Further evidence for a phycobilisome model from selective dissociation,fluorescence emission,immunoprecipitation, and electron microscopy

Phycobilisomes, isolated in 500 mM Sorensen's phosphate buffer pH 6.8 from the red alga, Porphyridium cruentum, were analyzed by selective dissociation at various phosphate concentrations. The results are consistent with a structural model consisting of an allophycocyanin core, surrounded by a hemispherical layer of R-phycocyanin, with phycoerythrin being on the periphery. Such a structure also allows maximum energy transfer.Intact phycobilisomes transfer excitation energy ultimately to a pigment with a fluorescence emission maximum at 675 nm. This pigment is presumed to be allophycocyanin in an aggregated state. Uncoupling of energy transfer among the pigments, and physical release of the phycobiliproteins from the phycobilisome follow a parallel time-course; phycoerythrin is released first, followed by R-phycocyanin, and then allophycocyanin. In 55 mM phosphate buffer, the times at which 50% of each phycobiliprotein has dissociated are: phycoerythrin 40 min, R-phycocyanin 75 min, and allophycocyanin 140 min.The proposed arrangement of phycobiliproteins within phycobilisomes is also consistent with the results from precipitation reactions with monospecific antisera on intact and dissociated phycobilisomes. Anti-phycoerythrin reacts almost immediately with intact phycobilisomes, but reactivity with anti-R-phycocyanin and anti-allophycocyanin is considerably delayed, suggesting that the antigens are not accessible until a loosening of the phycobilisome structure occurs. Reaction with anti-allophycocyanin is very slow in P. cruentum phycobilisomes, but is much more rapid in phycobilisomes of Nostoc sp. which contains 6–8 times more allophycocyanin. It is proposed that allophycocyanin is partially exposed on the base of isolated intact phycobilisomes of both algae, but that in P. cruentum there are too few accessible sites to permit a rapid formation of a precipitate with anti-allophyocyanin.Phycobilisome dissociation is inversely proportional to phosphate concentration (500 mM to 2 mM), and is essentially unaffected by protein concentration in the range used (30–200 μg/ml). Phycobiliprotein release occurs in the same order (phycoerythrin > R-phycocyanin > allophycocyanin) in the pH range 5.4–8.0.     

A new mechanism for adaptation to changes in light intensity and quality in the red alga,Porphyra perforata. I. Relation to State 1—State 2 transitions

Time courses of chlorophyll fluorescence and fluorescence spectra at 77 K after various light treatments were measured in the red alga, Porphyra perforata. Photosystem (PS) I or II light (light 1 or 2) induced differences in the fluorescence spectra at 77 K. Light 2 decreased the two PS II fluorescence bands (F-685 and F-695) in parallel, while light 1 preferentially increased F-695. Light 1 and 2 also produced different effects on the activities of PS I and II. Preillumination with light 1 increased PS II activity and decreased PS I activity. However, preillumination with light 2 decreased PS II activity with no effect on PS I activity. These results show that there are at least two mechanisms that can alter the transfer of light energy in P. perforata. The dark state in this alga was found to be State 2 and light 1 induced a State 2-State 1 transition which retarded the transfer of light energy from PS II to PS I. Light 2 induced another change (which we have called a State 2-State 3 transition) that was accompanied by a change only in PS II activity.     

Mechanism of the light state transition in photosynthesis. IV. Picosecond fluorescence spectroscopy of Anacystis nidulans and Porphyridium cruentum in state 1 and state 2 at 77 K

The sequential energy-transfer pathway through the phycobilin pigments to chlorophyll a was investigated as a function of the state transition in the cyanobacterium Anacystis nidulans and the red alga Porphyridium cruentum. The fluorescence decay kinetics of the phycobilin pigments and chlorophyll a were determined for cells frozen at 77 K in state 1 and state 2 using a single-photon timing fluorescence spectroscopy apparatus with picosecond resolution. Time-resolved 77 K fluorescence emission spectra were also obtained for both species in state 1 and state 2. In both A. nidulans and P. cruentum the transition to state 1 was accompanied by a large increase in the apparent fluorescent lifetime of chlorophyll a associated with PS II (emission peak at 695 nm). There were smaller increases in the lifetime of the terminal phycobilin emitter (685 nm) in both species and no change in phycocyanin (645 nm) or allophycocyanin (660 nm). Time-resolved spectra showed sequential emission from phycocyanin, allophycocyanin, the terminal phycobilin emitter and chlorophyll a. Spectral red shifts were observed with time for all emission peaks with the exception of the terminal phycobilin emitter. In A. nidulans this peak showed a small blue shift with time. The results are interpreted as evidence for an effective uncoupling of PS II chlorophyll a from subsequent energy transfer to PS I chlorophyll a upon transition to state 1. Our recently proposed model for the mechanism of the state transition in phycobilisome-containing organisms is discussed in terms of a decrease in the energy transfer overlap between PS II chlorophyll a and PS I chlorophyll a in state 1.     

Isolation and Function of Allophycocyanin B of Porphyridium cruentum

Allophycocyanin B was purified to homogeneity from the eukaryotic red alga Porphyridium cruentum. This biliprotein is distinct from the allophycocyanin of P. cruentum with respect to subunit molecular weights, and spectroscopic and immunological properties. The purified allophycocyanin B has a long wavelength absorption maximum at 669 nm at room temperature and at 675 nm at −196 C while the fluorescence emission maximum is at 673 nm at room temperature and 679 nm at −196 C. The emission spectrum of allophycocyanin shifted only 1 nm, from 659 to 660 nm, on cooling to −196 C, and was the same with allophycocyanin crystals as it was with pure solutions of the pigment. Phycobilisomes from P. cruentum have a major fluorescence emission band at 680 nm at −196 C which emanates from the small amount of allophycocyanin B present in the phycobilisomes. Light energy absorbed by the bulk of the biliprotein pigments is transferred to allophycocyanin B with high efficiency.     

Picosecond study of energy-transfer kinetics in phycobilisomes of Synechococcus 6301 and the mutant AN 112

Excitation-energy-transfer kinetics in isolated phycobilisomes from the cyanobacterium Synechococcus 6301 (Anacystis nidulans) and the mutant AN 112 (rods containing one hexameric C-phycocyanin unit only) was investigated by picosecond absorption and fluorescence techniques. The different chromophores in the phycobilisomes were selectively excited. A lifetime component of about 10 ps was found for both C-phycocyanin and allophycocyanin in both types of phycobilisomes. We assign these signals to a transfer of excitation energy from sensitizing (‘s’) to fluorescing (‘f’) chromophores within C-phycocyanin and allophycocyanin units. A 10 ps component was also observed in the anisotropy relaxation measurements. The anisotropy decay is attributed mainly to differently oriented transition dipole moments of ‘s’- and ‘f’-chromophores and partially to ‘f’ → ‘f’ transfer. An absorption recovery signal of τ ≈ 90 ps at λ ≤ 630 nm in phycobilisomes of Synechococcus 6301 is reduced to 40–50 ps in AN 112 phycobilisomes. This is rationalized in terms of a decreased rod → core transfer time in the shorter rods of AN 112. The 40–50 ps lifetime of fluorescence and absorption recovery in AN 112 phycobilisomes is assigned mainly to a rate-limiting transfer step between C-phycocyanin and the allophycocyanin core. A decay component of allophycocyanin τ ≈ 50 ps was observed both in absorption recovery measurements and in fluorescence decay. It is assigned to energy transfer to the terminal chromophores. The final emitter(s) of the phycobilisomes from AN 112 have fluorescence lifetimes of 1.9 and 1.3 ns. We find a good correlation in the fluorescence kinetics between the decay times of phycocyanin and allophycocyanin and the fluorescence risetimes of the terminal emitters.     

Fluorescence decay kinetics in phycobilisomes isolated from the bluegreen alga Synechococcus 6301

The picosecond fluorescence and energy-transfer kinetics of isolated phycobilisomes from Synechococcus 6301 were studied under low intensity excitation. Different combinations of excitation and emission wavelengths were used in order to monitor selectively the fluorescence of the pigments phycocyanin and allophycocyanin. The relatively long overall energy-transfer time of 120 ps from the phycocyanin rods to the allophycocyanin-core is rationalized in terms of the special structure of the rods being built up of several phycocyanin hexamers in this alga species. The fluorescence lifetime of the terminal chromophores in the core was determined to be 1.8–1.9 ns depending on the excitation wavelength. A fast decay component of 20 ± 10 ps which is most prominent at short emission wavelengths is assigned to arise mainly from energy transfer within the C-phycocyanin-units from ‘sensitizing’ to ‘fluorescing’ chromophores.     

Stabilization and modulation of the phycobilisome by calcium in the calciphilic freshwater red alga Bangia atropurpurea

The bangiophycean filamentous red alga Bangia atropurpurea is distributed in freshwater habitats such as littoral and splash zones of lakes or rapid currents distant from the sea. In these habitats, the distribution and growth of this alga appear to be related to hard water rich in calcium ions. To characterize the eco-physiological properties of this calciphilic red alga, we examined the effects of long-term and short-term Ca(2+) depletion on photosynthetic growth of the thallus and on the phycobilisome. Long-term culture experiments suggested that higher Ca(2+) concentrations (>50mgL(-1)) were required to sustain thallus growth and pigmentation of cells. In short-term Ca(2+)-depletion treatments, fluorescence derived from phycoerythrin (PE) fluctuated, although the absorption spectra of the thalli did not change. After 30 min of Ca(2+) depletion, the fluorescence lifetime of PE became markedly longer, indicating that the energy transfer from PE to phycocyanin (PC) was suppressed. The fluorescence lifetime of PE returned to its original value within a short time after 4h of Ca(2+) depletion, however, energy transfer from PE to PC was still suppressed. This suggested that the excitation energy absorbed by PE was quenched during prolonged Ca(2+) depletion. The efficient energy transfer from PC and allophycocyanin were unchanged during these treatments.     

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part II. In the isolated light harvesting complex (phycobilisomes)

The transfer of excitation energy between phycobiliproteins in isolated phycobilisomes has been observed on a picosecond time scale. The photon density of the excitation pulse has been carefully varied so as to control the level of exciton interactions induced in the pigment bed. The 530 nm light pulse is absorbed predominantly by B-phycoerythrin, and the fluorescence of this component rises within the pulse duration and shows a mean 1/e decay time of 70 ps. The main emission band, centred at 672 nm, is due to allophycocyanin and is prominent because of the absence of energy transfer to chlorophyll. Energy transfer to this pigment from B-phycoerythrin via R-phycocyanin produces a risetime of 120 ps to the fluorescence maximum. The lifetime of the allophycocyanin fluorescence is found to be about 4 ns using excitation pulses of low photon densities (10¹³ photons · cm^?2), but decreases to about 2 ns at higher photon densities. The relative quantum yield of the allophycocyanin fluorescence decreases almost 10 fold over the range of laser pulse intensities, 10¹³–10¹⁶ photons · cm^?2. Fluorescence quenching by exciton-exciton annihilation is only observed in allophycocyanin and could be a consequence of the long lifetime of the single exciton in this pigment.     

A New Mechanism for Adaptation to Changes in Light Intensity and Quality in the Red Alga Porphyra perforata: III. Fluorescence Transients in the Presence of 3-(3,4-Dichlorophenyl)-1,1-dimethylurea

In the red alga Porphyra perforata, the level of chlorophyll fluorescence in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) decreased during illumination of the thallus. The results showed that: (a) this decay was related to the photooxidative activity of photosystem I; (b) Q, the primary electron acceptor of photosystem II, became oxidized during the decay of the fluorescence; (c) reagents which inhibit the back reaction of photosystem II inhibited the decay.

From these results, it is suggested that, when conditions in the chloroplasts of this red alga become too oxidative, excess light energy can be converted to heat as a result of an accelerated back reaction of photosystem II. This may be one of the mechanisms by which this alga can cope with the high salt and high light conditions that can occur in its natural habitat.

     

Allophycocyanin B (λmax 671, 618 nm)

A hitherto undescribed red fluorescent phycobiliprotein (maximum emission at ∼ 680 nm), characterized by long wavelength absorption maxima in the visible region at 671 nm (ε=172000 M⁻¹·cm⁻¹ per monomer of mol. wt. 30600) and 618 nm, has been purified to homogeneity from a unicellular cyanobacterium, Synechococcus sp., and from a filamentous cyanobacterium, Anabaena variabilis. The name allophycocyanin B has been proposed for the new protein. A. variabilis allophycocyanin B is characterized by a native molecular weight of 89000 ± 5000 (in 0.05 M phosphate at pH 7.2), an isoelectric point of 5.09, and a subunit molecular weight, based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, of 15300. The protein contains one phycocyanobilin chromophore per subunit. In common with allophycocyanin from the same organism, allophycocyanin B does not contain either histidine or tryptophan. In other respects, the amino acid compositions of the two proteins are significantly different. Synechococcus sp. (Anacystis nidulans) allophycocyanin B gives two components of 16000 and 17000 mol. wt., of equal staining intensity, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Allophycocyanins B from both organisms cross-react with rabbit antisera directed against either Synechococcus sp. or Anabaena sp. allophycocyanin, but not with antisera against the phycocyanins of the same organisms. It is suggested that allophycocyanin B occupies a position between allophycocyanin and chlorophyll a in the energy transfer path from the accessory pigments to species of chlorophyll a with absorption maxima at λ>670 nm.     

Some properties of allophycocyanin from a thermophilic blue-green alga

Allophycocyanin was purified from the extremely thermophilic blue-green alga Synechococcus lividus. It was shown to be more stable to thermal or urea denaturation than allophycocyanin from a mesophilic organisms. Its amino acid composition and spectroscopic response to pH were investigated. An analysis was made of the relatively low fluorescence polarization of allophycocyanin compared to that of a comparable sized aggregate of the biliprotein, C-phycocyanin. A rather speculative conclusion was reached that suggests that the lower polarization of allophycocyanin may be caused by orientations or positioning of the chromophores that are more favorable for intra-protein energy transfer.     

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part I. In the intact alga

The wavelength-resolved fluorescence emission kinetics of the accessory pigments and chlorophyll a in Porphyridium cruentum have been studied by picosecond laser spectroscopy. Direct excitation of the pigment B-phycoerythrin with a 530 nm, 6 ps pulse produced fluorescence emission from all of the pigments as a result of energy transfer between the pigments to the reaction centre of Photosystem II. The emission from B-phycoerythrin at 576 nm follows a nonexponential decay law with a mean fluorescence lifetime of 70 ps, whereas the fluorescence from R-phycocyanin (640 nm), allophycocyanin (660 nm) and chlorophyll a (685 nm) all appeared to follow an exponential decay law with lifetimes of 90 ps, 118 ps and 175 ps respectively. Upon closure of the Photosystem II reaction centres with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination the chlorophyll a decay became non-exponential, having a long component with an apparent lifetime of 840 ps. The fluorescence from the latter three pigments all showed finite risetimes to the maximum emission intensity of 12 ps for R-phycocyanin, 24 ps for allophycocyanin and 50 ps for chlorophyll a.A kinetic analysis of these results indicates that energy transfer between the pigments is at least 99% efficient and is governed by an exp $\text{?At}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ transfer function. The apparent exponential behaviour of the fluorescence decay functions of the latter three pigments is shown to be a direct result of the energy transfer kinetics, as are the observed risetimes in the fluorescence emissions.     

Protection of Energy Transfer Process in Isolated Phycobilisome by "White Light" from Ultraviolet-B Induced Damage in the Cyanobacterium Spirulina Platensis

Effect of UV-B (1.9 W m^-2) alone or in combination with supplemental "white light". WL (20 W m^-2) exposure was studied on the energy transfer process of intact phycobilisomes isolated from Spirulina platensis. Exposure of UV-B or supplemental irradiation induced a decrease in room temperature fluorescence intensity and caused a shift towards shorter wavelengths. The low temperature fluorescence measurements showed that UV-B impairs energy transfer from phycocyanin to allophycocyanin B and the extent of damage may be reduced by the exposure to supplemental WL.     

Native and in vitro-associated phycobilisomes of Nostoc sp. Composition,energy transfer,and effect of antibodies

The relationship of the structure and function of the light-harvesting antennae in the blue-green alga Nostoc sp. was further elucidated by reconstitution experiments. Separated phycoerythrin-phycocyanin complexes and allophycocyanin fractions were reassociated as described earlier (Canaani, O., Lipschultz, C.A. and Gantt, E. (1980) FEBS Lett. 115, 225–229) into functional phycobilisomes with a 70% yield. Native and reassociated physobilisomes had molar ratios of about 1.4:1.1:1.0 of phycoerythrin:phycocyanin:allophycocyanim. Energy transfer was demonstrated by their fluorescence emission maximum at approx. 675 nm (20°C), and their excitation spectra (emission wavelength 680 nm) which reflected the contribution of the three constitutive phycobiliproteins. Scans of Coomassie blue-stained SDS-polyacrylamide gels showed that the polypeptide composition of native and reassociated phycobilisomes was virtually indistinguishable. Reassociation of phycobilisomes was dependent on the interaction of allophycocyanin and phycocyanin, because it could be blocked with antisera to phycocyanin and allophycocyanin, but not to phycoerythrin. In addition, reassociation did not occur when a 31 000 Da polypeptide, which is part of the phycoerythrin-phycocyanin complex, was reduced in size (by 4000 Da). These results suggest that at least two domains are required for functional reassociation of phycobilisomes involving phycocyanin and allophycocyanin.     

Photochromic pigments in akinetes and pigment characteristics of akinetes in comparison with vegetative cells of Anabaena variabilis

Since akinete germination is triggered by light and the action spectrum for this process has features in common with the spectra of the two photochromic pigments, phycochromes b and d, a search was made for the presence of these phycochromes in akinetes of the blue-green alga. Anabaena variabilis Kützing. Allophycocyanin-B was also looked for, since the action spectrum for akinete germination points to a possible participation of this pigment too. Isoelectric focusing was used for purification of the pigments. The different fractions were investigated for phycochromes b and d by measuring the absorbance difference spectra: for phycochrome b. 500 nm irradiated minus 570 nm irradiated, and for phycochrome d, 650 nm irradiated minus 610 nm irradiated. For determination of allophycocyanin-B. fourth derivative analysis of absorption spectra was made for some of the fractions from the isoelectric focusing column. Phycochrome b was also assayed for by measuring in vivo absorption difference spectra. The assays were positive for all three pigments. The complete photosynthetic pigment systems were also studied by in vivo fluorescence measurements on both akinetes and vegetative cells of Anabaena variabilis. Fluorescence emission and excitation spectra at selected emission wavelengths were measured at room temperature and liquid nitrogen temperature. The energy transfer from phycoerythrocyanin to phycocyanin is very efficient under all conditions, as is the energy transfer from phycocyanin to allophycocyanin at room temperature. At low temperature, however, phycocyanin is partly decoupled from allophycocyanin, particularly in the akinetes; the energy transfer from allophycocyanin to chlorophyll a is less efficient at low temperature in both types of cells, but especially in akinetes. Delayed light emission was measured for both types of cells and found to be very weak in akinetes compared to vegetative cells. From this study it would seem that akinetes lack an active photosystem II, although the 691 nm peak in the 570 nm excited low temperature fluorescence emission spectrum proves the presence of photosystem II chlorophyll, and also its energetic connection to the phycobilisomes.     

Analysis of photosynthetic pigments and chlorophyll fluorescence characteristics of different strains of Porphyra yezoensis

The levels of photosynthetic pigments and chlorophyll fluorescence of Porphyra yezoensis strains selected from high-light environments were investigated. Sutong and Sulian strains originated from the same high-light environment but were selected from different sites on the Yellow Sea coast of Jiangsu Province, China. In January (a low temperature period), the Sulian strain and the WT (a widely cultivated strain) had higher levels of chlorophyll a, phycoerythrin, phycocyanin, and allophycocyanin, and higher actual photochemical efficiency of PSII (ΔF/F _m′) than the Sutong strain. This indicated that Sulian and the WT may have better adaptation to low temperature. In March (an optimal temperature period), Sutong had higher levels of photosynthetic pigments and higher ΔF/F _m′ than the WT and Sulian strains. This suggested that Sutong had higher light use efficiency at optimal temperatures and that most energy absorbed by PSII was used for photosynthetic electron transport. The differing areas of origin of these strains may have resulted in these differences in temperature adaptation.     

Phycobilisomes from blue-green and red algae: isolation criteria and dissociation characteristics

A general procedure for the isolation of functionally intact phycobilisomes was devised, based on modifications of previously used procedures. It has been successful with numerous species of red and blue-green algae (Anabaena variabilis, Anacystis nidulans, Agmenellum quadruplicatum, Fremyella diplosiphon, Glaucosphaera vacuolata, Griffithsia pacifica, Nemalion multifidum, Nostoc sp., Phormidium persicinum, Porphyridium cruentum, P. sordidum, P. aerugineum, Rhodosorus marinus). Isolation was carried out in 0.75 molar K-phosphate (pH 6.8 to 7.0) at 20 to 23 C on sucrose step gradients. Lower temperature (4 to 10 C) was usually unfavorable resulting in uncoupling of energy transfer and partial dissociation of the phycobilisomes, sometimes with complete loss of allophycocyanin. Intact phycobilisomes were characterized by fluorescence emission peaks of 670 to 675 nanometers at room temperature, and 678 to 685 nanometers at liquid nitrogen temperature. Uncoupling and subsequent dissociation of phycobilisomes, in lowered ionic conditions, varied with the species and the degree of dissociation but occurred preferentially between phycocyanin and allophycocyanin, or between phycocyanin and phycoerythrin.     

Carotenoid-triggered energy dissipation in phycobilisomes of Synechocystis sp. PCC 6803 diverts excitation away from reaction centers of both photosystems

Cyanobacteria are capable of using dissipation of phycobilisome-absorbed energy into heat as part of their photoprotective strategy. Non-photochemical quenching in cyanobacteria cells is triggered by absorption of blue-green light by the carotenoid-binding protein, and involves quenching of phycobilisome fluorescence. In this study, we find direct evidence that the quenching is accompanied by a considerable reduction of energy flow to the photosystems. We present light saturation curves of photosystems’ activity in quenched and non-quenched states in the cyanobacterium Synechocystis sp. PCC 6803. In the quenched state, the quantum efficiency of light absorbed by phycobilisomes drops by about 30-40% for both photoreactions—P700 photooxidation in the photosystem II-less strain and photosystem II fluorescence induction in the photosystem I-less strain of Synechocystis. A similar decrease of the excitation pressure on both photosystems leads us to believe that the core-membrane linker allophycocyanin APC-L_CM is at or beyond the point of non-photochemical quenching. We analyze 77 K fluorescence spectra and suggest that the quenching center is formed at the level of the short-wavelength allophycocyanin trimers. It seems that both chlorophyll and APC-L_CM may dissipate excess energy via uphill energy transfer at physiological temperatures, but neither of the two is at the heart of the carotenoid-binding protein-dependent non-photochemical quenching mechanism.     

Further evidence for a phycobilisome model from selective dissociation, fluorescence emission, immunoprecipitation, and electron microscopy.

Phycobilisomes, isolated in 500 mM Sorensen's phosphate buffer pH 6.8 from the red alga, Porphyridium cruetum, were analyzed by selective dissociation at various phosphate concentrations. The results are consistent with a structural model consisting of an allophycocyanin core, surrounding by a hemispherical layer of R-phycocyanin, with phycoerythrin being on the periphery. Such a structure also allows maximum energy transfer. Intact phycobilisomes transfer excitation energy ultimately to a pigment with a fluorescence emission maximum at 675 nm. This pigment is presumed to be allophycocyanin in an aggreagated state. Uncoupling of energy transfer among the pigments, and physical release of the phycobiliproteins from the phycobilisome follow a parallel time-course; phycoerythrin is released first, followed by R-phycocyanin, and then allophycocyanin. In 55 mM phosphate buffer, the times at which 50% of each phycobiliprotein has dissociated are: phycoerythrin 40 min, R-phycocyanin 75 min, and allophycocyanin 140 min. The proposed arrangement of phycobiliproteins within phycobilisomes is also consistent with the results from precipitation reactions with monospecific antisera on intact and dissociated phycobilisomes. Anti-phycoertythrin reacts almost immediately with intact phycobilisomes, but reactivity with anti-R-phycocyanin and anti-allophycocyanin is considerably delayed, suggesting that the antigens are not accessible until a loosening of the phycobilsome structure occurs. Reaction wbilisomes, but is much more rapid in phycobilisomes of Nostoc sp. which contains 6-8 times more allophycocyanin. It is proposed that allophycocyanin is partially exposed on the base of isolated intact phycobilisomes of both algae, but that in P. cruentum there are too few accessible sites to permit a rapid formation of a precipitate with anti-allophyocyanin.