonium-induced state I transition.     

Nitrogenase activity and nitrate respiration inAzospirillum spp.

The interaction between nitrate respiration and nitrogen fixation inAzospirillum lipoferum andA. brasilense was studied. All strains examined were capable of nitrogen fixation (acetylene reduction) under conditions of severe oxygen limitation in the presence of nitrate. A lag phase of about 1 h was observed for both nitrate reduction and nitrogenase activity corresponding to the period of induction of the dissimilatory nitrate reductase. Nitrogenase activity ceased when nitrate was exhausted suggesting that the reduction of nitrate to nitrite, rather than denitrification (the further reduction of nitrite to gas) is coupled to nitrogen fixation. The addition of nitrate to nitrate reductase negative mutants (nr^-) ofAzospirillum did not stimulate nitrogenase activity. Under oxygen-limited conditionsA. brasilense andA. lipoferum were also shown to reduce nitrate to ammonia, which accumulated in the medium. Both species, including strains ofA. brasilense which do not possess a dissimilatory nitrite reductase (nir^-) were also capable of reducing nitrous oxide to N₂.     

Relationship between growth,nitrogen fixation and assimilation in a legume (Medicago sativa L.)

Summary Symbiotic N₂ fixation, NO ₃ ⁻ assimilation and protein accumulation in the shoots were measured simultaneously in alfalfa (Medicago sativa L.) grown in the field or in pots, in order to study how the balance between the two modes of nitrogen nutrition could be influenced by agronomic factors, such as harvest, mineral nitrogen supply and drought stress. During periods of rapid growth, fixation and assimilation may function simultaneously; they are antagonistic at the beginning and at the end of the growth cycle, when the nitrogen requirement of the plant is lower. When nitrogen nutrition does not limit growth, mineral nitrogen supply favours assimilation at the expense of fixation, but does not modify the amount of nitrogen accumulated, which is adjusted to the growth capacity of the plant. After cutting, nitrate assimilation compensated for the decrease in fixation and supplied the plant with the nitrogen required by the regrowth, the proliferation of which determined the fixation recovery. Drought stress decreased N₂ fixation much more than NO ₃ ⁻ assimilation. The latter made growth recovery possible when water supply conditions became normal again. These results suggested the existence of an optimum level of nitrate assimilation, which differed depending on the age of the plants and allowed both maximum growth and fixing activity.     

Optimization of carbon dioxide and nitrogen fixation by the blue-green alga Anabaena in freshwater blooms

Summary In the bloom-forming filamentous blue-green algae Anabaena, both carbon dioxide and nitrogen fixation share a dependence on light. During daylight, A. reduces direct competition between these processes for light-generated reductant by optimizing carbon fixation during late morning hours while optimizing nitrogen fixation during afternoon hours.Sequential optimization was examined from both biochemical and ecological perspectives. Biochemically, it is sound to optimize carbon prior to nitrogen fixation, due to the higher sensitivity of the former to afternoon increases in dissolved oxygen levels which commonly occur in blooms. It is also advantageous to first assure adequate supplies of fixed carbon prior to incorporating fixed nitrogen. Ecologically, the sequence represents optimal use of radiant energy. A. is able to shift energy flow from a highly inhibited to a less inhibited process, thereby maintaining a sink for photo-generated reductant. Both A. and a non-nitrogen fixing diatom community show similar carbon fixation efficiencies during morning and midday hours. During afternoon however, A. diverts significant portions of photo-reductant from carbon to nitrogen fixation. This allows A. to optimize carbon fixation while maintaining access to nitrogen during periods of ambient nitrogen shortage. Accordingly, A. blooms usually appear during summer months when both nitrogen starvation and photosynthetically-active radiation inputs are maximal.     

CHANGES IN CELL COMPOSITION AND LIPID METABOLISM MEDIATED BY SODIUM AND NITROGEN AVAILABILITY IN THE MARINE DIATOM PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

The effects of nitrogen starvation in the presence or absence of sodium in the culture medium were monitored in batch cultures of the marine diatom Phaeodactylum tricornutum Bohlin. During nitrogen starvation in the presence of sodium, cell nitrogen and chlorophyll a decreased, mainly as a consequence of continued cell division. These decreases were accompanied by decreases in the rates of photosynthesis and respiration. There was no change in either cell volume or carbohydrate, but both carbon and lipid increased. During nitrogen starvation in the absence of sodium, cell division ceased. Cell nitrogen and chlorophyll a remained constant, and respiration did not decrease, but the changes in the photosynthetic rate and the lipid content per cell were similar to cultures that were nitrogen-starved in the presence of sodium. The carbon-to-nitrogen ratio increased in both cultures. Nitrogen, in the form of nitrate, and sodium were resupplied to cultures that had been preconditioned in nitrogen- and sodium-deficient medium for 5 d. Control cultures to which neither nitrate or sodium were added remained in a static state with respect to cell number, volume, and carbohydrate but showed slight increases in lipid. Cells in cultures to which 10 mM nitrate alone was added showed a similar response to cultures where no additions were made. Cells in cultures to which 50 mM sodium alone was added divided for 2 d, with concomitant small decreases in all measured constituents. Cell division resumed in cultures to which both sodium and nitrate were added. The lipid content fell dramatically in these cells and was correlated to metabolic oxidation via measured increases in the activity of the glyoxylate cycle enzyme, isocitrate lyase. We conclude that lipids are stored as a function of decreased growth rate and are metabolized to a small extent when cell division resumes. However, much higher rates of metabolism occur if cell division resumes in the presence of a nitrogen source.