Regulation of lipid synthesis from acetate in diploid fibroblast cultures —variation with passage level and stage of cell growth

Regulation of lipid synthesis from acetate in human diploid fibroblast cultures has been studied at various passage levels and at different stages of cell growth. When cultures were transferred to lipid free medium, a stimulation of [¹⁴C]acetate incorporation into lipid occurred within three to six hours after removal of exogenous lipid. In early passage cultures, this stimulation was observed whether cells were transferred to protein-free medium or medium supplemented with delipidized serum protein. However, in late passage cultures the presence of delipidized serum protein was required for the stimulation of lipid synthesis. When logarithmically dividing and stationary phase cultures were compared, the cultures in log phase showed stimulation of acetate incorporation into lipid in the presence or absence of delipidized serum protein, whereas in the stationary cultures the delipidized serum protein was required. When cultures were partially synchronized by a thymidine block, stimulation of acetate incorporation into lipid in the blocked cells only occurred in the presence of delipidized serum protein; in released cells stimulation occurred in protein free medium. When inhibition of lipid synthesis from acetate was compared in young vs. old or dividing vs. stationary cultures, however, no differences were observed. The data indicate the response of diploid fibroblast cultures to change in exogenous lipid is dependent on passage level and state of growth.     

Change in quantity of lipids and cell size during intracytoplasmic membrane formation in Gluconobacter oxydans.

Electron microscopy previously revealed that Gluconobacter oxydans differentiates by forming quantities of intracytoplasmic membranes at the end of exponential growth. It was also shown that the formation of these membranes appears concurrently with an increased rate of polyol oxidation. In the present study, exponential-phase cells devoid of intracytoplasmic membranes were harvested and the quantity of free lipid was determined. This quantity was compared with that extracted from cells harvested 4 and 16 h into the stationary phase that contained intracytoplasmic membranes. Cells harvested 4 and 16 h into the stationary phase contained 58 and 43% more free lipid per 100 mg of cell weight than found in undifferentiated exponential-phase cells. These same cultures were used to compare the quantity of lipid extracted per cell. This analysis revealed 89 and 142% more lipid per cell in 4 and 16 h stationary-phase cells. Further study demonstrated that cells increased in length and decreased in density with time after they entered the stationary phase. We estimated, however, that intracytoplasmic membrane development in G. oxydans is accompanied by a 57 to 62% increase in free-lipid that cannot be attributed to a change in cell size. These results suggest that the traditional expression of extracted lipid per milligram of cellular dry weight should not be used for comparative purposes during differentiation in gram-negative bacteria, unless it is first established that both cell size and cell density remain constant throughout differentiation.     

Changes in Membrane Fatty Acid Composition and Δ12-Desaturase Activity during Growth of Acanthamoeba castellanii in Batch Culture

ABSTRACT. Growth of Acanthamoeba castellanii in batch culture at 30° C was associated with marked changes in cellular fatty acid composition. The largest change occurred in the linoleate to oleate ratio, which was maximal in early- to mid-exponential phase cultures but decreased approximately 10-fold as cells approached stationary phase. The higher degree of lipid unsaturation in young cultures was accentuated by a greater proportion of 20-carbon polyunsaturated fatty acids than in stationary phase cultures. The unsaturation index (average number of double bonds per fatty acid) was maximal in mid-exponential phase cultures after 24 hours growth. Incorporation of [1-¹⁴C]acetate into polyunsaturated fatty acids in short-term (2 hour) experiments was high in 12 and 24 hour old cultures, where linoleate and eicosadienoate accounted for up to 26% of total labelled fatty acids. Incorporation of [1-¹⁴CJacetate into these fatty acids was negligible in stationary phase cultures. These results were correlated with changes in the specific activity of the Δ12-desaturase. Δ12-Desaturase activity was greatest in microsomal membranes isolated from early- to mid-exponential phase cells, but declined by approximately 50% as cultures progressed towards stationary phase. Membrane fractionation studies revealed that although some differences in fatty acid composition between plasma-membrane, mitochondrial (enriched), and microsomal membrane fractions were evident, the large changes in lipid unsaturation in whole cells of A. castellanii could not be accounted for by differential development of particular subcellular membranes.     

Ultrastructural changes in <Emphasis Type="Italic">Yarrowia lipolytica</Emphasis> cells under stress conditions

Ultrastructural organization of the aerobic yeast Yarrowia lipolytica was studied under conditions of oxidative, heat, and ethanol stresses. It was shown that the following uniform changes in cell ultrastructure did not depend on the type of stress: enlargement of mitochondria, enhanced number and enlargement of peroxisomes, and formation of lipid granules. Similar ultrastructural changes also occurred during the transition of cells to the stationary growth phase. It was shown for the first time that accumulation of polyphosphate granules occurred as a stress response in yeasts. Moreover, numerous globular structures of unknown nature appeared on the cell wall surface under oxidative or heat stress. Under ethanol stress, the cells developed clearly marked deep invaginations of the cytoplasmic membrane. (The same changes in the cytoplasmic membrane were observed in the cells grown on ethanol.) Variations of the cell envelope structure along with the formation of polyphosphate granules were not observed in the stationary growth phase. Ultrastructural changes in the cells under stress conditions are in agreement with the previous data on survival, respiratory activity, and variations of the antioxidant systems.     

The Mechanism of Onset of the Stationary Phase in Euglena gracilis Grown with 10 mM Succinate: Intracellular pH Values*

SYNOPSIS. When Euglena gracilis were grown with 10mM succinate at pH 3.5 the extracellular pH averaged 3.62 and the cultures had produced 6 × 10⁵ cells/ml when the stationary phase began. Oxygen consumption values reached a maximum of 30 μliters/10⁶ cells/hr. Total protein and dry weights per cell remained constant during the logarithmic phase and began to decline when the late logarithmic phase was reached. Added succinate caused the cultures in stationary phase to commence logarithmic growth once more. Onset of the stationary phase in cultures grown at pH 3.5 was due to depletion of succinate. When cultures were grown at pH 6.9 the extracellular pH averaged 7.62 and the cultures produced 3 × 10⁵ cells/ml when the stationary phase began. Oxygen consumption values reached a maximum of 20 μliters/10⁶ cells/hr during the logarithmic phase. The decline in total protein and dry weights per cell began at the beginning of the logarithmic phase and continued into the stationary phase of growth. Cultures grown at pH 3.5 should produce a larger number of cells/ml than cultures grown at pH 6.9 if the cells are responding to the unionized moiety of succinate and not the ionized moiety. At pH 3.5 83% of the succinate is unionized, whereas at pH 6.9 0.20% of the succinate is unionized. The onset of the stationary phase in cultures grown at pH 3.5 and pH 6.9 is due to lack of an adequate amount of extracellular unionized succinate. Intracellular pH values were determined in cultures grown at pH 6.9 using the weak acid DMO (5.5-dimethyl-2,4-oxazolidinedione). As the extracellular pH increased from 6.90 to 7.62, the intracellular pH increased from 5.89 to 6.89. As the extracellular pH increased from 7.62 to 8.44, the intracellular pH increased from 6.89 to 7.50.     

Effects of temperature and growth phase on lipid and biochemical composition of Isochrysis galbana TK1

The lipid and biochemical composition of the haptophyte Isochrysis galbana TK1 was examined. Cultures were grown at 15 °C and 30 °C, and harvested in the exponential and early stationary growth phases. Carbohydrate and protein content varied at the two culture temperatures and growth phases. The highest protein content was found at the exponential growth phase at 15 °C, and the highest carbohydrate content was found at the stationary phase at the same culture temperature. Lipid accumulated in the stationary growth phase and its content was higher at 30 °C than at 15 °C regardless of the growth phase. The neutral lipids were the major class of lipid found in all the cultures. The stationary phase culture had a higher proportion of neutral lipids than the exponential phase culture and the proportion decreased slightly when culture temperature was increased from 15 °C to 30 °C. Phospholipid levels remained constant at the two temperatures, but slightly decreased in the stationary phase. Glycolipids in the exponentially growing cells were higher than those from stationary growth phase and increased with temperature. Polyunsaturated fatty acids (PUFAs) predominated in glycolipids and phospholipids. Cells grown at 15 °C contained higher proportion of 18:3 (n–3) and 22:6 (n–3) with a corresponding decrease in 18:2 (n–6), monounsaturated and saturated fatty acids. This revised version was published online in August 2006 with corrections to the Cover Date.     

Osmotic and Metabolic-Induced Changes in Light Scattering of Leishmania donovani as Measured by Flow Cytometry

Leishmania donovani promastigotes were collected from cultures in log and stationary phases of growth and resuspended in Hank's Balanced Salt Solution containing 1 mM sodium acetate. Changes in the forward and side scattering of the cells were measured by flow cytometry in response to acute changes in osmolality and to the addition of several different substrates. Forward and side scattering of cells from log phase cultures decreased when the osmolality was decreased by the addition of H₂O and increased when the osmolality was increased by the addition of NaCl. Cells from stationary phase cultures had about the same forward scatter as cells from log phase cultures, but almost a four-fold lower side scatter, and their side scatter values did not change significantly in response to a reduction in osmolality. Microscopic observation showed that both log and stationary cells got longer and thinner, on average, in response to hyperosmolality. The light scattering properties of log (but not of stationary) cells changed in a reproducible manner when substrates were added to the buffer. The ratio of forward to side scatter increased in the following order: controls in balanced salt solution >aspartate >glutamate, glucose or 2-deoxyglucose >alanine >proline. Thus the light scattering properties of L. donovani promastigotes change with culture age, in response to changes in osmolality, and, in log phase cells, in response to the presence of several substrates.     

DOPAMINERGIC PROPERTIES OF A SOMATIC CELL HYBRID LINE OF MOUSE NEUROBLASTOMA X SYMPATHETIC GANGLION CELLS

Abstract— Studies were made on the regulation of dopamine metabolism in a cell line derived by hydridization of a non-tyrosine-hydroxylase-containing line of murine neuroblastoma cells with a neur-onally-enriched population of murine embryonic sympathetic ganglion cells. Hybrid subclones with tyrosine hydroxylase activity were selected by exposure to tyrosine-free medium. The cells also exhibited DOPA decarboxylase activity and the subclone (named T28) with the highest specific activities of both enzymes was further characterized. The hybrid T28 line did not contain dopamine-β-hydroxylase activity. The specific activity of tyrosine hydroxylase as well as of DOPA decarboxylase increased significantly in T28 cultures when the cells entered the stationary phase of growth. Both of these enzymes were also induced after several days of exposure to 1 m m -dibutyryl cyclic AMP in culture medium containing either 5% or 0.8% serum. However, maintenance in medium containing 0.8% serum alone, which inhibited cell multiplication, did not induce either enzyme. The dopamine content of T28 cells was also regulated as a function of cell density. High density (stationary phase) cultures of T28 cells contained about 300 pmol dopamine per mg protein and at least half of this endogenous amine appeared to he stored in vesicles or granules (as judged by depletion with reserpine or α-methyl- m -tyramine). The T28 and other neuronal hybrid lines appear to be useful model systems for neuro-chemical studies.     

Effect of cell density on binding and uptake of low density lipoprotein by human fibroblasts

The effect of cell density on low density lipoprotein (LDL) binding by cultured human skin fibroblasts was investigated. Bound LDL was visualized by indirect immunofluorescence. Cellular lipid and cholesterol were monitored by fluorescence in cells stained with phosphine 3R and filipin, respectively. LDL binding and lipid accumulation were compared in cells in stationary and exponentially growing cultures, in sparsely and densely plated cultures, in wounded and non-wounded areas of stationary cultures, and in stationary cultures with and without the addition of lipoprotein-deficient serum. We conclude that LDL binding and cholesterol accumulation induced by LDL are influenced by cell density. It appears that, compared to rapidly growing cells, quiescent (noncycling) human fibroblasts exhibit fewer functional LDL receptors.     

Development of mitochondrial membranes in anaerobically grown yeast cells.

Biochemical analyses of mitochondrial marker substances, especially cardiolipin and oligomycin-sensitive ATPase [EC 3.6.1.3], as well as electron microscopic observations were carried out to eludicate the process of mitochondrial development in annaerobic yeast cells. Cardiolipin was found to be localized in the mitochondria in anaerobic cells. Its cellular content was a little higher in the stationary phase than in the exponential phase in glucose-grown cells and increased further in galactose-grown cells. The lipid content of the mitochondrial preparation obtained from glucose-grown stationary cells was nearly as high as that from galactose-grown cells. It was also comparable to that of aerobic cells in the stationary phase, where mitochondria are fully developed. Both cellular and mitochondrial levels of oligomycin-sensitive ATPase activity were also found to rise markedly in galactose-grown anaerobic cells, although not in stationary phase cells grown anaerobically on glucose. These high levels of the mitochondrial markers indicate a developmental change in mitochondrial structure even in anaerobically grown cells, which lack mitochondrial cytochromes. In the process of aerobic adaptation, respiratory system formation was observed to occur much faster in galactose-grown cells than in glucose-grown cells, and not to be inhibited by chloramphenicol and high concentrations of glucose structure in anaerobic cells. The developmental change was also corroborated by electron microscopic observations, which revealed the occurrence of two types of mitochondria in anaerobic cells. One was found in glucose-repressed cells and was characterized by the presence of numerous electron-dense granules in the matrix. In contrast, the other type, found in glucose-derepressed cells, had an electron-lucent matrix. No crista membrane was seen in either type of mitochondria in anaerobic cells, although the infoldings of the inner membrane, which partition the matrix into two parts and therefore are called "septum membranes," appeared frequently in the stationary phase cells. On the basis of these results, the process of mitochondrial development in yeast cells is discussed.     

Ultrastructural Observations on Aging of Stationary Cultures and Feeding in Ochromonas*

SYNOPSIS. Changes accompanying aging of stationary cultures of Ochromonas danica were examined with the electron microscope. The cultures included light- and dark-grown populations ranging in age from 3 days to 5 weeks. Cells from the youngest cultures contained minimal amounts of lipid and a distinct leucosin vacuole. After 1 week, the number of lipid globules in the cytoplasm increased. The amount of lipid increased progressively in cells from older cultures until the leucosin vacuole was obliterated by the coalescing spheres. Cells from cultures older than 3 weeks showed a general breakdown of cytoplasmic integrity. An area of pinocytotic activity was also present; a relationship between this anterior region and blebs arising from the cell membrane is suggested.     

Physiological and morphological characteristics of stationary phase vibrio cells able to support phase growth

Growth of phase alpha 3a on stationary phase Vibrio cultures requires micro-aerophilic conditions and is inhibited by aeration. Since pre-conditioning of the bacteria by allowing them to stand for 24 h after shaking for 3 d is an important aspect of the stationary phase phage growth system, various physiological and morphological characteristics of the stationary phase cells during the transition from shaking to standing were investigated. Shaken stationary phase cells were less viable and more sensitive to ultraviolet irradiation and heat than standing stationary phase cells. During pre-conditioning the small, non-flagellated cells present in shaken stationary phase cultures underwent morphological changes and became large, flagellated rods which resembled exponential phase cells. The transition of stationary phase cells from shaking to standing was associated with a marked increase in total RNA synthesis but a rapid and large decrease in total protein synthesis. Intracellular concentrations of ATP in shaken stationary phase cells were 53% lower than those in standing stationary phase cells. Studies on leucine uptake indicated that its transport was inhibited by isoleucine and that the major part (90%) of the total leucine uptake was due to a shared system for uptake of both amino acids. Shaken stationary phase cells transported less leucine than standing stationary phase cells. Inhibition of phage growth in aerated stationary phase cultures was not due to the prevention of phase absorption by shaking. It is suggested that the observed differences between shaken and standing stationary phase cells could be due to aeration affecting the template specificity of the Vibrio RNA polymerase.     

A Neoplastic Connective Tissue Mast Cell Capable of Continuous Growth In Tissue Culture

A neoplastic connective tissue mast cell from a dog mast cell sarcoma has been grown in tissue culture for 50 passages over a period of 2 years. The cells were grown as monolayer cultures in glass bottles, using Eagle's basal medium fortified with calf serum. The cultures were contaminated with an Alkaligenes sp. for 10 months but finally were sterilized bacteriologically by treatment with specific antiserum combined with antibiotics. The cells grow in a fibroblastic pattern, and contain mitochondria, mast cell granules, and lipid granules or droplets. The mast cell granules stain basophilic with Giemsa's stain and metachromatically with azure A or toluidine blue. They also stain with Sudan black B and with periodic acid-Schiff stain. The interphase nuclei are vesicular, contain from 1 to 20 nucleoli, and frequently show bizarre outlines. Multinucleate cells are often seen, as are mitotic figures. Extracellular fibrous material occurs in all cultures and apparently originates from the cell surface. This material does not have the structure of connective tissue fibers and has not been identified. The cells develop an increased number of metachromatic granules when grown in medium containing heparin and an increased number of sudanophilic granules when grown in medium containing stearic acid. Only small amounts of histamine were present in the tumor from which this cell line was derived and in the cells grown in tissue culture.     

The size,density, and relative area of chromatic granules (“hydrogenosomes”) in Trichomonas vaginalis donne from cultures in logarithmic and stationary growth

Summary Electron micrographs of Trichomonas vaginalis cells were examined morphometrically by means of an electronic image analyser. Cells from cultures in logarithmic growth contained smaller chromatic granules (hydrogenosomes ) and a higher proportion of sausage- or dumb-bell-shaped granules than did cells in stationary growth.These findings support the view that chromatic granules replicate by division of pre-existing granules and that this involves elongation and attenuation of their mid-region.The total area of cross sectioned granules occupied the same percentage of the cytoplasm of cells from cultures in logarithmic and stationary growth. The density of chromatic granules in the cytoplasm was identical for the two groups of cells.     

SILICATE DEFICIENCY AND LIPID SYNTHESIS OF MARINE DIATOMS1,2

Lipid synthesis of three marine diatoms was studied with a ¹⁴CO₂ incorporation technique in silicate limited batch cultures. Growth rates were independent of the silicate concentration but the cellular yields were proportional to the initial amount of silicate. At the beginning of the stationary growth phase, lipid synthesis rates per unit culture volume increased by 1.7 times for Chaetoceros gracilis, 3.1 times for Hantzschia sp., and 2.8 times for Cyclotella sp., respectively compared to those during the exponential growth phase. Lipid carbon accounted for as much as 57% of the carbon in C. gracilis, 71% in Hantzschia sp., and 65% in Cyclotella sp., respectively. Additional enrichment with silicate during stationary growth phase allowed the cultures to grow further. Lipid synthesis rates were reduced during the subsequent growth phase, and the growth rates themselves were dependent on the level of biomass achieved during the previous stationary phase. However, the cellular yields were similar and probably controlled by light.     

EFFECTS OF HARVEST STAGE AND LIGHT ON THE BIOCHEMICAL COMPOSITION OF THE DIATOM THALASSIOSIRA PSEUDONANA1

The marine diatom Thalassiosira pseudonana (Hustedt, clone 3H) Hasle and Heimdal was cultured under three different light regimes: 100 μmol photon · m^?2· s^?1 on 12:12 h light : dark (L:D) cycles; 50 μmol photon · m^?2· s^?2 on 24:0 h L:D; and 100 μmol photon · m^?2· s^?1 on 24:0 h L:D. It was harvested during logarithmic and stationary phases for analysis of biochemical composition. Across the different light regimes, protein (as % of organic weight) was highest in cells during logarithmic phase, whereas carbohydrate and lipid were highest during stationary phase. Carbohydrate concentrations were most affected by the different light regimes; cells grown under 12:12 h L:D contained 37–44% of the carbohydrate of cells grown under 24:0 h L:D. Cells in logarithmic phase had high proportions of polar lipids (79 to 89% of total lipid) and low triacylglycerol (≤10% of total lipid). Cells in stationary phase contained less polar lipid (48 to 57% of total lipid) and more triacylglycerol (22 to 45% of total lipid). The fatty acid composition of logarithmic phase cells grown under 24:0 h L:D were similar, but the 100 μmol photon · m^?2· s^?1 (12:12 h L:D) cells at the same stage contained a higher proportion of polyunsaturated fatty acids (PUFAs) and a lower proportion of saturated and monounsaturated fatty acids due to different levels of 16:0, 16:1(n-7), 16:4(n-1), 18:4(n-3), and 20:5(n-3). With the onset of stationary phase, cells grown at 100 μmol photon · m^?2· s^?1 (both 12:12 and 24:0 h L:D) increased in proportions of saturated and monounsaturated fatty adds and decreased in PUFAs. Concentrations (% organic or dry weight) of 14:0, 16:0, 16:1(n-7), 20:5(n-3), and 22:6(n-3) increased in cells of all cultures during stationary phase. The amino acid compositions of cells were similar irrespective of harvest stage and light regime. For mariculture, the recommended light regime for culturing T. pseudonana will depend on the nutritional requirements of the animal to which the alga is fed. For rapidly growing bivalve mollusc larvae, stationary-phase cultures grown under a 24:0 h L:D regime may provide more energy by virtue of their higher percentage of carbohydrate and high proportions and concentrations of energy-rich saturated fatty acids.     

Cytochemical Changes in Aging Ochromonas; Evidence for an Alkaline Phosphatase

SYNOPSIS. Changes accompanying aging in light-grown stationary cultures of Ochromonas danica were examined cytochemically. Succinate dehydrogenase activity increased during the log phase and decreased steadily during stationary and later phases. Acid phosphatase, alkaline phosphatase and lipase activities increased during the several phases of growth, as did accumulation of lipid. These results imply loss of mitochondrial activity and a gain in lysosomal activity with aging of the cell population. Alkaline phosphatase, widely distributed in animals and believed absent from most photosynthetic organisms and bacteria, is here reported in the photosynthetic genus Ochromonas.     

Monitoring Rhodotorula glutinis CCMI 145 physiological response and oil production growing on xylose and glucose using multi-parameter flow cytometry

Flow cytometry was used to monitor the lipid content, viability and intrinsic light scatter properties of Rhodotorula glutinis CCMI 145 cells growing on batch cultures using xylose and glucose as carbon sources. The highest lipid content was observed for cells grown on glucose, at the end of the exponential phase (17.8% w/w). The proportion of cells stained with PI attaining 77% at the end of the glucose growth. Cells growing on xylose produced a maximum lipid content of 10.6% (w/w), at the stationary phase. An increase in the proportion of cells stained with PI was observed, reaching 29% at the end of xylose growth. Changes in the side and forward light scatter detected during the yeast batch cultures supported that R. glutinis cells grown on glucose experienced harsher conditions, resulting in a high level of cytoplasmic membrane damage, which did not occur when R. glutinis cells grew on xylose.     

Analysis of growth phase regulated KatA and CatE and their physiological roles in determining hydrogen peroxide resistance in Agrobacterium tumefaciens

Agrobacterium tumefaciens possesses two catalases, a bifunctional catalase-peroxidase, KatA and a homologue of a growth phase regulated monofunctional catalase, CatE. In stationary phase cultures and in cultures entering stationary phase, total catalase activity increased 2-fold while peroxidase activity declined. katA and catE were found to be independently regulated in a growth phase dependent manner. KatA levels were highest during exponential phase and declined as cells entered stationary phase, while CatE was detectable at early exponential phase and increased during stationary phase. Only small increases in H2O2 resistance levels were detected as cells entering stationary phase. The katA mutant was more sensitive to H2O2 than the parental strain during both exponential and stationary phase. Inactivation of catE alone did not significantly change the level of H2O2 resistance. However, the katA catE double mutant was more sensitive to H2O2 during both exponential and stationary phase than either of the single catalase mutants. The data indicated that KatA plays the primary role and CatE acts synergistically in protecting A. tumefaciens from H2O2 toxicity during all phases of growth. Catalase-peroxidase activity (KatA) was required for full H2O2 resistance. The expression patterns of the two catalases in A. tumefaciens reflect their physiological roles in the protection against H2O2 toxicity, which are different from other bacteria.     

Oxidative stress and antioxidant content in Chlorella vulgaris after exposure to ultraviolet-B radiation

Growth of Chlorella vulgaris was measured in cultures irradiated with 0, 0.8, 2.0 and 4.4 kJ m² UV-B. Growth expressed as chlorophyll content, declined significantly with increased UV-B dose. Ultraviolet-B irradiated cultures in log phase of growth showed a 284% increase in oxygen radical generation and a 145% increase in lipid peroxidation compared with unirradiated cultures, whereas cultures in the stationary growth phase showed no significant changes in these parameters. The activities of superoxide dismutase and catalase increased by 40 and 500%, respectively, after exposure to a UV-B dose of 4.4 kJ m⁻². Contents of the lipophilic antioxidants α-tocopherol and β-carotene increased by 180 and 63 amol cell⁻¹ respectively, between log and stationary phases in unirradiated cultures; but in UV-B-irradiated cultures these increases were significantly depressed. Photoreducing capacities of chloroplasts were decreased following UV-B irradiation of both isolated chloroplasts and those isolated from irradiated algae. Cells exposed to UV-B exhibited increased size and starch accumulation. These results suggest that oxidative stress conditions related to UV-B exposure trigger an antioxidant response that includes an increase in the activity of the antioxidant enzymes (superoxide dismutase and catalase).