Internal conversion in the photosynthetic mechanism of blue-green algae

1. In Chroococcus a quantum of light absorbed by phycocyanin has 90 per cent the chance of doing photosynthesis that a quantum absorbed by chlorophyll has. 2. By a process analogous to internal conversion in radioactivity (but with the linear dimensions and the wave length 10⁴ times larger) there will be transferred from phycocyanin to chlorophyll See PDF for Equation (a number of the order of 100) quanta for every one emitted as fluorescent light by the phycocyanin in the Chroococcus cell. 3. The yield of fluorescent light in Chroococcus is between 1 and 2 per cent. 4. The transfer of energy by internal conversion can account for the photosynthesis by phycocyanin observed by Emerson and Lewis.     

QUANTITATIVE STUDIES ON THE PHOTOLETHAL EFFECTS OF QUARTZ ULTRA-VIOLET RADIATION UPON PARAMECIUM

Paramecia grown under controlled conditions were irradiated at known intensities of light of wave-lengths 2537, 2654, 2804, 3025, and 3130 A. The approximate absorption of the light by the Parmecia was found to be greatest and of the same order of magnitude at the three shortest wave-lengths, considerably less at 3025, and least at 3130 A. Paramecia did not die when irradiated with high dosages of intense light of wave-length 3130 A. At the other wave-lengths 50 per cent vesiculation occurred when between 10¹² and 10¹³ quanta had been absorbed by a Paramecium. This would indicate that a very large number of molecules in a Paramecium are affected before vesiculation occurs.     

Seasonal changes in the lipids of the sea anemone, Metridium senile (L.)

Total lipids were extracted from freshly collected Metridium senile (L.) in September, November, February, and June. The neutral lipid profile as well as total content was determined in each of these months. With the exception of June, the larger anemones contained less lipid as a percentage of the wet weight than did the smaller animals. In June the anemones contained the highest level of total lipid and a large part of this was sterol. In February, total lipid was also higher as a result of the increased wax ester content. Triglyceride levels remained relatively constant throughout the year while the wax ester content was found to vary both with season and temperature. Both triglyceride and wax ester levels were low in June, when sterol content was high, suggesting sexual reproduction at this time of the year. It is postulated that triglycerides serve as the primary lipid energy reserve in Metridium, while wax esters function in a secondary capacity, being called upon in times of metabolic stress such as sexual reproduction.     

Increased photoreversal of ultraviolet injury by flashing light

1. Photoreversal of ultraviolet (UV) injury was studied in the ciliate protozoan Tetrahymena pyriformis (geleii) strain W, cultured in the absence of other living organisms. The division pattern of progeny of single animals was followed in hanging drop preparations. 2. A sublethal dose of 450 ergs/mm.² of monochromatic UV of wave length 2654 A produces a lag before the first division followed by a period of cessation of fission after the second division. This cessation sometimes lasts as long as 6 weeks, during which time the animals become smaller and rounder and more opaque. Organisms about to resume division increase in size and transparency; after a few divisions the animals regain their normal division rate. 3. The effect of UV ranging in intensity from 5 to 15 ergs/mm.²/sec. was found to obey the reciprocity law quite well for the UV effect on the division pattern of T. pyriformis. However, the same dose at lower and at higher intensities was less effective. 4. The effect of a dose of UV delivered at high intensity (19 ergs/mm.²/sec.) could be increased by flashing the light, indicating that the system became saturated in the continuous light. 5. A photoreversing dose of monochromatic blue light of wave length 4350 A was found to be more effective when delivered as continuous light at a low intensity, or as intermittent light at a high intensity, rather than as continuous light at the high intensity—indicating that a dark mechanism participates in photoreversal. 6. The time for the dark reaction was determined to be of the order of a few hundredths of a second in experiments in which different lengths of dark period were used while maintaining a constant light period of 0.0025 second. 7. For Colpidium colpoda the efficiency of a given dose of photoreversing light was increased by flashing the light. 8. The present experiments are interpreted in terms of data available in the literature.     

Effects of light intensity on membrane differentiation in Rhodopseudomonas capsulata

Cells of Rhodopseudomonas capsulata, strain 37b4, leu^?, precultivated anaerobically under low light intensity, were exposed to high light intensity (2000 W · m^?2). The cells grew with a mass doubling time of 3 h. The synthesis of bacteriochlorophyll (BChl) began after two doublings of cell mass. Reaction center and light-harvesting BChl I (B-875) were the main constituents of the photosynthetic apparatus incorporated into the membrane. The size of the photosynthetic unit (total BChl/reaction center) decreased and light-harvesting BChl I became the dominating BChl species. Concomitant with the appearance of the different spectral forms of BChl the respective proteins were incorporated into the membrane, i.e. the three reaction center polypeptides, the polypeptide associated with light-harvesting BChl I, the two polypeptides associated with BChl II. A polypeptide of an apparent molecular weight of 45 000 was also incorporated. A lowering of the light intensity to 7 W · m^?2 resulted in a lag phase of growth for 6 h. Afterwards, the time for doubling of cell mass was 11 h. The concentration of all three BChl complexes (reaction center, light-harvesting BChl I and II complexes)/cell and per membrane protein increased immediately. Also the size of the photosynthetic unit and the amount of intracytoplasmic membranes/cell increased.The activities of photophosphorylation, succinate dehydrogenase, NADH dehydrogenase and NADH oxidation (respiratory chain)/membrane protein are higher in membrane preparations isolated from cells grown at high light intensities than in such preparations from cells grown at low light intensities.     

The interplay o light and heat in bleaching rhodopsin

Rhodopsin, the pigment of the retinal rods, can be bleached either by light or by high temperature. Earlier work had shown that when white light is used the bleaching rate does not depend on temperature, and so must be independent of the internal energy of the molecule. On the other hand thermal bleaching in the dark has a high temperature dependence from which one can calculate that the reaction has an apparent activation energy of 44 kg. cal. per mole. It has now been shown that the bleaching rate of rhodopsin becomes temperature-dependent in red light, indicating that light and heat cooperate in activating the molecule. Apparently thermal energy is needed for bleaching at long wave lengths where the quanta are not sufficiently energy-rich to bring about bleaching by themselves. The temperature dependence appears at 590 mµ. This is the longest wave length at which bleaching by light proceeds without thermal activation, and corresponds to a quantum energy of 48.5 kg. cal. per mole. This value of the minimum energy to bleach rhodopsin by light alone is in agreement with the activation energy of thermal bleaching in the dark. At wave lengths between 590 and 750 mµ, the longest wave length at which the bleaching rate was fast enough to study, the sum of the quantum energy and of the activation energy calculated from the temperature coefficients remains between 44 and 48.5 kg. cal. This result shows that in red light the energy deficit of the quanta can be made up by a contribution of thermal energy from the internal degrees of freedom of the rhodopsin molecule. The absorption spectrum of rhodopsin, which is not markedly temperature-dependent at shorter wave lengths, also becomes temperature-dependent in red light of wave lengths longer than about 570 to 590 mµ. The temperature dependence of the bleaching rate is at least partly accounted for by the temperature coefficient of absorption. There is some evidence that the temperature coefficient of bleaching is somewhat greater than the temperature coefficient of absorption at wave lengths longer than 590 mmicro;. This means that the thermal energy of the molecule is a more critical factor in bleaching than in absorption. It shows that some of the molecules which absorb energy-deficient quanta of red light are unable to supply the thermal component of the activation energy needed for bleaching, so bringing about a fall in the quantum efficiency. The experiments show that there is a gradual transition between the activation of rhodopsin by light and the activation by internal energy. It is suggested that energy can move freely between the prosthetic group and the protein moiety of the molecule. In this way a part of the large amount of energy in the internal degrees of freedom of rhodopsin could become available to assist in thermal activation. Assuming that the minimum energy required for bleaching is 48.5 kg. cal., an equation familiar in the study of unimolecular reaction has been used to estimate the number of internal degrees of freedom, n, involved in supplying the thermal component of the activation energy when rhodopsin is bleached in red light. It was found that n increases from 2 at 590 mµ to a minimum value of 15 at 750 mµ. One wonders what value n has at 1050 mµ, where vision still persists, and where rhodopsin molecules may supply some 16 kg. cal. of thermal energy per mole in order to make up for the energy deficit of the quanta.     

Fluorescence decline as a function of redox potential and actinic light intensity in spinach thylakoids

Excitation of isolated thylakoids with sufficiently strong actinic light increases the fluorescence quantum yield up to a maximum level, F_max, followed by a slower decline under certain experimental conditions. In this study the latter effect was analyzed as a function of the ambient redox potential and the actinic light intensity. Two different types of fluorescence decrease were found. (a) In the presence of specific quinones widely used as redox mediators a fast and comparatively small decrease (30% of F_max), referred to as ΔF_SQ, was observed at moderate redox potentials (−300 <E_m < + 200 mV). ΔF_SQ disappears at positive values with E_{m, 7.5} = + 110 mV, whereas the decrease at negative redox potential depends on the midpoint potential of the quinone. (b) A more pronounced fluorescence decline was observed at redox potentials below −300 mV, which comprises 65–70% of the maximum fluorescence. The full expression of this effect, referred to as ΔF^max_LP, requires markedly higher actinic light intensities than ΔF^max_SQ. The extent of ΔF^max_LP as a function of the redox potential is dependent on the presence of redox mediators. In their absence the full expression of ΔF^max_LP can be only observed below −400 mV. Based on the hypothesis of Pheo⁻ photoaccumulation being responsible for the fluorescence decline at low redox potentials (Klimov, V.V., Klevanik, A.V. and Shuvalov, V.A. (1977) FEBS Lett. 82, 182–186), a reaction scheme is presented that qualitatively describes the time course of ΔF_LP at different actinic light intensities and redox potentials. Based on this analysis, the rate of Pheo^⨪ reoxidation is inferred to be limited by the reaction center apoprotein acting as a barrier to redox equilibration. The implications for the interpretations of redox titration curves are briefly discussed.     

Photoinhibition and P700 in the Marine Diatom Amphora sp

The marine diatom Amphora sp. was grown at a light intensity of 7.0 × 10¹⁵ quanta centimeter⁻² second⁻¹. Light saturation of photosynthesis for these cells was between 6.0 and 7.0 × 10¹⁶ quanta centimeter⁻² second⁻¹. At light intensities greater than saturation, photosynthetic ¹⁴CO₂ fixation was depressed, while P700 unit size (chlorophyll a concentration/P700 activity) increased and number of P700 units per cell decreased. After a 1-hour exposure of Amphora sp. to a photoinhibitory light intensity of 2.45 × 10¹⁷ quanta centimeter⁻² second⁻¹, there was a 45 to 50% decrease in the rate of ¹⁴CO₂ fixation relative to the rate at the culture light intensity. There also was a 25% increase in P700 unit size and a 30% reduction in the number of P700 units per cell but no change in total chlorophyll a concentration. Following this period of photoinhibition, the cells were returned to a light regime similar to that in the original culture conditions. Within 1 hour, both number of P700 units per cell and P700 unit size returned to levels similar to those of cells which were kept at the culture light intensity. The rates of photosynthesis did not recover as rapidly, requiring 2 to 3 hours to return to the rate for the nonphotoinhibited cells. Our results indicate that a decrease in P700 activity (with a resultant increase in P700 unit size) may be partially responsible for the photoinhibition of algal photosynthetic carbon dioxide fixation.     

Modulating Effect of Far-Red Light on Activities of Alternative Electron Transport Pathways Related to Photosystem I

Barley (Hordeum vulgare L.) leaves were irradiated with far-red (FR) light of various intensities after different periods of dark adaptation in order to investigate activities of alternative electron transport pathways related to photosystem I (PSI). Photooxidation of P700, the primary electron donor of PSI, was saturated at FR light intensity of 0.15 μmol quanta/(m² s). As the photon flux density was raised in this range, the slow and middle components in the kinetics of P700⁺ dark reduction increased, whereas the fast component remained indiscernible. The amplitudes of the slow and middle components diminished upon further increase of FR photon flux density in the range 0.15–0.35 μmol quanta/(m² s) and remained constant at higher intensities. The fast component of P700⁺ reduction was only detected after FR irradiation with intensities above 0.15 μmol quanta/(m² s); the light-response curve for this component was clearly sigmoid. In dark-adapted barley leaves, three stages were distinguished in the kinetics of P700 photooxidation, with the steady state for P700⁺ achieved within about 3 min. In leaves predarkened for a short time, the onset of FR irradiation produced a very rapid photooxidation of P700. As the duration of dark exposure was prolonged, the amplitude of the first peak in the kinetic curve of photoinduced P700 photooxidation was diminished and the time for attaining the steady-state oxidation level was shortened. After a brief dark adaptation of leaves, ferredoxin-dependent electron flow did not appreciably contributed to the kinetics of P700⁺ dark reduction, whereas the components related to electron donation from stromal reductants were strongly retarded. It is concluded that FR light irradiation, selectively exciting PSI, suffices to modulate activities of alternative electron transport routes; this modulation reflects the depletion of stromal reductants due to continuous efflux of electrons from PSI to oxygen under the action of FR light. __________ Translated from Fiziologiya Rastenii, Vol. 52, No. 6, 2005, pp. 805–813. Original Russian Text Copyright ? 2005 by Egorova, Drozdova, Bukhov.     

Light-Dependency of Growth and Secondary Metabolite Production in the Captive Zooxanthellate Soft Coral Sinularia flexibilis

The branching zooxanthellate soft coral Sinularia flexibillis releases antimicrobial and toxic compounds with potential pharmaceutical importance. As photosynthesis by the symbiotic algae is vital to the host, the light-dependency of the coral, including its specific growth rate (μ day⁻¹) and the physiological response to a range of light intensities (10–1,000 μmol quanta m⁻² s⁻¹) was studied for 12 weeks. Although a range of irradiances from 100 to 400 μmol quanta m⁻² s⁻¹ was favorable for S. flexibilis, based on chlorophyll content, a light intensity around 100 μmol quanta m⁻² s⁻¹ was found to be optimal. The contents of both zooxanthellae and chlorophyll a were highest at 100 μmol quanta m⁻² s⁻¹. The specific budding rate showed almost the same pattern as the specific growth rate. The concentration of the terpene flexibilide, produced by this species, increased at high light intensities (200–600 μmol quanta m⁻² s⁻¹).     

Effect of light on the initiation of pileus formation in a basidiomycete, Favolus arcularius

Pileus formation in Favolus arcularius is induced by light,but no photoinduction occurred in young epileate stipes. Thestipes usually had to attain a length of about 5 mm to be photosensitive.Synchronous pileus formation could be induced by exposure tolight using epileate stipes which had been preincubated in darknessfor 48 to 72 hr. The pileus primordium formed about 24 hr afterthe start of illumination, however, continuous illuminationwas not necessary to produce this effect. A dark period givenbetween 1 and 8 hr after the start of illumination did not retardpileus formation. The photoinduction of pileus formation involvedtwo light-requiring processes, one occurring during the firsthour (the first light process) and the other from the 8th tothe 24th hr (the second light process). The photoresponse inthe first light process was saturated with 5 lux of light, buta light intensity below 1 lux was essentially ineffective. Onthe other hand, the reaction in the second light process couldbe started by less than 2 lux, and was accelerated by increasingthe light intensities up to about 150 lux. Further increasesin light intensity did not improve any significant effect. (Received April 30, 1974; )     

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.     

Regulation of the Hill reaction by cations and its abolishment by uncouplers

Concurrent measurements of P-700 turnover and the reduction of K₃Fe(CN)₆ revealed an identical relative quantum yield for both reactions in isolated pea chloroplasts as well as chloroplast particles from wild type Scenedesmus. On the other hand, chloroplast particles of wild type Scenedesmus showed the same relative quantum yield for the Hill reaction as those of the P-700-free mutant No. 8, indicating that P-700 is not required for ferricyanide reduction.Several metal ions, such as Mg²⁺, Ca²⁺, Na⁺ and K⁺ stimulated the reduction of K₃Fe(CN)₆. In short wavelength light, the stimulation was a function of light intensity, varying in magnitude from an approximate doubling of the yield in low intensities to only a slight increase at light saturation. P-700 was totally unaffected by the cations.The effect of the metal salts was abolished in the presence of uncouplers of photophosphorylation.The data reconcile several divergent results concerning the effect of divalent cations on the reduction of ferricyanide which have been reported in the recent literature. On the whole the experiments suggest that the Hill acceptor can be reduced at two sites. The stimulation of the Hill reaction by metal ions is proposed to be due to an activation of Photosystem II and a more efficient utilization of quanta at the expense of radiationless de-excitation.     

The responses of the pupil of Gekko gekko to external light stimulus

1. The responses of the pupil of a nocturnal gecko (Gekko gekko) to external light stimulus were studied. 2. The responses of the pupil are determined by light entering the pupil and not by light acting directly on the iris. 3. The responses of the pupil are very uniform in sensitivity including spectral sensitivity for light coming in different directions to the eye. 4. The possible change in area of the pupil is more than 300-fold and probably represents an effort to shield the pure rod retina from saturating light intensities. 5. The pupil continues to contract sharply for changes in external light intensity which give retinal illuminations corresponding to 10⁶ quanta/sec. striking a retinal rod. 6. There is a large degree of spatial summation of the response; circular external light fields subtending 5 and 140° giving the same illumination at the pupil give approximately the same pupil response. 7. The spectral sensitivity curve agrees with the absorption curve of an extracted pigment from a closely related gecko described by Crescitelli in the followig paper. It is similar to the human scotopic curve but its maximum is displaced about 20 to 30 mµ towards the red end of the spectrum. The fall in sensitivity towards the red end of the spectrum is described by the equation See PDF for Equation     

Physiology and Phylogeny of Green Sulfur Bacteria Forming a Monospecific Phototrophic Assemblage at a Depth of 100 Meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter⁻¹. High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter⁻¹) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 μmol quanta m⁻² s⁻¹ at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 μmol quanta m⁻² s⁻¹ for photosynthetic ¹⁴CO₂ fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.     

Sedimentary nitrogen uptake and assimilation in the temperate zooxanthellate sea anemone Anthopleura aureoradiata

The sea anemone Anthopleura aureoradiata (Carlgren), which harbours symbiotic dinoflagellates (zooxanthellae), is abundant on mudflats and rocky shores around New Zealand. We measured the potential for particulate nitrogen uptake from sediment by A. aureoradiata and the subsequent consequences of this uptake on the nitrogen status of its zooxanthellae. Sediment was rinsed, labelled with (¹⁵NH₄)₂SO₄, and provided to anemones at low (0.23 g ml^− 1) and high (1.33 g ml^− 1) sediment loads for 6 h. Both anemone tissues and zooxanthellae became enriched with ¹⁵N. Enrichment of anemone tissues was similar at both high and low sediment loads, but the zooxanthellae became more enriched at the lower load. This was presumably because the uptake of ammonium, arising from host catabolism, by zooxanthellae is light driven and because the anemones at the lower load were able to extend their tentacles into the light while those at the higher load were not. The influence of sediment uptake on the nitrogen status of the zooxanthellae was determined by measuring the extent to which 20 μM NH₄⁺ enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the ammonium enhancement ratio (AER) was expressed as [dark NH₄⁺ rate/dark FSW rate], where ‘rate’ refers to C fixation and a ratio of 1.0 or less indicates nitrogen sufficiency. When anemones were starved with and without rinsed sediment in nitrogen-free artificial seawater for 8 weeks, zooxanthellar nitrogen deficiency became apparent at 2-4 weeks and reached similar levels in both treatments (AER = ~ 2). In contrast, anemones fed 5 times per week for 8 weeks with Artemia nauplii were nitrogen sufficient (AER = 1.03). In the field, zooxanthellae from mudflat anemones were largely nitrogen sufficient (AER = 1.26), while nitrogen deficient zooxanthellae were present in anemones from a rocky intertidal site (AER = 2.93). These results suggest that, while there was evidence for particulate nitrogen uptake, dissolved inorganic nitrogen (especially ammonium) in interstitial pore water may be a more important source of nitrogen for the zooxanthellae in mudflat anemones, and may explain the marked difference in nitrogen status between the mudflat and rocky shore populations.     

Probability of Occurrence of Discrete Potential Waves in the Eye of Limulus

Discrete potential waves can be recorded from cells in the eye of Limulus both in darkness and in dim illumination. With constant illumination the frequency of these waves is linearly related to light intensity and the distribution of intervals between waves follows an exponential function. The latency of waves evoked by short flashes of light is usually long and variable and the number of waves evoked by a flash varies randomly, obeying approximately a Poisson distribution. The results of experiments with flashes of light have been compared with the predictions derived from the hypotheses that one, two, or three quanta of light are required for production of one wave. The agreement of the data with the theory can be considered acceptable for the "one quantum" hypothesis, is less satisfactory for the "two quanta" hypothesis, and is very poor for the "three quanta" hypothesis.     

Night-interruption experiments and action spectra for dawn and dusk in relation to the photoperiodic clock of the cabbage whitefly,Aleyrodes proletella (Homoptera: Aleyrodidae)

The dark period (scotophase) is the most photoperiodically important part of a light-dark cycle in Aleyrodes proletella. Night-interruption studies have revealed three distinct dark stages: the photosensitive stage 1 lasts for about 3 h after dusk and 1-h light breaks both stop and re-set the photoperiodic clock; stage 2 also lasts about 3 h, but is photorefractory to some degree; stage 3 is photosensitive, but short light breaks do not re-set the clock although a 4-h light break (equivalent to a main photophase) does restore the capacity to respond to a normal critical night length in the post-interruption scotophase.Action spectra revealed peak photoperiodic sensitivity to blue light (410–430 nm) with 50% responses., at 1.5 μWcm⁻² and 2.5 μWcm⁻² for the dusk and dawn peaks respectively. These data are consistent with the view that the photopigment is a carotenoprotein.The results are interpreted in terms of the photoperiodic clock in A. proletella operating on the hour glass principle.