THE PHOTOSYNTHETIC EFFICIENCY OF PHYCOCYANIN IN CHROOCOCCUS,AND THE PROBLEM OF CAROTENOID PARTICIPATION IN PHOTOSYNTHESIS

The absorption spectra of the principal pigment components extracted from Chroococcus cells have been measured, and their sum compared with the absorption of a suspension of living cells. The agreement was sufficiently close so that it was concluded the absorption spectra of the extracted and separated pigment components could be used to obtain estimates of the relative absorption of the various components in the living cells. The quantum yield of Chroococcus photosynthesis was measured at a succession of wave lengths throughout the visible spectrum, and the dependence of yield on wave length was compared with the proportions of light absorbed by the pigment components. This comparison showed beyond reasonable doubt that the light absorbed by phycocyanin is utilized in photosynthesis with an efficiency approximately equal to that of the light absorbed by chlorophyll. The light absorbed by the carotenoid pigments of Chroococcus seems for the most part to be unavailable for photosynthesis. The results leave open the possibility that light absorbed by the carotenoids is active in photosynthesis, but with an efficiency considerably lower than that of chlorophyll and phycocyanin. It is also possible that the light absorbed by one or a few of the several carotenoid components is utilized with a high efficiency, while the light absorbed by most of the components is lost for photosynthesis.     

Relation between pigment content and photosynthetic characteristics in a blue-green algae

1. The blue-green alga Anacystis nidulans was cultured under steady state conditions at 25 and 39°C. and under several different light intensities to give five different types of cells. 2. Cells were submitted to pigment analysis based upon acetone extracts and aqueous extracts obtained by sonic disintegration. The different cell types show a threefold range of chlorophyll content and a fourfold range of phycocyanin content with only minor changes in the chlorophyll/phycocyanin ratio. Cells of highest pigment content were estimated to contain 2.8 per cent chlorophyll a and 24 per cent phycocyanin, the latter on a total chromoproteid basis. 3. Light intensity curves of photosynthesis were obtained for each of the cell types at 25 and at 39°C. The slopes of the light-limited regions of the curves are approximately linear functions of chlorophyll and phycocyanin contents. Maximum light-saturated rates of photosynthesis at 25 and 39° show no simple relation to pigment content.     

The fluorescence spectra of red algae and the transfer of energy from phycoerythrin to phycocyanin and chlorophyll

1. The fluorescence spectra of the alga Porphyridium have been recorded as energy distribution curves for eleven different incident wave lengths of monochromatic incident light between wave lengths 405 and 546 mµ. 2. In these spectra chlorophyll fluorescence predominates when the incident light is in the blue part of the spectrum which is strongly absorbed by chlorophyll. 3. For blue-green and green light the spectrum excited in Porphyridium contains in addition to chlorophyll fluorescence, the fluorescence bands characteristic of phycoerythrin and of phycocyanin. 4. From these spectra the approximate curves for the fluorescence of the individual pigments phycoerythrin, phycocyanin, and chlorophyll in the living material have been derived and the relative intensity of each of them has been obtained for each of the eleven incident wave lengths. 5. The effectiveness spectrum for the excitation of the fluorescence of these three pigments in vivo has been plotted. 6. From comparisons of the effectiveness spectrum for the excitation of each of these pigments it appears that both phycocyanin and chlorophyll receive energy from light which is absorbed by phycoerythrin. 7. It is suggested that phycocyanin may be an intermediate in the resonance transfer of energy from phycoerythrin to chlorophyll. 8. Since phycoerythrin and phycocyanin transfer energy to chlorophyll, it appears probable that chlorophyll plays a specific chemical role in photosynthesis in addition to acting as a light absorber.     

Incompetence of dark-synthesized phycocyanin in excitation transfer to photosystem II chlorophyll

Summary When cells of Anabaena variabilis, all the phycobilin pigments of which had been newly synthesized in the dark, were excited by light absorbed in phycocyanin, the fluorescence emission spectrum showed a peak corresponding to the emission from allophycocyanin, but no emission from chlorophyll. These cells were active in photosynthesis and, when excited by light absorbed by chlorophyll, the emitted fluorescence was characteristic of photosystem II chlorophyll. This indicates that dark synthesized phycocyanin is capable of excitation transfer to allophycocyanin but not to photosystem II chlorophyll.Abbreviation CMU 3-(p-chlorophenyl)-1,1-dimethylurea     

Photosynthetic efficiency of marine plants

Multicellular marine plants were collected from their natural habitats and the quantum efficiency of their photosynthesis was determined in the laboratory in five narrow wave length bands in the visible spectrum. The results along with estimates of the relative absorption by the various plastid pigments show a fairly uniform efficiency of 0.08 molecules O₂ per absorbed quantum for (a) chlorophyll of one flowering plant, green algae, and brown algae, (b) fucoxanthol and other carotenoids of brown algae, and (c) the phycobilin pigments phycocyanin and phycoerythrin of red algae. The carotenoids of green algae are sometimes less efficient while those of red algae are largely or entirely inactive. Chlorophyll a of red algae is about one-half as efficient (_o2 = 0.04) as either the phycobilins, or the chlorophyll of most other plants. These results as well as those of high intensity and of fluorescence experiments are consistent with a mechanism in which about half the chlorophyll is inactive while the other half is fully active and is an intermediate in phycoerythrin- and phycocyanin-sensitized photosynthesis.     

Photoinhibition of chloroplast reactions. I. Kinetics and action spectra

A study was made of photoinhibition of spinach chloroplast reactions. The kinetics and spectral characteristics of the photoinhibition over a range between 230 and 700 mμ have been examined. The decline of activity due to preillumination was independent of wavelength, and dependent upon the number of quanta applied, not upon the rate of application. The effectiveness spectra of photoinhibition indicate that active ultraviolet light is absorbed by a pigment which is not a normal light absorber for photosynthesis and acts with a high quantum efficiency (> 0.1) for photoinhibition.

Active visible light is absorbed by the pigments which sensitize photosynthesis (chlorophyll, carotenoids). A very low quantum efficiency (about 10⁻⁴) was observed for the photoinhibition with visible light.

The action spectrum of the photoinhibition of dye reduction by chloroplasts and lyophylized Anacystis cells indicated that the damage caused by visible light is due to quanta absorbed by photosystem II. However, since system I might not be involved in dye reduction, the spectra may reflect only damage to photosystem II.

     

Photosynthetic action spectra of marine algae

A polarographic oxygen determination, with tissue in direct contact with a stationary platinum electrode, has been used to measure the photosynthetic response of marine algae. These were exposed to monochromatic light, of equal energy, at some 35 points through the visible spectrum (derived from a monochromator). Ulva and Monostroma (green algae) show action spectra which correspond very closely to their absorption spectra. Coilodesme (a brown alga) shows almost as good correspondence, including the spectral region absorbed by the carotenoid, fucoxanthin. In green and brown algae, light absorbed by both chlorophyll and carotenoids seems photosynthetically effective, although some inactive absorption by carotenoids is indicated. Action spectra for a wide variety of red algae, however, show marked deviations from their corresponding absorption spectra. The photosynthetic rates are high in the spectral regions absorbed by the water-soluble "phycobilin" pigments (phycoerythrin and phycocyanin), while the light absorbed by chlorophyll and carotenoids is poorly utilized for oxygen production. In red algae containing chiefly phycoerythrin, the action spectrum closely resembles that of the water-extracted pigment, with peaks corresponding to its absorption maxima (495, 540, and 565 mµ). Such algae include Delesseria, Schizymenia, and Porphyrella. In the genus Porphyra, there is a series P. nereocystis, P. naiadum, and P. perforata, with increasingly more phycocyanin and less phycoerythrin: the action spectra reflect this, with increasing activity in the orange-red region (600 to 640 mµ) where phycocyanin absorbs. In all these red algae, photosynthesis is almost minimal at 435 mµ and 675 mµ, where chlorophyll shows maximum absorption. Although the chlorophylls (and carotenoids) are present in quantities comparable to the green algae, their function is apparently not that of a primary light absorber; this role is taken over by the phycobilins. In this respect the red algae (Rhodophyta) appear unique among photosynthetic plants.     

Transfer of the Excitation Energy in Anacystis nidulans Grown to Obtain Different Pigment Ratios

The blue-green alga, Anacystis nidulans, was grown in lights of different colors and intensities, and its absorption and fluorescence properties were studied. Strong orange light, absorbed mainly by phycocyanin, causes reduction in the ratio of phycocyanin to chlorophyll a; strong red light, absorbed mainly by chlorophyll, causes an increase in this ratio. This confirms the earlier findings of Brody and Emerson (12) on Porphyridum, and of Jones and Myers (8) on Anacystis. Anacystis cultures grown in light of low intensity show, upon excitation of phycocyanin, emission peaks at 600 mmu and 680 mmu, due to the fluorescence of phycocyanin and chlorophyll a, respectively. Changes in the efficiency of energy transfer from phycocyanin to chlorophyll a are revealed by changes in the ratios of these two bands. A decrease in efficiency of energy transfer from phycocyanin to chlorophyll a seems to occur whenever the ratio of chlorophyll a to phycocyanin deviates from the normal. Algae grown in light of high intensity show, upon excitation of phycocyanin, only a fluorescence band at 660 mmu and no band at 680 mmu. This suggests reduced efficiency of energy transfer from phycocyanin to the strongly fluorescent form of chlorophyll a (chlorophyll a(2)) and perhaps increased transfer to the weakly fluorescent form of chlorophyll a (chlorophyll a(1)).     

Light production by green plants

1. Green plants have been found to emit light of approximately the same color as their fluorescent light for several minutes following illumination. This light is about 10^–3 the intensity of the fluorescent light, about one-tenth second after illumination below saturation or 10^–6 of the intensity of the absorbed light. 2. The decay curve follows bimolecular kinetics at 6.5°C. and reaction order 1.6 at 28°C. 3. This light saturates as does photosynthesis at higher light intensities and in about the same intensity range as does photosynthesis. 4. An action spectrum for light emitted as a function of the wave length of exciting light has been determined. It parallels closely the photosynthetic action spectrum. 5. The intensity of light emission was studied as a function of temperature and found to be optimal at about 37°C. with an activation energy of approximately 19,500 calories. Two-temperature studies indicated that the energy may be trapped in the cold, but that temperatures characteristic for enzymatic reactions are necessary for light production. 6. Illumination after varying dark periods showed initial peaks of varying height depending on the preceding dark period. 7. 5 per cent CO₂ reversibly depresses the amount of light emitted by about 30 per cent. About 3 minutes are required for this effect to reach completion at room temperatures. 8. Various inhibitors of photosynthesis were tested for their effect on luminescence and were all inhibitory at appropriate concentrations. 9. Irradiation with ultraviolet light (2537A) inhibits light production at about the same rate as it inhibits photosynthesis. 10. This evidence suggests that early and perhaps later chemical reactions in photosynthesis may be partially reversible.     

Studies on the Second Emerson Effect in the Hill Reaction in Algal Cells

This paper shows that the “second Emerson effect”¹ exists not only in photosynthesis, but also in the quinone reduction (Hill reaction), in Chlorella pyrenoidosa and Anacystis nidulans. The peaks at 650 mμ, 600 mμ, 560 mμ, 520 mμ, and 480 mμ, observed in the action spectrum of this effect in the Hill reaction in Chorella, are attributable to chlorophyll b; the occurrence of an additional peak at 670 mμ, 620 mμ, and of two (or three) peaks in the blueviolet region suggests that (at least) one form of chlorophyll a contributes to it. In analogy to suggestions made previously in the interpretation of the Emerson effect in photosynthesis, these results are taken as indicating that excitation by light preferentially absorbed by one (or two) forms of chlorophyll a (Chl a 690 + 700), needs support by simultaneous absorption of light in another form of chlorophyll a (Chl a 670)—directly or via energy transfer from chlorophyll b—in order to produce the Hill reaction with its full quantum yield. In Anacystis, the participation of phycocyanin in the Emerson effect in the Hill reaction is revealed by the occurrence, in the action spectrum of this effect, of peaks at about 560 mμ, 610 mμ, and 640 mμ; a peak at 670 mμ, due to Chl a 670, also is present.     

Studies with cyanidium caldarium,an anomalously pigmented chlorophyte

Summary Cyanidium caldarium, an alga found in acid hot springs troughout the world, has a morphology and developmental history resembling those of Chlorella, but contains C-phycocyanin and no chlorophyll other than chlorophyll a. The reasons for considering it to be a member of the Chlorophyta are reviewed. Cyanidium is also remarkable for its thermal and acid tolerance. It grows readily in the dark on sugar media. However, light is required for the formation of chlorophyll and phycocyanin except in occasional variant cells which can form limited amounts of these pigments in the dark. Light-grown Cyanidium carries out normal green plant photosynthesis but resembles the red and some of the blue-green algae in that chlorophyll-absorbed light is used with lower efficiency than that absorbed by phycocyanin.The possible significance of the unusual pigmentation of Cyanidium is discussed.Contribution no.23 from the Laboratory of Comparative Physiology and Morphology of The Kaiser Foundation.     

Untersuchung der Beziehung zwischen Pigmentzusammensetzung und CO2-Gaswechsel bei der Blaualge Anacystis nidulans

Summary Changes in culture conditions caused strong changes in the pigment composition in the blue-green alga Anacystis nidulans. Under normal illumination (white light; 0.6·10³ erg/cm²·sec) the relation between the amounts of chlorophyll a and phycocyanin was 1:6.6. In a high light intensity (20.8·10³ erg/cm²·sec) the phycocyanin content was reduced and the relations thus changed to 1:1.9. Growing the algae in red light of high intensity (20·10³ erg/cm²·sec) increased the phycocyanin content; the chlorophyll a: phycocyanin relation was then 1:12.1.The action spectrum of apparent photosynthesis showed a minimum at 473 nm in all three cultures. The maximum of photosynthesis in low light cultures fell in the absorption region of phycocyanin at 621 nm. The action spectrum of the red light culture showed a reduced rate of photosynthesis in the same region. The strong light culture had an action spectrum similar to that of the red light culture with a maximum at 651 nm. The differing action spectrum of the low light culture may be a result of interruption in the energy transfer from phycocyanin to chlorophyll a within pigment system II.The transients of CO₂ exchange are independent of the pigment composition. Two different types of transients were found depending on the wavelength of the incident light. In red light of 550–650 nm a higher stationary rate was reached after a maximum of photosynthesis at the beginning of the illumination period. In blue and far red light a lower rate was found after the first maximum. Following a illumination period in blue or far red light a CO₂ evolution in the dark was observed. On the other hand, this CO₂ evolution was not found after illumination with red light. These effects are possiblt caused by a decarboxylation reaction (photorespiration) which occurs only in blue and far red light.     

Photosynthesis Regulation by Glucohexaose Through Redox Changes in Cucumis sativus

To investigate the effects of glucohexaose (P6) on cucumber, leaf CO₂ assimilation, chlorophyll fluorescence parameters, chlorophyll content, and carbohydrate metabolism were examined in cucumber plants. The net photosynthetic rate (P _n ) of cucumber leaves was enhanced after being treated with 10 μg mL^?1 P6. The increase was correlated with increases in transpiration rate (E) and stomatal conductance (G _s), whereas the intercellular CO₂ concentration (C _i) was not different from the control plants. Chlorophyll content, absorption of light energy per unit area (ABS/CS), capture of light energy per unit area (TR_o/CS), quantum yield of electron transport per unit area (ET_o/CS), maximum photochemical efficiency of PSII (φP _o), quantum yield of photosynthetic institution electron transfer (φE _o), probability of other electron acceptors that captured exciton-transferred electrons to the electronic chain which exceeds QA (ψ _o), number of reaction centers per unit leaf area (RC/CS_o), and the performance index on absorption basis (PI_ABS) were improved, but heat dissipation per unit area (DI_o/CS) and maximum quantum yield of non-chemical quenching (φD _o) were reduced. In addition, increases in sucrose, soluble sugars, and starch contents were observed in P6-treated plants. However, H₂O₂ scavenger (DMTU) or NADPH oxidase inhibitor (DPI) pretreatment significantly abolished the effect of P6 on photosynthesis. The results demonstrated that ROS played a critical role in P6-induced photosynthesis. The increase in chlorophyll content together with efficient light absorption, transmission, and conversion in P6-treated plants is important for increasing photosynthesis.     

The photochemical and fluorescence properties of whole cells,spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum

The photochemical activities and fluorescence properties of cells, spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum were compared. The photochemical activities were measured in a whole range of wavelengths and expressed as quantum yield spectra (quantum yield vs. wavelength). The following reactions were measured: Photosynthesis (O₂ evolution) in whole cells; Hill reaction (O₂ evolution) with Fe(CN)₆^3? and NADP as electron acceptors (Photosystem II and Photosystem II+Photosystem I reactions); electron transfer from reduced 2,6-dichlorophenolindophenol to diquat (Photosystem I reaction). The fluorescence properties were emission spectra, quantum yield spectra and the induction pattern.On the basis of comparison between the quantum yield spectra and the pigments compositions the relative contribution of each pigment to each photosystem was estimated. In normal cells and spheroplasts it was found that Photosystem I (Photosystem II) contains about 90 % (10 %) of the chlorophyll a, 90 % (10 %) of the carotenoids and 15 % (85 %) of the phycocyanin. In spheroplast particles there is a reorganization of the pigments: they loose a certain fraction (about half) of the phycocyanin but the remaining phycocyanin attaches itself exclusively to Photosystem I (!). This is reflected by the loss of Photosystem II activity, a flat quantum yield vs. wavelength dependence and a loss of the fluorescence induction.The fluorescence quantum yield spectra conform qualitatively to the above conclusion. More quantitative estimation shows that only a fraction (20–40 %) of the chlorophyll of Photosystem II is fluorescent. Total emission spectrum and the ratio of variable to constant fluorescence are in agreement with this conclusion.The fluorescence emission spectrum shows characteristic differences between the constant and variable components. The variable fluorescence comes exclusively from chlorophyll a; the constant fluorescence is contributed, in addition to chlorophyll a, by phycocyanine and an unidentified long wavelength component.The variable fluorescence does not change in the transition from whole cells to spheroplasts. However, the constant fluorescence increases considerably. This indicates the release of a small fraction of pigments from the photosynthetic photochemical apparatus which then become fluorescent.     

The quantum efficiency of photosynthesis in macroalgae and submerged angiosperms

Summary Photon absorption and photosynthesis under conditions of light limitation were determined in six temperate marine macroalgae and eight submerged angiosperms. Photon absorption and photosynthetic efficiency based on incident light increased in proportion to chlorophyll density per area and approached saturation at the highest densities (300 mg chlorophyll m^–2) encountered. Absorption and photosynthetic efficiency were higher in brown and red algae than in green algae and angiosperms for the same chlorophyll density because of absorption by accessory pigments. Among thin macroalgae and submerged angiosperms chlorophyll variations directly influence light absorption and photosynthesis, whereas terrestrial leaves have chlorophyll in excess and thus there is only a minor influence of pigment variability on light-limited photosynthesis. The quantum efficiency of photosynthesis averaged 0.062±0.019 (±SD) mol O₂ mol^–1 photons absorbed for macroalgae and, significantly less, 0.049±0.016 mol O₂ mol^–1 photons for submerged angiosperms. Of the measurements 80% were between 0.037 and 0.079 mol O₂ mol^–1 photons. The results are lower than values given in the literature for unicellular algae and terrestrial C₃ species at around 0.1 mol O₂ mol^–1 photons, but resemble values for other marine macroalgae and terrestrial C₄ species. The reason for these differences remains unknown, but may be sought for in differential operation of cyclic photophosphorylation and photorespiration.     

Acclimation Processes in the Light-Harvesting System of the Cyanobacterium Anacystis nidulans following a Light Shift from White to Red Light

Cyanobacteria acclimate to changes in light by adjusting the amounts of different cellular compounds, for example the light-harvesting macromolecular complex. Described are the acclimatization responses in the light-harvesting system of the cyanobacterium Anacystis nidulans following a shift from high intensity, white light to low intensity, red light.

The phycocyanin and chlorophyll content and the relative amount of the two linker peptides (33 and 30 kilodaltons) in the phycobilisome were studied. Both the phycocyanin and chlorophyll content per cell increased after the shift, although the phycocyanin increased relatively more. The increase in phycocyanin was biphasic in nature, a fast initial phase and a slower second phase, while the chlorophyll increase was completed in one phase. The phycocyanin and chlorophyll responses to red light were immediate and were completed within 30 and 80 hours for chlorophyll and phycocyanin, respectively. An immediate response was also seen for the two phycobilisome linker peptides. The amount of both of them increased after the shift, although the 33 kilodalton linker peptide increased faster than the 30 kilodalton linker peptide. The increase of the content of the two linker peptides stopped when the phycocyanin increase shifted from the first to the second phase. We believe that the first phase of phycocyanin increase was due mainly to an increase in the phycobilisome size while the second phase was caused only by an increase in the amount of phycobilisomes. The termination of chlorophyll accumulation, which indicates that no further reaction center chlorophyll antennae were formed, occurred parallel to the onset of the second phase of phycocyanin accumulation.

     

A proportion of photosystem II core complexes are decoupled from the phycobilisome in light-state 2 in the cyanobacterium Synechococcus 6301

Cells of Synechococcus 6301 were briefly exposed to a phycocyanin-absorbed light in the presence of DCMU. PS II trap closure was then estimated from fluorescence induction measurements with excitation light absorbed predominantly either by chlorophyll or by phycocyanin. In cells adapted to light-state 2, the exposure to light absorbed by phycocyanin closed only a proportion of the PS II centres that could be closed by exposure to light absorbed by chlorophyll. This distinction was reduced in cells adapted to light-state 1. We conclude that a proportion of PS II core complexes become decoupled from the phycobilisomes during the transition to light-state 2.     

CHLOROPHYLL CATALYSIS AND EINSTEIN'S LAW OF PHOTOCHEMICAL EQUIVALENCE IN PHOTOSYNTHESIS

We describe the catalysis of photosynthesis by chlorophyll: (a) chlorophyll bound to carbonic acid (that is, the photolyte) absorbs 1 quantum of light and is thereupon split into oxygen, reduced carbonic acid, and free chlorophyll; (b) the photolyte (that is, chlorophyll bound to carbonic acid) is resynthesized from carbonic acid and free chlorophyll with the help of the energy derived from the reoxidation of two-thirds of the reduced carbon formed in the light reaction. For calculation of the true quantum requirement of photosynthesis from experimental measurements, it is necessary to measure the light absorbed by the photolyte only, since the light energy absorbed by free chlorophyll is not used in the oxygen development of photosynthesis. To eliminate the loss of the light absorbed by the free chlorophyll, the factor ɛ = photolyte/total chlorophyll must be introduced into the calculation. Failure to take ɛ into consideration has led to discrepancies of 1,000% between the quantum yields obtained in different laboratories. These discrepancies are now removed. The quantum requirement of the splitting of the photolyte is always 1.     

THE TOTAL LUMINOUS EFFICIENCY OF LUMINOUS BACTERIA

Methods are described for measuring the light emitted by an emulsion of luminous bacteria of given thickness, and calculating the light emitted by a single bacterium, measuring 1.1 x 2.2 micra, provided there is no absorption of light in the emulsion. At the same time, the oxygen consumed by a single bacterium was measured by recording the time for the bacteria to use up .9 of the oxygen dissolved in sea water from air (20 per cent oxygen). The luminescence intensity does not diminish until the oxygen concentration falls below 2 per cent, when the luminescence diminishes rapidly. Above 2 per cent oxygen (when the oxygen dissolving in sea water from pure oxygen at 760 mm. Hg pressure = 100 per cent) the bacteria use equal amounts of oxygen in equal times, while below 2 per cent oxygen it seems very likely that rate of oxygen absorption is proportional to oxygen concentration. By measuring the time for a tube of luminous bacteria of known concentration saturated with air (20 per cent oxygen) to begin to darken (2 per cent oxygen) we can calculate the oxygen absorbed by one bacterium per second. The bacteria per cc. are counted on a blood counting slide or by a centrifugal method, after measuring the volume of a single bacterium (1.695 x 10^–12 cc.). Both methods gave results in good agreement with each other. The maximum value for the light from a single bacterium was 24 x 10^–14 lumens or 1.9 x 10^–14 candles. The maximum value for lumen-seconds per mg. of oxygen absorbed was 14. The average value for lumen-seconds per mg. O₂ was 9.25. The maximum values were selected in calculating the efficiency of light production, since some of the bacteria counted may not be producing light, although they may still be using oxygen. The "diet" of the bacteria was 60 per cent glycerol and 40 per cent peptone. To oxidize this mixture each mg. of oxygen would yield 3.38 gm. calories or 14.1 watts per second. 1 lumen per watt is therefore produced by a normal bacterium which emits 14 lumen-seconds per mg. O₂ absorbed. Since the maximum lumens per watt are 640, representing 100 per cent efficiency, the total luminous efficiency if .00156. As some of the oxygen is used in respiratory oxidation which may have nothing to do with luminescence, the luminescence efficiency must be higher than 1 lumen per watt. Experiments with KCN show that this substance may reduce the oxygen consumption to 1/20 of its former value while reducing the luminescence intensity only ¼. A partial separation of respiratory from luminescence oxidations is therefore effected by KCN, and our efficiency becomes 5 lumens per watt, or .0078. This is an over-all efficiency, based on the energy value of the "fuel" of the bacteria, regarded as a power plant for producing light. It compares very favorably with the 1.6 lumens per watt of a tungsten vacuum lamp or the 3.9 lumens per watt of a tungsten nitrogen lamp, if we correct the usual values for these illuminants, based on watts at the lamp terminals, for a 20 per cent efficiency of the power plant converting the energy of coal fuel into electric current. The specific luminous emission of the bacteria is 3.14 x 10^–6 lumens per cm². One bacterium absorbs 215,000 molecules of oxygen per second and emits 1,280 quanta of light at λ_max = 510µµ. If we suppose that a molecule of oxygen uniting with luminous material gives rise to the emission of 1 quantum of light energy, only 1/168 of the oxygen absorbed is used in luminescence. On this basis the efficiency becomes 168 lumens per watt or 26.2 per cent.     

Growth under Red Light Enhances Photosystem II Relative to Photosystem I and Phycobilisomes in the Red Alga Porphyridium cruentum

Acclimation of the photosynthetic apparatus to light absorbed primarily by photosystem I (PSI) or by photosystem II (PSII) was studied in the unicellular red alga Porphyridium cruentum (ATCC 50161). Cultures grown under green light of 15 microeinsteins per square meter per second (PSII light; absorbed predominantly by the phycobilisomes) exhibited a PSII/PSI ratio of 0.26 ± 0.05. Under red light (PSI light; absorbed primarily by chlorophyll) of comparable quantum flux, cells contained nearly five times as many PSII per PSI (1.21 ± 0.10), and three times as many PSII per cell. About 12% of the chlorophyll was attributed to PSII in green light, 22% in white light, and 39% in red light-grown cultures. Chlorophyll antenna sizes appeared to remain constant at about 75 chlorophyll per PSII and 140 per PSI. Spectral quality had little effect on cell content or composition of the phycobilisomes, thus the number of PSII per phycobilisome was substantially greater in red light-grown cultures (4.2 ± 0.6) than in those grown under green (1.6 ± 0.3) or white light (2.9 ± 0.1). Total photosystems (PSI + PSII) per phycobilisome remained at about eight in each case. Carotenoid content and composition was little affected by the spectral composition of the growth light. Zeaxanthin comprised more than 50% (mole/mole), β-carotene about 40%, and cryptoxanthin about 4% of the carotenoid pigment. Despite marked changes in the light-harvesting apparatus, red and green light-grown cultures have generation times equal to that of cultures grown under white light of only one-third the quantum flux.