Culture conditions and the development of the photosynthetic mechanism; influence of light intensity on photosynthetic characteristics of Chlorella

1. Chlorella pyrenoidosa has been grown in a continuous-culture apparatus under various light intensities provided by incandescent lamps, other conditions of culture being maintained constant. Light intensity curves for cells immersed in the No. 11 Warburg buffer and in Knop''s solution + 4.4 per cent CO₂ at a saturating light intensity were determined as characteristics of the photosynthetic mechanism. These characteristics were referred to the centrifuged cell volume as an index of quantity of cellular material. 2. Cells grown at intensities in the range of about 35 f.-c. develop a capacity for a high rate of photosynthesis (c.mm. O₂/hour/c.mm. cells). At culture intensities above or below this range the cells produced have a lower capacity for photosynthesis. A similar effect is observed for rate of photosynthesis per unit dry weight or rate per unit cell nitrogen. 3. The rate of photosynthesis per cell or rate per unit chlorophyll shows no maximum at any light intensity of culture but increases continuously throughout the range of light intensities studied. 4. Maximum rate of growth is attained at a light intensity of about 100 f.-c. The hypothesis is advanced that at culture intensities above that needed to give maximum rate of growth (100 f.-c.) a mechanism is developed which opposes the photosynthetic process and removes the photosynthetic products. 5. The low capacity for photosynthesis shown by cells grown at culture intensities below 35 f.-c. finds no immediate explanation. 6. The shape of the light intensity curve is markedly affected by the light intensity at which the cells have been cultured. Cells grown at lower intensities give light intensity curves approaching the Blackman type with a short transitional region between light limitation and light saturation.     

The Adaptation of Plankton Algae

The various aspects of the adaptation of plankton algae lo light and temperature are discussed. The shape of a light intensity-photosynthesis curve is shown to be an important means of describing the physiological adjustment of an algal population. If the algae are not exposed to adverse influences such as poisons, pronounced nutrient deficiency or light shocks, the rate of real photosynthesis per mg chlorophyll a at 1 Klux (incandescent light) should be about 0.4–0.6 mg C/hour. Hence this rate presents an excellent means of judging the quality of experiments. Experiments are presented where Chlorella pyrenoidosa was adapted to light intensities between 0.32 klux and 21 Klux. This alga adapts to different light intensities by varying the amount of pigments per cell. Algae grown at 1 Klux have about 10 times more chlorophyll per cell than those grown at 21 klux. Other species of algae—but by no means all—are shown to behave in the same way. The problem of algal resistance to photo-oxidation at high light intensities is discussed. Adaplation is shown to he one of the mechanisms which make the algae resistent. “Chlorophyll inactivation” is another. Experiments with the diatom Skeletonema costatum concerning adaptation to different temperatures have been performed. The fact that the alga has essentially the same rate of photosynthesis per cell at all light intensities at 20°C and 7°C, may be attributed to an increase of all the enzymes at the low temperature. The amount of protein per cell was twice as high at 7°C as at 20°C.     

Monochromatic Light Saturation Curves for Photosynthesis in Chlorella

We used a small oxygen electrode and a grating monochromator of 10 mmu half-band width to determine light-saturation curves of photosynthesis for films of Chlorella pyrenoidosa no more than 1 cell thick. All curves were referenced to the lightsaturated rate observed in 5 mw/cm(2) of 680 mmu. To a close approximation (+/- 2%) the light-saturated rate was independent of wavelength over the region in which light intensity was sufficient to make the test (450-705 mmu).At wavelengths of high absorption we obtained intensities sufficient to cause photoinhibition. As a measure of photoinhibition we used the breakpoint, the lowest intensity at which rate of photosynthesis decreased with time. In general, the saturated rate was slightly lower and the estimated breakpoint was considerably lower at wavelengths of high absorption. Maximum rate of absorption of quanta at the breakpoint was highest in the far-red (>/= 700 mmu), lower and relatively constant in the near-red (630-680 mmu), and lowest in the blue. At 435 and 450 mmu the breakpoint occurred below saturation. We attribute small deviations in maximum rate of photosynthesis to effects of photoinhibition.Independence of wavelength observed for the light-saturated rate is consistent with models of photosynthesis in which maximum rate is limited by the same dark reaction at all wavelengths.     

SUN-SHADE ECOTYPES OF A BLUEGREEN ALGA IN A HOT SPRING1,2

Clone cultures of the thermophilic alga Plectonema notatum Schmidle were established from cells collected from the high and low light intensity regions of the algal mat which developed in Jerry Johnson Hot Spring, Idaho. Clones isolated from cither high, or low light intensify zones were grown at light intensities of 8000 and 400 ft-c. The existence of specialized and genetically fixed sun or shade ecotypes was evidenced, by the ability of low light intensity clones to synthesize more light harvesting chlorophyll a when grown under low light conditions than the high light clones. High light clones showed light saturation of photosynthesis at higher light intensities with higher carboxylating enzyme activities and less chlorophyll a than low light clones when both were cultured at 8000 ft-c. These clones displayed infraspecific variation along the light intensity gradient and therefore exist as a mosaic of light, intensity ecotypes.     

Photosynthetic Activity during the Life Cycle of Synchronous Skeletonema Cells

A culture of Skeletonema costatum grown at a light intensity of 3 klux and at 20°C was synchronized in diurnally intermittent illumination of 12 hour light and 12 hour dark. The culture was hardly fully synchronous as the cell division period lasted about 9 hours. The cell division started in the middle of the light period. The concentration of the pigments: chlorophyll a, chlorophyll 6 and fucoxanthin and the rate of light-saturated photosynthesis were followed every hour during the 24 hour period. Both the concentration of pigments and the photosynthetic activity showed a rhythmical variation. The concentration per cell of all three pigments examined increased during the development of the cells and decreased automatically during the period of cell division. An increase in the pigment concentration was found only in the light period. The rate of light-saturated photosynthesis calculated per unit of cell number increased during the cell development and decreased during the division period. The increase in the photosynthetic activity at light-saturation started about 4 hours after the end of cell division, which was 4 hours before the light was turned on while the increase in the concentration of chlorophyll a first started 1–2 hours after this moment. The variation in photosynthetic activity was compared with that found by other workers. The results found with Chlorella ellipsoidea by Japanese scientists (Nihci et al.) was explained as an inhibition phenomenon because the cells were not adapted to the experimental conditions.     

The Adaptation of Plankton Algae

The variation in Skeletonema cells grown at 3 klux continuous illumination and 20°C is reported. Four different types of lamps gave no difference in the photosynthetic characteristics. The average diameter of the cells decreased from 8–3.5 μ during their six months vegetative period. The ratio between the pigment content in the largest and the smallest cells was about 2:1. A good correlation between cell volume and chlorophyll a content was found for this species. The content of chlorophyll c generally varied between 4 and 17 per cent of the chlorophyll a content. — A distinct correlation between the chlorophyll a content and the rate of photosynthesis per unit of cells at low light intensity was found. The rate of photosynthesis, in mg C per mg chlorophyll a and hour at 1 klux, varied between 0.40 and 0.70 for all 60 experiments with an average value of 0.56. The corresponding value for cells deficient in phosophorus was 0.19 and for cells deficient in nitrogen 0.09. — The material also showed a good correlation between the rate of photosynthesis per cell at 1 klux and the light-saturated rate of photosynthesis. I_k varied between 7 and 13 klux.     

Metabolic conditions in Chlorella

1. The effect of nitrate reduction and assimilation on the CO(2)/O(2) quotient of gas exchange has been used as an index of the relative rates of carbon and nitrogen assimilation in Chlorella pyrenoidosa. Changes in over-all metabolism induced by starvation, high light intensity, and nitrogen deficiency have been studied in comparison with the metabolism of cells growing at light-limiting intensities. 2. Starvation, which results in depletion of carbohydrate reserves, gives rise to a high CO(2)/O(2) quotient ( approximately 0.9) during photosynthesis and, therefore, a high C/N assimilation ratio. Starved cells apparently restore their normal C/N ratio before becoming growing cells. 3. Under photosynthesis-saturating light intensities cells show the high CO(2)/O(2) quotient (0.9) indicative of a high C/N assimilation ratio. Return to low light intensities is followed by the abnormally low CO(2)/O(2) quotient ( approximately 0.4) of a low C/N assimilation ratio. High light intensity apparently gives rise to a condition of a limiting rate of nitrogen assimilation and to an overflow metabolism analogous to that found in other microorganisms. 4. Nitrogen deficiency leads to a completely carbohydrate metabolism in short time experiments and makes still more pronounced the effects characteristic of high light intensity alone. 5. Considerations of nutritional economy sustain the experimental evidence in establishing the metabolism of cells growing under light-limiting intensities as the normal or reference metabolic condition in Chlorella.     

EFFECTS OF ENVIRONMENTAL FACTORS ON PHOTOSYNTHESIS PATTERNS IN PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE). II. EFFECT OF OXYGEN1

Oxygen inhibited the rate of light-saturated photosynthesis of the marine diatom Phaeodactylum tricornutum Bohlin. However, inhibition could only be detected with O₂ concentrations approaching 100%. Atmospheric concentrations of O₂ (21%) had little effect on photosynthesis. In this, Phaeodactylum more closely resembles the so-called C-4 plants which show low rates of photorespiration. The results presented here agree with others in showing increased O₂ inhibition at reduced bicarbonate concentrations. The biochemical mechanism of photorespiration in Phaeodactylum appears to be similar to that reported for other photosynthetic systems. The activity of ribulose-1,5 diphosphate (RuDP) carboxylase in cell-free extracts was also inhibited, by oxygen. Inhibition by O₂ was optimal at pH 9.2 as was the RuDP-dependent O₂ uptake. RuDP carboxylase/oxygenase ratios decreased with increasing pH and were greater in cells grown at lower light intensities. Carboxylase levels were less affected by the light intensity for growth than were the levels of the oxygenase. Short-term incorporation of NaHCO₃-¹⁴C by cells grown at high light intensities showed increased labelling of glycolate and glycine plus serine under O₂ compared with nitrogen. There was a concomitant decrease in the radioactivity found in phosphoglyeric acid (PGA) and sugar phosphates in the presence of O₂. The effects of O₂ on the short-term pattern of photosynthesis were less marked when the alga was previously grown at low light intensities.     

Further Studies on Differentiation of Photosynthetic Properties in Sun and Shade Ecotypes of Solidago virgaurea

Measurements of the fraction of the incident light absorbed by diverse Solidago leaves revealed that differences in light harvesting capacity cannot explain the differences in efficiency of utilization of weak light in photosynthesis that have previously been shown to exist between sun and shade ecotypes when these have been grown in strong light and between identical clones of shade ecotypes when grown at different light intensities. Photosynthesis measurements at low and normal oxygen concentrations, provided no evidence that a different degree of inhibition of photo-synthetic CO₂ uptake by atmospheric oxygen is responsible for the observed differences in photosynthetic efficiency, at low or high light intensities. These results support the conclusion that the markedly less efficient use of weak light by shaded habitat clones grown in strong as compared with weak light is caused primarily by damage to the photosystems, or to a site close to them. Measurements of Emerson enhancement and of light-induced absorbance changes provide some evidence that photoreaction II is more affected than I. Enzyme extracts prepared from clones native to an exposed habitat were found to contain considerably higher activities of carboxydismutase (ribulosc-l,5-diphos-phate carboxylase) than from clones native to a shaded habitat when the plants were previously grown at a moderately high light intensity. Exposed habitat clones apparently have a genetically determined, higher capacity to produce the carboxyla-tion enzyme than shaded habitat clones. The high degree of correlation found when the light-saturated rate of CO₂ uptake in vivo of a number of individual Solidago leaves is plotted against the carboxydismutase activities found in the extracts of these same leaves suggests that low carboxydismutase activity is one of the intrinsic properties responsible for the low capacity for light-saturated photosynthesis of clones from shaded habitats. It is concluded from this and other investigations that differentiation between plants from habitats with contrasting light intensities, whether unrelated species or ecotypos of the same species, probably involves the capacity of several component steps of the photosynthetic process.     

Adaptation to quantum flux by the emerson photosynthetic unit

The size of the Emerson photosynthetic unit was measured in Chlorella pyrenoidosa strain no. 252 grown at light intensities between 50 and 1000 foot candles. The Emerson photosynthetic unit changed from a minimum size of 1970 molecules chlorophyll a + b/O₂ per flash in cells grown at 1000 foot candles to a maximum size of 3150 molecules chlorophyll a + b/O₂ per flash for cells grown at 50 foot candles. The size changes were interpreted as a partial adaptation where the trapping center antenna responded to changes in incident light intensity. Light-induced changes in chlorophyll content and size of the Emerson photosynthetic unit were directly related.     

Effect of Light Intensity on Adaptation of Dunaliella to Very High Salt Concentrations

Light intensity was found to have a strong effect on the adaptationto high salt concentrations of a green microalga, Dunaliellaparva, normally grown at low and medium salt concentrations.At high light intensities (200 µmol m^–2 s^–1)the cell glycerol content increased in parallel with an increasein external salt concentration; protein synthesis and cell divisioncontinued with no period of arrest. At low light intensitiesno glycerol synthesis occurred as the external salt concentrationwas raised; protein synthesis and cell division were equallyarrested. The importance of high light intensity during theinitial phase of increase of salt concentration was demonstratedand was found to be a requirement for protein synthesis andcell division. In experiments designed to discover the effectof light intensity on cells growing in media in which the saltconcentration was kept constant, it was confirmed that the lightintensity needed for growth increases as the salt concentrationof the medium is increased. Key words: Dunaliella, salt concentration, light intensity, growth rates     

Photosystem II Excitation Pressure and Development of Resistance to Photoinhibition (I. Light-Harvesting Complex II Abundance and Zeaxanthin Content in Chlorella vulgaris)

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 - qp (photochemical quenching). We established that Chlorella vulgaris exposed to either 5[deg]C/150 [mu]mol m-2 s-1 or 27[deg]C/2200 [mu]mol m-2 s-1 experienced a high PSII excitation pressure of 0.70 to 0.75. In contrast, Chlorella exposed to either 27[deg]C/150 [mu]mol m-2 s-1 or 5[deg]C/20 [mu]mol m-2 s-1 experienced a low PSII excitation pressure of 0.10 to 0.20. Chlorella grown under either regime at high PSII excitation pressure exhibited: (a) 3-fold higher light-saturated rates of O2 evolution; (b) the complete conversion of PSII[alpha] centers to PSII[beta] centers; (c) a 3-fold lower epoxidation state of the xanthophyll cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll a/b; and (e) a lower abundance of light-harvesting polypeptides than Chlorella grown at either regime at low PSII excitation pressure. In addition, cells grown at 5[deg]C/150 [mu]mol m-2 s-1 exhibited resistance to photoinhibition comparable to that of cells grown at 27[deg]C/2200 [mu]mol m-2 s-1 and were 3- to 4-fold more resistant to photoinhibition than cells grown at either regime at low excitation pressure. We conclude that increased resistance to photoinhibition upon growth at low temperature reflects photosynthetic adjustment to high excitation pressure, which results in an increased capacity for nonradiative dissipation of excess light through zeaxanthin coupled with a lower probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.     

Photosynthetic response to varying light intensity in ecotypes of Solanum dulcamara L. from shaded and exposed habitats

Summary Within the widespread species Solanum dulcamara, contrasting ecotypes were found which are physiologically adapted to the light intensities prevailing in their natural habitats. When grown under a high light intensity, an ecotype from a shaded habitat exhibits signs of damage. Another one from an exposed habitat has higher rates of photosynthetic CO₂ uptake when grown under strong as compared to weak light and does not show damage. This differential response becomes even more evident when leaves of both ecotypes are grown to maturity under weak light and are subsequently subjected to strong light for some time. The quantum requirement for photosynthesis increases in the shade-, but not in the sun-ecotype. The sun type increases its rate of photosynthesis under saturating light intensities after a few days in strong light.No significant difference in physical resistances to gas diffusion could be found to explain the highly differing rates of photosynthesis. With the increase in photosynthetic capacity in leaves of the sun type, protein content, activity of RuDP carboxylase, and concentration of Fraction I protein increased likewise. It is suggested that de novo synthesis of photosynthetic enzymes in fully expanded leaves of the sun ecotype following treatment with strong light is the cause of its increased capacity for CO₂ fixation.     

EFFECTS OF CANOPY POSITION AND IRRADIANCE ON THE LEAF PHYSIOLOGY AND MORPHOLOGY OF PENTACLETHRA MACROLOBA (MIMOSACEAE)

Pentaclethra macroloba (Willd.) Kuntze (Mimosaceae) is a dominant late-successional tree species in the Atlantic lowland forests of Costa Rica. Leaves of P. macroloba from three heights in the forest canopy were compared with leaves of seedlings grown in controlled environment chambers under four different irradiance levels. Changes in leaf characteristics along the canopy gradient paralleled changes resulting from the light gradient under controlled conditions. The effect of light or canopy position on light-saturated photosynthesis was small, with maximum photosynthesis increasing from 5 to 6.5 μmol m^−-2 s^−-1 from understory to canopy. Both chamber grown and field leaves showed large adjustments in photosynthetic efficiency at low light via reductions in dark respiration rates and increases in apparent quantum yields. Light saturation of all leaves occurred at or below 500 μmol m^−-2 s^−-1. Leaf thickness, specific leaf weight, and stomatal density increased to a greater extent than saturated photosynthesis with higher irradiance during growth or height in the canopy. As a result, there was a poor correspondence between leaf thickness and light-saturated photosynthesis on an area basis. It is concluded that Pentaclethra macroloba possesses the characteristics of a typical shade-tolerant species.     

Light adaptation of cyclic electron transport through Photosystem I in the cyanobacterium Synechococcus sp. PCC 7942

Photosystem I-driven cyclic electron transport was measured in intact cells of Synechococcus sp PCC 7942 grown under different light intensities using photoacoustic and spectroscopic methods. The light-saturated capacity for PS I cyclic electron transport increased relative to chlorophyll concentration, PS I concentration, and linear electron transport capacity as growth light intensity was raised. In cells grown under moderate to high light intensity, PS I cyclic electron transport was nearly insensitive to methyl viologen, indicating that the cyclic electron supply to PS I derived almost exclusively from a thylakoid dehydrogenase. In cells grown under low light intensity, PS I cyclic electron transport was partially inhibited by methyl viologen, indicating that part of the cyclic electron supply to PS I derived directly from ferredoxin. It is proposed that the increased PSI cyclic electron transport observed in cells grown under high light intensity is a response to chronic photoinhibition.Abbreviations DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - ES energy storage - MV methyl viologen - PA_m photoacoustic thermal signal with strong non-modulated background light added - PA_s photoacoustic thermal signal without background light added CIW/DPB Publication No. 1205.     

Adaptation of the Photosynthetic Apparatus of Scenedesmus obliquus to Strong and Weak Light Conditions

Homocontinuous cultures of the unicellular green alga Scenedesmus obliquus were grown under strong (28 W/m²～28,000 lux) and weak (5 W/m²～5000 lux) light conditions to simulate the conditions of ‘sun’ and ‘shade’ plants. As in higher plants the cells adapted to strong light had less chlorophyll but demonstrated a higher photosynthetic capacity and a higher respiration rate, so that their compensation point was reached at three times higher energy than in the cells grown under low light intensities. The CO₂ fixation rate and the RuDP carboxylase activity under saturating light intensities were both higher in the cells grown in strong light. In spite of the differences in the pigment content and in the light saturated photosynthetic capacities for both cultures, the quantum yields of photosynthetic oxygen evolution were equal. As documented for some species of higher plants Scenedesmus is not genetically determined to be either a ‘sun’ or ‘shade’ organism but can adapt its photosynthetic apparatus to the different light intensities.     

Selective Formation of Photosystem 1 under Illumination at Low Intensity

Analyses of chlorophylls a and b and P700 in the wheat leaves grown for 8 days under illumination with white light at different intensities suggested selective formation of photosystem 1 of the photosynthesis at low light intensities. This was confirmed for the two types of chloroplasts isolated from leaves grown at light intensities of 1.1 and 240 μ W/cm², respectively, by measuring their pigment compositions, activities of photosystems 1 and 2, and absorption and fluorescence spectra. The chloroplasts developed at the low intensity showed properties only of photosystem 1 while those developed at the high intensity showed properties of both photosystems 1 and 2. Only photosystem 1 particles were obtained by fractionation of low intensity chloroplasts by treatment with digitonin followed by centrifugation, while high intensity chloroplasts could be fractionated into photosystem-1 and photosystem-2 particles. When the leaves grown at low light intensity were illuminated with strong light, photosystem 2 was developed. The fluorescence emission spectrum of low intensity chloroplasts at 77°K showed two peaks at 685 and 734 nm, and the spectrum of high intensity chloroplasts showed three peaks at 685, 697 and 740 nm.     

Immuno-electron microscopic quantification of the fucoxanthin chlorophyll a/c binding polypeptides Fcp2, Fcp4, and Fcp6 of Cyclotella cryptica grown under low- and high-light intensities.

The diatom Cyclotella cryptica was grown under low- and high-intensity white light of 50 and 500 micromol photons m-2 s-1, respectively. Western immunoblotting showed that the diatom adapted its light-harvesting apparatus, giving rise to different amounts of distinct fucoxanthin chlorophyll a/c binding polypeptides (Fcp). The amount of Fcp2 was approximately two-fold higher under low-light than under high-light conditions, whereas the amount of Fcp6 increased four- to five-fold under high-light conditions. For Fcp4, no significant differences were detected in response to either light regime. Cells of Cyclotella grown under high- and low-light intensity were subjected to immunoelectron microscopy. Quantification of the gold label, expressed as gold particles per microm2, confirmed the results obtained by Western immunoblotting. Exposure to low light resulted in the detection of approximately six times more Fcp2-bound gold particles per microm2 in thylakoid membranes, whereas in cells grown under high light the number of Fcp6-bound gold particles increased ten-fold. For Fcp4, similar amounts of gold particles per microm2 were counted under the two light regimes. These immunocytochemical results confirmed molecular data derived from phylogenetic analyses of the sequences of genes encoding fucoxanthin chlorophyll a/c binding polypeptides (fcp genes) and from measurements of steady-state fcp mRNA concentrations. The results show that Fcp2 and Fcp6 accumulate under low- and high-light intensity, respectively, whereas Fcp4 seems to be constitutively synthesized.     

The desert green algae Chlorella ohadii thrives at excessively high light intensities by exceptionally enhancing the mechanisms that protect photosynthesis from photoinhibition

Although light is the driving force of photosynthesis, excessive light can be harmful. One of the main processes that limits photosynthesis is photoinhibition, the process of light-induced photodamage. When the absorbed light exceeds the amount that is dissipated by photosynthetic electron flow and other processes, damaging radicals are formed that mostly inactivate photosystem II (PSII). Damaged PSII must be replaced by a newly repaired complex in order to preserve full photosynthetic activity. Chlorella ohadii is a green microalga, isolated from biological desert soil crusts, that thrives under extreme high light and is highly resistant to photoinhibition. Therefore, C. ohadii is an ideal model for studying the molecular mechanisms underlying protection against photoinhibition. Comparison of the thylakoids of C. ohadii cells that were grown under low light versus extreme high light intensities found that the alga employs all three known photoinhibition protection mechanisms: (i) massive reduction of the PSII antenna size; (ii) accumulation of protective carotenoids; and (iii) very rapid repair of photodamaged reaction center proteins. This work elucidated the molecular mechanisms of photoinhibition resistance in one of the most light-tolerant photosynthetic organisms, and shows how photoinhibition protection mechanisms evolved to marginal conditions, enabling photosynthesis-dependent life in severe habitats.     

Regulation of the Size of Light-Harvesting Antennae in Response to Light Intensity in the Green Alga Chlorella pyrenoidosa

Changes in size of the light-harvesting Chl-protein complex(LHC) induced by changes in light intensity were studied withthe green alga Chlorella pyrenoidosa. The Chi a/b ratio, whichis correlated with the size of LHC, varied over a wide range(2.5–5.0) when the light intensity for autotrophic growthwas changed. Comparison of properties of LHC II isolated fromcells grown under light of high and low intensity indicatedthat the large difference in Ch1 a/b ratios in cells grown underlight of different intensities is due mainly to changes in levelsof LHC. Reduction in levels of LHC under light of high intensitydid not occur when proliferation of cells was suppressed. Thisresult indicates that reduction in levels of LHC is not attributableto acceleration of the degradation of LHC under light of highintensity. Stimulation of formation of LHC occurred even underlight of high intensity when formation of photosystems was suppressedby chloram-phenicol (CAP). Analysis of the CAP-induced formationof LHC indicated that (1) such formation of LHC was regulatedby light intensity, being less active under higher intensity,and (2) the suppressive effect of gabaculine, an inhibitor ofthe synthesis of porphyrin, and thus of Ch1, was greater underlight of high intensity while the suppressive effect of cycloheximide,an inhibitor of the synthesis of apoprotein, was slightly greaterunder light of low intensity. The results described in thisreport indicate that (1) intensity-induced changes in the sizeof LHC in Chlorella pyrenoidosa are due to regulation of theassembly of LHC and (2) the regulation occurs primarily at thelevel of the synthesis of Ch1. (Received June 1, 1989; Accepted August 21, 1989)