Time-Resolved Spectroscopy of Chlorophyll-a like Electron Acceptor in the Reaction Center Complex of the Green Sulfur Bacterium Chlorobium tepidum

The absorption changes of chlorophyll (Chl) a-like pigments(C670) were studied by ns-ms laser spectroscopy at 77 K in theuntreated and urea-treated homodimeric reaction center (RC)complex of the green sulfur bacterium Chlorobium tepidum. Theuntreated RC complex contained 9 molecules of C670 in additionto 41 molecules of Bchl a and 0.9 molecules of menaquinone-7per one primary electron donor Bchl a dimer (P840). Upon photo-oxidationof P840, C670 showed an absorption change of a red-shift withan isosbestic wavelength at 668 nm. The absorption change ofP840 decayed with time constants (t_1/e) of 55 and 37 ms at 283and 77 K, respectively, and was assigned to represent the chargerecombination between P840⁺ and FeS^–. In the urea-treatedRC complex, a bleach peaking at 670 nm with a shoulder peakat 662 nm, which is ascribable to the reduced primary electronacceptor A₀^–, was detected after the laser excitationin addition to the shift at 668 nm indicating the formationof the P840⁺A₀^– state. The P840⁺A₀^– state decayedwith a t_1/e of 43 ns at 77 K and produced a triplet state p840^Tdue to the suppression of the forward electron transfer. Theseresults indicate the two different types of C670 species inthe RC complex; the one peaking at 670 nm functions as A⁰, whilethe other peaking at 668 nm shows the electrochromic shift,which presumably functions as the accessory pigment locatedin the close vicinity of P840. (Received May 17, 1999; Accepted July 14, 1999)     

Reconstitution of the Photosystem I Secondary Quinone Acceptor (A1) in the P700-Fx Core Isolated from Synechococcus PCC 6301

By treating a F_A/F_B-depleted P700-F_x core from SynechococcusPCC 6301 with diethylether, most of the phylloquinone was removedwithout loss of P700. The 1 ms decay of P700⁺ in the originalcore was replaced by the 25 ns decay, which was interpretedas the backreaction occurring in a P700⁺     

Photoactive Photosystem I Particles with a Molar Ratio of Chlorophyll a to P700 of 9

Lyophilized photosystem I particles from spinach were treatedwith diethyl ether that contained various organic solvents withdifferent dielectric constants. More pigments were extractedas the dielectric constant of the solvent added to ether increased.The reaction-center chlorophylldimer, P700, was more resistantto extraction than the rest of the chlorophyll. Particles thatcontained only 6 chlorophylls in addition to P700 and the primaryelectron acceptor (A₀), in a single reaction-center unit, wereprepared by extraction with a mixture of ether and acetaldehyde.A distinct shoulder at 695 nm due to P700 or at 686 nm due toP700⁺ was observed in the absorption spectra of the reducedor oxidized particles, respectively, even at room temperature.No secondary acceptor phylloquinone remained in the particles.Stable charge separation was restored upon the addition of 2-amino-anthraquinone,even though the particles had the lowest molar ratio of chlorophyllto P700 of any reported particles. (Received February 20, 1995; Accepted May 8, 1995)     

Light absorption by phytoplankton: development of a matching parameter for algal photosynthesis under different spectral regimes

A spectral matching parameter (absorption efficiency, A_e) wasdeveloped to quantify the relationship between the light absorptionspectra of phytoplankton communities and the spectral irradianceof their ambient light field. A_e was defined as the ratio betweenthe amount of radiation absorbed by the phytoplankton in situand the amount absorbed in a spectrally flat light regime. Thisapproach was applied to our measurements of spectral absorptionfor the phytoplankton communities in six lakes in High ArcticCanada that spanned a range of bio-optical conditions. A_e valueswere calculated for the light spectrum down through the watercolumn and for 11 types of artificial light source. Spectralmatching varied among lakes and with depth. There was a significantlinear relationship between the relative change in A_e with depthand the diffuse attenuation coefficient K_d (r² = 0.52, P = 0.012for K_d for the 400–700 nm waveband; r² = 0.78, P = 0.0003for K_d at 440 nm). The tabulated values for the matching parameterA_e allow the comparison of photosynthesis versus irradiance(P versus E) curves among studies using different light sources.A_e estimates also facilitate the evaluation of chromatic adaptationin natural waters, and the calculation of spectrally adjusted,in situ primary production down through a water column fromP versus E relationships under a single spectral regime.     

Reaction of Reconstituted Acceptor Quinone and Dynamic Equilibration of Electron Transfer in the Photosystem I Reaction Center

Electron transfer mechanism in the spinach photosystem I reactioncenter that contains artificial quinones in place of phylloquinone(2-methyl-3-phytyl-1,4-naphthoquinone, vitamin K₁) as the secondaryelectron acceptor, Q_ø (or A₁) was discussed. (1) Mostof the reconstituted quinones oxidized the primary acceptorchlorophyll a, A⁰, at a rate rapid enough to compete againstthe charge recombination between A₀^– and the oxidizeddonor chlorophyll P700⁺. (2) The pathway of electron transferfrom the semiquinone^– varied depending on the redox potentialvalue of each semiquinone^– /quinone couple. Low potentialquinones reduced the tertiary acceptor iron-sulfur center, F_x,while the high potential ones reduced P700⁺ directly with a200-µs halftime. (3) The E_m value of each semiquinone^–/quinone couple in situ in the reaction center was estimatedto be shifted by about 0.3 volt to the negative side from theirhalf wave redox potential values that were measured polarographicallyin dimethylformamide. The shift seems to represent the acceptorproperty of the protein environment at the Q_ø site. (4)The E_m of reconstituted phylloquinone was estimated to be 50–80mV more negative than that of F_x. (5) The mechanism of efficientelectron transfer in the reaction center was discussed basedon the dynamic equilibria between the electron transfer componentsand on the estimated E_m values. (Received April 9, 1994; Accepted July 7, 1994)     

The rate of formation of P700+—A0 - in photosystem I particles from spinach as measured by picosecond transient absorption spectroscopy

Photosystem I particles containing 30–40 chlorophyll a molecules per primary electron donor P700 were subjected to 1.5 ps low density laser flashes at 610 nm resulting in excitation of the antenna chlorophyll a molecules followed by energy transfer to P700 and subsequent oxidation of P700. Absorbance changes were monitored as a function of time with 1.5 ps time resolution. P700 bleaching (decrease in absorbance) occurred within the time resolution of the experiment. This is attributed to the formation of ¹P700.^* This observation was confirmed by monitoring the rise of a broad absorption band near 810 nm due to chlorophyll a excited singlet state formation. The appearance of the initial bleach at 700 nm was followed by a strong bleaching at 690 nm. The time constant for the appearance of the 690 nm bleach is 13.7±0.8 ps. In the near-infrared region of the spectrum, the 810 nm band (which formed upon the excitation of the photosystem I particles) diminished to about 60% of its original intensity with the same 13.7 ps time constant as the formation of the 690 nm band. The spectral changes are interpreted as due to the formation of the charge separated state P700⁺—A₀ ^-, where A₀ is the primary electron acceptor chlorophyll a molecule.     

Femtosecond primary charge separation in Synechocystis sp. PCC 6803 photosystem I

The ultrafast (< 100 fs) conversion of delocalized exciton into charge-separated state between the primary donor P700 (bleaching at 705 nm) and the primary acceptor A₀ (bleaching at 690 nm) in photosystem I (PS I) complexes from Synechocystis sp. PCC 6803 was observed. The data were obtained by application of pump-probe technique with 20-fs low-energy pump pulses centered at 720 nm. The earliest absorbance changes (close to zero delay) with a bleaching at 690 nm are similar to the product of the absorption spectrum of PS I complex and the laser pulse spectrum, which represents the efficiency spectrum of the light absorption by PS I upon femtosecond excitation centered at 720 nm. During the first ∼ 60 fs the energy transfer from the chlorophyll (Chl) species bleaching at 690 nm to the Chl bleaching at 705 nm occurs, resulting in almost equal bleaching of the two forms with the formation of delocalized exciton between 690-nm and 705-nm Chls. Within the next ∼ 40 fs the formation of a new broad band centered at ∼ 660 nm (attributed to the appearance of Chl anion radical) is observed. This band decays with time constant simultaneously with an electron transfer to A₁ (phylloquinone). The subtraction of kinetic difference absorption spectra of the closed (state P700⁺A₀A₁) PS I reaction center (RC) from that of the open (state P700A₀A₁) RC reveals the pure spectrum of the P700⁺A₀⁻ ion-radical pair. The experimental data were analyzed using a simple kinetic scheme: An* [(PA₀)*A₁ P⁺A₀⁻A₁] P⁺A₀A₁⁻, and a global fitting procedure based on the singular value decomposition analysis. The calculated kinetics of transitions between intermediate states and their spectra were similar to the kinetics recorded at 694 and 705 nm and the experimental spectra obtained by subtraction of the spectra of closed RCs from the spectra of open RCs. As a result, we found that the main events in RCs of PS I under our experimental conditions include very fast (< 100 fs) charge separation with the formation of the P700⁺A₀⁻A₁ state in approximately one half of the RCs, the ∼ 5-ps energy transfer from antenna Chl* to P700A₀A₁ in the remaining RCs, and ∼ 25-ps formation of the secondary radical pair P700⁺A₀A₁⁻.     

Triplet States in a Photosystem I Reaction Center Complex. Inhibition of Radical Pair Recombination by Bipyridinium Dyes and Naphthoquinones

Flash-induced absorption changes between 400 and 570 nm werestudied in a P700-chlorophyll a-protein complex from the thermophiliccyanobacterium Synechococcus sp. that lacked the bound secondaryelectron acceptors A₂ and P430. A positive peak at 520 nm, whichincreased linearly with the flash intensity and independentlyof the redox state of P700, is ascribed to a carotenoid triplet.Bleaching at 430 nm, which decayed with a half time of about10 µs, was abolished when P700 was oxidized with ferricyanideand saturated at a high flash intensity, indicative of its dependenceon the primary photochemistry of photosystem I. Several bipyridinium dyes and naphthoquinones suppressed the10 µs decay of the 430 nm signal in a way indicating thatthe 10 µs component represents the P700 triplet generatedby the back reaction between the reduced primary electron acceptorand oxidized P700 and that the added oxidants oxidize the reducedprimary acceptor so rapidly that back electron transfer to oxidizedP700 is prevented. Our results also show that the primary electronacceptor is located in a lypophilic environment in the chlorophyll-bindingsubunits of the photosystem I complexes. In a reaction centercomplex containing the secondary electron acceptors, the exogenousoxidants accept electrons only via P430. (Received February 23, 1984; Accepted May 1, 1984)     

Dibromothymoquinone (DBMIB) Replaces the Function of QA at 77 K in the Isolated Photosystem II Reaction Center (D1-D2-Cytochrome b559) Complex: Difference Spectrum of the P680+(DBMIB-) State

Transient absorbance changes of the primary electron donor chlorophylla (P680) and acceptor pheophytin a (H) were measured at 77 Kby nanosecond laser spectroscopy in the D1-D2-cytochrome b₅₅₉photosystem II reaction center complex containing dibromomethylisopropylbenzoquinone (DBMIB). After the laser excitation of the reactioncenter in the presence of DBMIB, only the P680⁺-(DBMIB^-) statewas detected. P680⁺ mainly decayed with a t_1/e of 11 ms. Inthe absence of DBMIB, the excitation produced the P680⁺H^- radicalpair. The radical pair produced the triplet state (P680^T) witha t_1/e of 50 ns, and P680^T then decayed with a t_1/e of 2.1 ms.It was concluded that H_- was oxidized by DBMIB in a time rangefaster than the detecting time resolution (3.5 ns) even at 77K. The rapid oxidation of H^- by DBMIB was also confirmed bythe suppression of delayed fluorescence with a decay t_1/e of50 ns. The P680⁺(DBMIB^-)/P680(DBMIB) difference spectrum exhibiteda Q_y, band with a peak at 682 nm with a shoulder at 673 nm.The spectral shape was almost temperature insensitive between77 and 265 K. The feature of this spectrum in the wavelengthrange between 330 and 720 nm was compared with that of P680_T/P680or H^-/H at 77 K. (Received May 8, 1996; Accepted June 24, 1996)     

The Photoinhibition Site of Photosystem I in Isolated Chloroplasts under Extremely Reducing Conditions

Under anaerobic atmosphere where the gas phase was simply replacedby N₂, photo-inhibition of PS I of isolated spinach chloroplastswas insignificant. However, when dithionite was included inthe irradiation mixture, severe photoinhibition of the NADP⁺and the MV photo-reduction occurred. Neither P700 determinedby continuous illumination-induced difference spectroscopy,Fe-S centers determined by EPR under cryogenic temperatures,nor vitamin K-l determined by HPLC analysis were significantlydecreased under these photoinhibition conditions. Although photobleachingof antenna chlorophylls occurred to more or less extent, NADP⁺photoreduction activities were markedly depressed even undersaturating actinic light. The maximal amplitude of the flashinduced absorbance changes of P700 in ms range decreased almostin parallel with the loss of NADP⁺ photoreduction activity.These results indicate that although the Fe-S centers of thephotoinhibited chloroplasts were reducible by continuous illumination,to almost the same extents as that of the control chloroplasts,the efficiency of reduction by each flash was much lower thanthat of the control chloroplasts. The site of photoinhibitionin PS I under extremely reducing conditions is between A₀ andFe-S X. (Received July 28, 1988; Accepted October 31, 1988)     

Regulation of K+ current in human airway epithelial cells by exogenous and autocrine adenosine

The regulatory actions ofadenosine on ion channel function are mediated by four distinctmembrane receptors. The concentration of adenosine in the vicinity ofthese receptors is controlled, in part, by inwardly directed nucleosidetransport. The purpose of this study was to characterize the effects ofadenosine on ion channels in A549 cells and the role of nucleosidetransporters in this regulation. Ion replacement and pharmacologicalstudies showed that adenosine and an inhibitor of human equilibrative nucleoside transporter (hENT)-1, nitrobenzylthioinosine, activated K⁺ channels, most likely Ca²⁺-dependentintermediate-conductance K⁺ (I_K)channels. A₁ but not A₂ receptor antagonistsblocked the effects of adenosine. RT-PCR studies showed that A549 cellsexpressed mRNA for I_K-1 channels as well asA₁, A_2A, and A_2B but notA₃ receptors. Similarly, mRNA for equilibrative (hENT1 andhENT2) but not concentrative (hCNT1, hCNT2, and hCNT3) nucleosidetransporters was detected, a result confirmed in functional uptakestudies. These studies showed that adenosine controls the function ofK⁺ channels in A549 cells and that hENTs play a crucialrole in this process.

     

pH and Temperature Dependence of Tyrosine Z Decay Kinetics in Tris-Treated PSII Particles Studied by Time-Resolved EPR

The decay kinetics of tyrosine Z (Y_z) in Tris-treated PhotosystemII particles were measured by time-resolved EPR at differentpH values (pH 5.5 to 7.5) between 230 and 297 K. Y_z inducedby laser flashes decayed in a biphasic wav; t values were about100 ms in the fast phase and about 1 s in the slow one, respectively,at room temperature. The fast phase was attributed to a recombinationof charges on Y_z and Q^–_A. The activation energies forthe reaction of Y_z with Q^–_A between 245 and 297 K havebeen estimated to be more than 38 kJ mol^–1 at pH (>6.5)and less than 32 kJ mol^–1 at pH (<6.0), respectively.The activation energy gap between high pH (>6.5) and lowpH (<6.0) ranges was found to be E=4.4 kJ mol^–1. Judgingfrom the fact that pH 6.2 appears to be the border between thehigh and low pH regions, we suggest that the kinetics of Y_zis strongly influenced by the dissociation of the nearby histidineresidue. (Received February 21, 1997; Accepted June 24, 1997)     

Variation and Estimation of the Differential Absorption Coefficient of P-700 in Spinach Photosystem I Preparations

Treatment of spinach Photosystem I particles with detergentsinduced an apparent blue shift of chlorophyll at 690 nm inthe difference spectrum of P-700. As a consequence of the bandshift, the differential absorption coefficient of P-700 wasincreased from 63 to 87 mM^–1 .cm^–1. A curve waspresented, with which changes in the apparent differential absorptioncoefficient of P-700 can be estimated by measuring magnitudesof light-induced absorption decreases at 680 nm and 700 nm.The curve was compared with that for cyanobacterial preparationsand the mechanism of the detergent-induced band shift was discussed. (Received July 11, 1990; Accepted August 23, 1990)     

Preparation and Characterization of P700-Enriched Photosystem-I Complexes from the Thermophilic Cyanobacterium, Synechococcus sp.

P700 was enriched relative to antenna pigments by treating thethylakoid membranes from thermophilic cyanobacterium (Synechococcussp.) with diethylether. The total Chi a/P700 ratio of the membraneswas 147 but decreased to 13 after the treatment with ether whichhad been 70% saturated with water. Vitamin K₁ and carotenoidswere completely removed by this treatment. The low chlorophylla to P700 ratio was retained in photosystem I reaction-centercomplexes purified from the ether-extracted membranes with TritonX-100. The midpoint potential of P700 was considerably loweredby the ether treatment of the thylakoid membranes but was partiallyreversed by the further treatment of the extracted membraneswith Triton X-100. Photo-oxidation of P700 in the purified complexeswas extremely slow under continuous illumination, indicatingthat there is no significant leakage of electrons from the primaryelectron acceptor to other bound acceptors or O₂. The photooxidationof P700 was appreciably accerelated on addition of vitamin K₃(or K₁). (Received June 23, 1988; Accepted November 25, 1988)     

Factors controlling primary production in a hypertrophic lake (Hartbeespoort Dam, South Africa)

The physical factors controlling algal primary production weredemonstrated from data collected for a hypertrophic lake. A_maxranged between 12.4 and 5916 mg C m^–3 h^–1. Arealrates (A) varied between 46.9 and 3381 mg C m^–2 h^–1.The factors permitting and controlling production were subjectivelyseparated into two categories. In category 1, nutrients (N +P), which were in overabundance, permitted large standing cropsof Microcystis aeruginosa to develop (>1000 µg chla 1^–1). Wind patterns determined the dramatic spatialand temporal changes in algal standing crop which could dropto 2.7 µg chl a 1^–1. In category 2 were the factorswhich affected the rate processes. The buoyancy mechanism ofMicrocystis usually kept the alga in the euphotic zone. A powerrelationship (r = 0.92, n = 54) between A and A_max/_min showedthat with increasing phytoplankton vertical stratification,A_max was increasingly important in the integral. The saturationparameter I_K and photosynthetic capacity were temperature dependent.Variations of A were significantly related to changes in watercolumn stability (g cm cm^–2) because both axes of thephotosynthesis depth-profile were affected by stability changes.     

The Effects of Vitamin K3 and Dicumarol on the Plasma Membrane Redox System and H+ Pumping Activity of Zea mays L Roots Measured over a Long Time Scale

The effects of vitamin K₃ or dicumarol on plasma membrane boundhexacyanoferrate (III) and hexabromoiridate (IV) reductase activityand on the H⁺ pumping rate were investigated. Incubation withvitamin K₃ followed by intense rinsing stimulated the subsequentreduction of hexabromoiridate (IV) and hexacyanoferrate (III)as well as proton secretion induced by external electron-acceptors,while pretreatment with dicumarol inhibited proton secretioninduced by redox activity and hexacyanoferrate (III) reductionrate, but not the effects of hexabromoiridate (IV). A 30 minincubation in 0·2 mM K₃ or dicumarol, followed by rinsing,inhibited H⁺ secretion for about 2 d. Incubation for more than12 h in 0·1 mM dicumarol or 0·2 mM K₃ caused lethalinjury to the root cells. Key words: Vitamin K.₃, dicumarol, plasmalemma redox system, Zea mays L., proton pump     

Adenosine stimulates depolarization and rise in cytoplasmic [Ca2+] in type I cells of rat carotid bodies

During hypoxia, the level of adenosine in the carotid bodies increases as a result of ATP catabolism and adenosine efflux via adenosine transporters. Using Ca²⁺ imaging, we found that adenosine, acting via A_2A receptors, triggered a rise in cytoplasmic [Ca²⁺] ([Ca²⁺]_i) in type I (glomus) cells of rat carotid bodies. The adenosine response could be mimicked by forskolin (but not its inactive analog), and could be abolished by the PKA inhibitor H89. Simultaneous measurements of membrane potential (perforated patch recording) and [Ca²⁺]_i showed that the adenosine-mediated [Ca²⁺]_i rise was accompanied by depolarization. Ni²⁺, a voltage-gated Ca²⁺ channel (VGCC) blocker, abolished the adenosine-mediated [Ca²⁺]_i rise. Although adenosine was reported to inhibit a 4-aminopyridine (4-AP)-sensitive K⁺ current, 4-AP failed to trigger any [Ca²⁺]_i rise, or to attenuate the adenosine response. In contrast, anandamide, an inhibitor of the TWIK-related acid-sensitive K⁺-1 (TASK-1) channels, triggered depolarization and [Ca²⁺]_i rise. The adenosine response was attenuated by anandamide but not by tetraethylammonium. Our results suggest that adenosine, acting via the adenylate cyclase and PKA pathways, inhibits the TASK-1 K⁺ channels. This leads to depolarization and activation of Ca²⁺ entry via VGCC. This excitatory action of adenosine on type I cells may contribute to the chemosensitivity of the carotid body during hypoxia. O₂ sensing; A_2A receptor; cAMP; protein kinase A; TWIK-related acid-sensitive K⁺ channel     

FA/FB Protein from the Spinach Photosystem I Complex: Isolation in a Native State and Some Properties of the Iron-Sulfur Clusters

The F_A/F_B protein of the photosystem I complex was isolatedfrom spinach leaves in a native state by use of anaerobic systems.The protein contained 8.5 non-heme iron atoms and 8.0 acid-labilesulfur atoms per molecule, consistent with the current conceptthat it has two [4Fe-4S] clusters. Its absorption spectrum wasvery similar to those of bacterial-type ferredoxins. The ratioof the absorbance at 390 nm to that at 280 nm was 0.6, and themolar extinction coefficient at 390 nm was 32,000 M^–.cm^–.Theoxidation-reduction properties of the iron-sulfur clusters wereexamined by redox potentiometry and EPR spectroscopy. The twoclusters were distinguishable in terms of their oxidation-reductionmidpoint potentials; their E_m values were determined to be about-470mV and-560 mV, respectively. (Received July 16, 1990; Accepted October 8, 1990)     

Effects of Nitrate Supply and Deprivation and/or Defoliation on Potassium Absorption and Distribution in Ryegrass

Absorption and distribution of K⁺ by two ryegrass genotypesfrom steady-state supplies at low external concentrations werefollowed through periods of NO₃^– deprivation and afterdefoliation. Electron microprobe analysis was also used to examineK⁺ concentrations across the root cortex and the effects onvacuolar/cytoplasmic K⁺ concentrations in the cortex. In manyrespects, the two genotypes behaved in a similar way. Totalquantities of K+ absorbed were less than 50% of those previouslyrecorded for NO₃^–, and NO₃^– deprivation slightlyreduced cumulative K+ uptake. Unit absorption rates (_K) declinedwith time in the control plants, and removal of NO₃^– resultedin a progressive decline in _K, with an initial rapid responsewhich was independent of any effects of growth. In one genotypeabsorption rates did not recover but gradually did so in theother. Defoliation reduced _K only slightly for 6 d, thereafter_K, increased almost linearly so that values for plants withsustained NO₃^– supply were 2 ? those of entire plants.There was no evidence of an oscillatory cycle of K⁺ unit absorptionrates of the kind previously shown for NO₃^– during therecovery periods. Concentrations of K⁺ on both a dry matterand tissue water basis remained within a narrow range irrespectiveof treatment or time. The immediate decline in K⁺ unit absorption rate after NO₃^–withdrawal indicated that at least a proportion of their uptakeswere coupled. Calculated values for the length of the diffusivepathway indicated that the primary site of absorption into thesymplasm was either the epidermis or the outermost corticalcell. This was supported by evidence from the electron microprobeanalysis which indicated a strong gradient of K⁺ concentrationswhich declined from outer to inner cortex and was much steeperin plants with a sustained NO₃^– supply. Concentrationsin the cytoplasm of most cortical cells were generally greaterthan in the vacuoles: this difference was greater in low N plants.The changes in K+ in the cortex were related to the role ofK+ in the transport of NO₃^– in the xylem and effects onrecycling to the roots in the phloem. Key words: Cytoplasm, defoliation, diffusion, electron probe X-ray microanalysis, Lolium, nitrate, potassium, S.E.M.,, unit absorption, vacuole     

Transmembrane Ferricyanide Reduction and Membrane Properties in the Euryhaline Charophyte Lamprothamnium papulosum

The euryhaline charophyte Lamprothamnium papulosum has the abilityto reduce the extracellular electron acceptor ferricyanide (Fe³⁺Cy).Addition of 0.5 mol m^–3 Fe³⁺Cy stimulated H⁺-efflux ata rate of 0.8 H⁺/Fe³⁺Cy-reduced and increased K⁺-efflux intoa potassium-free medium at a rate of 0.66 K⁺/Fe³⁺Cy-reduced.0.5 mol m^–3 Fe³⁺Cy-induced maximum membrane depolarizationfor cells with resting potentials more negative than the diffusionpotential. The peak value of Fe³⁺Cy-induced depolarizationswas similar to the potential obtained by poisoning the electrogenicpump with DCCD. The value of maximum depolarization was determinedby (K⁺)₀. E_m tended to more positive values with increasing(K⁺)₀. Depolarizations coincided with a decrease in membraneresistance (R_m) from a resting value of 1.5 m² to 0.2 m² inthe depolarized state. Depolarization increased the sensitivityof the membrane potential (E_m) to (K⁺)₀. The resting potentialwas only slightly changed when (K⁺)₀ was increased from 3 to15 mol m^–3. The Fe³⁺ Cy-induced depolarized Em changedin a Nernstian fashion when (K⁺)₀ was increased. It is concludedthat Fe³⁺Cy reduction causes a net depolarization current flowacross the plasmalemma. The depolarization shifts the membranefrom a hyperpolarized pump dominated state into a depolarizedK⁺ diffusion state. Key words: Ferricyanide reduction, membrane potential, Lamprothamnium