High irradiance responses involving photoreversible multiple photoreceptors as related to photoperiodic induction of cell division in Euglena

Little is known about the photoreceptors involved in the photoperiodism of unicellular organisms, which we elucidated by deriving their action spectra. The flagellated alga Euglena gracilis exhibits photoperiodism, with a long-day response in cell reproduction. The underlying clock is a circadian rhythm with photoinductive capability, peaking at subjective dusk and occurring at the 26th hour in continuous darkness (DD) when transferred from continuous light (LL); it regulates photoinduction, a high-irradiance response (HIR), of a dark-capability of progressing through cell division. We derived the action spectra by irradiating E. gracilis with monochromatic light for 3h at around the 26th hour; the action maxima occurred at 380, 450-460, 480, 610, 640, 660, 680, and 740nm. Except for the maximum at 450-460nm, which was always a major maximum, the maxima greatly depended on the red (R)/far-red (FR) ratio of the prior LL. The high R/FR ratio resulted in a dominant major peak at 640nm and minor peaks at 480 and 680nm, whereas the low ratio resulted in dominant major peaks at 610 and 740nm and minor peaks at 380 and 660nm; the critical fluence was minimally about 60mmolm(-2). These HIRs resulted from the accumulation of corresponding low-fluence responses (LFRs) because we found that repetition of a 3-min light/dark cycle, with critical fluences of 1mmolm(-2), lasting for 3h resulted in the same photoinduction as the continuous 3-h irradiation. Moreover, these LFRs expressed photoreversibility. Thus, photoperiodic photoinduction involves Euglena-phytochrome (640 and 740nm) and blue photoreceptor (460nm). Although 380, 480, 610, 660, and 680nm may also represent Euglena-phytochrome, a definite conclusion awaits further study.     

Light irradiation induces fragmentation of the plasmodium, a novel photomorphogenesis in the true slime mold Physarum polycephalum: action spectra and evidence for involvement of the phytochrome

A new photomorphogenesis was found in the plasmodium of the true slime mold Physarum polycephalum: the plasmodium broke temporarily into equal-sized spherical pieces, each containing about eight nuclei, about 5 h after irradiation with light. Action spectroscopic study showed that UVA, blue and far-red lights were effective, while red light inhibited the far-red-induced fragmentation. Difference absorption spectra of both the living plasmodium and the plasmodial homogenate after alternate irradiation with far-red and red light gave two extremes at 750 and 680 nm, which agreed with those for the induction and inhibition of the fragmentation, respectively. A kinetic model similar to that of phytochrome action explained quantitatively the fluence rate-response curves of the fragmentation. Our results indicate that one of the photoreceptors for the plasmodial fragmentation is a phytochrome.     

A monochromatic action spectrum for the photoinduction of the UV-absorbing mycosporine-like amino acid shinorine in the red alga Chondrus crispus

To determine the action spectrum for photoinduction of the ultraviolet (UV)-absorbing mycosporine-like amino acid shinorine, specimens of the marine red alga Chondrus crispus were irradiated with monochromatic light of various wavelengths using the Okazaki large spectrograph at the National Institute for Basic Biology, Okazaki, Japan. Fluence response curves were determined for the wavelengths between 280 and 750 nm, by irradiating the algae with monochromatic light for 10 h, followed by 4 h of 25 micromol m(-2) s(-1) photosynthetically active radiation and 10 h darkness. Samples were taken after the second exposure interval. A linear correlation between fluence rate and accumulated shinorine concentration was detected for wavelengths between 350 and 490 nm in the fluence rate range of 20-30 micromol m(-2) s(-1), whereas there was no induction above 490 nm. Below 350 nm a decline in shinorine concentration could be observed at fluence rates above 30 micromol m(-2) s(-1), probably due to an inhibition of photosynthetic activity and a subsequent impairment of shinorine biosynthesis. The constructed action spectrum indicated that the photoreceptors mediating shinorine photoinduction might be an unidentified UV-A-type photoreceptor with absorption peaks at 320, 340 and 400 nm.     

Stentor coeruleus SHOWS POSITIVE PHOTOKINESIS

Abstract— Stentor coeruleus responds to a sudden increase in light intensity with a step-up photophobic response (avoiding reaction), and to collimated light with negative phototaxis. The peaks of the action spectra for the photophobic response and for the phototaxis are in common, 610 nm.
5. coeruleus showed changes in its steady-state swimming velocity induced with varying intensities of light (photokinesis). The cells swam fast in light regions but slowly in dark ones (positive photokinesis); the mean velocity of swimming was about 0.6 mm/s at 100 lx but reached about 1.0 mm/s at 50000 lx. The peak of the action spectrum for this photokinesis was about 680 nm.
The organism is the first protozoan cell reported to show three types of photoresponse: photophobic response, phototaxis and photokinesis.     

Phototaxis in the ciliated protozoan Ophryoglena flava: dose-effect curves and action spectrum determination

The sensitivity of positive phototactic orientation of cells of the ciliated protozoan Ophryoglena flava has been measured for white light, broad-band blue and red light, and narrow-band monochromatic light, using a laboratory-developed computer aided system. The white-light fluence rate-response curve shows that there is no negative phototaxis in the fluence rate range investigated (0-15 W/m2) and no adaptation phenomena; it is very well fitted by a hyperbolic function; the fluence rate curves under broad band blue and red light (full width at half maximum, FWHM= 100 nm) can be fitted by the same model. The saturation level is, within experimental errors, the same for the three curves, indicating that there are no chromaticity effects and that if there is more than one photoreceptor pigment, they act independently of each other. The fluence rate-response curves determined under narrow band monochromatic light (FWHM = 10 nm) can also be fitted by the same model and show, within experimental errors, the same saturation level. An action spectrum for positive phototaxis at 10-nm intervals has been calculated from fluence rate-response curves: it shows three maxima, at 420, 540 and 590 nm. This action spectrum is significantly different from the ones for photomotile responses in Blepharisma japonicum, Stentor coeruleus and Chlamydodon mnemosyne, whereas it resembles the ones of Paramecium bursaria and Fabrea salina.     

Antagonistic blue- and red-light regulation of cab-gene expression during photosynthetic adaptation in Scenedesmus obliquus.

During adaptation of the photosynthetic apparatus of the green alga Scenedesmus obliquus to various light qualities, the accumulation of chlorophylls and pigment-protein complexes (with specific consideration of chlorophyll a/b-binding (Cab) proteins) and cab-gene expression were determined. The fluence rate dependences for chlorophyll accumulation and cab-gene expression were very different. Very low fluence rates of violet (404 nm), blue (461 nm) and red (650 nm) light below the photosynthetic threshold, i.e. between 10(-3) and 10(-1) mumol m-2 s-1, inhibited all of these reactions in cells grown under heterotrophic conditions. At elevated fluence rates (above 1 mumol m-2 s-1), red light retained its negative regulation, whereas blue light stimulated pigment accumulation. Under autotrophic conditions the pattern was more complex, because chlorophyll accumulation was unaffected by light below the photosynthetic threshold. However, the expression of cab-genes was inhibited by red light but stimulated by blue light. Cells adapted to fluence rates, which ensured photosynthetic energy supply (above 1 mumol m-2 s-1), showed an increase in chlorophyll accumulation, blue light being more effective than red light. The results confirm and extend our previous discovery of two antagonistically acting photoreceptors in Scenedesmus which mediate and coordinate the complex functional and structural changes associated with photosynthetic adaptation. One of these receptor pigments is a blue-light receptor with positive action; the other is a violet-red-light receptor which can operate far below the photosynthetic threshold and exerts a negative regulation.     

ANTAGONISTIC EFFECTS OF RED AND FAR-RED LIGHTS ON THE STABILITY OF PHOTOSYSTEM II IN PEA LEAVES EXPOSED TO HEAT

Abstract— The presence of light during exposure of intact pea leaves to high temperature (40°C) protects Photosystem II (PSII) against inactivation, as indicated by the preservation of the maximal variable 685 nm chlorophyll fluorescence and the photosynthetic oxygen evolution. This photoprotection was observed (i) to be saturated at low fluence rates ( ca 10 W m^-2) and (ii) to be strongly dependent on the spectral characteristics of the light. It was specifically induced by red light (630–670 nm) whereas other wavelengths were much less protective. A strong antagonism between red and far-red lights was also observed, with PSII stabilization by red light being partially cancelled by additional far-red light.     

Photomovement of the Gliding Cyanobacterium Synechocystis sp. PCC 6803

Abstract— Using a computerized videomicroscope motion analysis system, we investigated the photomovements of two Synechocystis sp. (PCC 6803 and ATCC 27184). Synechocystis sp. PCC 6803 displays a relatively slow gliding motion. The phototactic and photokinetic speeds of this cyanobacterium in liquid media were 5μm/min and 15.8 μm/min, respectively, at 3μmol/m²/s of stimulant white light. Synechocystis sp. PCC 6803 senses light direction rather than intensity for phototaxis. Synechocystis sp. ATCC 27184 showed a weak photokinesis but no phototaxis. Analysis of Synechocystis sp. ATCC 27184 suggests that the loss of phototaxis results from spontaneous mutation during several years of subculture. When directional irradiation was applied, the cell population of Synechocystis sp. PCC 6803 began to deviate from random movement and reached maximum orientation at 5 min after the onset of stimulant white light. Synechocystis sp. PCC 6803 showed high sensitivity to the stimulant white light of fluence rates as low as 0.002 |unol/m²/s. Neither 1,3-dichlorophenyldimethyl urea nor cyanide affected phototactic orientation, whereas cyanide inhibited gUding speed. This result suggests that the phototaxis of Synechocystis sp. PCC 6803 is independent of photosynthetic phosphorylation and that its gliding movement is primarily powered by oxidative phosphorylation. In the visible wavelength region, 560 nm, 660 nm and even 760 nm caused positive phototaxis. However, 360 nm light induced strikingly negative phototaxis. Therefore, at least two independent photoreceptors may exist to control phototaxis. The photoreceptor for positive phototaxis appears likely to be a phytochrome-like tetrapyrrole rather than chlorophyll a .     

PHOTOTAXIS IN DICTYOSTELIUM DISCOIDEUM AMOEBAE

Abstract— An apparatus has been developed to measure phototactic movement in a population of amoebae of Dictyostelium discoideum. Fluence–response curves in white light show a positive phototaxis to light below 100mW/m². Higher intensities cause a negative phototaxis. An action spectrum, based on the zero-crossing points in fluence–response curves for monochromatic light, shows a major peak at about 405nm and secondary maxima at about 450, 520, 580 and 640nm. This action spectrum resembles the action spectra for accumulations of amoebae in and dispersal from light traps and that of inhibition of aggregation by light, but is distinctly different from the action spectrum for phototaxis by D. discoideum pseudoplasmodia.     

A POSSIBLE CONTROL BY A PHYTOCHROME-LIKE PHOTORECEPTOR OF CHLOROPHYLL SYNTHESIS IN THE GREEN ALGA Ulva rigida

Chlorophyll synthesis is stimulated by red light pulses in the green alga Ulva rigida C. Aghard. Chlorophyll synthesis in darkness is greater after longer red light pulses (30 min) than after shorter red light pulses (5 min). Chlorophyll synthesis was higher after red light pulses of 14 Wm_-2 fluence rate than after those of 7 Wm_-2. The effect of red light showed some far-red reversibility. The reversion by far-red light was higher after red light pulses of 4 min than after those of 30 min. These results indicate the existence of a rapid induction of chlorophyll synthesis during the red light pulses and a fast escape from photoreversibility. The percentage of reversion is also affected by the fluence rate of the light pulses. The reversion was reduced by about 15% when the photon fluence rate was increased from 7 to 14 Wm_-2. Reversion was also observed when red and far-red light pulses were applied successively. Thus, phytochrome or a phytochrome-like photoreceptor could be involved in the induction of chlorophyll synthesis in Ulva rigida.     

THE ACTION SPECTRUM OF DAPHNIA MAGNA (CRUSTACEA) PHOTOTAXIS IN A SIMULATED NATURAL ENVIRONMENT

Abstract— The action spectrum of phototaxis in Daphnia magna (Crustacea) was measured in a chamber which simulated a natural angular distribution of underwater light. A 17% step-down in irradiance was used to stimulate the phototactic response at all wavelengths and irradiances tested. Peaks in the spectral response curves depended on the fluence rate to which the zooplankton were acclimated. The wavelength of maximum response (Z_max) shifted from yellow-green at the highest acclimation fluence rate (5.1 × 10⁻² Wm⁻²) to blue-violet at moderate rates. At low acclimation fluence rates, the blue-violet maximum was retained and another maximum developed in the red. At the lowest fluence rate (1.6 × 10⁻⁵ Wm⁻²), the blue-violet and red maxima were lost and another maximum developed in the near ultraviolet. The action spectrum indicates the presence of three, and possibly four, photopigments with Z_max, at ∼405, 440, 570 and 690nm. The 440 and 690nm maxima may belong to the same photopigment; however, this was not tested. Changes in zooplankton swimming speed, caused either by large changes in irradiance or by mechanical stimuli, were accompanied by changes in the strength of the phototactic response to the −17% stimulus at any irradiance level for white and monochromatic light, and indicated the presence of a mechanism connecting swimming speed and photosensitivity.     

ON THE EMERSON ENHANCEMENT EFFECT IN THE FERRICYANIDE HILL REACTION IN CHLOROPLAST FRAGMENTS*

Abstract— The Emerson effect is demonstrated in the ferricyanide Hill reaction when the rates of steady-state oxygen evolution are measured in spinach chlorplast fragments exposed to red (650 nm) and far-red (700 nm) light of high but not saturating intensity. However, at very low light intensity, the Emerson effect could not be observed. These experiments suggest that ferricyanide can be reduced at two sites. At low light intensity, the rate at one site predominates and at this site one photochemical system is active. At high light intensity, however, the action at a site that is dependent on the cooperation of two photochemical systems predominates. The action spectra of the ferricyanide Hill reaction measured in the presence of an excess of 650 nm or in the excess of 700 nm light show two peaks: one at 650 nm due to chlorophyll b and the other around 675 nm due to chlorophyll a. The ratio of chlorophyll a to chlorophyll b peaks is about 1.4 when 650 nm background light is used; the same ratio is about 0.7 with 700 nm background light. The two pigment systems seem to contain both chlorophyll a and chlorophyll b but in different proportions.     

ACTION SPECTRA FOR PHOTOTROPIC BALANCE IN Phycomyces blakesleeanus: DEPENDENCE ON REFERENCE WAVELENGTH and INTENSITY RANGE

Abstract— Action spectra for phototropic balance of Phycomyces blakesleeanus sporangiophores were measured for various reference wavelengths and intensity ranges. Balance action spectra were made at fluence rates of 10^-4 W m^-2 with reference wavelengths of 450 nm, 394 nm, 507 nm, and broadband blue light. For broad-blue light and 450 nm light as references, typical flavin-like action spectra were found with a ma jor peak at 455 nm, a secondary peak at 477 nm, and a minor peak at 383 nm; these peaks are wider for broad blue than for 450 nm light. With the 394 nm reference, there is a major peak at 455 nm, a secondary peak at 477 nm and a minor peak at 394 nm. An action spectrum with 507 nm reference has a major peak at 455 nm and a minor peak at 383 nm, but no peak at 477 nm. A balance action spectrum was made with 450 nm reference light near threshold intensity (2 times 10^-8 W m^-2); there, the 386 nm peak is greatly reduced, while the 455 nm peak is enhanced. The intensity dependence of the 386 nm peak was studied in detail for reference light of 450 nm. We found that the relative quantum efficiency of the 386 nm light increases with the logarithm of the 450 nm fluence rate; in the high intensity range (0.3 W m^-2) the relative quantum efficiency of the 386 nm light is 1.3 and approaches zero at 10^-9 W m^-2. These findings indicate that P. blakesleeanus phototropism is mediated by multiple interacting pigments or by a photochromic photoreceptor.     

Phytochrome-like responses in Euglena: A low fluence response that reorganizes the spectral dependence of the high irradiance response in long-day photoperiodic induction of cell division

Irradiance spectra change spatiotemporally, and angiosperms adapt accordingly, mainly through phytochromes. This study challenges the long-held belief that the flagellated alga Euglena gracilis lacks phytochromes and is therefore unaffected by spectral changes. We photoautotrophically cultured the alga under continuous light (LL), then transferred it to darkness. After about 26h in darkness, different irradiations for 3h enabled cell division in dark-arrested G2 cells evoking a high-irradiance response (HIR). The spectral characteristics of the irradiation during the LL period (pre-irradiation) defined the spectral sensitivity in the subsequent dark period. LL with light rich in the red spectrum led to a HIR to the red spectrum (R-HIR), whereas light rich in the far-red spectrum (FR) led to a FR-HIR. Finishing the period of pre-irradiation consisting of continuous cool-white fluorescent light (rich in R) by a FR pulse enhanced the characteristics of the FR-HIR 26h later. By contrast, a R pulse given at the end of the pre-irradiation rich in FR potentiated the R-HIR. The effects were completely photoreversible between R and FR with critical fluences of about 2mmolm(-2), satisfying the classic diagnostic feature of phytochromes. The action spectrum of the FR effect at the end of pre-irradiation consisting of continuous cool-white fluorescent light (rich in R) had a main peak at 740nm and a minor peak at 380nm, whereas antagonization of the FR effect had a main peak at 640nm and a minor peak at 480nm. Wavelengths of 610 and 670nm appeared in both spectra. We also demonstrated the photoreversibility of 380/640, 480/740, and (610 and 670)/(640 and 740) nm. We conclude that Euglena displays phytochrome-like responses similar to the 'shade avoidance' and 'end-of-day FR' effects reported in angiosperms.     

Intraperitoneal Photodynamic Therapy: Comparison of Red and Green Light Distribution and Toxicity

Abstract— The aim of this study was to compare red (652 nm) and green (514 nm) light for photodynamic therapy (PDT) of the peritoneal cavity with emphasis on light distribution and toxicity. Red-light PDT was limited by intestinal toxicity and it was hypothesized that less penetrating green light would allow higher light doses to be used in the peritoneal cavity. Female non-tumor-bearing rats were photosensitized with mTHPC (meta-tetrahydroxyphenylchlorin, Foscan®) intravenously or intraperitoneally and the peritoneum was illuminated using a minimally invasive technique. For both red and green light, the time of illumination was varied to give the required dose. Light fluence rate was measured in situ at multiple sites within the abdominal cavity. The toxicity experiments were carried out with a total of 160 J incident red or 640 J incident green light and a drug dose of 0.15 mg/kg Foscan® For red light a mean fluence rate of 55.2 38.5 mW cm ₂ was measured, with a peak fluence rate of 128 mW cm ₂ on the intestines. For green light the mean and peak fluence rates were 8.2 9.0 (i.e. including zero fluence rate measurements) and 28 mW cm ₂, respectively. Intestines were most vulnerable to red light illumination. The intravenous injection route resulted in increased toxicity for red light, but for green light there were no major differences between intravenous and intraperitoneal routes. The 4 h interval between drug and illumination resulted in very little toxicity for both wavelengths. We conclude that for intraperitoneal PDT green light allows higher light doses than red light, but the light distribution over the peritoneum is much less favorable and may not be suitable for whole peritoneal illumination using a minimal-access technique.     

AN ACTION SPECTRUM FOR PHOTOTAXIS BY PSEUDOPLASMODIA OF Dictyostelium discoideum

Abstract— The sensitivity of phototactic orientation of pseudoplasmodia (slugs) of the cellular slime mold Dictyosrelium discoideum has been measured for white light and monochromatic light using computer aided directional statistics. The zero threshold for white light was found at about 10^-5 Ix. An action spectrum for positive phototaxis has been calculated from fluence rate-response curves; it shows two major maxima at about 420 and 440 nm and secondary peaks at 560 and 610 nm. This action spectrum is significantly different from the one for phototactic orientation in Dictyostelium amoebae.     

FLUENCE RATE COMPENSATION OF THE PERCEPTION OF RED: FAR-RED RATIO BY PHYTOCHROME IN LIGHT-GROWN SEEDLINGS*

Abstract— The hypothesis that phytochrome functions as a sensor of vegetational shade through the perception of the red: far-red photon fluence rate ratio requires that the mechanism of perception be compensated for wavelength-independent fluctuations in fluence rate (Smith, 1982). This paper seeks to establish the lower limit of fluence-rate compensation and to assess whether or not compensation is effective at the total fluence rates typical of herbaceous canopies. Using specially-designed cabinets, Sinapis alba L. (white mustard) seedlings were grown from germination under a range of total photosynthetically-active radiation (PAR = 400 to 700 nm) values and a range of red: far-red ratios. The data indicate that fluence-rate compensation is effective above a PAR value of ca. 60 μ.mol m² s'. Pretreating seedlings at high red: far-red ratio and a PAR level of 300 (μmol m₂S_-1for increasing periods of time led to an extension of fluence rate compensation to lower fluence rates. The results are discussed in relation to the photosynthetic competence of the seedlings grown under these conditions.     

Action Spectrum for Sporulation and Photoavoidance in the Plasmodium of Physarum polycephalum, as Modified Differentially by Temperature and Starvation

Abstract— The Plasmodium of the myxomycete Physarum polycephalum sporulates in bright natural environments, suggesting a relationship between photobehavior and sporulation. Thus, the action spectra for two light-dependent phenomena as well as the effects of other environmental conditions have been studied. Sporulation like photo-avoidance responded to UVC (near 270 nm) and near IR (near 750 nm) in addition to the well-documented UVA (near 350 nm) and blue (near 460 nm) regions. Sporulation and photoavoidance had similar sensitivities in the shorter wavelengths, while the former was about 100 times more sensitive in near IR. The Plasmodium moved away from light in a wide spectral range. Starvation and high temperature at 31°C (25°C in standard conditions) reduced photoavoidance to UVA and to blue light, respectively. A high fluence rate of UVC suppressed the rhythmic contraction of the Plasmodium, and the action spectrum peaked at 270 nm. These results indicate that the Physarum Plasmodium may stay at brighter places not by positive phototaxis but by weakening the negative phototaxis to sunlight or by other possible taxes such as hydrotaxis. There may be at least four different photo-systems in the Plasmodium.     

Phototaxis in the cyanobacterium Synechocystis sp. PCC 6803: role of different photoreceptors

The second cyanobacterial phytochrome Cph2 from Synechocystis sp. PCC 6803 was suggested as a part of a light-stimulated signal transduction chain inhibiting movement toward blue light. Cph2 has the two bilin binding sites, cysteine-129 and cysteine-1022, that might be involved in sensing of red/far-red and blue light, respectively. Here, we present data on wavelength dependence of the phototaxis inhibition under blue light, indicating that Cph2 itself is the photoreceptor for this blue light response. We found that inhibition of blue-light phototaxis in wild-type cells occurred below the transition point of about 470 nm. Substitution of cysteine-1022 with valine led to photomovement of the cells toward blue light (cph2(-) mutant phenotype). Analysis of mutants lacking cysteine-129 in the N-terminal chromophore binding domain indicated that this domain is also important for Cph2 function or folding of the protein. Furthermore, putative blue-light and phytochrome-like photoreceptors encoded by the Synechocystis sp. PCC 6803 genome were inactivated in wild-type and cph2 knockout mutant background. Our results suggest that none of these potential photoreceptors interfere with Cph2 function, although inactivation of taxD1 as well as slr1694 encoding a BLUF protein led to cells that reversed the direction of movement under blue light illumination in mutant strains of cph2.     

The Photoreceptor for Phototaxis in the Photosynthetic Flagellate Euglena gracilis

The unicellular flagellate Euglena gracilis shows positive phototaxis at low fluence rates (≤10 W m ²) and negative phototaxis at high fluence rates (≥100 W m ²). Currently, retinal or flavins/pterins are discussed as chromo-phores of the photoreceptor. When grown in the presence of 4 mM nicotine, a retinal inhibitor, for several generations, the cells still showed both responses, indicating that retinal is unlikely to be the chromophoric group of the photoreceptor responsible for phototaxis. The native flavin(s) can be substituted by growing the cells in roseo-flavin dissolved in the medium. The absorption spectrum of roseoflavin extends well beyond the action spectrum for phototaxis (up to 600 nm). Excitation at wavelengths >550 nm does not cause phototactic orientation in control cells but causes both positive and negative phototaxis in roseoflavin-grown cells, indicating an uptake and assembly of the chromophore in the photoreceptor complex. The white mutant strain 1224-5/1f, induced by streptomycin treatment, lacks flavins as indicated by fluorescence spectroscopy. The phototaxis-deficient pheno-type cannot be complemented by the addition of external riboflavin. Fluorescence spectra of intact paraxonemal bodies (PAB) indicate that both pterins and flavins are involved in photoperception and that the excitation energy is efficiently funneled from the pterins to the flavins. This energy transfer is disrupted by solubilization of the PAB. In intact PAB flavins are not accessible to reducing or oxidizing substances, indicating that they are located inside the structure, while pterins are accessible, so that their localization can be assumed to be on the surface. The results described above are discussed with regard to the potential involvement of flavins and pterins as well as retinal in photoperception.